4./5. Replication

Question 1

(DNA)-Replication

Answer 1

Def./Loc.:

• > process in which DNA produces exact copy of itself
• takes place in S-phase of cell cycle

Why does it happen?
-> basis for reproduction of cells: passing of genetic information to daughter cells. For that happen, genetic informt. have to duplicate before each cell division

Question 2

Replication of linear DNA/ eukaryotic DNA

Answer 2

1. Unwinding of DNA by DNA-helicase
   - disrupts hydrogen bonds btw base pairs -> replication fork is formed
   - DNA-Topoisomerase I avoid supercoiling of replication fork
2. Primer Binding
   - RNA-primer binds to 3’end of leading strand (3’-5’) -> starting point for repl.
   (Primer are generated by DNA-primase α)
3. Elongation/Replication
   - DNA-polymerase δ binds to primer and begins adding complementary DNA-nucleotides in 5’-3’ direction (=continuous)
   - at lagging strand(5’-3’) DNA-polymerase has to work antiparallel -> Okazaki-fragments and RNA-Primer are added by DNA-polymerase α (=discontinuous)
4. Termination
   - Exonuclease(DNA-polymerase β) removes all RNA-Primers, which are replaced by complementary DNA-ncltds. (=Proofreading) *
   - DNA-Ligase joins Okazaki-fragments with new DNA-ncltds.

*replication problem, explained on next card

Question 3

Replication problem at the end (regardless to linear DNA)

Answer 3

• after removing of RNA-Primers, a 3‘-ending is missed -> DNA- Polymerase cannot linked -> means Telomere (=endings of Chrmsm) get lost during every replication -> shorted DNA
• > but the enzyme Telomerase (kind of reverse Transikriptase) avoids shortening of DNA and can replicate the ends of eukaryotic Chrmsm

Question 4

DNA repair

-> Def. of Fidelity of replication

Answer 4

Fidelity of replc.

• Def: means faithful replc. of DNA and production of accurate daughter DNA using parental DNA as template
• high fidelity is achieved by high activity of DNA-polymerases (helping to select correct base for insertion into newly synthesised DNA)

Question 5

DNA repair

• > repair mechanisms/ components
• > two diseases (both autosomal recessive)

Answer 5

1. Photoreactivation — uses Photolyase to split cyclobutane, pyrimidine diner formed by UV light
2. Alkyltransferase — DNA repair protein, remove alkyl groups from guanine and thymine, transfers it to itself, causing its inactivation
3. Excision repair
   - base-exc. — replace just the defective base
   - nucleotide exc. — cut out segment of DNA around damaged base
4. Mismatch repair — correct mispaired base pairs wh escape proofreading
5. Hereditary repair defects (mutations in excision repair genes or DNA-ploysm.):
6. Cockayne syndrome — poor growth, premature aging, sensivity to sunlight
7. Xeroderma pigmentosum — impairment of repairing damage from UV, skin cancer in young age

Question 6

Differences btw linear/eukaryotic and circular/prokaryotic DNA

Answer 6

Differences
1. Location
Eu: inside nucleus
Pro: in cytoplasm

2. Initiation
   Eu: multiple origins/replicons (otherwise it would lasts a month)
   Pro:contain a single origin/replicon
3. Enzymes
   Eu:
   - DNA-polymerase α,δ,ε (β,γ)
   - usually Topoismerase I (breaks one of DNA-strand during movement of replc. fork)
   Pro:
   - DNA-polymerase III (DNA-Synthese) and I (DNA-repair)
   - uses Topoisomerase II clld. DNA-gyrase (introduces a nick in both DNA strands)
4. Termination
   Eu: have several termination sides, unique end-replication problem (telomeres)
   Pro: have a single termination site (the two replication forks meet at this site and stopping replic process)
5. Duration
   Eu: slower -> more and denser DNA, includes histonse
   Pro: fast

Similarities:

• semi-conservative
• bidirectional
• semi-discontinues (leading & lagging strand)