NAGE 2 - DNA Replication, Cell Cycle & Mitosis

Question 1

What is a faithful copy of DNA.

Answer 1

Before each cell division, a copy of the DNA is made to ensure genetic continuity to each daughter cell.

Question 2

What does semi-conservative replication mean?

Answer 2

Each daughter cell inherits one old and one new strand.

Question 3

Describe the process of DNA synthesis.

Answer 3

1) DNA helix is unzipped by DNA helicase
2) DNA primase synthesises RNA primers which bind to the DNA
3) DNA polymerase extends the chains from the primers, working in the 5’ to 3’ direction. dNTPs are used - energy released from hydrolysis of the triphosphate drives the reaction.
4) Leading strand is synthesised continuously, lagging strand requires many primers and forms okasaki fragments.
5) A ribonuclease removes the primers and replaces with DNA nucleosides, repair DND polymerase replaces the RNA with DNA, and DNA ligase joins the fragments using ATP.

Question 4

What do DNA polymerases require?

Answer 4

A template strand, an oglionucleotide primer, a supply of deoxynucleotide triphosphates.

Question 5

What are nucleoside analogs?

Answer 5

Molecules woth no 3’-OH group, which act as chain terminators.

Question 6

What is the replication fork?

Answer 6

The site of DNA synthesis

Question 7

What are sliding clamps?

Answer 7

Proteins that bind to DNA polymerase and stop it falling off the DNA

Question 8

What is the proofreading mechanism used in DNA replication?

Answer 8

DNA polymerase uses its 3’ to 5’ exonuclease activity to break phosphodiester bonds and remove incorrect bases.

Question 9

How does replication of the E.coli chromosome occur?

Answer 9

Chromosom replicates from an origin site called OriC - the DNA replicates in a bidirectional replication form, and replication finishes when the two forks meet at the other side of the circular chromosome.

Question 10

How does replication of the eukaryotic chromosome occur?

Answer 10

There are multiple replication origins distributed at intervals of 100 kilobase pairs. Each origin gives bidirectional replication forks, and replication finishes when all the forks have met.

Question 11

What are the stages of the cell cycle, and their relative time periods?

Answer 11

G1 phase (10 hours)
S phase is DNA synthesis (9 hours)
G2 phase (4 hours)
M phase is mitosis (1 hour)
G0 is the cells that have stopped dividing

Question 12

How do chromosomes appear in the cell cycle?

Answer 12

G1 - DNA of each chromosome are present as a single linear double helix of DNA
G2 - each chromosome has two identical sister chromtids
Mitosis - chromosomes separate

Question 13

What happens in the cell cycle?

Answer 13

Interphase - chromosomes not visible, cell growth
Prophase - condensed chromosomes, nucleolus and nuclear envelope dissolve, centrioles move to opposite poles of the cell.
Metaphase - chromosomes line up along the metaphase plate and attach to spindle fibres
Anaphase - centrioles contract and chromatids are separated
Telophase - DNA decondenses again, cytokinesis begins