The Cell Cycle

Unit 5, Part 1: Mitosis

Life From Life• Remember the story about the maggots on the hamburger meat?

In case you forgot, it was really gross. A guy left meat out and a week later, it was infested with nasty maggots. He thought the maggots were born from the meat.

• He called this “spontaneous generation.” This means that living things were produced spontaneously, or by nonliving things.

Biogenesis• Now, we’re a little smarter, and if you remember the cell theory,

we know that living things only come from other living things.• This is called “biogenesis.” It means that new life cannot come

from absence of life - it is created by another living organism.• In this unit, we will study how these new cells are made to produce

new tissues, organs, organisms, and communities.

Reproduction of Body Cells• Body cells, or somatic cells, are the cells that make up everything in

the body, except the egg and sperm cells used in sexual reproduction.

• This includes blood cells, skin cells, bone cells, and every other type of cell you can come up with.

• The process by which new somatic cells are produced is called mitosis.

The Cell Cycle

• The cell cycle is made up of several stages, each with its own major events.

• G1 phase - cell growth • S phase - DNA replication• G2 phase - preparation for mitosis• M phase - mitosis and cytokinesis

Mitosis and the Cell Cycle

• Most of a cell’s lifetime, which varies from cell type to cell type, is spent in a lazy stage. We call that inactive phase interphase.

• Remember, inter- means between, so this is the phase between reproductive cycles.

Major Events in Mitosis• About midway through interphase, the chromosomes of each cell

(located in the nucleus, which carry genes) begin the process of replication to prepare for reproduction.

• During replication (which means to make a copy), a whole second set of chromosomes is produced. These will be used in new cells.

Interphase

Events of Prophase

• During prophase, the thick strands of chromatin (in the nucleus) are condensing, growing thicker and shorter.

• The nucleolus disappears and the nuclear membrane breaks down.

• Chromosomes can be seen clearly.• Microtubules form between the two pairs of centrioles and

become spindle fibers.• Each chromosome has replicated and becomes a double-

stranded structure. Each strand is called a chromatid, and a pair are called sister chromatids.

Prophase

Events of Metaphase

• This is a short portion of the cell cycle. At the beginning of the phase, the chromosomes are scattered in the area formerly known as the nucleus.

• Later, they move towards the spindle fibers’ equator. • The microtubules that are attached to the centromere

change in length.• At the end of this phase, the chromosomes are aligned

on the spindle’s equator.

Metaphase

Events of Anaphase• The centromeres split at the beginning of the stage. • The two sister chromatids separate from one another and

move apart.• The microtubules join and begin to lengthen. The poles are

pushed apart and sister chromatids are pulled to opposite poles of the cell.

• At the end of this stage, there is one set of single stranded chromosomes at each end of the cell.

Anaphase

Events of Telophase

• The final phase of mitosis begins with one cell splitting into two cells. This is called cytokinesis.

• The plasma membrane of animal cells begins to pinch together at the cell’s center.

• This stage ends with the reverse processes that are present in prophase.

• The chromosomes de-condense into chromatin, the nuclear membrane re-appears, and the nucleolus re-appears.

Telophase

Cell Reproduction in Prokaryotes

• Since prokaryotes are single-celled organisms, they only have one chromosome.

• They don’t have a cell cycle per se, but they do replicate in a process called binary fission.

• During binary fission, prokaryotes like bacteria have a chromosome that is attached to the inside of the cell membrane. There, a duplicate chromosome is formed.

• The new chromosome also attaches to the cell membrane and a new cell membrane forms.

• This new membrane or cell wall (in algae) pinches off and forms two separate cells, each containing one chromosome.

The Cell Cycle

Unit 5, Part 2: Meiosis

Reproductive Reproduction

• The reproduction of reproductive cells (egg and sperm) is called meiosis.

• Meiosis is very similar to mitosis, except that the result is not a cloned cell that is identical to its parent cell.

• This is the reason for the variation within a species.

Haploid v. Diploid

• A haploid cell is one which has only half the number of chromosomes as a duplicated cell. In most cases, this is the number of chromosomes that a sex cell has. It is represented by the letter n.

• In humans, all sex cells have 23 chromosomes. So, for humans, n = 23.

• A diploid cell has twice as many chromosomes as a sex (haploid) cell, which is equal to the number of chromosomes in a duplicated cell.

• This is represented by 2n.• That means for humans, a diploid cell contains 46

chromosomes, or 2n = 46.

• Haploid cells cannot be produced by mitosis.• Instead, haploid cells are produced from normal

(diploid) nuclei. These haploid cells are called gametes, or sex cells.

• Meiosis only occurs in reproductive tissues to produce sex cells, and has two main stages.

Steps of Meiosis

• Meiosis is divided into two parts: meiosis I and meiosis II. There is an interphase, along with DNA replication before meiosis I. They are separated by a cell division, but no DNA replication.

• Meiosis II is very similar to mitosis. The steps are even named the same.

• Meiosis I is where the big changes occur.

Prophase I• During prophase I, the homologues (half of a

chromosome) move close together. • Each pair of homologues comes together so that

matching sections along the length of their chromatids lie side by side.

• During this time, the non-sister chromatids of two homologues may be wound around one another, as seen on the next slide.

• Because two homologous chromosomes pair up, and each chromosome is made of two chromatids, we now have a quadruple structure.

Prophase I

• The top illustration shows a reproductive cell in prophase I.

• The bottom picture shows how non-sister chromatids might cross over one another when near each other, providing for more variations.

Metaphase I

• The quadruple structures created during prophase I are called tetrads.

• During metaphase I, the tetrads line up at the equator of the spindle, much like in metaphase of mitosis.

• By this time, the nuclear membrane has already disappeared and the spindles have formed during metaphase I.

Metaphase I

Anaphase I

• During anaphase I of meiosis, the centromeres do not divide and sister chromatids do not separate.

• Instead, one homologous pair of chromosomes goes to each pole.

• The sister chromatids stay attached to one another.

Anaphase I

Telophase I

• With the tetrads separated, the cell membrane begins to pinch inward. In telophase I, the original cell divides to form two cells, each of which contains one homologous pair from every tetrad, making them diploid (2n).

• However, each chromosome is still double-stranded and consists of two chromatids that are still joined together.

Telophase I

Meiosis II

• After a short interphase in which there is no DNA replication, meiosis II begins. Meiosis II is much like mitosis.

Meiosis II• Each of the two cells may skip early events.• Chromosomes move directly to the equator.• The centromere of each chromosome splits.• Sister chromatids are pulled toward opposite poles.• Separation of the chromatids occurs in the second meiotic

division.• Each new nucleus contains single-stranded chromosomes.

Meiosis in Males v. Females

• The overall outcome in males is the formation of four haploid cells (immature sperm), from one diploid cell. Each of the sperm have half as many chromosomes as the original cell.

• In a female, only one egg develops from each cell that undergoes meiosis. The other three cells become polar bodies and die, giving all their nutrients to the one egg that survives.

• If each sex cell is haploid, what do you get when you join them?

Variation Through Meiosis

• Since mitosis produces exact clones of the parent cells, it allows for no variation except through mutation.

• Meiosis allows for variety because it is a process that “re-shuffles” chromosomes, allowing them to mix and match, change order, etc.

• Each chromosome has a 50% chance of being passed on to each daughter cell made during meiosis.

Mitosis v. Meiosis

Mitosis Meiosis

Occurs in… All body cells Sex cells

Produces… Two cells Four cells, though three may die

Parent cell has… Diploid number of chromosomes

Diploid number of chromosomes

Daughter cell has…

Diploid number Haploid number

Good for… Genetic continuity Genetic variation

The Cell Cycle

Unit 5, Part 3: Genetics

Principles of Genetics: Objectives

• Discuss Mendel’s experiments and his results.• Explain the three principles of genetics

Mendel obtained through his experiments.• Solve genetics problems using Punnett

squares.

What is Genetics?• We know that like produces like. That means cats

don’t give birth to birds, but to cats. Flies have flies, dogs have dogs, and humans have humans.

• But are parent and offspring exactly alike? We know that because of meiosis and sexual reproduction, there are variations from one generation to the next.

• The passing of traits from parent to offspring is called heredity.

• Genetics is the science of heredity -predicting traits, studying genes, and understanding variations.

A History Lesson

• Gregor Mendel was a monk in a monastery in Czechoslovakia, living from 1822 - 1884. After studying many years of science and math, he returned to the monastery to work in the gardens.

• Thus began a beautiful experiment.• Mendel’s work during this time is the basis of

everything we know about genetics and heredity today.

Gregor Mendel, The Father of Genetics

Peas in a Pod?

• Mendel spent most of his time working with pea plants because they are easy to grow and they produce many offspring.

• Pea plants also have distinct characteristics, like flower color, stem length, and seed color.

• Mendel’s experiments lasted 8 years, giving him enough results to make his work valid.

Mendel’s Experiments

• Mendel studied seven distinct traits of pea plants over his eight years.

• Those traits were: seed shape, seed color, flower color, flower position, pod color, pod shape, and plant height.

• He studied the offspring of many generations, realizing (because he knew step 7 of the scientific method) that repetition produces more valid results.

The 7 Traits of Pea Plants• Dominant Traits• Seed shape: round• Seed color: yellow• Flower color: purple• Flower position: axial (sides)• Pod color: green• Pod shape: inflated• Plant height: tall

• Recessive Traits• Seed shape: wrinkled• Seed color: green• Flower color: white• Flower position: terminal

(tips)• Pod color: yellow• Pod shape: constricted• Plant height: short

Mendel’s Results

• After crossing plants with different traits, he observed many variations among the generations. Each generation had a name. For example, the first generation was called F1, or filial generation one. The second was F2, then F3, and so on.

• He noticed that some traits that were possessed by the parent plants did not show up in the offspring, while some traits showed up in offspring that were not visible in the parents.

Dominant v. Recessive

• Mendel called the traits that were present in both the parent and offspring plants dominant traits.

• The ones the appeared in offspring plants but were not visible in parent plants, or that appeared in parent plants but not in offspring plants, recessive traits.

Symbolizing Traits

• Dominant traits are usually symbolized by capital letters. For example, since the dominant trait for plant height is tall, the symbol could be a T.

• Recessive traits are symbolized by the same letter, only lower case. If a plant were short, its gene symbol would be t.

Gene Segregation: Is That Constitutional?

• In Mendel’s experiment, the pure plants (the ones that have the same features as their parents) each had two identical genes (characteristics) for a trait. That means both parents passed on the same gene for that trait. Tall plants had the genes TT and short plants had the genes tt.

• He suggested that the two genes separate during meiosis and each cell (one egg and one sperm) gives ones of its genes to the offspring.

• Therefore, the gametes (sex cells) of tall plants would have the T gene and the gametes of short plants would have the t gene.

• If you crossed a tall plant and a short plant, the offspring would have a the genes Tt.

Punnett Squares

• Punnett squares are a method of crossing the genes of each parent to determine the gene outcomes for their offspring.

• It is written as a chart in which along the top of the square are written the genes for one parent and along the side of the square are written the genes of the other parent.

• The square is then filled in with the possible gene combinations.

• Along the top are the genes for a particular trait from one parent.

• Down the left side are the genes for the same trait from the other parent.

• Each box contains the combination of the genes, listing all the possibilities for that particular trait.

Punnett Practice

• If you crossed a TT plant with a tt plant, what would the outcome be?

• If you crossed a TT plant with a Tt plant, what would the outcome be?

• If you crossed a tt plant with a Tt plant, what would the outcome be?

• If you crossed a TT plant with a TT plant, what would the outcome be?

• If you crossed a tt plant with a tt plant, what would the outcome be?

Terminology

• Many traits have different forms. For example, for height, there is the form of short and tall.

• For the trait of seed color, there is the form of yellow or green.

• Each form of a trait is called an allele. Each allele is expressed by a letter, such as T or t.

• The combination of alleles for one trait is called an organism’s genotype. The genotype for a short plant would be tt. The genotype for a tall plant would be either TT or Tt.

• The expression of genes is called phenotype. Phenotype tells how the genes make the organism look. If a cell had a genotype of tt, its phenotype would be short. If it were TT or Tt, its phenotype would be tall.

• If a cell has a genotype where both alleles are the same (TT or tt), it is said to be homozygous for that trait.

• If a cell has a genotype where the alleles are different (Tt), it is heterozygous.

• If it is homozygous with both dominant alleles (TT), it is homozygous dominant.

• If it homozygous with both recessive alleles (tt), it is homozygous recessive.

Two Traits• After Mendel’s initial experiments, he studied the

inheritance of two traits at once. For example, what type of offspring would be produced by crossing a plant with round seeds that was tall (RrTt) with another one just like it (RrTt)?

• According to the law of segregation, each gamete must possess one allele for each trait. That means the gametes of the tall, round seeded plants must have alleles R,r, t, and T.

A Two-Trait Problem

So many letters!

• The previous diagram, though it was only a two-trait cross, had 16 squares. This is because each parent has four possible gene combinations for that trait.

• If a parent had genes R, r , T, and t, the possible combinations would be RT, Rt, rT, and rt.

Incomplete Dominance

• There are a few traits that exhibit a pattern called incomplete dominance. This means that a dominant genotype would be one thing (such as red flower color), a recessive genotype would be another (white flower color), and a heterozygous genotype would be a blending of the other two (pink flower color).

Multiple Alleles

• In most cases, there are only a few alleles for a trait. The most any cell can have is two, but there may be many more than that possible. This may result in one allele not being dominant over any other.

• Blood type is one of these. Like incomplete dominance, codominance results when a cell receives two different alleles, neither of which is dominant over the other. Parents can either donate IA , IB, or i alleles to offspring cells.

Blood Groups in HumansPhenotypes (Blood Types)

Genotypes

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

Sex Determination

• Drosophilia, common fruit flies, are commonly used in studies of sex determination.

• Sex chromosomes are symbolized by X and Y.• All gametes produced by a female fruit fly contain an

X chromosome, while male gametes will contain X and Y.

• The probability of receiving one gene over the other can be figured the same way as any other trait.

Male XY X Y

Female X XX

X

XX XY

XX XY

Sex-Linked Inheritance

• A sex-linked trait is one that is attached to a sex chromosome. Some examples that are found on the X chromosomes (and therefore, only show up in males, passing from mother to son) are hemophilia, baldness, color-blindness and certain types of anemia.