TOPIC 11: DNA replication Flashcards
Explain how ‘complementary base pairing’ related to DNA replication
Complementary base pairing: During the process of DNA replication when the two strands of DNA unwind as a result there are new complementary bases placed in the strand.
Explain how ‘semi-conservative replication’ is related to DNA replication
Semi-conservative replication: This is where we have two strands of DNA: one is the parent DNA template or strand (initial DNA strand) and the other DNA strand is the synthesised complementary strand (daughter DNA).
Explain the role of the enzymes and proteins required for DNA replication
Primase: Function: It synthesizes small pieces of RNA sequences (primers) that are complementary to that of a single stranded piece of DNA
DNA Polymerase lll: Function: Synthesises and proofreads new strand of DNA (check to see if there is any error in the incorporation of the nucleotides).
DNA polymerase l: Function:
It synthesizes nucleotides (so helps with the polymerisation of the nucleotides)
Also has a role in cutting off certain nucleotides from the strand of the DNA. This is during the proofreading stage where the polymerase l checks for any errors such as an error in the incorporation of a nucleotides(s), if there is however the nucleotide will as a result be cut off from the strand in order to prevent any mutations in the DNA from occurring.
DNA ligase: Function:
It is a DNA-joining enzyme in which the ligase joins fragments of DNA if the nucleotides of the strands are complementary.
If there is a breakage in the double strand in DNA, DNA ligase will be able to repair it.
DNA Helicase: Function:
Breaks the hydrogen bonds between DNA strands (unwinds the DNA)
Single-stranded binding proteins (SSBPs): Function:
Keeps the parental strands apart
Describe the process of DNA replication under the following headings
- Initiation
- Elongation
- Termination
• Initiation
This is where the double helix DNA molecule unwinds. This is done by the enzyme helicase (the helicase breaks the hydrogen bonds between the complementary base pairs).
• Elongation
Leading strand
• A primer comes along and binds to the end of the leading strand. The primer functions as the starting point of the DNA replication.
• DNA polymerase binds to the leading strand and adds new complimentary nucleotide bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction
Lagging strand
• There are several of RNA primers are made by the primase enzyme and bind at various sites of the lagging strand
• Chunks of DNA, called okazaki fragments are then added to the lagging strand also in the 5’ to 3’ direction. This type of replication is called discontinuous. But these okazaki fragments are joined later.
• Termination
- When all of the bases are matched up (AT, GC) an enzyme called exonuclease strips away the primer(s). The gaps where the primers were are then filled by more complementary nucleotides
- The new strand is proofread to make sure there are no mistakes in the new DNA sequence
- DNA ligase seals up the sequence of DNA into two continuous double strands.
- The new DNA automatically winds up into a double helix after it has been sealed up by the DNA ligase.
Draw, label and explain a simple diagram of the ‘replication fork’
Diagram of ‘replication fork’
Describe the differences between the leading and lagging strand
Leading strand
• Does not require DNA ligase
• Its template is in the 3’ to 5’ direction
• The development of a leading strand begins immediately at the begging of replication
• Only a single primer is required
• Growth is continuous
Lagging strand
• Does not require ligase for joining okazaki fragments
• Its template is in the 5’ to 3’ direction
• The development of a lagging strand begins a bit later than the leading strand
• Starting of each okazaki fragment requires a new RNA primer
• Growth is discontinuous
List the proteins/enzymes involved in the process of replication
- DNA ligase
- DNA polymerase l
- DNA polymerase lll
- RNA primase
- Helicase
- SSBP
How does replication start? What prevents the unwound DNA for twisting back?
DNA replication starts with the unwinding of the DNA molecule. This is achieved by the enzyme helicase.
Which enzyme is the key player in replication? What is this enzyme’s limitation? How is this limitation overcome?
DNA polymerase. Weakness: It cannot work in the 5’ to 3’ direction. Overcome: By inserting okazaki fragments into one strand (lagging strand) by phosphoester bonds.
Why do the two strands of the helix have to be elongated by two slightly different mechanisms?
This is due to the two strands of DNA having different directions or orientations.
Leading strand: 3’ to 5’
Lagging strand: 5’ to 3’
Explain the elongation stage of replication-your answer should include a discussion of the leading strand, lagging strand and the contents of the replisome.
Leading strand
• A primer comes along and binds to the end of the leading strand. The primer functions as the starting point of the DNA replication.
• DNA polymerase binds to the leading strand and adds new complimentary nucleotide bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction
Lagging strand
• There are several of RNA primers are made by the primase enzyme and bind at various sites of the lagging strand
• Chunks of DNA, called okazaki fragments are then added to the lagging strand also in the 5’ to 3’ direction. This type of replication is called discontinuous. But these okazaki fragments are joined later.
How are Okazaki fragments on the lagging strand joined into one continuous strand?
DNA ligase joins the okazaki fragments with phosphodiester bonds to produce a continuous strand.
Describe what would happen if a cell was lacking:
a) RNA primase
b) SSPB
a) RNA primase:
If the cell was lacking RNA primase it won’t be able to produce primers to initiate the DNA replication process as primase is required for the production of primers.
b) SSPB:
If this was lacking, the parental strands would not have anything to keep them apart so as a result these two strands may come in contact and prevent the process of DNA replication from occurring.
What important process occurs in the termination phase? (to protect the DNA)
The new strand is proofread to make sure there are no mistakes in the new DNA sequence.
How do eukaryotes speed up the process of replication-since they have multiple long chromosomes?
Ask teacher for answer
