Cell Cycle - DNA Replication

Question 1

What are the important enzymes in DNA replication

Answer 1

• Helicases
• Binding proteins
• DNA polymerase
• Primase
• DNA ligases

Question 2

Explain helicases

Answer 2

Unwind and unzip as well as breaks hydrogen bonds

Question 3

Explain binding proteins

Answer 3

Stabilise each strand at the replication fork

Question 4

Explain DNA polymerase

Answer 4

Adds new DNA nucleotides and can only be added to existing strands

Question 5

Explain primase

Answer 5

Builds a short RNA strand to attract DNA polymerase

Question 6

Explain DNA ligase

Answer 6

Sticks pieces of DNA together forming covalent bonds

Question 7

What must happen before a cell divides

Answer 7

Its whole genome must first replicate in S-stage

Question 8

Where does replication start in prokaryotes

Answer 8

One point known as the origin ( where replication fork is formed )

Question 9

Explain the unwinding of the DNA

Answer 9

• The enzyme helicase breaks the hydrogen bonds that keep the two strands together from the origin,
• The individual strands, now known as templates, are stabilized by single-strand binding proteins (SSB)
• DNA-primase which is an RNA-polymerase, will align a short stretch of RNA along the template as the DNA-polymerase cannot start the replication process, DNA polymerase can only add nucleotides to an existing strand

Question 10

Explain complementary base pairing

Answer 10

• DNA-polymerase will bring complementary nucleotides to the template, i.e. Adenine next to Thymine and Guanine next to Cytosine in a 5/ → 3/ direction to form the new DNA strand
• Hydrogen bonds form between the complementary bases it is suggested that this enzyme fits like a clamp around the template and that the strand moves through this clamp like a thread through a needle
• The one strand grows continuously in the 5/ → 3/ direction on the one template and is called the leading strand
• The other strand is added in short stretches and is called the lagging strand which forms loops in order to have the process go in a 5/ → 3/ direction these short stretches of DNA are called the Okazaki-fragments.
• The enzyme DNA polymerase digests the primers and extends the preceding Okazaki fragment in a 5/ → 3/ direction
  The linkage between Okazaki fragments to form a continuous chain is achieved with the aid of the enzyme DNA ligase

Question 11

Explain the end of DNA replication

Answer 11

• The end result is two new DNA molecules looking exactly the same as the parent DNA, sometimes mistakes can occur in the addition of the new nucleotides, i.e. wrong pairs are formed
• DNA polymerase, will proofread the newly-formed DNA to look for any miss-paired groups and replace the incorrect ones
• It might happen that the nucleotides on the template are removed instead of the one on the newly formed strand and this will result in mutations taking place
• The newly formed DNA will consist of one parent strand and one new strand, and we call this type of replication semi-conservative replication, one “old” strand is conserved in the new molecule

Question 12

Explain DNA replication in eukaryotes

Answer 12

• The basic process of replication is the same in eukaryotes as in prokaryotes
• However, due to the multitude of chromosomes and their much larger sizes the process is a bit more complex (8-10 hours to replicate)
• Replication starts at numerous positions along the DNA molecule which allows for the rapid replication of DNA
• These origins will be determined by the DNA structure