Cell Bio- DNA replication

What were the early DNA experiments?

Chargaff’s rules (1947), DNA composition varies between species

In every species:

-Number of A = Number of T

-Number of 6 = Number of C

In DNA replication, the parent strand serves as a _____ for newly made strands.

Template

What did Watson and Crick note?

That base pairing suggested a possible mechanism for copying genetic material

What are two strands of DNA?

Complementary

Each strands acts as a template for building a new strand in replication

What ends are nucleotides added to?

3’ of DNA only

DNA replication is?

Semiconservative

What does each new daughter DNA double helix contain?

One old strand and one new strand

What is DNA replication?

Parent molecule unwinds, each parent strand acts as a template to build a new DNA strand using base-pairing rules

What is the origin of replication?

The special DNA sequence where DNA replication begins

What has one origin of replication?

Bacterial chromosomes that are small and circular

What has many origins of replication?

Eukaryotic chromosomes that are long and linear

What is a replication fork?

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

What is a leading strand?

3’ end is pointing towards the fork

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

What is a lagging strand?

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

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

What is Okazaki fragments?

When DNA must be synthesized as a series of segments

What is Helicase?

Unzips parent DNA at the replication fork, separating the DNA strands

What is a single-strand binding protein?

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

What is Topoisomerase?

Prevents overwinding of DNA ahead of the replication fork

What is primase?

An enzyme that makes RNA primers

What is an RNA primer?

A short piece of RNA onto which DNA nucleotides are added

What can only ne added to an existing chain of nucleotides?

DNA polymerase

What can be linked together without a primer?

RNA nucleotides

What is DNA polymerase?

An enzyme that adds DNA nucleotides to the 3’ end only of a growing strand

Requires a primer

Requires a DNA template

What is DNA polymerase III?

The main polymerase enzyme that elongates DNA starting from RNA primers

Used on leading and lagging strands

What is DNA polymerase I?

Removes the RNA primer (chews away from 5’ end) and replaces it with DNA (adds to the 3’ end)

What is DNA ligase?

Joins the Okazaki fragments together on the lagging strand

Makes a covalent (phosphodiester) bond between the 3’ end of one fragment and the 5’ end of another

Why Okazaki fragments?

On the lagging strand, the 5’ ends points to the fork

Nucleotides cannot be added in the 5’ end, only 3’ end, which is pointing in the opposite direction of synthesis

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