T4 Module 2

Question 1

proposed models of DNA replication

Answer 1

Semi-conservative model
- complementary strands, each a template to synthesize a new daughter strand
- HB broken between them to replicate

Conservative model
- 1 replication identical, 1 new

Dispersive model
- patchwork DNA replication

Question 2

explain proof for semi-conservative model of DNA replication

Answer 2

Mendelson + Stahl (50s)

• cultured E-coli cells in radioactive N15, collect DNA
• transfer cells to N14 medium, after cel division collect DNA
• another cell division and DNA collection in N14
• centrifuge samples to separate samples by density

FINDINGS
- conservative model rejected: after 1st replication, found only medium density DNA (not heavy and light)
- dispersive model rejected: after multiple divisions, only medium and light density DNA appear (not just medium density)

Question 3

DNA replication differences in prokaryotes vs eukaryotes

Answer 3

key difference in INITIATION

Prokaryotes
- starts in S-phase @ ONE origin of replication on DNA
- Continues around circular chromosome
- faster
- different DNAP used
- no gap in new DNA after primer removed

Eukaryotes
- starts in S-phase @ multiple origins
- slower
- DNAP 3 (elongation) and 1 (removed primer, replaces with DNA nucleotides)
- DNA ligase bridges gaps in okazaki fragments (new nucleotides + phosphodiest. bond)

Question 4

Replication complex

Answer 4

DNA helicase
- binds to parental DNA @ origin of replication, initiates unwinding
- breaks HB between nucleotide pairs
Single stranded binding pr-
- bind to stabilize each parent strand before replication begins
Topoisomorases
- bind upstream of replication fork
- minimize strain that occurs there
RNA primase
- synthesizes RNA primers
DNA polymerases
- in prokaryotes: 1 elongates, 3 removes primers

in eukaryotes
DNA ligase
- “glues” okazaki fragments together by bringing in new nucleotides

Question 5

process of DNA replication

Answer 5

• replication fork created by DNA helicase
• SSBPr- stabilize parents strands
• topoisomerase minimized strain upstream of replication fork

DNA synthesized 5’-3’ direction
DNAP proofreads for incorrect nucleotides, can replace them

leading strand: smooth work
lagging strand: okazaki fragments made
- eukaryotes need DNA ligase to attatch fragments

eukaryotes
- primer on lagging strand leaves free nucleotides that need to be cut
- telomeres prevent DNA becoming shorter (repeated sequence TTAGGG)
- for stem and germ cells, telomerase extends telomeres by adding sequence repeats