DNA replication, the cell cycle and mitosis

Question 1

Initiation steps of DNA replication

Answer 1

• Topoisomerase unwinds DNA.

- DNA helicase unzips two strands, uses ATP to break H bonds between base pairs.

Question 2

Leading strand synthesis process

Answer 2

• RNA primer placed at origin of replication
• DNA polymerase adds to RNA primer and synthesises continuously in a 5’ to 3’ direction.
• RNA primer is removed

Question 3

Lagging strand synthesis process

Answer 3

• DNA primase synthesises multiple short RNA fragment primers
• DNA polymerase adds to RNA primer, starting Okazaki fragment
• Ribonuclease removes RNA primer after fragment completion.
• Repair DNA polymerase replaces RNA with DNA
• DNA ligase joins the two fragments together using ATP to make newly synthesised strand continuous

Question 4

Name of starting point for replication

Answer 4

Origin of replication

Question 5

Name of site of DNA synthesis

Answer 5

Replication fork

Question 6

Role of sliding clamp in DNA replication

Answer 6

Ensures DNA polymerase stays attached during synthesis

Question 7

Role of single-stranded DNA-binding proteins in DNA replication

Answer 7

Ensures DNA remains extended for replication

Question 8

What is semi-conservative replication

Answer 8

Double-stranded daughter copies of parental DNA are made up of one strand from the parental DNA and a newly synthesised strand, complementary to the parental strand.

Question 9

Process of proof-reading and correction in DNA replication

Answer 9

• Before a new nucleotide is added, previous nucleotide is checked for correct base-pairing
• Any incorrect bases are removed by 3’ to 5’ (opposite direction to synthesis) exonuclease activity of DNA polymerase
• New correct nucleotide added

Question 10

Process of eukaryotic chromosome replication (and diagram see notes)

Answer 10

• Eukaryotic chromosomes are linear and very long
• Multiple origins of replication established around 100 kilo base pairs apart
• Each replication origin gives bi-directional replication forks
• Replication forks eventually meet to synthesis entirely new chromosome

Question 11

E.coli chromosome replication process (and diagram see notes)

Answer 11

• Replication starts at unique origin
• Two replication forks proceed simultaneously in opposite directions
• Bacterial chromosomes are circular and continuous so replication forks eventually meet at the other side of the chromosome to where the origin is located

Question 12

List cell cycle phases in order

Answer 12

M phase
G1 phase
- G0 phase
S phase
G2 phase

Question 13

M phase features

Answer 13

Mitosis and cell division

Question 14

G1 phase features

Answer 14

Gap phase 1

DNA in the form of a single linear double helix

Question 15

S phase features

Answer 15

Synthesis phase

DNA synthesis and replication takes place

Question 16

G2 phase features

Answer 16

Gap phase 2

Each chromosome has two identical sister chromatids produced by S phase ready for mitosis (M phase)

Question 17

G0 phase features

Answer 17

Exits from G1 phase

Cells which have stopped dividing, apoptotic cells etc.

Question 18

Mitosis phases in order

Answer 18

Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Interphase

Question 19

Late prophase chromosome condition

Answer 19

Chromosomes condensed, each contains two sister chromatids

Question 20

Metaphase chromosome condition

Answer 20

Chromosomes condensed, now aligned on the central plane of the spindle

Question 21

Anaphase chromosome condition and process

Answer 21

Sister chromatids move to opposite poles of spindle. Centromere splits, ensuring each new cell gets one half of new chromosomes

Question 22

Telophase chromosome condition and process

Answer 22

Two daughters cells formed with half of total chromosomes in each

Question 23

Cytokinesis process

Answer 23

Division of cytoplasm

Question 24

Interphase G1 begins after telophase, main feature

Answer 24

Condensation of chromosomes is reversed

Question 25

Interphase G2 before prophase of mitosis, main feature

Answer 25

Chromosomes uncondensed and not visible but about to undergo condensation