DNA Flashcards
What are the key features of DNA?
Double-stranded helix
Right-handed helix
Antiparallel
Strands held together by complementary base pairing
Outer edges of the bases exposed in major and minor grooves
Hydrogen bonding between complementary base pairs
Sugar-phosphate backbones form a coil around outside of helix, nitrogenous bases point towards the centre
Van dee Waals forces occur between adjacent bases on the same strand
What is the direction of DNA strands determined by?
Direction determined by sugar phosphate bonds
Phosphate groups connect the 3’ C of one sugar with the 5’ C of the next
One strand has a free 5’ phosphate group- the 5’ end
Other strand has a free 3’ hydroxyl group- the 3’ end
Why is DNA replication semi conservative?
Each parent strand is a template, new molecules have one old and one new strand
What are the 3 steps of DNA replication?
Initiation
Elongation
Termination
What happens in initiation of DNA replication?
Starts at the ori site
DNA helicase uses energy from ATP hydrolysis to unzip DNA
Single strand binding proteins keep the strands from getting back together
Primase synthesises a shirt RNA strand primer that binds to the unzipped DNA
DNA polymerase comes to the primed site and begins elongation
What happens in elongation of DNA replication?
At the replication fork, DNA opens continuously in one direction
As the fork opens, leading strand grows at 3’ end
On the lagging strand, an unreplicated gap forms
Synthesis of the lagging strand occurs in small, discontinuous stretches called Okazaki fragments
Each fragment requires its own primer
DNA polymerase III adds nucleotides to the 3’ end, until reaching primer of previous fragment
DNA polymerase I comes to replace RNA primer with DNA
DNA ligase joins the Okazaki fragments together
Polymerase DNA complex is stabilised by sliding DNA clamp
What happens in termination of DNA replication?
In prokaryotes, DNA replication starts at the ter site. Eukaryotes don’t have an equivalent site, at the end of their chromosomes are telomeres (repetitive sequences that bind proteins and prevent the DNA repair system from recognising chromosome ends as breaks)
On lagging strands, when the terminal Okazaki primed is removed, no DNA can be synthesized to replace it
A short piece of single stranded DNA is removed and the chromosome becomes shorter with each replication
After many divisions, genes may be lost and the cell dies
How can DNA be damaged?
DNA polymerase mistakes
Chemicals
UV radiation
Etc
What are the mechanisms of repair in DNA?
Proof reading
Mismatch repair
Excision repair
Homologous recombination
What is proof reading as a DNA repair mechanism?
DNA polymerase recognises mismatched pairs and removes incorrectly paired bases
What is mismatch repair as a DNA repair mechanism?
Newly replicated DNA is scanned for mistakes by other proteins and mismatches can be corrected
What is excision repair as a DNA repair mechanism?
Enzymes scan DNA for damaged bases- they are excised and DNA polymerase I adds the correct ones
What is homologous recombination as a DNA repair mechanism?
Breaks in DNA strands can be repaired by copying from the sister chromosome
What is PCR?
Automated process makes multiple copies of short DNA sequences for genetic manipulation and research
What does a PCR reaction mixture contain?
A sample of double stranded DNA
Two artificially synthesized primers
Four dNTPs
DNA polymerase that can tolerate high temperatures
Salts and pH butter
What are the steps of PCR?
Denaturing- heat separates DNA strands
Annealing- lower temperature, primers bind
Extension- temperature increased, DNA polymerase begins the extension
