DNA Replication & Repair
by Stephanie Knol
What is DNA?
Deoxyribonucleic acid is a molecule that carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses.
Structure:
Sugar-phosphate back bone
Hydrogen bonding
Four nitrogen bases, two pairs
Adenine and Thymine
Guanine and Cytosine
The two strands run in opposite directions
DNA Replication
Semi-conserved replication (made of one old, conserved strand, and one new replicated strand)
โพ In the leading strand (5'-3') nucleotides can be added continuously
โพIn the lagging strand (3'-5') nucleotides must be added in ๐ข๐ธ๐ฎ๐๐ฎ๐ธ๐ถ ๐ณ๐ฟ๐ฎ๐ด๐บ๐ฒ๐ป๐๐
Different enzymes come in to help with the process of replication:
1. In order for replication to occur, the two strand must separate
Helicase: "Unzips" the two strands from each other, making the nitrogenous bases available to bond with other bases
Replication fork creating a template for a new strand of DNA
Single stranded binding proteins: attach to the separated strands and prevent them from re-annealing
DNA gyrase: Untwists strands to relieve tension
Primase: The initializer/ primer that creates RNA primers to initiate replication in the DNA sequence
DNA polymerase:
III: Adds nucleotides to the 3' end of the DNA sequence.
I: Removes RNA primers, and replaces them with DNA
Ligase: "Glues" bonds to join all the fragments together
DNA Repair
A collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome
Damage is detected by simply looking for a bulge in the helix of the DNA, detected by the cell itself.
Once damage is localized, specific DNA repair molecules are triggered and they bind at or near the site of damage and repair can start to take place
Single strand repair mechanisms:
Nucleotide excision repair: entire nucleotide is excised (cut out) about 24, and they are replaced
Base excision repair: One enzyme snips out the damages base, and other enzymes come to trim around the site and replace the nucleotide
Mismatch repair: Fix and rematch unmatched nucleotide
Double strand repair mechanisms:
Ionizing Radiation: Gamma rays and x-rays
Non- homologous end joining: ligase connects to split ends of DNA and fuse them back together (isn't as accurate as homologous recombination but is useful when sister DNA isn't present)
Microhomology- mediated end joining: Ligating mismatched hanging strands of DNA, removing overhanging nucleotides and filling extra spaces
Homologous recombination: uses undamaged section of similar DNA as a template. Enzymes inter-lay the damaged and undamaged strands, get them to exchange sequences of nucleotide, and fill them in, ending in two complete double segments
DNA repair process is a constantly active response to damage in the structure of DNA
1 million instances/cell/day!
Proofreading
Sometimes an error occurs (mismatch), about one per million; proofreader enzymes identify and correct most if not all the mistakes (about 3000 per cell division)
Mismatch repair:
IF a base is mismatched or another error occurs, the DNA replication pauses.
The mismatched base enters an exonucleus site on the enzyme polymerase.
Removal occurs in the 3'-5' direction
After the mismatched base is removed, DNA replication continues
Photo References:
https://5mindna.com/product/info-for-5-min-total-nucleic-acid-extraction-kit/