OBJ - Nucleic Acid Structure/DNA Replication and Repair Flashcards
Describe the structure of DNA and its monomeric unit.
Double helix made of 2 strands of linked nucleotides (made of nucleoside + phosphate + ribose)
Sugar phosphate backbone
A-> T 2 H bonds
C->G 3 H bonds
Pyrimidines = cYtosine & tYrosone & uracil Purines = A & G
Describe the key structural features of B-form DNA.
B-form is most common form found in vivo cells
R handed helix
what we know of as double helix
A form = 11
B form = 10 base pairs/turn
Z form = 12
Explain the concepts of base pairing and antiparallelism and polarity in DNA.
2 strands run opposite directions 5' -> 3' 3' -> 5' Polarity = chain direction overall DNA is negatively charged
Describe the structural differences between DNA and RNA.
DNA:
- Deoxyribose
- Thymine
- Double stranded
- Nucleus only
- Self replicating
RNA
- Ribose (more reactive)
- Uracil
- Single Stranded
- Nucleus/Cytoplasm/Ribosome
- Needs DNA to replicate
- A-Form helix
- more resistant to UV damage
Describe the various steps involved in eukaryotic DNA replication.
Synthesis ALWAYS starts reading at the 3’ end (making at the 5’ end)
1) Splitting of the double helix (HELICASE)
creates replication fork/origin of replication
2) RNA Primase lays down anchoring sequence of nucleotides on 3’ -> 5’ strand
3) Elongation from 3’ C to 5’ C by DNA Polymerase
Leading Strand
Lagging Strand - Okazaki fragments from 3’-5’ but linked together in net 5’-3’
4) Smoothing:
DNA Pol I exonuclease removes RNA Primers & fills in missing nucleotides
DNA Ligase adds phosphate to sugar backbone/gaps on Okazaki fragments (by linking & adding nucleotides)
5) Termination
DNA Polymerase reaches Telomere
**Nucleases go through and remove wrong nucleotides & DNA Polymerase fills in gap
Explain the function of topoisomerase in replication and why it is important.
Enzymes that regulate the overwinding/underwinding of DNA that occurs when the double helix is unwound to copy DNA
Moves ahead of helicase & relives superhelical stress by temporarily cuts the phosphate backbone allowing it to unwind and then reseals it
Explain what a telomerase is and why it is important for DNA replication.
Telomerase is an enzyme that adds to the 3’ end of DNA chromosome @ telomere
TTAGGG
**shortening of Chromosome -> aging
Explain the various mechanisms of DNA repair. What are the diseases that result from the impairment of DNA repair processes?
Base Pair excision - remove wrong nucleotide with endonuclease then DNA Polymerase fills in & DNA Ligase smoothes out & joins
Nucleotide Excision repair = wrong nucleotide pair (UV Light)
Mismatch Repair - the newer (daughter strand) mismatched nucleotide is removed by endonuclease, followed by DNA Polymerase & DNA Ligase
-ID’s parent strand by presence of methylation on it (posttrasncriptional modification to allow histones to open & DNA to be copied)
Semiconservative replication
each daughter double helix has 1 parent & 1 daughter strand
Bidirectional replication
replication proceeds away from the replication origin in both directions
in humans 100’s of origins/replication forks
DNA Polymerase
- Homodimer Beta2 subunit - acts as a sliding clamp before DNA Polymerase
- ## Enzyme requires a primer annealed to a template
Helicase
Separate 2 annealed nucleic acids (DNA/RNA)
bind to 1 of the DNA strands – unzips
Bound by Single strand Binding (SSB) protein – to keep unzipped
DNA Ligase
Links the phosphodiester bonds of the sugar backbond
RNA Primer/Primase
Laid down to start/anchor; then DNA polymerase takes over & lays down DNA
Reverse transcriptase to jump start DNA Polymerase
Exonuclease vs Endonuclease
EXOnuclease (@ the 3’ end & moves to 5’)
ENDOnuclease (in the middle of the chain)
Both proof read – cleaves nucleotides 1 at a time by hydrolizing phosphodiester bond
