Lecture 8 DNA Synthesis and Repair

DNA Strand Orientation

-DNA strands are antiparallel to one another

•5' ----->3'

3'

Adenine

always pairs with thymine

Cytosine

always pairs with guanine

Conservative model for DNA Replication

The parental double helix remains intact and an all new copy is made

Semiconservative model for DNA Replication

The the strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand

Disperative model for DNA Replication

Each strand of both daughter molecules contains a mixture of old and newly synthesized parts.

Unwinding DNA

Eukaryotic chromosomes have replication bubbles and multiple origins of replication

Addition of Nucleotides

- New nucleotides add to 3' end ONLY

-DNA synthesis is 5' —> 3'

The Replication Bubble

This creates a "Leading" and a "Lagging" strand of DNA to be replicated within the bubble

Helicase

Catalyzes the breaking of hydrogen bonds between base pairs to open the double helix

Single-strand DNA-binding proteins

Stabilizes single-stranded DNA

Topoisomerase

Breaks and rejoins the DNA double helix to relieve twisting forces caused by the opening of the helix

Opening the helix

1. DNA is opened by helicase

2. SSBP stabilizes the single stranded DNA

3. Topoisomerase relieves stress of twisting

Primase

Catalyzes the synthesis of the RNA primer

DNA polymerase III

extends the leading strand

Sliding clamp

Holds DNA polymerase in place during strand extension

The Proteins - Opening the Helix

1. Helicase

2. Single-strand DNA-binding proteins

3. Topoisomerase

The Proteins - Leading Strand Synthesis

1. Primase

2. DNA Polymerase III

3. Sliding Clamp

DNA Polymerase I

removes the RNA primer and replaces it with DNA

DNA ligase

Catalyzes the joining of Okazaki fragments into a continuous strand

The Proteins - Lagging Strand Synthesis

1. DNA polymerase I

2. DNA Ligase

Lagging strand synthesis

1. primer added

2. first fragment synthesized

3. second fragment synthesized

4. primer replaced

5. gap closed

Leading Strand Synthesis

1. DNA is opened, unwound, and primed

2. Synthesis of leading strand begins

- sliding clamp holds DNA polymerase in place

- DNA polymerase III works in 5' -> 3' direction, synthesizing leading strand

Telomeres

Comprised of repetitive sequences of nucleotides - TTAGGG

Do not code for any genes

A buffer that protects the rest of the chromosome

Telomerase can restore telomeres

1. End is unreplicated

2. Telomerase extends unreplicated end

3. Again, telomerase extends unreplicated end

4. Lagging strand is completed

Correcting Mistakes in DNA Synthesis

- DNA replication is very accurate

- error rate is about one mistake per billion bases

- DNA polymerase matches bases with high accuracy

---inserts an incorrect base about once every 100,000 bases

- repair enzymes remove defective bases and replace them with the correct one

Repairing Damaged DNA

-DNA can be damaged by radiation and chemicals

-Thymine produce a kink in the DNA strand that blocks DNA replication