DNA STRUCTURE, REPLICATION, AND REPAIR

DNA-Structure  Double helix Hydrophobic nitrogen bases on the inside Hydrophilic phosphate-sugar backbones on outside Nitrogenous bases pair in specific manner  Purine pairs with pyrimidine to make “rungs” of the ladder equal widths

DNA-Structure

DNA structure

DNA-Structure  Nitrogen base pairs 2 hydrogen bonds between Adenine and Thymine 3 hydrogen bonds between guanine and cytosine  Between each “rung” of the ladder, Van der Waals forces hold the stacks together

DNA REPLICATION  Look at animations—they really help!  Biologycorner.com  Google search: DNA replication animation—there’s lots of them!  biocoach

Meselson and Stahl experiment

DNA REPLICATION  Starts at origin of replication “bubble forms”  In long pieces of DNA, there are many origins of replication  At the ends of each bubble is a replication fork

DNA REPLICATION

DNA REPLICATION--Elongation  The bubbles unzip (open) HELICASE enzyme untwists the double helix as replication occurs  DNA replication occurs in both directions, but by different process in each direction.  DNA polymerases catalyze the process of making new strands

DNA REPLICATION  DNA strands are antiparallel

DNA REPLICATION  One strand runs in 3’  5’ direction; other strand runs in 5’  3’ direction 3’ carbon has a –OH group 5’ carbon has the phosphate group

DNA REPLICATION

 DNA polymerase can only add nucleotides to the 3’ end of the growing strand Continuous elongation can only occur in the 5’  3’ direction This new strand is called the LEADING STRAND

DNA REPLICATION—leading strand

DNA REPLICATION  The other new strand is called the LAGGING STRAND (3’  5’ direction) Replicates in short pieces, called OKAZAKI FRAGMENTS DNA LIGASES then join together all of the Okazaki fragments to create the complete strand

DNA REPLICATION

 DNA polymerase adds to the 3’ end of a strand, but it can’t START the process An RNA primer is necessary to start replication process Only one primer is required for leading strand Each Okazaki fragment requires an RNA primer (on the lagging strand)

Priming DNA synthesis

DNA REPLICATION

DNA REPLICATION: eukaryotes  The ends of the strands have telomeres Short, repetitive, non-coding nucleotide sequences Protects from progressive shortening of DNA strands

Telomeres:why are they necessary?

Telomeres

DNA REPAIR  DNA polymerase acts as a “proofreader” Checks new nucleotide against original If incorrect, the nucleotide is removed, replaced, and then replication continues on This system is not 100% accurate

DNA REPAIR  MISMATCH REPAIR Enzymes fix incorrectly paired nucleotides Can repair nucleotides damaged by environmental agents Monitoring is continuous

DNA REPAIR  NUCLEOTIDE EXCISION REPAIR NUCLEASE  Enzyme that “cuts out” the damaged segment DNA POLYMERASES AND LIGASES  Fill in the gaps

NUCLEOTIDE EXCISION REPAIR