Molecular Genetics - DNA Replication

Question 1

Helicase function

Answer 1

• “Unzips” DNA molecule
• Produces replication fork (template for replication)

Question 2

Single stranded binding proteins (SSBPs) function

Answer 2

Bond to strand to prevent the strands from reconnecting by preventing formation of hydrogen bonds

Question 3

Topoisomerase function

Answer 3

• After strands are separated, tension is created in still joined sections
• Cuts one of the strands near replication fork to allow them to untangle
• Cut strand is then rejoined

Question 4

RNA primase function

Answer 4

• In order for new strand of DNA to be synthesized, RNA primer must be added (starting point for synthesis)
• Places RNA primers
• On lagging strand, new primer must be places each time the replication fork is extended

Question 5

DNA polymerase III function

Answer 5

• During elongation, it adds new nucleotides to RNA primer
• Can only build new strand in 5’ to 3’ direction
• Moves continuously along leading strand, but on lagging strand it eventually runs into primer placed ahead

Question 6

DNA polymerase I function

Answer 6

Removes RNA primers and replaces with DNA nucleotides

Question 7

DNA ligase function

Answer 7

• Joins Okazaki fragments once leading + laggings strands are formed and RNA primers are removed
• Catalyzes formation of phosphodiester bond between last nucleotide of one fragment and first nucleotide of the next

Question 8

DNA polymerase II function

Answer 8

• Reads newly formed strands and checks for distortions (areas w/ mismatched base pairs)
• If distortion detected, it will remove a small section of the DNA
• Section will be replaced with appropriate nucleotides and fragments are joined by DNA ligase

Question 9

Order of enzymes used in DNA replication

Answer 9

Helicase, SSBPs, topoisomerase, RNA primase, DNA polymerase III, DNA polymerase I, DNA ligase, DNA polymerase II

Question 10

What is semiconservative replication?

Answer 10

• One strand from the parent DNA becomes half (semi) of each of the new DNA molecules
• Parent strands are used as templates to build new strands by matching base pairs

Question 11

What are Okazaki fragments and why do they form? How do they affect rate of replication?

Answer 11

• Lagging strand is made up of multiple primers and short DNA fragments
• Form since DNA polymerase III can only synthesize 5’ to 3’ (lagging strand runs 3’ to 5’) so new primer must be places each time replication fork is extended
• Lagging strand replicates slower because it takes time to remove primers, replace with nucleotides, and join fragments

Question 12

What are two ways to correct errors that occur in replication?

Answer 12

• DNA polymerase III cannot move forward if base pairs are mismatched, so it backs up tp replace the incorrect bases and then continues
• DNA polymerase II checks newly formed strands for distortions and cuts out small DNA section where they are detected. That section is then replaced with proper nucleotides and joined by DNA ligase