3.1.5.2 DNA replication Flashcards
What is the first step of DNA replication?
The double helix unwinds.
What enzyme is responsible for unwinding the DNA double helix?
DNA helicase.
What does DNA helicase do during DNA replication?
DNA helicase breaks hydrogen bonds between complementary base pairs.
What happens after the hydrogen bonds between DNA strands are broken?
Each strand acts as a template for complementary base pairing.
What is the role of free DNA nucleotides in DNA replication?
They are attracted to exposed bases on the template strands by complementary base pairing.
What is complementary base pairing?
Adenine pairs with thymine and cytosine pairs with guanine.
What enzyme joins adjacent nucleotides in the newly forming DNA strand?
DNA polymerase.
How does DNA polymerase join nucleotides together?
DNA polymerase catalyses a condensation reaction forming phosphodiester bonds.
What is the result of semi-conservative replication?
Each new DNA molecule consists of one original strand and one newly synthesised strand.
Why is DNA replication described as semi-conservative?
Each new DNA molecule retains one original strand and one newly synthesised strand.
What was the role of Meselson and Stahl’s experiment?
They provided experimental evidence supporting the semi-conservative model of DNA replication.
How did Meselson and Stahl confirm semi-conservative replication?
They used nitrogen isotopes (15N and 14N) and density gradient centrifugation to show that new DNA molecules contained one old and one new strand.
Why was Watson and Crick’s model of DNA replication initially doubted?
Other models such as conservative and dispersive replication had not yet been disproven.
What scientific method did Watson and Crick use to develop their model?
They used X-ray diffraction images and model building based on Chargaff’s base-pairing rules.
What is the significance of the 3’ and 5’ ends of a DNA strand?
DNA strands are antiparallel meaning one runs 5’ to 3’ while the other runs 3’ to 5’.
Why does DNA polymerase only add nucleotides to the 3’ end?
DNA polymerase can only work in the 5’ to 3’ direction.
How does DNA polymerase synthesise the leading strand?
It continuously adds nucleotides in the 5’ to 3’ direction towards the replication fork.
How does DNA polymerase synthesise the lagging strand?
It synthesises short Okazaki fragments discontinuously in the 5’ to 3’ direction away from the replication fork.
What enzyme joins Okazaki fragments together?
DNA ligase.
Why is DNA ligase needed in DNA replication?
It forms phosphodiester bonds between Okazaki fragments on the lagging strand.
Describe the structure of DNA.
- Polymer of nucleotides;
Accept ‘Polynucleotide’
Accept for ‘phosphate’. phosphoric acid
- Each nucleotide formed from deoxyribose, a phosphate (group) and
an organic/nitrogenous base; - Phosphodiester bonds (between nucleotides);
- Double helix/2 strands held by hydrogen bonds;
- (Hydrogen bonds/pairing) between adenine, thymine and
cytosine, guanine;
Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule.
- Condensation (reaction)/loss of water;
- (Between) phosphate and deoxyribose;
- (Catalysed by) DNA polymerase;
Reject if DNA polymerase joins AT/GC OR
complementary nucleotides/bases OR forms
hydrogen bonds
Name the protein associated with DNA in a chromosome.
Histone;
Describe how the separation of strands occurs.
- DNA helicase;
- Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases
Describe the role of DNA polymerase in the semi-conservative replication
of DNA.
- Joins (adjacent DNA) nucleotides;
Reject suggestions that it forms hydrogen bonds or
joins complementary bases.
Reject ‘nucleotide bases’.
- (Catalyses) condensation (reactions);
- (Catalyses formation of) phosphodiester bonds (between adjacent
nucleotides);
Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication.
Watson and Crick
Give two features of DNA and explain how each one is important in the
semi-conservative replication of DNA.
- Weak / easily broken hydrogen bonds between bases allow two
strands to separate / unzip;
may appear in the same feature - Two strands, so both can act as templates;
may appear in the same feature - Complementary base pairing allows accurate replication;
Allow description of complementary base pairing
and accurate replication.
Contrast the structures of ATP and a nucleotide found in DNA to give two
differences.
- ATP has ribose and DNA nucleotide has
deoxyribose; - ATP has 3 phosphate (groups) and DNA
nucleotide has 1 phosphate (group); - ATP – base always adenine and in DNA
nucleotide base can be different / varies;
