M. Evans Biology Midterm Review Chapter 1 Introduction to Biology Vocabulary:

natural selection

population

homologous structure

analogous structure

vestigial structure

variation

Adaptation

xxx

biosphere ecosystem dependent variable

biodiversity homeostasis constant

species evolution theory

biology adaptation microscope

organism observation Gene

cell data

metabolism hypothesis

DNA experiment

System

evolution

Species

1.1 KEY CONCEPT Biologists study life in all its forms. Biology is the scientific study of all forms of life. Living things are found almost

everywhere on Earth, from very hot environments to very cold

environments and from the dry deserts to the ocean floor. The types of

living things found in a particular

region depend on which can survive there. Those living things that can

survive in an environment can differ greatly in size and shape.

All of the living things on Earth, and all of the places in which they live, make up the

biosphere. The variety of living things in a certain area, or across the entire biosphere,

is called biodiversity. Biodiversity can be measured in terms of the number of species

in an area or across the biosphere. Although there are several definitions of the term

species, you can think of a species as a certain type of living things that

can reproduce by interbreeding.

At least two million species exist on Earth. Each individual living

thing, no matter the species, is an organism. Every organism, from

any species, shares certain characteristics of life.

• All organisms are made of one or more cells. A cell is the basic unit of life on Earth.

• All organisms need chemical energy to carry out all of their cell functions. Energy

is used for metabolism, which is all of the chemical processes that

build up or break down materials.

• All organisms respond to physical factors, or stimuli, in their environment.

• Members of a species must be able to reproduce so that the species will survive.

When organisms reproduce they pass on their genetic material, which is called

DNA, to their offspring.

1.2 KEY CONCEPT Unifying themes connect concepts from many fields of biology

Several major concepts run through all of biology. These underlying ideas, or unifying

themes, demonstrate the relationships among all organisms and help to connect one

f ield of biology to others.

• Systems: A system is a group of related parts that interact to form a whole.

Groups of molecules can interact to form the cellular machinery for a

particular process. Groups of cells can interact to form an organ, such

as the heart. Groups

of organisms can interact within an ecosystem. An ecosystem is a

certain area in which living and nonliving things interact.

• Structure and Function: The biological function of a part of an

organism is directly related to that part’s structure. This relationship is

found in molecules within cells, among different types of cells, and

across different species. Different organisms

have different specialized structures that perform functions

specialized to that species.

• Homeostasis: All organisms must keep their internal conditions stable

in order to stay alive. Homeostasis is the maintenance of these

conditions. Homeostasis is necessary because the cells of all organisms

function best within a particular range

of conditions. If conditions vary too far from the ideal set, cells will not

be able to function normally. When internal conditions change, a

negative feedback system often acts to return the condition to normal.

• Evolution: Evolution is the process by which the genetic makeup of a

population changes over time. One way in which evolution occurs is

through the natural selection of genetic traits that give an individual an

advantage in a particular environment. Individuals with these

advantageous traits, or adaptations, are more likely to survive and

reproduce.

Heritability is the ability of a trait to be inherited, or passed down,

from one generation to the next.

Natural selection is a mechanism by which individuals that have

inherited beneficial adaptations produce more offspring on average than do other

individuals.

Natural selection is based upon four principles:

• Variation: the heritable differences that exist in every population

• Adaptation: a certain characteristic that allows an individual to survive better than

other individuals it competes against for resources

• Anatomy also provides insight into evolution. Homologous structures are features

that are similar in structure but appear in different organisms and have different

functions. Vestigial structures are remnants of organs or structures that had a

function in an early ancestor. Both homologous structures and vestigial structures

point to a shared ancestry among organisms that share them.

1.3 KEY CONCEPT Science is a way of thinking, questioning, and gathering evidence. Scientists do not use one scientific method, but all science is based on the same

principles: curiosity, critical and logical evaluation of evidence, and the

open and honest exchange of data. In any scientif ic inquiry, scientists

• Make observations: Scientists use their senses and various

measurement tools to collect information, or make observations, about

the world.

• Form hypotheses: Scientists propose answers to scientific questions, or form

hypotheses, based on observations they, or other scientists, made.

• Test hypotheses: Scientists devise methods of observing and

experimenting to test their predictions.

• Analyze data: Scientists use statistics to analyze data. This analysis

tells a scientist whether a hypothesis is supported or not supported by the

data.

• Evaluate results: Scientists evaluate both their own results and the

results from other scientists.

Scientists use experiments to test hypotheses. A scientific experiment

uses tightly controlled conditions to test a possible cause-and-effect

relationship between variables.

In an experiment, there are constants and two types of variables:

the independent variable and dependent variables.

• Independent variables: An independent variable is a condition that is manipulated

by a scientist to determine its effect on a dependent variable. An

independent variable is the “cause.”

• Dependent variables: A dependent variable is measured by a

scientist to study the effect of the independent variable. It is the

“effect” and depends on the independent variable.

• Constants: Factors that are controlled so that they do not change are constants.

A hypothesis is a proposed explanation for a single observation. A

scientific theory, however, is a proposed explanation for a wide range

of observations and experimental results, and is supported by a wide

range of evidence.

Chapter 2 CHEMISTRY OF LIFE Vocabulary

atom adhesion carbohydrate bond energy

element solution lipid equilibrium

compound solvent fatty acid activation energy

ion solute protein exothermic

ionic bond acid amino acid endothermic

covalent bond base nucleic acid catalyst

molecule pH chemical reaction enzyme

hydrogen bond monomer reactant substrate

cohesion polymer product

2.1 KEY CONCEPT All living things are based on atoms and their interactions

All matter, whether living or nonliving, is made of the same tiny building blocks, called

atoms. An atom is the smallest basic unit of matter. All atoms have

the same basic structure, composed of three smaller particles.

• Protons: A proton is a positively charged particle in an atom’s

nucleus. The nucleus is the dense center of an atom.

• Neutrons: A neutron has no electrical charge, has about the same

mass as a proton, and is also found in an atom’s nucleus.

• Electrons: An electron is a negatively charged particle found outside the nucleus.

Electrons are much smaller than either protons or neutrons.

Different types of atoms are called elements, which cannot be broken

down by ordinary chemical means. Which element an atom is depends on

the number of protons in the atom’s nucleus. For example, all hydrogen

atoms have one proton, and all oxygen atoms have 16 protons. Only about

25 different elements are found in organisms. Atoms of different elements

can link, or bond, together to form compounds. Atoms form bonds

in two ways.

• Ionic bonds: An ion is an atom that has gained or lost one or more

electrons. Some atoms form positive ions, which happens when an atom

loses electrons. Other

atoms form negative ions, which happens when an atom gains electrons.

An ionic bond forms through the electrical force between oppositely

charged ions.

• Covalent bonds: A covalent bond forms when atoms share one or

more pairs of electrons. A molecule is two or more atoms that are held

together by covalent bonds.

2.2 KEY CONCEPT Water’s unique properties allow life to exist on Earth. The structure of the water molecule gives water unique properties. Water is a polar

molecule, which means that it has a region with a slight negative charge

(the oxygen atom), and a region with a slight positive charge (the hydrogen

atoms). The oppositely charged regions of water molecules interact to

form hydrogen bonds. A hydrogen bond

is an attraction between a slightly positive hydrogen atom and a slightly

negative atom. Hydrogen bonds are responsible for several important

properties of water.

• High specific heat: Water resists changes in temperature; it must

absorb a large amount of heat energy to increase in temperature.

• Cohesion: The attraction among molecules of a substance is called cohesion.

Cohesion due to hydrogen bonds makes water molecules “stick” together.

• Adhesion: The attraction among molecules of different substances

is called adhesion. Water molecules “stick” to many other materials

because of hydrogen bonds.

Many compounds that are important for life dissolve in water. Water

is the largest component of cells’ interiors, and chemical reactions in

the cell take place in this

water. When one substance dissolves in another, a solution is formed.

The substance present in the greatest amount is called the solvent.

Substances that are present in

lower amounts and dissolve in the solvent are called solutes. Polar

solvents, such as water, dissolve polar molecules and ions.

When some substances dissolve in water they break up into ions. A

compound that releases a hydrogen ion (a proton) when it dissolves in

water is an acid. Bases are compounds that remove, or accept,

hydrogen ions. A solution’s acidity, or its hydrogen ion concentration, is

measured on the pH scale. An acid has a low pH (pH below 7)

and a high hydrogen ion concentration. A base has a high pH (pH above

7) and a low hydrogen ion concentration. Organisms must maintain a

stable pH. Even a small change

in pH can disrupt many biological processes.

2.3 KEY CONCEPT Carbon-based molecules are the foundation of life. Carbon atoms are the basis of most molecules that make up living things. Many

carbon-based molecules are large molecules called polymers that are made of

many smaller, repeating molecules called monomers. There are four

main types of carbon-based molecules in living things.

• Carbohydrates include sugars and starches, and are often broken down as a source

of chemical energy for cells. Some carbohydrates are part of cell

structure, such as cellulose, which makes up plant cell walls.

• Lipids include fats and oils and, like carbohydrates, are often broken down as

a source of chemical energy for cells. One type of lipid, called a

phospholipid, makes up most of all cell membranes.

• Proteins have a large number of structures and functions. Some

proteins are needed for muscle movement; another protein, called

hemoglobin, transports oxygen in blood. Another type of proteins,

called enzymes, speed up chemical reactions in cells.

• Nucleic acids are molecules that store genetic information and build proteins.

DNA stores genetic information in cells, and RNA helps to build the

proteins for which DNA codes.

2.4 KEY CONCEPT Life depends on chemical reactions. At the most fundamental level, every process that takes place in an organism depends

on chemical reactions. In a chemical reaction, substances are

changed into different substances by the breaking and forming of

chemical bonds. The substances that are present at the start of a

chemical reaction, and are changed by the reaction, are called

reactants. The substances that are formed by a chemical reaction

are the products.

Chemical bonds must be broken in the reactants and new ones must be

formed in the products. Energy must be added to break chemical bonds.

In contrast, energy is always released when new bonds form. The amount

of energy needed to break a bond, or the amount of energy released

when a bond forms, is called bond energy.

All chemical reactions require the input of at least a small amount of

energy in order for bonds to break in the reactants and for the

reaction to start. The energy needed to start a chemical reaction is

the activation energy. In general, there are two types of

energy changes that can occur during a chemical reaction.

• Exothermic reaction: An exothermic chemical reaction releases more energy than

it absorbs. The bonds that are broken in the reactants of an exothermic reaction

have a higher bond energy than the new bonds that form in the products.

Energy is usually released as heat or light.

• Endothermic reaction: An endothermic chemical reaction absorbs

more energy than it releases. The bonds that are broken in the

reactants of an endothermic reaction have a lower bond energy than the

new bonds that form in the products. The energy that is absorbed

makes up for the difference.

Chapter 3 CELL STRUCTURE AND FUNCTION

Vocabulary

cell theory vacuole concentration gradient

cytoplasm lysosome osmosis

organelle centriole isotonic

prokaryotic cell cell wall hypertonic

eukaryotic cell chloroplast hypotonic

cytoskeleton cell membrane facilitated diffusion

nucleus phospholipid active transport

endoplasmic reticulum fluid mosaic model endocytosis

ribosome selective permeability phagocytosis

Golgi apparatus receptor exocytosis

vesicle passive transport

mitochondrion diffusion

3.1 KEY CONCEPT Cells are the basic unit of life. The invention of the microscope in the late 1500s revealed to early scientists a whole

new world of tiny cells. Most cells are so small that they cannot be seen

without a microscope. The discoveries of scientists from the 1600s

through the 1800s led to the cell theory, which is a unifying concept of

biology. The cell theory has three major principles:

• All organisms are made of cells.

• All existing cells are produced by other living cells.

• The cell is the most basic unit of life.

All cells can be divided into two major groups: prokaryotic cells or eukaryotic cells.

The main differences between the two kinds of cells are in their structure:

• Eukaryotic cells have a nucleus defined by a membrane, while

prokaryotic cells have no nucleus.

• In eukaryotic cells, the DNA, or genetic information, is found in the

nucleus. In prokaryotic cells, the DNA is found in the cytoplasm, the

jellylike substance

that fills both types of cells.

• Eukaryotic cells have organelles, structures that perform jobs for a

cell. Most organelles are surrounded by membranes. Prokaryotic cells do

not have organelles surrounded by membranes.

Prokaryotic cells make up organisms called prokaryotes. All

prokaryotes are tiny and consist of single cells. Bacteria are

prokaryotic cells. Eukaryotic cells make

up eukaryotes. You are a eukaryote, as are plants and some types

of single-celled organisms. All multicellular organisms, or organisms

that have many cells, are eukaryotes.

3.2 KEY CONCEPT Eukaryotic cells share many similarities Plants, animals, and some single-celled organisms are eukaryotes. Eukaryotic cells have an organized internal structure and organelles that are surrounded by membranes. Organelles look different from each other and have different functions. Several have a specific job in making and processing proteins so that a cell can live, function, and reproduce. Plant and animal cells have a lot of the same parts, but

a few of their parts are different.

The list below tells you what each cell part does. Part Job and Description

nucleus double membrane layer that stores and protects DNA; includes the nucleolus, a dense region where ribosomes are assembled.

endoplasmic reticulum (ER)

network of thin folded membranes that help produce proteins and lipids; two kinds of ER: smooth and rough

ribosomes tiny round organelles that link amino acids together to form proteins; may be in the cytoplasm or on the ER, which makes it look rough

Golgi apparatus stacked layers of membranes that sort, package, and deliver proteins

vesicles little sacs that carry different molecules where they’re needed; made and broken down as needed by the cell

mitochondria bean-shaped organelles that release energy from sugars for the cell centrioles found in animal cells; organize microtubules to form cilia and flagella

vacuoles sacs that store materials for the cell; the materials might be water, food molecules, ions, and enzymes

cell walls strong layer that protects, supports, and gives shape to plant cells; not found in

animal cells

chloroplasts change energy from the sun into chemical energy for the plant; not found in animal

cells

cytoplasm jellylike substance that fills a cell cell membrane double-layer of phospholipids that forms a boundary between a cell and its

surrounding environment lysosomes membrane-bound organelles that contain enzymes

3.3 KEY CONCEPT The cell membrane is a barrier that separates a cell from the external

environment. The cell membrane forms a boundary that separates the inside of a cell from the outside

environment. It plays an active role by controlling the passage of materials into and

out of a cell and by responding to signals. The membrane is made of

molecules called phospholipids, which consist of three parts: (1) a

charged phosphate group; (2) glycerol; (3) two fatty acid chains.

The structure of phospholipids gives them distinct chemical properties. The

phosphate group and glycerol form a polar “head.” The fatty acid chains form

a nonpolar “tail.” Cells are both surrounded by water and contain water. In

the cell membrane, phospholipids

form a double layer, or bilayer. In this way, the polar heads interact with

the polar water molecules outside and inside a cell. The nonpolar tails are

sandwiched together inside

the bilayer, away from the water.

The cell membrane also includes a variety of molecules that give the membrane properties

it would not otherwise have.

• Cholesterol molecules make the membrane stronger.

• Proteins help molecules and ions cross the membrane and can act as receptors,

proteins that detect a signal and respond by performing an action.

• Carbohydrates help cells distinguish one cell type from another.

The fluid mosaic model describes the characteristics and makeup of the

cell membrane. The phospholipids can slip past each other like a fluid.

The membrane is made up of

many different molecules, like a mosaic.

The cell membrane has a property called selective permeability,

which means that it allows some molecules to cross but blocks

others. Selective permeability helps a cell maintain homeostasis.

Cells have receptors both in the cell membrane and inside the cell.

Receptors help cells communicate with other cells and respond to the

environment.

• Membrane receptors bind to signals that cannot cross the cell membrane.

They cross the membrane and transmit a message inside the cell by

changing shape.

• Intracellular receptors are located inside a cell and bind to molecules

that can cross the cell membrane. They may interact with DNA to control

certain genes.

3.4 KEY CONCEPT Materials move across membranes because of concentration differences. Cells are continuously exchanging materials with their environment across the cell

membrane. Passive transport is the movement of molecules across a

cell membrane that does not require energy input by the cell.

Diffusion, a type of passive transport, is

the movement of molecules from an area of higher concentration to an

area of lower concentration. This difference in concentration from one area

to another is called a concentration gradient. When a molecule diffuses,

it can be described as moving down

its concentration gradient.

Not all molecules can cross the cell membrane. Facilitated diffusion is

the diffusion of molecules across a membrane through transport proteins,

proteins that form channels across the membrane.

Diffusion is a result of the natural energy of molecules. When molecules are

in solution, they collide and scatter. Over time, these molecules will become

evenly spread throughout the solution, which means that the molecules

have reached dynamic equilibrium. The molecules continue to move, but

their concentration remains equal.

Water also moves from a higher water concentration to a lower water

concentration. The diffusion of water is called osmosis. The higher the

concentration of dissolved particles that are in a solution, the lower the

concentration of water molecules. The reverse is also true. That is, the

lower the concentration of dissolved particles that are in a solution, the

higher the concentration of water molecules.

Scientists have developed terms to compare the concentration of

solutions with some reference point. Here, our reference point is the

concentration of particles in a cell.

• An isotonic solution has the same concentration of dissolved particles as a cell. A

cell in an isotonic solution will not change.

• A hypertonic solution has a higher concentration of dissolved particles than a cell.

A cell in a hypertonic solution will shrivel.

• A hypotonic solution has a lower concentration of dissolved particles than a cell. A

cell in a hypotonic solution will swell.

3.5 KEY CONCEPT Cells use energy to transport materials that cannot diffuse across the

membrane. Cells use active transport to obtain materials they need that they could not get by means

of diffusion or facilitated diffusion. Active transport is the movement of a

substance against its concentration gradient by the use of transport

proteins embedded in the cell membrane and chemical energy. The

transport proteins used in active transport are often called pumps. Most

often, the chemical energy that is used comes from breakdown of a

molecule called ATP. A cell may use this energy directly or indirectly.

• The sodium-potassium pump directly uses energy from the

breakdown of ATP to pump two potassium ions into a cell for every

three sodium ions it removes from the cell.

• The proton pump indirectly uses energy from the breakdown of ATP to

remove hydrogen ions (protons) from a cell. This action creates a charge

gradient, which is a form of stored energy. This charge gradient can then be

used to drive other pumps

to transport molecules such as sucrose.

Some molecules are too large to be transported through proteins. These

molecules can be moved in vesicles, so they never actually have to cross

the membrane. The movement of these vesicles also requires energy

from a cell.

• Endocytosis is the process of taking liquids or large molecules into a

cell by engulfing them in a vesicle. During endocytosis, the cell membrane

makes a pocket around the material to be brought in. The pocket pinches

together around the

material and breaks off, forming a vesicle, inside the cell. This vesicle then joins

with a lysosome, which breaks down the contents if needed and recycles the vesicle.

Phagocytosis is a type of endocytosis and means “cell eating.”

• Exocytosis is the process of releasing materials from a cell by fusion of a vesicle

with the cell membrane. In this process, a vesicle forms around select

materials. The vesicle is moved to the cell surface, and it fuses with the cell

membrane, releasing

the contents. Exocytosis plays an important role in releasing hormones and

digestive enzymes and in transmitting nerve impulses.

Chapter 5 CELL GROWTH AND DIVISION

Vocabulary

cell cycle prophase metastasize

mitosis metaphase carcinogen

cytokinesis anaphase asexual reproduction

chromosome telophase binary fission

histone growth factor tissue

chromatin apoptosis organ

chromatid cancer organ system

centromere benign cell differentiation

telomere malignant stem cell

5.1 KEY CONCEPT Cells have distinct phases of growth, reproduction, and normal functions. Cells have a regular pattern of growth, DNA duplication, and division that is called the

cell cycle. In eukaryotic cells, the cell cycle consists of four stages: gap 1 (G1), synthesis

(S), gap 2 (G2), and mitosis (M). G1, S, and G2 are collectively called interphase.

• During gap 1 (G1), a cell carries out its normal functions. Cells may also

increase in size and duplicate their organelles during this stage. Cells must pass a checkpoint before they can progress to the S stage.

• During synthesis (S), cells duplicate their DNA. At the end of the S

stage, a cell contains two complete sets of DNA.

• During gap 2 (G2), a cell continues to grow and carry out its normal

functions. Cells must pass a checkpoint before they can progress to the M

stage.

• The mitosis (M) stage consists of two processes. Mitosis divides the

cell nucleus, creating two nuclei that each have a full set of DNA.

Cytokinesis divides the cytoplasm and organelles, resulting in two

separate cells.

Cells divide at different rates to accommodate the needs of an

organism. For example, cells that receive a lot of wear and tear, such

as the skin, have a life span of only a few days. Cells making up many

of the internal organs have a life span of many years.

Cells tend to stay within a certain size range. To maintain a suitable size

range, cell growth must be coordinated with cell division. Cell volume

increases much faster than cell surface area for most cells. All materials

that a cell takes in or secretes enter and

exit through the membrane. The cell’s surface area must be large

enough relative to its overall volume in order for the cell to get its

necessary materials. Therefore, most cells tend to be very small.

5.2 KEY CONCEPT Cells divide during mitosis and cytokinesis. During interphase, a cell needs access to its DNA to make use of specific genes and to

copy the DNA. During mitosis, however, the DNA must be condensed and

organized so that it can be accurately divided between the two nuclei.

DNA is a long polymer made

of repeating subunits called nucleotides. Each long continuous thread of DNA is called

a chromosome, and each chromosome has many genes.

During interphase, DNA wraps around organizing proteins called

histones and is loosely organized as chromatin, which looks sort of like

spaghetti. As a cell prepares for mitosis, however, the DNA and histones

start to coil more and more tightly until

they form condensed chromosomes. Each half of the duplicated chromosome is called a

chromatid. Both chromatids together are called sister chromatids, which are attached at

a region called the centromere. The ends of DNA molecules form

telomeres, structural units that do not code for proteins. Telomeres help

prevent chromosomes from sticking

to each other.

Mitosis is a continuous process, but scientists have divided it into

phases for easier discussion.

• During prophase the chromatin condenses into chromosomes,

the nuclear envelope breaks down, and spindle fibers start to

assemble.

• During metaphase spindle fibers align the chromosomes along the

middle of the cell.

• During anaphase spindle fibers pull the sister chromatids away from

each other and toward opposite sides of the cell.

• During telophase, the nuclear membranes start to form around each

set of chromosomes, the chromosomes start to uncoil, and the spindle

fibers fall apart.

• Cytokinesis divides the cytoplasm into two separate cells. In animal

cells, the cell membrane pinches together. In plant cells, a cell plate

forms between the two

nuclei. It will eventually form new cell membranes for the cells and a new cell wall.

5.3 KEY CONCEPT Cell cycle regulation is necessary for healthy growth. The cell cycle is regulated by both external and internal factors. External factors come

from outside the cell. These include cell–cell contact, which prevents

further growth of normal cells, and chemical signals called growth factors.

Growth factors stimulate

cells to divide. Most cells respond to a combination of growth factors, not just one.

Some growth factors affect many different types of cells. Others

specifically affect one cell type. Internal factors come from inside the cell.

Very often, an external factor triggers the activation of an internal factor.

A cyclin is a type of internal factor. It activates kinases, which in turn, add

a phosphate group to other molecules that help

drive the cell cycle forward.

Cells not only regulate growth, but also death. Apoptosis is

programmed cell death. Apoptosis plays important roles in

development and metamorphosis.

When a cell loses control over its cycle of growth and division, cancer

may result. Cancer cells can continue to divide despite cell–cell contact or

a lack of growth factors. Cancer cells form disorganized clumps of cells

called tumors. Benign tumors tend to remain clumped together and may

be cured by removal. Malignant tumors have cells

that break away, or metastasize, and spread to other parts of the body,

forming new tumors. Malignant tumors are more difficult to treat than

benign tumors. Radiation therapy and chemotherapy are common

treatments for cancer. However, both treatments kill healthy cells as well

as cancer cells.

Cancer cells can arise from normal cells that have experienced damage

to their genes involved in cell cycle regulation. Damage may arise from

inherited errors in genes,

from mutations carried by viruses, and from carcinogens. Carcinogens

are substances known to produce or promote the development of cancer.

These include substances such

as tobacco smoke and other air pollutants.

5.4 KEY CONCEPT Cells work together to carry out complex functions. Your body began as a single fertilized egg. Since that time, your cells have not only

gone through millions of cell divisions, but those cells have also undergone the process

of cell differentiation by which unspecialized cells develop into their

mature form and function. Groups of cells that work together to

perform a similar function make

up tissues. Groups of tissues that work together to perform a similar function make up

organs. Groups of organs that carry out related functions make up organ systems.

The interaction of multiple organ systems working together helps

organisms maintain homeostasis.

An organism’s body plan is established in the very earliest stages of

embryonic development. In both animals and plants, a cell’s location

within the embryo helps determine how that cell will differentiate. In

animals, cells migrate to specific areas that will determine how they

specialize. Plant cells cannot readily migrate because of their

cell walls. However, the cells remain very adaptable throughout the life of the plant.

Stem cells are a unique type of body cell characterized by three features:

• They divide and renew themselves for long periods of time.

• They remain undifferentiated in form.

• They can develop into a variety of specialized cell types.

Because of their ability to develop into other types of cells, stem cells

offer great hope for curing damaged organs and currently untreatable

diseases. However, they also raise many ethical concerns. Stem cells

can be categorized by their developmental potential,

as totipotent, pluripotent, or multipotent. Stem cells can also be classified by origin,

as adult or embryonic.

Chapter 6

Vocabulary:

somatic cell egg genotype

gamete polar body phenotype

homologous chromosome trait dominant

autosome genetics recessive

sex chromosome purebred Punnett square

sexual reproduction cross monohybrid cross

fertilization law of segregation testcross

diploid gene dihybrid cross

haploid allele law of independent assortment

meiosis homozygous probability

gametogenesis heterozygous crossing over

sperm genome genetic linkage

6.1 KEY CONCEPT Gametes have half the number of chromosomes that body cells have.

Your body is made of two basic cell types. One basic type are somatic cells, also called

body cells, which make up almost all of your tissues and organs. The second basic

type are germ cells, which are located in your reproductive organs. They are the cells

that will undergo meiosis and form gametes. Gametes are sex cells. They

include eggs and sperm cells.

Each species has a characteristic number of chromosomes per cell. Body cells are diploid, which means that

each cell has two copies of each chromosome, one from each parent. Gametes are haploid, which means that

each cell has one copy of each chromosome. Gametes join together during fertilization, which is the actual

fusion of egg and sperm, and restores the diploid number.

The diploid chromosome number in humans is 46. Your cells need both copies of each chromosome to function

properly. Each pair of chromosomes is called homologous. Homologous chromosomes are a pair of

chromosomes that have the same overall appearance and carry the same genes. One comes from the mother, and

one comes from the father. Thus, one chromosome from a pair of homologous chromosomes might carry a gene

that codes for green eye color, while the other carries a gene that codes for brown eye color. For reference, each

pair of homologous chromosomes has been numbered, from largest to smallest. Chromosome pairs 1 through 22

are autosomes. Autosomes are chromosomes that contain genes for characteristics not directly related to sex.

The two other chromosomes are sex chromosomes, chromosomes that directly control the development of sexual

characteristics. In humans, a woman has two X chromosomes, and a man has an

X and a Y chromosome. The Y chromosome is very small and carries few genes.

Meiosis is a form of nuclear division that reduces chromosome number from diploid to haploid. Each haploid cell

produced by meiosis has 22 autosomes and 1 sex chromosome.

6.2 KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result

in haploid cells.

Meiosis occurs after a cell has already duplicated its DNA. Cells go through two rounds of

cell division during meiosis. During the first round, meiosis I, homologous chromosomes

separate from each other. During the second round, meiosis II, sister chromatids separate

from each other. Meiosis produces genetically unique haploid cells that will go through

more steps to form mature gametes.

Meiosis is a continuous process, but scientists have divided it into phases.

• Prophase I: The nuclear membrane breaks down, and the spindle fibers assemble.

The duplicated chromosomes condense, and homologous chromosomes pair up.

The sex chromosomes also pair together.

• Metaphase I: The homologous chromosome pairs randomly line up along the middle

of the cell. Because this is random, there are a mixture of chromosomes from both

parents on each side of the cell equator.

• Anaphase I: The paired homologous chromosomes separate from each other and

move to opposite sides of the cell.

• Telophase I: The nuclear membrane forms in some species, the spindle fibers break

apart, and the cell undergoes cytokinesis. Each cell has 23 duplicated chromosomes.

• Prophase II: The nuclear membrane breaks down if necessary and the spindle fibers

assemble again.

• Metaphase II: The chromosomes line up along the middle of the cell.

• Anaphase II: The sister chromatids are pulled apart from each other and move to

opposite sides of the cell.

• Telophase II: The nuclear membranes form again, the spindle fibers break apart, and

the cell undergoes cytokinesis.

The haploid cells produced by meiosis are not capable of fertilization. They must undergo

additional steps to form mature gametes. During gametogenesis, sperm cells—the

male gametes—and eggs—the female gametes—become specialized to carry out their

functions. Sperm cells lose much of their cytoplasm and develop a tail. Eggs receive

almost all of the cytoplasm during the divisions in meiosis. This is necessary for an

embryo to have all the materials needed to begin life after fertilization. The smaller

cells produced by meiosis in the female are called polar bodies, and they are eventually

broken down.

6.3 KEY CONCEPT Mendel’s research showed that traits are inherited as discrete units.

Traits are inherited characteristics, and genetics is the study of the biological inheritance

of traits and variation. Gregor Mendel, an Austrian monk, first recognized that traits are

inherited as discrete units. We call these units genes. Mendel conducted his experiments

with pea plants, which were an excellent choice because they are easily manipulated,

produce large numbers of offspring, and have a short life cycle. Mendel made three

important decisions that helped him to see patterns in the resulting offspring.

• Use of purebred plants: Mendel used pea plants that had self-pollinated for so

long that they had become genetically uniform, or purebred. This meant that the

offspring looked like the parent plant. Because of this characteristic, Mendel knew

that any differences he observed in the offspring were the result of his experiments.

• Control over breeding: At the start of his experiments, Mendel removed the male

flower parts from the pea plants. He then pollinated the female flower part with

pollen from a plant of his choosing, which produced offspring referred to as the

F1 generation.

• Observation of “either-or” traits: Mendel studied seven traits that appeared in only

two forms. For example, flowers were white or purple; peas were wrinkled or round.

Mendel observed that when he mated, or crossed, a purple-flowered plant with a

white-flowered plant, for example, all of the F1 offspring had purple flowers. Mendel

next allowed the F1 offspring to self-pollinate; that is, the plant mated with itself. In the

resulting offspring, the F2 generation, approximately three-fourths of the flowers were

purple and one-fourth were white. Mendel continued to find this 3:1 ratio for each of his

crosses, regardless of the specific trait he was examining.

Based on his results, Mendel concluded that traits are inherited as discrete units. He

also developed what is known as Mendel’s first law, or the law of segregation. This

law states the following:

• Organisms inherit two copies of each unit (gene), one from each parent.

• The two copies separate, or segregate, during gamete formation. As a result,

organisms donate only one copy of each unit (gene) in their gametes.

6.4 KEY CONCEPT Genes encode proteins that produce a diverse range of traits.

A gene is a segment of DNA that tells the cell how to make a particular polypeptide. The

location of a gene on a chromosome is called a locus. A gene has the same locus on

both chromosomes in a pair of homologous chromosomes. In genetics, scientists often

focus on a single gene or set of genes. Genotype typically refers to the genetic makeup

of a particular set of genes. Phenotype refers to the physical characteristics resulting

from those genes.

An alternative form of a gene is an allele. The pea plants that Mendel worked with had

two alleles for each gene. For example, there was an allele for round peas and an allele for

wrinkled peas. Genes are not limited to two alleles, however. Some genes are found in

many different forms throughout a population.

Your cells have two alleles for each gene regardless of how many alleles are present in

a population. Suppose there were 64 alleles of a hair color gene present in the human

population. Your cells would only have two of those alleles, one from your mother and

one from your father. If the two alleles are the same, they are homozygous. If the two

alleles are different, they are heterozygous.

Some alleles are dominant over others.

• A dominant allele is expressed when two different alleles or two dominant alleles

are present. Therefore, both homozygous dominant and heterozygous genotypes

can produce the dominant phenotype.

• A recessive allele is expressed only when both alleles are recessive. Therefore, only

the homozygous recessive genotype can produce the recessive phenotype.

Alleles may be represented using letters. Uppercase letters represent dominant alleles.

Lowercase letters represent recessive alleles.

6.5 KEY CONCEPT The inheritance of traits follows the rules of probability.

The possible genotypes resulting from a cross can be predicted using a Punnett square.

A Punnett square is a grid. The axes are labeled with the alleles of each parent

organism. The grid boxes show all of the possible genotypes of the offspring resulting

from those two parents.

A monohybrid cross is used when studying only one trait. A cross between a

homozygous dominant organism and a homozygous recessive organism produces

offspring that are all heterozygous and have the dominant phenotype. A cross

between two heterozygous organisms results in a 3:1 phenotypic ratio in the offspring,

where three-fourths have the dominant phenotype and one-fourth have the recessive

phenotype. The genotypic ratio resulting from this cross is 1:2:1 of homozygous

dominant:heterozygous:homozygous recessive.

A testcross is a cross between an organism with an unknown genotype (dominant

phenotype) and an organism with the recessive phenotype. If the organism with the

unknown genotype is homozygous dominant, the offspring will all have the dominant

phenotype. If it is heterozygous, half the offspring will have the dominant phenotype,

and half will have the recessive phenotype.

A dihybrid cross is used when studying the inheritance of two traits. Mendel’s dihybrid

crosses helped him develop the law of independent assortment, which basically states

that different traits are inherited separately. When two organisms that are heterozygous

for both traits are crossed, the resulting phenotypic ratio is 9:3:3:1.

Probability is the likelihood that a particular event, such as the inheritance of a

particular allele, will happen. The events of meiosis and fertilization are random, so

hereditary patterns can be calculated with probability.

6.6 KEY CONCEPT Independent assortment and crossing over during meiosis result

in genetic diversity.

In organisms that reproduce sexually, the independent assortment of chromosomes during

meiosis and the random fertilization of gametes creates a lot of new genetic combinations.

In humans, for example, there are over 64 trillion different possible combinations of

chromosomes. Sexual reproduction creates genetically unique offspring that have a

combination of both parents’ traits. This uniqueness increases the likelihood that some

organisms will survive or even flourish in changing conditions.

Genetic diversity is further increased through crossing over. Crossing over is the

exchange of segments of chromosomes between homologous chromosomes. It happens

during prophase I of meiosis I when homologous chromosomes pair up with each other

and come into very close contact. At this stage, the chromosomes have already been

duplicated. Part of a chromatid from each homologous chromosome may break off and

reattach to the other chromosome.

Crossing over is more likely to occur between genes that are far apart from each other on

a chromosome. The likelihood that crossing over will happen is much less if two genes

are located close together. Thus, genes that are located close together on a chromosome

have a tendency to be inherited together, which is called genetic linkage. Most of the

traits that Mendel studied were located on separate chromosomes, and so they assorted

independently. When genes are on the same chromosome, however, their distance from

each other is a large factor in how they assort. If they are far apart, crossing over is likely

to occur between them and so they will assort independently. If they are close together,

they are unlikely to be separated by crossing over and so they will not assort independently.

Chapter 8

Vocabulary

RNA polymerase mutagen

nucleotide messenger RNA (mRNA) mutation

double helix ribosomal RNA (rRNA) point mutation

base pairing rules transfer RNA (tRNA) frameshift mutation

replication translation

DNA polymerase codon

central dogma stop codon

RNA start codon

transcription anticodon

8.2 KEY CONCEPT DNA structure is the same in all organisms.

DNA is a chain of nucleotides. In DNA, each nucleotide is made of a phosphate group,

a sugar called deoxyribose, and one of four nitrogen-containing bases. These four bases

are cytosine (C), thymine (T), adenine (A), and guanine (G). Two of the bases, C and T,

have a single-ring structure. The other two bases, A and G, have a double-ring structure.

Although scientists had a good understanding of the chemical structure of DNA by the

1950s, they did not understand its three-dimensional structure. The contributions of

several scientists helped lead to this important discovery.

• Erwin Chargaff analyzed the DNA from many different organisms and realized that

the amount of A is equal to the amount of T, and the amount of C is equal to the

amount of G. This A = T and C = G relationship became known as Chargaff ’s rules.

• Rosalind Franklin and Maurice Wilkins studied DNA structure using x-ray

crystallography. Franklin’s data suggested that DNA is a helix consisting of two

strands that are a regular, consistent width apart.

James Watson and Francis Crick applied Franklin’s and Chargaff ’s data in building a

three-dimensional model of DNA. They confirmed that DNA is a double helix in

which two strands of DNA wind around each other like a twisted ladder. The sugar and

phosphate molecules form the outside strands of the helix, and the bases pair together in

the middle, forming hydrogen bonds that hold the two sides of the helix together. A

base with a double ring pairs with a base with a single ring. Thus, in accordance with

Chargaff ’s rules, they realized that A pairs with T, and C pairs with G. The bases always

pair this way, which is called the base pairing rules.

8.3 KEY CONCEPT DNA replication copies the genetic information of a cell.

Every cell needs its own complete set of DNA, and the discovery of the

three-dimensional structure of DNA immediately suggested a mechanism by which the

copying of DNA, or DNA replication, could occur. Because the DNA bases pair in

only one way, both strands of DNA act as templates that direct the production of a new,

complementary strand. DNA replication takes place during the S stage of the cell cycle.

The process of DNA replication is very similar in both eukaryotes and prokaryotes,

but we will focus on eukaryotes.

• During the S stage of the cell cycle, the DNA is loosely organized in the nucleus.

Certain enzymes start to unzip the double helix at places called origins of

replication. The double helix unzips in both directions along the strand. Eukaryotic

chromosomes are very long, so they have many origins of replication to help speed

the process. Other proteins hold the two strands apart.

• The unzipping exposes the bases on the DNA strands and enables free-floating

nucleotides to pair up with their complementary bases. DNA polymerases bond

the nucleotides together to form new strands that are complementary to the original

template strands.

• The result is two identical strands of DNA. DNA replication is described as

semiconservative because each DNA molecule has one new strand and one original

strand.

DNA polymerase not only bonds nucleotides together. It also has a proofreading

function. It can detect incorrectly paired nucleotides, clip them out, and replace them

with the correct nucleotides. Uncorrected errors are limited to about one per 1 billion

nucleotides.

8.4 KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.

DNA provides the instructions needed by a cell to make proteins. But the instructions

are not made directly into proteins. First, a DNA message is converted into RNA in a

process called transcription. Then, the RNA message is converted into proteins in a

process called translation. The relationship between these molecules and processes is

summed up in the central dogma, which states that information flows in one direction,

from DNA to RNA to proteins.

Like DNA, RNA is a nucleic acid. It is made of nucleotides that consist of a phosphate

group, a sugar, and a nitrogen-containing base. However, RNA differs in important ways

from DNA: (1) RNA contains the sugar ribose, not deoxyribose; (2) RNA is made up of

the nucleotides A, C, G, and uracil, U, which forms base pairs with A; (3) RNA is usually

single-stranded. This single-stranded structure enables RNA to fold back on itself into

specific structures that can catalyze reactions, much like an enzyme.

During transcription, a gene is transferred into RNA. Specific DNA sequences and a

combination of accessory proteins help RNA polymerase recognize the start of a gene.

RNA polymerase is a large enzyme that bonds nucleotides together to make RNA.

RNA polymerase, in combination with the other proteins, forms a large transcription

complex that unwinds a segment of the DNA molecule. Using only one strand of DNA

as a template, RNA polymerase strings together a complementary RNA strand that has

U in place of T. The DNA strand zips back together as the transcription complex moves

forward along the gene.

Transcription makes three main types of RNA.

• Messenger RNA (mRNA) is the intermediate message between DNA and proteins.

It is the only type of RNA that will be translated to form a protein.

• Ribosomal RNA (rRNA) forms a significant part of ribosomes.

• Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome

during translation.

The DNA of a cell therefore has genes that code for proteins, as well as genes that code

for rRNA and tRNA.

8.5 KEY CONCEPT Translation converts an mRNA message into a polypeptide, or

protein.

Translation is the process that converts an mRNA message into a polypeptide, or protein.

An mRNA message is made up of combinations of four nucleotides, whereas proteins

are made up of twenty types of amino acids. The mRNA message is read as a series of

non-overlapping codons, a sequence of three nucleotides that code for an amino acid.

Many amino acids are coded for by more than one codon. In general, codons that code

for the same amino acid share the same first two nucleotides. Three codons, called stop

codons, signal the end of the polypeptide. There is also a start codon, which both signals

the start of translation and codes for the amino acid methionine. This genetic code is the

same in almost all organisms, so it is sometimes called the universal genetic code.

Although tRNA and rRNA are not translated into proteins, they play key roles in helping

cells translate mRNA into proteins. Each tRNA molecule folds up into a characteristic

L shape. One end has three nucleotides called an anticodon, which recognize and bind

to a codon on the mRNA strand. The other end of the tRNA molecule carries a specific

amino acid. A combination of rRNA and proteins make up the ribosome. Ribosomes

consist of a large and small subunit. The large subunit has binding sites for tRNA. The

small subunit binds to the mRNA strand.

At the start of translation, a small subunit binds to an mRNA strand. Then the large

subunit joins. A tRNA molecule binds to the start codon. Another tRNA molecule binds

to the next codon. The ribosome forms a bond between the two amino acids carried by the

tRNA molecules and pulls the mRNA strand by the length of one codon. This causes the

first tRNA molecule to be released and opens up a new codon for binding. This process

continues to be repeated until a stop codon is reached and the ribosome falls apart.

8.7 KEY CONCEPT Mutations are changes in DNA that may or may not affect phenotype.

A mutation is a change in an organism’s DNA. Although a cell has mechanisms to deal

with mutations, exposure to mutagens may cause mutations to happen more quickly than

the body can repair them. Mutagens are agents in the environment that can change

DNA. Some occur naturally, such as UV light from the Sun. Many other mutagens are

industrial chemicals.

Mutations may affect individual genes or an entire chromosome. Gene mutations include

point mutations and frameshift mutations.

• A point mutation is a substitution in a single nucleotide.

• A frameshift mutation involves the insertion or deletion of a nucleotide or

nucleotides. It throws off the reading frame of the codons that come after the

mutation.

Chromosomal mutations include gene duplications and translocations. Gene duplication

is the result of improper alignment during crossing over. It results in one chromosome

having two copies of certain genes, and the other chromosome having no copies of

those genes. Translocation is the movement of a piece of one chromosome to another,

nonhomologous chromosome.

Mutations may or may not affect phenotype. Chromosomal mutations affect many

genes and tend to have a large effect on an organism. They may also cause breaks in

the middle of a gene, causing that gene to no longer function or to make a hybrid with

a new function. The effect of a gene mutation can also vary widely. For example, a

point mutation may occur in the third nucleotide of a codon and have no effect on the

amino acid coded for. Or the mutation may occur in an intron and thus have no effect.

However, the mutation might result in the incorporation of an incorrect amino acid that

messes up protein folding and function. Or it might code for a premature stop codon.

Even mutations that occur in noncoding regions of DNA can have significant effects if

they disrupt a splice site or a DNA sequence involved in gene regulation. For a mutation

to affect offspring, it must occur in an organism’s germ cells.

Part A :short Essay Questions: Answer any 10 , answer in complete sentences and on your Scantron Sheet , each worth 3 points each.

1. Describe 2 of the 4 unifying themes in Biology and give an example of each.

2. An Enzyme is a catalyst in a living thing. What function do enzymes carry out?

3. What is the primary role of DNA in the cell?

4. What are two characteristics of eukaryotic cells?

5. Why don’t cells all divide at the same rate?

6. In terms of structure and function describe the relationship between; genes, DNA and chromosomes.

7. Explain why gametes such as egg and sperm have a haploid number of chromosomes instead of a diploid

number.

8. Certain types of small molecules cross the plasma membrane without the cell exerting any energy , so when is

energy required to move molecules across the cell membrane?

9. The word facilitate means “to make easier.” How does this meaning apply to facilitated diffusion?

10. What is homeostasis and why is it important to the survival of an organism??

11. How is DNA related to reproduction?

12. In terms of structure and function, what role do cell walls play in a plant?

13. Describe two of the three parts of the cell theory?

14. Describe a mutation can be an advantage to an animal to better inhabit its environment.

15. The police are called to a Susquehanna Twp. Home because bones were found buried in the ground and the

home owner is worried they may be human. Describe the experiment you perform in order determine the identity

of the bones.

16. Where in the biosphere is there the greatest biodiversity? Explain why this location has the greatest

biodiversity.

17. Why is it important that a scientist’s results are evaluated by other scientists?

Part B-Long Essay (answer on the space provided on your answer sheet- use complete sentences ) Some have

described the cellular processes of Photosynthesis and Respiration as mirror images of each other. Go into detail to

compare and contrast the cellular processes of Photosynthesis and Respiration using; chemical equations, chemical

reactions, chemical formulas and examples of reactants and products. Additionally, describe where in the cell these

processes occur and how the differing locations tell us something about structure and function.

Word Bank: ATP, ADP, Glucose, Carbon dioxide, water, cellular respiration, aerobic, anaerobic, endothermic,

exothermic, glycolysis, Krebs cycle, mitochondria, electron transport chain, stroma , NADPH, NADP+, NAD+, NADH,

FAD2+ , FADH2,phate ,chemosynthesis, photosynthesis, chloroplasts, chlorophyll, thylakoid, light-dependent reactions,

light-independent reactions, photosystem, electron transport system, ATP-synthase, Calvin cycle , Oxygen, membrane,

matrix, Yields