DNA replication - Eukaryotic DNA replication

20 important questions on DNA replication - Eukaryotic DNA replication

The assembly of which complex is needed for DNA replication in eukaryotes?

The assembly of the prereplication complex (preRC).

What acts as the first initiator of preRC assembly?

The origin recognition complex (ORC), which is a part of the preRC.

What does ORC promote?

The binding of certain proteins and a group of proteins called MCM helicase.
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To what does the binding of MCM helicase to the leading strand complete?

A process called DNA replication licensing.

Which of the proteins of the prereplication complex (preRC) do also play a role in the elongation?

Only MCM helicase. Cdc6, Cdt1 and ORC are released.

Which 3 types of DNA polymerase play a role in DNA replication within the cell nucleus?

Alfa, epsilon and sigma.

What does DNA polymerase alfa do?

It associates with primase, which then synthesizes a RNA-DNA primer.

What happens after the primer is made by DNA polymerase alfa and primase?

The DNA polymerase alfa / primase complex dissociates from the replication fork and is replaced by DNA polymerase epsilon or sigma.

How is the exchange DNA polymerase alfa to DNA polymerase epsilon or sigma, called?

Polymerase switch.

What is the role of DNA polymerase beta?

The removal of incorrect bases from damaged DNA.

What are translesion-replicatiing polymerases?

Polymerases that synthesize a complementary region over a piece of damaged DNA (which can not be replicated by DNA polymerase alfa, sigma and epsilon).

How are the RNA primers removed in bacteria? And in eukaryotes?

- In bacteria: DNA polymerase I.
- Eukaryotes: flap endonuclease.

What causes the flaps in primers (figure 11.21)?

The elongation by DNA polymerase sigma.

Why are telomeric repeat sequences needed?

Because DNA polymerase is unable to replicate the 3' ends of DNA strands (because there is no room for a primer).

What happens when the problem of the inability of DNA polymerase to replicate the 3' end is not solved?

The chromosome would become shorter with each round of DNA replication.

How is this problem solved?

Additional DNA sequences are attached to the ends of telomeres by an enzyme telomerase.

In which three phases does telomere lengthening occur?

1. The binding of telomerase: telomerase has an RNA sequence known as TERC which contains a sequence complementary to a sequence in the telomeric repeat sequence;
2. Polymerization: the RNA sequence beyond the binding site functions as a template for the lengthening of the 3' overhang;
3. Translocation: every time telomerase moves further and adds new nucleotides to the 3' end.

Which subunits of telomerase catalyze telomere lengthening?

Telomerase reverse transcriptase (TERT).

What happens when people age?

The telomerase activity decreases, so the telomeres become shorter. When they are too short, the cells become senescent, they cannot divide.

How do cancer cells keep dividing when telomeres become shorter?

Most cancer cells have mutations that increase the activity of telomerase

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