DNA Replication: Eukaryotes

Question 1

DNA polymerase in Eukaryotes

Answer 1

• Slower than bacterial counterparts
• synthesises DNA at 100-200 bp (size of Okazaki fragments) per second
• Several types (alpha, gamma, sigma etc.)

Question 2

DNA Replication in Eukaryotes

Answer 2

• Mnay origins of replication per chromosome, each generating bidirectional replication forks
• Linear chromosomes –> problem completing lagging strand on each end of chromosome

Question 3

How replicating linear chromosomes shortens DNA

Answer 3

• Results in shortening of one 5’ end of each daughter DNA molecule as DNA polymerase can’t fill gap left by last Okazaki removed (due to primer)
• Repeated DNA replication results in shorter DNA

Question 4

Preserving ends of chromosomes - Telomeres

Answer 4

• Terminal end of chromosome with a highly repeated sequence (TTAGGG)

Question 5

Telomerase

Answer 5

• A specialised reverse transcriptase
• Special RNA containing it can add copies of the telomere repeat sequence to replace what’s lost during replication
• RNA acts as a template for DNA synthesis

Question 6

How Telomerase works

Answer 6

(1) Binds the lagging strand, the telomerase RNA acts as template for extension of lagging strand for another Okazaki fragment
(2) Telomerase translocates
(3) Restores telomere length

Question 7

Not all cells have telomerase activity

Answer 7

• Only germ cells and other actively dividing cells do
• Germ cells can replicate indefinitely while somatic cells don’t have T activity

Question 8

Length of telomeres

Answer 8

• At birth, 10,000 bp in length - enough repeats to allow replication for whole lifespan

Question 9

Telomeres + Cell death

Answer 9

• If too short, apoptosis is triggered + may be important in determining life span of organism
• T activity present in cancer cells, allowing indefinite division
• T inhibitors are in clinical trials for treatment