DB Lecture 29: DNA Replication

Question 1

Why does each chromosome have multiple sites at which DNA initiates?

Answer 1

large size

need to complete replication within a limited time in the cell cycle

Question 2

What is an ori?

Answer 2

origins of replication

located on average 100,000 bp apart

Question 3

What is origin firing?

Answer 3

initiation of DNA replication from an origin

highly coordinated at all origins simultaneously

Question 4

In what direction does nucleic synthesis occur?

Answer 4

5’ to 3”

Question 5

How does DNA synthesis begin?

Answer 5

from a pre-existing “primer” that provides a free 3’ hydroxyl onto which DNA polymerase adds the next nucleotide

Question 6

What is DNA primase?

Answer 6

subunit of DNA pol alpha

RNA polymerase that lays down a short RNA primer which is then extended by DNA polymerase

Question 7

How are RNA primers removed from Okazaki fragments?

Answer 7

the “old” RNA primer located ahead of a new primer extension product is displaced by DNA polymerase so that it is a “flap” (no longer annealed to the DNA template strand) which is easily cleaved off by FEN1 (flap endonuclease)
the nick between the 5’ end of the old Okazaki DNA and the 3’ end of the new Okazaki DNA is sealed by DNA ligase

Question 8

List the DNA polymerases.

Answer 8

DNA pol epsilon (synthesizes the leading strand)
DNA pol delta (synthesizes the lagging strand)
DNA pol alpha (synthesis of RNA primer and primer extension)
DNA pol beta (DNA repair)
DNA pol gamma (mitochondrial DNA replication)

Question 9

How do antivirals exploit the process of viral DNA replication?

Answer 9

they incorporate nucleoside analogues (e.g. azidothymidine and dideoxythymidine) that do not have a 3’ hydroxyl onto which subsequent nucleotides can be added

Question 10

What are telomers?

Answer 10

ends of chromosomes with repeat sequence of TTAGGG

fold into t-loop to distinguish it from broken DNA end and prevent end-to-end joining of chromosomes

Question 11

What are shelterin complexes?

Answer 11

complex of telomere-specific proteins that bind telomere repeat sequences that protects the end of the telomere

Question 12

How do we solve the end replication problem?

Answer 12

telomerase (ribonucleoprotein complex that carries its own RNA template) extends the lagging strand

Question 13

What is the Hayflick limit?

Answer 13

creates a limit to the number of cell divisions that can occur before cells become senescent
determined by telomere length
normal cells do not have telomerase activity