6.4.1 Events at the Replication Fork: The Leading Strand

Question 1

note

Answer 1

• The prokaryotic cell polymerizes 500 ucleotides/second. The eukaryotic cell synthesizes only 50 nucleotides/second. The eukaryotic cell, however, contains between four and five billion nucleotides! The prokaryotic cell chromosome is circular, and replication occurs efficiently. Eukaryotic cells employ the use of multiple replication bubbles, greatly speeding up replication.
  • Each replication bubble has two y-shaped replication forks.
• DNA replication occurs at sites on the molecule called
  replication bubbles, and both strands are replicated
  simultaneously at a y-shaped structure called the
  replication fork. The position of the fork is constantly
  moving as replication proceeds. The illustrations on the left show replication bubbles and forks.
• Synthesis of both complementary strands occurs in the 5’ to 3’ direction. The strand that is growing towards the
  replication fork grows continuously and is called the
  leading strand. Note that the newly formed leading strand is antiparallel to the parental strand that served as its template.

Question 2

DNA replication

Answer 2

• continuous in one strand and discontinuous in the other. Events basic to the process of DNA replication
  and specific to the leading strand are discussed below

Question 3

The overall events by which both strands are replicated are as follows:

Answer 3

1. Strands are separated and unwound by DNA helicase.
2. The strands are prevented from coming together by helix-destabilizing proteins.
3. The enzymes that catalyze the synthesis of the new complementary strands are called DNA polymerases. DNA polymerase can only add nucleotides to the 3’ end of the growing nucleotide strand.
4. RNA primer is required initiate synthesis of either new strand to provide the first 3’ end that the enzyme requires. The RNA primer is later removed and substituted with DNA.

Question 4

During DNA synthesis, elongation of the leading strand:

Answer 4

• Depends on the action of DNA polymerase.

Question 5

________________________ is the enzyme that catalyzes the polymerization of new DNA.

Answer 5

• DNA polymerase

Question 6

The __________ is at the center of the replication bubble.

Answer 6

• origin of replication

Question 7

The purpose of multiple origins of replication is to

Answer 7

• speed up replication of eukaryotic chromosomes

Question 8

True or false?

DNA polymerase can only polymerize reactions in the 3′ to 5′ direction of the template DNA strand.

Answer 8

• true

Question 9

The slow strand-synthesis rate in prokaryotes is overcome by:

Answer 9

• Prokaryotes actually have a relatively rapid strand synthesis rate

Question 10

DNA polymerase performs all of the following activities and functions except

Answer 10

• relax DNA supercoiling.