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

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

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 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 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