DNA replication

When does DNA replication take place in the cell cycle?

occurs during the S-phase

What are the three hallmarks of DNA replication?

1) semi-conservative

2) bi-directional

3) semi-discontinous

What is an origin of replication>

a specific sequence of DNA where duplication of the genome begins

What enzyme separates the strands of DNA at the replication forks?

Helicase

What proteins coat the single stranded DNA?

single-stranded binding proteins (SBB)

What does DNA gyrase do?

moves ahead of the helicase to relieve supercoiling stress

What does primase do? Why is it necessary?

• primase makes RNA primer

• it makes short RNA sequences

Is primer made of DNA or RNA?

it is made of RNA

where does DNA polymerase III add new nucleotides?

it adds new nucleotides to the 3’ end of a growing DNA strand

What is the direction of polymerization?

polymerization proceeds in the 5’-3’ direction

What is the difference between the leading strand and the lagging strand?

• leading strand synthesizes DNA in one long piece

• lagging strand lays down new RNA primer and synthesizes another

What are Ozaki fragments? Which strand are they synthesized on?

short sequences of DNA that are synthesized on the lagging strand during DNA replication

What does DNA polymerase I do?

removes RNA primer

what does DNA ligase do?

covalently links the ends together, resulting in a seamless strand of DNA

How does the proofreading activity of DNA polymerase reduce the error rate?

It detects and cuts mismatched nucleotides immediately after they are added

In which direction does proofreading occur?

3’-5’ direction

Why does this mean polymerization only occurs in the 5’-3’ direction?

because DNA polymerase can only attach nucleotides to the 3’ hydroxyl on the growing strand

What is the end replication problem?

the inability of cells to completely copy the very tips of their DNA strands

On which strand does the DNA shorten?

the lagging strand

How do telomeres solve the end replication problem?

they act as repetitive DNA buffers at the end of chromosomes

What is added to the end of telomeres?

telomerase

what does telomerase use as a template?

• TTAGGG on one DNA strand

• AATCCC on the other matching strand

what is the sequence of the human telomere repeat?

5’ TTAGGGG 3’

How do telomeres protect the ends of chromosomes?

they act as disposable buffer zones

Where is telomerase usually expressed?

germ cells, stem cells, cancer cells

What are some conditions that are associated with shortened telomeres?

bone marrow failure, anemia

Why do cancer cells express telomerase?

so they can divide indefinitely without dying