DNA Replication Flashcards
Who first hypothesised the basic principle: Base pairing to a template strand?
The Basic Principle: Base Pairing to a Template Strand
• First hypothesized by Francis Crick and James Watson
What happens when a double helix replicates?
• When a double helix replicates, each of the two daughter molecules will have one old strand, from the parental molecule, and one newly made strand—> semiconservative model.
What is the initiation step in DNA replication? (6)
- The replication of a chromosome begins at particular sites called origins of replication—>these are short stretches of DNA having a specific sequence of nucleotides
- Bacteria have circular chromosomes with a single origin
- Proteins that initiate DNA replication recognize this sequence and attach to the DNA, separating the two strands and opening up a replication ‘bubble’
- Replication of DNA then proceeds in both directions until the entire molecule is copied
- A eukaryotic cell may have hundreds or thousands of origins—>multiple bubbles form and eventually fuse, speeding up the process of replication in very long strands
- At the end of each replication bubble is a replication fork, is a Y-shaped region where parental strands of DNA are being unwound
What is helicase? (2)
- Helicase is an enzyme that untwists the double helix at the replication forks, separating the parental strands and making them available as template strands
- After the parental strands are separated, single-strand binding proteins bind to the unpaired DNA strands, stabilizing and keeping them from re-pairing
What is the function of topoisomerase?
The unwinding/untwisting of the DNA strand causes tension and strain ahead of the replication fork—>
topoisomerase helps relieve the strain by breaking, swivelling and re-joining the DNA strands. The unwound sections of parental DNA strands are now available to serve as templates for the synthesis of new complementary DNA strands.
Can the enzymes that synthesize DNA initiate the synthesis of a polynucleotide?
However, the enzymes that synthesize DNA cannot initiate the synthesis of a polynucleotide—>they can only add DNA nucleotides to the end of an already existing strand that is base-paired with a template strand.
What is the initial nucleotide chain that is produced during DNA synthesis? (3)
The initial nucleotide chain that is produced during DNA synthesis is actually a short stretch of RNA.
o This RNA strand is called a primer and is synthesized by primase.
o Primase starts a complementary RNA chain from a single RNA nucleotide, using the DNA strand as a
template
o The completed primer(5-10nucleotideslong)is base-paired to the template strand and the new
DNA strand will start from the 3’ end of the primer
What is the elongation step in DNA replication? (4)
• Enzymes called DNA polymerases to catalyse the synthesis of new DNA by adding nucleotides to a pre-existing chain
• Two of these enzymes play a major role in replication—>DNA polymerase I and III
• Most of the DNA polymerases require a primer and template strand—>along which complementary
nucleotides are lined up
• In bacteria DNA pol. III adds a DNA nucleotide to the RNA primer and then continues to add nucleotides to the growing end of the new DNA strand (500 per second in bacteria, 50 in humans)
What is antiparallel elongation? (5)
- As noted the two ends of a DNA strand are different, giving each strand directionality, like a one-way street
- In addition, the two strands of DNA in a double helix are antiparallel, meaning they are orientated in opposite directions of each other
- Therefore, the two new strands formed during DNA replication must also be antiparallel to their template strands
- Because of the structures of DNA polymerases, they can only add nucleotides to the 3’ prime end of a primer or growing DNA strand, never to the 5’ end
- Thus a new DNA strand can elongate only in the 5’—>3’ direction
Describe what happens on the leading strands. (4)
• Along one template strand, DNA pol III can synthesize a complementary strand continuously
by elongating the new DNA in the mandatory 5’—>3’ direction
• DNA poll III remains in the replication fork on that template strand and continuously adds nucleotides
to the new complementary strand as the fork progresses
• Strand made by this mechanism is known as the leading strand
• Only one primer is required for DNA poll III to synthesize an entire leading strand
Describe what happens to the lagging strand. (5)
• To elongate the other new strand of DNA in the mandatory 5’—>3’ direction, DNA poll III must work along the other template strand in the direction away from the replication fork
• The DNA strand elongating in this direction is called the lagging strand
• Even though synthesis of both strands occurs simultaneously and at the same rate, it is known as the lagging strand because synthesis is delayed slightly relative to the synthesis of the leading strand
——-> enough template strand needs to be exposed first before elongation can begin
• Unlike the leading strand (continuous), the lagging strand is synthesized discontinuously, as a series of segments
• These segments on the lagging strand are called Okozaki fragments
Where does energy for the polymerisation of nucleotides into a strand of DNA come from? (2)
> an incoming nucleotide has three phosphates before it becomes part of a DNA strand
with a similar molecule, ATP provides energy for metabolism
the hydrolysis of two of its three phosphates provides free energy to do work in the
same way incoming nucleotides can supply energy for their own polymerization reactions
How the incoming nucleotide provides the free energy needed for its addition to the new
strand: (5)
- Two phosphates of the incoming nucleotide are first released together as pyrophosphate and pyrophosphate is further hydrolyzed into two inorganic phosphate
- these 2 exorganic reactions are energetically coupled to the polymerization reaction that adds the nucleotide to the new strand
- A nucleotide has been added to the growing DNA strand by DNA polymerase
- After the new nucleotide has been added it provides the next 3’ hydroxyl group that’s available to covalently bond to the next new nucleotide
- This process continues until the entire template strand has a new complementary strand
What does Watson and Crick’s model predict about DNA replication?
Watson and Crick’s model predicts that when a double helix replicates, each of the two daughter molecules will have one old strand, from the parental molecule, and one newly made strand. This semiconservative model can be distinguished from a conservative model of replication, in which the two parental strands somehow come back together after the process (that is, the parental molecule is conserved). In yet a third model, called the dispersive model, all four strands of DNA following replication have a mixture of old and new DNA.
