Chapter 16 DNA Replication

Lecture 29

Template

A single strand of DNA used as a guide to build a new complementary strand of DNA (or RNA)

What is DNAs strand and parallel

Complementary and antiparallel

Nucleotides are added to the —- ends of DNA only

3’

Base pairing for DNA

A pairs with T

C pairs with G

3 Alternative models of DNA replication

Conservative model

Semiconservative model

Dispersive model

Meselson and Stahl expirement 1957 (mem)

1. Bacteria cultured in medium containing 15 N

2. Bacteria transferred to medium containing 14 N

3. Results:

4. DNA sample centrifuged after 20 min (after 1st replication)

5. DNA sample centrifuged after 20 min (after 2nd replication)

DNA replication (mem)

Semiconservative

Each new daughter DNA double helix contains one old strand and one new strand

-Parent molecule unwinds (strands separate)

-Each parent strand acts as a template to build a new DNA strand using base pairing rules

-Described as the most beautiful experiment in biology (Meselson and Stahl experiment 1957) 

Origin of Replication

Special DNA sequence where DNA replication begins

2 Chromosomes in the origin of replication

Bacterial chromosomes are small and circular: one origin of replication

Eukaryotic chromosomes are long and linear: many origins of replication per DNA molecule

DNA replication bubble

At the end of each replication bubble is a replication fork, a Y shaped region where new DNA strands are elongating

DNA replication fork

Leading strand, lagging strand and okazaki fragments

Leading strand 

3’ end is pointing towards the fork

Made in one continuous strand, adding nucleotides to the 3 end as the fork unwinds

Lagging strand 

The 3 end is pointing away from the fork, so must be made in the opposite direction of the fork

Okazaki fragments 

As the fork unwinds, a new fragment is started and extended in the direction away from the fork

8 enzymes in DNA replication

Helicase

Single strand binding protein

Topoisomerase

Primase

DNA polymerase

DNA polymerase III

DNA polymerase I

DNA ligase 

Helicase

unzips parent DNA at the replication fork separating the DNA strands

Single strand binding protein

Binds to single-stranded DNA to prevent annealing (keeps DNA strands apart)

Topoisomerase

Prevents overwinding of DNA ahead of the replication fork

Primase

Enzyme that makes RNA primers

RNA primer

Short piece of RNA onto which DNA nucleotides are added

DNA polymerase can only add to a nucleotide of ————

existing chains

RNA nucleotides unlike DNA can be linked together with out a ——

primer

DNA polymerase

Enzyme that adds DNA nucleotides to the 3’ end only of the growing strand

Requires a primer

Requires a DNA template

DNA polymerase III

Main polymerase enzyme that elognates DNA strands from RNA primers

Used on leading and lagging strand

DNA polymerase I

Removes the RNA primer (chews away from the 5’ end)

Replaces the RNA primer with DNA (adds to the 3’ end)

DNA ligase

Joins Okazaki fragments together on a lagging strand

Makes a covalent bond between the 3 end of one fragment and the 5 end of another

Lagging strand- why okazaki fragments? (know)

On the lagging strand the 5 end points to the fork

Nucleotides cant be added to the 5 end, only 3 end which points opposite directioin of synthesis 

Have to replicate lagging strand in the direction opposite of the fork unwinding direction

What are Chargaff’s rules? 

DNA composition varies between species. Number of A= number of T, Number of G= number of C, evidence for DNA base pairing and DNA structure.

What do imply about DNA structure?

It implies a double-helix structure with specific complementary base pairing.