6.4.2 Events at the Leading Strand, Part II

Question 1

replication bubble

Answer 1

• occurs at an origin of replication and consists of two replication forks. Replication proceeds in opposite directions along each of the replication forks

Question 2

note

Answer 2

• Review: The DNA strands are antiparallel meaning that their 5’ to 3’ orientation runs in opposite directions.
  • Review: The strand that grows continuously towards the replication fork is called the leading strand. The strand that grows discontinuously away from the replication fork is called the lagging strand.
• Review: The replication bubble enables DNA replication to occur in two directions. Each replication bubble consists of two replication forks. The enzyme DNA polymerase attaches to each replication fork.
• At each replication fork, the original DNA strands (the labeled strands in the illustration) are unwound. The arrow underneath the illustration points left indicating the overall direction of unwinding and replication.
• The illustration shows that the two DNA strands are
  antiparallel, meaning that their 5’ to 3’ orientations run in
  opposite directions.
• DNA polymerase can build new strands in the 5’ to 3’
  direction only. The limitation means that the new strands
  will be synthesized in opposite directions.
• The strand that is synthesized continuously in the direction of the replication fork is called the leading strand. The strand that grows opposite the direction of the replication fork is called the lagging strand.
• The illustration shows molecular details concerning the
  polymerization of the new DNA strand.
• Review: The nucleotide is the building block of the new DNA strand. A nucleotide consists of a sugar, a phosphate, and a nitrogenous base. The 5’ and 3’ designations refer to carbons on the sugar of the nucleotide.
• DNA polymerase requires an open 3’ OH group to -attach a new nucleotide. When the DNA polymerase is just starting, that OH group is provided by the 3’ end of an RNA primer strand that has been added by primase. The first DNA nucleotide is added to this open 3’ OH group. Subsequent nucleotides are added to the previously added nucleotide. The illustration shows a new nucleotide that will join the strand. Notice that the orientation of the new strand is opposite of the orientation of the parental strand, meaning that the
  strands are antiparallel.

Question 3

True or false?

RNA primer is an enzyme that synthesizes RNA to initiate synthesis of a DNA strand.

Answer 3

• false

Question 4

We know that a DNA polymerase can only catalyze a reaction if there is a free OH group on a nucleotide. Why is this?

Answer 4

• DNA polymerase is sensitive to the shape of the OH part of the nucleotide molecule

Question 5

Primase does not:

Answer 5

• Depend on the 3′ OH group of an existing strand

Question 6

A template DNA strand has the sequence 5′ ATTGC 3′. A possible RNA primer sequence is

Answer 6

• 5′ GCAAU 3′

Question 7

Which of the following statements concerning replication bubbles and forks is not true:

Answer 7

• Both prokaryotes and eukaryotes have multiple replication bubbles.