Chapter 3

The Biological Basis of Life

Chapter Outline The Cell DNA Structure DNA Replication Protein Synthesis What is a Gene? Cell Division: Mitosis and Meiosis

Cells Basic units of life. Complex life forms have billions of cells.

An adult human= as many as 1,000 billion cells.

Life on earth can be traced back 3.7 b.y.a. Prokaryotic cells (i.e. bacteria and blue-

green algae). Eukaryotic cells (1.2 billion years ago).

Types of Cells Somatic cells are components of body

tissue. Gametes are sex cells.

Ova are produced in female ovaries. Sperm are produced in male testes. A zygote is the union between a

sperm and an ovum.

DNA Structure Cellular function and an organism’s inheritance. DNA is composed of two chains of nucleotides. A nucleotide consists of a sugar, a phosphate, and one

of four nitrogenous bases. The two chains are held together by bonds formed on

their bases with their complement on the other chain. Adenine (A) is the complement of Thymine(T) Guanine(G) is the complement of Cytosine(C)

Proteins The major structural components of tissue. Enzymes are proteins that serve as catalysts,

initiating chemical reactions in the body. Amino acids are the building blocks of protein. Proteins differ according to number of amino

acids and the sequence in which they are arranged.

Protein Synthesis Ribosomes help convert the genetic

message from the DNA into proteins. Messenger RNA (mRNA) carries the

genetic message from the cell nucleus to the ribosome.

Transfer RNA (tRNA),found in the cytoplasm, binds to one specific amino acid.

RNA and DNA

RNA differs from DNA in three important ways:

1. It’s usually single-stranded.

2. It contains a different type of sugar.

3. It contains the base uracil as a substitute for the DNA base thymine.

Protein Synthesis: Transcription The process of coding a genetic message

for proteins by formation of mRNA. A portion of the DNA unwinds and serves

as a template for the formation of a mRNA strand.

Transcription

Protein Synthesis: Translation The mRNA travels through the nuclear

membrane to the ribosome. tRNAs arrive at the ribosome carrying their

specific amino acids. The base triplets on the tRNA match up with the

codons on the mRNA. As each tRNA line up in the sequence of

mRNA codons their amino acids link to form a protein.

Assembly of an Amino Acid Chain in Protein Synthesis

As the ribosome binds to the mRNA, tRNA brings a particular amino acid, specified by the mRNA codon, to the ribosome.

Assembly of an Amino Acid Chain in Protein Synthesis

The tRNA binds to the first codon while a second tRNA–amino acid complex arrives at the ribosome.

Assembly of an Amino Acid Chain in Protein Synthesis

The ribosome moves down the mRNA, allowing a third amino acid to be brought into position by another tRNA molecule. Note that the first two amino acids are now joined together.

Genes A gene is the entire sequence of DNA bases

responsible for the synthesis of a protein. A mutation occurs when the sequence of

bases in a gene is altered. Mutations may interfere with an organisms

ability to produce vital protein and may lead to a new variety within the species, hence, evolution.

Cell Division: Mitosis and Meiosis Cell division results in production of new

cells. During cell division:

Cells are involved with normal cellular and metabolic processes.

The cell’s DNA becomes tightly coiled. DNA is visible under a microscope as

chromosomes.

Chromosome Structure A chromosome is composed of a DNA molecule

and associated proteins. During normal cell functions, chromosomes

exist as single-stranded structures. During cell division, chromosomes consist of

two strands of DNA joined at the centromere. Since the DNA molecules have replicated, one

strand of a chromosome is an exact copy of the other.

Chromosomes and Genetics Each species is characterized by a

specific number of chromosomes. Humans have 46 chromosomes.

Chromosome pairs are called homologus: These carry genetic information that

influences the same traits. They are not genetically identical.

Types of Chromosomes Autosomes - govern all physical

characteristics except sex determination. Sex chromosomes - X and Y

chromosome. Mammal females have two X

chromosomes. Mammal males have one X and

one Y chromosome.

Mitosis Mitosis is cell division in somatic cells. Mitosis occurs during growth and

repair/replacement of tissues. The result of mitosis is two identical

daughter cells that are genetically identical to the original cell.

Meiosis Production of gametes (sex cells). 2 divisions result in 4 daughter cells. Each daughter cell contains 23 chromosomes. Resulting gamete may unite with another

gamete to create a zygote. The zygote inherits the DNA, half from each

parent, to develop and function normally.

Evolutionary Significance of Meiosis Meiosis and sexual reproduction are

highly important evolutionary innovations. Meiosis increases genetic variation at a

faster rate than mutation. Offspring in sexually reproducing species

represent the combination of genetic information from two parents.

Problems With Meiosis Errors in meiosis may lead to miscarriage. Nondisjunction occurs when chromosomes don’t

separate during meiosis. A gamete containing one less chromosome

that fuses with a normal gamete will produce a zygote containing 45 chromosomes.

A gamete containing one extra chromosome that fuses with a normal gamete will produce a zygote containing 47 chromosomes.

Mitosis and Meiosis Compared

Quick Quiz

1. The entire sequence of DNA bases on the chromosome that code for a particular polypeptide chain is a(n)

a) ribosome.

b) amino acid.

c) gene.

d) polypeptide chain.

Answer: c The entire sequence of DNA bases on

the chromosome that code for a particular polypeptide chain is a gene.

2. Which of the following is NOT true about RNA?

a) It is single stranded.

b) Some forms of RNA are involved with protein synthesis.

c) It has a different type of sugar than DNA has.

d) It contains the base thymine.

Answer : d The following is NOT true about RNA.

It contains the base thymine.

3. Which of the following is true for meiosis?

a) It has only one division that duplicates the parent cell exactly.

b) It produces gametes.

c) When a mutation occurs it affects only the individual.

d) It has no effect on evolution.

Answer: b The following is true for meiosis:

It produces gametes.

4. Chromosomes that carry genes for the same traits are _________________ pairs.

Answer: homologous• Chromosomes that carry genes for the

same traits are homologous pairs.

5. Somatic cell division is termed _________________ .

Answer: mitosis Somatic cell division is termed mitosis.