2- Dna Replication And Mitosis

Question 1

Describe how a new dna strand is synthesised

Answer 1

“DNA polymerases add deoxynucleotide tri-phosphates to the 3’ end of a DNA molecule.
Synthesis is driven by the release of energy from the hydrolysis of the tri-phosphate.
DNA polymerases need a template and a primer. They cannot start a new chain from scratch.
”

Question 2

Describe discontinuous dna synthesis on the lagging strand

Answer 2

As the replication fork moves forward, single-stranded DNA is exposed on the lagging strand.
When about 100-200 bases are exposed, a short RNA primer of about 5-10 bases is synthesised by an RNA polymerase called primase.
The primer is extended by a DNA Polymerase until the last RNA primer is reached forming an Okazaki fragment.
A ribonuclease removes the RNA primer using a 5’ to 3’ exonuclease activity.
DNA polymerase then synthesises DNA through the RNA primer region.
DNA ligase joins the two adjacent strands of DNA together using ATP.

Question 3

How is dna accuracy maintained

Answer 3

“Proof reading: DNA Polymerase has 3’ to 5’ exonuclease activities for proof reading.
RNA Primers: Inaccurate RNA primers are replaced by accurate DNA

Question 4

Describe replication of E. Coil

Answer 4

“there is a single replication origin and the two forks meet on the other side of the circular chromosome

Question 5

Describe eukaryotic dna replication

Answer 5

“Eukaryotic chromosomes are linear and very long.
Multiple replication origins are distributed at intervals of about 100 kb.
Each replication origin gives bidirectional replication forks.
Replication is finished when all the forks have met.

Question 6

What does the sliding clamp do

Answer 6

Make sure DNA polymerase is in the correct place

Question 7

Describe the interphase phases

Answer 7

G1 phase, material is accumulated, cell grows
In S phase the entire genome is replicated
In G2 phase, the cell is prepared for mitosis, cell organelles are duplicated etc.