Replication, initiation, and telomeres

How many origins of transfer do bacterial DNA have?

one

How is replication in bacteria initiated?

binding of initiator proteins to replication origin

Is there any loss of DNA in replication in bacteria?

no

Give one feature of replication origins

A-T rich which makes them easier to break (2 H-bonds)

Replication in bacteria (5 steps)

1. initiator proteins wrap the DNA around the complex and cause the helix beside it (replication origin) to destabilise

2. helicases are loaded by loading proteins and attach to the helix

3. loading proteins detach and helicase initiates the replication cycle

4. primase creates first RNA primers

5. rest of complex assembles and replication starts

How many origins of replication do eukaryotes have?

multiple proportional to their size

G phase:

origins are loaded with replication machinery (replisome)

S phase: 4 points

• replisome is activated

• no new helicase loading

• phosphoryaltion of proteins prevents new assemby of replication complex until next cell cycle

• histones are produced to form new nucleosomes

Why are origins activated at different times during S phase?

to ensure early replication stalling or mistakes can be repaired

State 2 problems that arise as a result of replication at the ends of chromosomes

• ends look like broken chromosomes to DNA repair machinery

• progressive shortening of lagging strand

Progressive shortening of lagging strand

because of the incomplete replication of the linear DNA, the DNA at the very end of the chromosome cannot be fully copied in each round of replication

How is the issue of progressive shortening of lagging strand fixed?

telomere replication by telomerase

How is the issue of seemingly broken chromosome ends fixed?

protecting chromosome ends via shelterin complex

What are telomeres?

repetitive ends of chromosomes

How are telomeres extended? (2 points)

• replication by telomerase, forming a repetitive DNA motif

• contains an RNA template to replicate that sequence

Telomere replication by telomerase (3 points)

1. telomerase pairs up with parent strand using its integral piece of RNA

2. the RNA is used as a template to extend the parent strand

3. this is done 7 times and new RNA primer can be inserted and DNA polymerase can finish the end of the chromosome

What is the shelterin complex?

DNA binding proteins that tuck the single stranded 3’ overhang of telomere into the double strand, resulting in a T-loop

What is the main benefit of the shelterin complex?

hides chromosome ends from DNA repair machinery, preventing erroneous repair