Section 10 – 1 Objectives

• Explain the principal function of DNA.

• Describe the structure of DNA.

• Define the term complementary base pairing.

• Explain the role of complementary base pairing in the replication of DNA.

• Summarize the main features of DNA replication.

Structure of DNA

• Building blocks called nucleotides

1. Five carbon sugar molecule (pentose) - deoxyribose

2. Phosphate group – P atom surrounded by O atoms

3. Nitrogen Base

Nitrogen Bases

• Purines - adenine(A) & guanine(G)

(1) contain 2 rings of carbon and nitrogen atoms

Nitrogen Bases

• Pyrimidines – cytosine(C) & thymine(T)

(1) contain 1 ring of carbon and nitrogen atoms

The Double Helix

1. Rosalind Franklin and Maurice Wilkins

- provided X-ray photographs of DNA crystals

2. James Watson and Francis Crick (1953)

- deciphered the structure of DNA

The Double Helix

Complementary Base Pairing

1. Erwin Chargaff – determined that

%A = %T and %G = %C in any given DNA sample

2. Chargaff’s Rule/Base Pairing Rules

a. Adenine pairs with thymine

b. Cytosine pairs with guanine

CBP

3. Bases are held together by hydrogen bonds

a. There are 2 hydrogen bonds between adenine and thymine.

b. There are 3 hydrogen bonds between cytosine and guanine.

CBP

4. The complementary nucleotide chains in the DNA model led to suggestion for how DNA may copy itself.

Why must exact copies of DNA be passed on to daughter cells?

Replication of DNA

Replication of DNA

1. Replication - is the process of copying DNA in a cell

2. Replication fork - is the point at which the two strands separate

- there may be thousands of these along a single DNA molecule to permit faster replication

Replication of DNA

3. Many enzymes are involved

a. DNA helicase – unwinds the DNA and breaks the hydrogen bonds between complementary bases

b. DNA polymerase – responsible for adding new nucleotides to the growing strand

DNA Replication

4. Replication is semi-conservative

- Each new DNA molecule consists of one original strand and one new strand.

DNA Replication

5. Accuracy and Repair• a. Approximately one error in every

10,000 paired nucleotides

b. Enzymes proofread DNA and repair errors.

c. Mutation - a change in the DNA (nitrogen base)

- caused by mutagens such as:

1. chemicals 2. radiation 3.viruses

Section 10-2 Objectives

• Explain the primary function of RNA

• Compare the structure of RNA with that of DNA.

• Describe the structure and function of each type of RNA.

• Summarize the process of transcription.

Structure of RNA

A. Building blocks called nucleotides.

DNA vs. RNA

DNA RNA

Name of pentose

Nitrogen bases

# of strands

Location in cell

# of types

Types of RNA

1. messenger RNA (mRNA) – carries the code from the nucleus to the cytoplasm

2. transfer RNA (tRNA) – carries amino acids to the ribosome

3. ribosomal RNA (rRNA) – makes up part of the ribosome

Transcription

A. Process by which genetic information is copied from DNA to RNA

Steps of Transcription

1. Initiation

- The enzyme RNA polymerase synthesizes RNA using the DNA as a template.

- binds to regions of DNA called promoters, which mark the beginning of the DNA to be transcribed.

Transcription

2. Complementary base pairing

3. Termination

- sequence of 3 bases on DNA marks the end of the gene

- RNA polymerase then releases the RNA and the DNA molecule “rezips”

Products of Transcription

1. Called transcripts.

2. Include mRNA, tRNA, and rRNA.

3. Following transcription, mRNA leaves the nucleus through the nuclear pores and attaches to a ribosome where it will direct protein synthesis

Section 10-3 Objectives

• Describe the genetic code.

• Distinguish between a codon and anticodon and state where each is found.

• Explain the roles of the start codon and stop codons.

• Summarize the process of translation.

Protein Structure and Composition

A Review of Proteins

1. Proteins are polymers of amino acids that are linked together by peptide bonds.

2. There are 20 different amino acids.

3. The sequence of these amino acids determines how the polypeptide will twist and fold into the 3-D structure of a protein.

The Genetic Code

A. Codon

1. Sequence of 3 bases on mRNA that codes for an amino acid.

2. There are 64 different codons.

3. Special Codons

a. The start codon is AUG

b. The stop codons are UAA, UGA, and UAG.

Translation

• the process of assembling polypeptides from information encoded of mRNA

tRNA and Anticodons

1. tRNA Structure

a. Amino acid attachment site at the top

b. anticodon - loop of 3 nucleotides at the bottom

- this will pair with a codon on the mRNA

Ribosomes

1. Composition

a. proteins

b. rRNA

Protein Assembly

1. Ribosome attaches to the start codon (AUG) on the mRNA.

2. The tRNA with the anticodon (UAC) pairs with AUG. This tRNA is carrying

the amino acid methionine.

3. The ribosome moves along the mRNA transcript.

Protein Assembly

4. The next tRNA carrying its amino acid positions itself beside the tRNA carrying methionine. A peptide bonds forms between the two amino acids.

5. The ribosome continues to move along the mRNA while each mRNA codon is paired with a tRNA anticodon.

6. A stop codon is reached, bringing an end to translation.

7. Many ribosomes may simultaneously translate the same mRNA, creating a structure called a polyribosome.

Protein Assembly