6.4.3 Events at the Replication Fork: The Lagging Strand

Question 1

note

Answer 1

• DNA polymerase adds nucleotides only to the 3’ end of a growing strand. The limitation means that only one of the new complementary strands can grow continuously in the direction of the moving replication fork. The strand that grows continuously is called the leading strand. The strand that grows away from the replication fork is called the lagging strand.

Question 2

Reiji Okazaki

Answer 2

• is attributed with discovering the phenomenon
  of the lagging strand.
• As the replication fork opens, the leading strand comprises a continuous strand of new nucleotides growing in the direction of the opening fork. The lagging strand grows away from the opening fork.
• The reason that the two strands grow in opposite direction is that DNA polymerase can add nucleotides only to the 3’ end of a strand. As the fork unwinds, the lagging strand is synthesized in as many 100 to 1000 nucleotide pieces that are called Okazaki fragments. Each Okazaki fragmentrequires an RNA primer to initiate its synthesis. The primers are removed, and DNA polymerase fills in the gap with DNA nucleotides. The fragments are joined together by ligase.
• Each of the parental strands has served as a template for the synthesis of a new complementary strand. The end product of DNA replication is two identical DNA molecules.

Question 3

Which of the following is not true of DNA polymerase?

Answer 3

• DNA polymerase only moves in the direction of the replication fork

Question 4

A biologist extracts DNA from mouse cell cultures, and isolates and purifies the various components needed for replication of the DNA. After DNA replication, she separates the strands and notices segments of DNA a few hundred nucleotides long. What component should she have added?

Answer 4

• Ligase

Question 5

Which of the following is true only concerning the lagging strand and not the leading strand?

Answer 5

• It involves the synthesis of more than one RNA primer

Question 6

DNA polymerase I is an enzyme that

Answer 6

• assists in removing RNA primers from the DNA strand and replaces them with DNA.

Question 7

At first, it seemed that this strand was synthesized (added to) in the 3′ to 5′ direction. The strand is also called __________.

Answer 7

• Lagging strand

Question 8

The leading and the lagging strands of DNA differ because

Answer 8

• DNA polymerase can only attach to the 3′ OH of an existing strand