Genetics DNA replication

Question 1

Describe the start of DNA rep & enzymes involved

Answer 1

Rep of DNA begins at ORIGIN OF REPLICATION (short stretches of DNA w specific nucleotide sequences)
Euk 2 ori per DNA molecule
Prok only 1

1. Helicase UNWINDS & UNZIPS DNA double helix into single strands
   - Breaking of H bonds betw. comple. base pairs
   - Forms rep. bubble & a rep. fork at each end
   - Parental strand acts as template for daughter strand
2. Single Stranded Binding proteins SSBPs stabilise single strands & prevent them from reannealing
3. Topoisomerase relieves strain ahead of rep. fork by Breaking, Swivelling & Rejoining each strand

Question 2

Describe priming of DNA rep

Answer 2

Primase catalyses synthesis of short [RNA primer] of 10 ribonucleotides complementary to DNA parental strands (template)

PROVIDE 3’OH GRP for DNA polymerase 3 to add new deoxyribonucelotides

Question 3

Describe elongation

Answer 3

DNA poly. 3 recognises & binds to FREE 3’OH end of primer, reads template strand in 3’ to 5’ direction selecting free deoxyribonucleotides thru CBP to 3’ OH grp via H bonds

Synthesises daughter strand in 5’ to 3’ direction by catalysing formation of phosphodiester bonds between adjacent deoxyribonucleotides of daughter strand.

Elongates daughter strand, proofreads(deoxyribonucleotide) ensure correct one added

DNA pol 3 will excise & replace if wrong

Question 4

Why can’t DNA pol 3 directly synthesise DNA?

Answer 4

DNA pol 3 can only add to existing 3’OH grp

->active site of DNA POL 3 only complementary to free 3’OH (primer provides)

Question 5

Synthesis of leading strand

Answer 5

• continuous from 5’ to 3’ direction
• DNA pol 3 towards rep fork
• only 1 primer needed

Question 6

Synthesis of lagging strand

Answer 6

Discontinuous, also 5’ to 3’
Okazaki fragments
DNA pol 3 moves away from rep fork
Many primers, 1 for each O fragment

Question 7

DNA proofreading + replacement of RNA primers + DNA ligation

Answer 7

DNA pol 1 proofreads synthesised region proofreading synthesized region on daughter strand via CBP

Excises and replaces deoxyribonucleotides

DNA ligase ligates Okazaki fragments via phosphodiester bonds

Question 8

Describe end of DNA rep & semi conservative process

Answer 8

Complementary parental & daughter strands rewind progressively into new double helix

SEMI CONSERVATIVE
- 2 strands of DNA separate by breaking hydrogen bonds betw base pairs of strands
- EACH strand acts as TEMPLATE -> form new strand
- NEW hydrogen bonds form betw based of old & new
- Daughter cell inherits resulting DNA molecule resulting in 1 new & 1 old

Gen 0 N¹⁵ N¹⁵ (100%)
Gen 1 N¹⁴ N¹⁵ (100%)
Gen 2 N¹⁴ N¹⁵ (50% each)
Gen 3 N¹⁴ (75%) N¹⁵ (25%)
Gen 4 N¹⁴ (87.5%) N¹⁵ (12.5%)