Topic 7- DNA Replication

Primary Structure of DNA

Phosphate group attached to the 5’ carbon, hydroxyl on the, and a nitrogen base attached to the 1’ carbon

Purine

Double-ring structure (adenine and guanine)

Pyrimidine

Single-ring structure (thymine and cytosine)

Primary structure of a DNA strand

Phosphate-sugar backbone of DNA strand; phosphodiester bond links deoxyribonucleotides

Watson and Crick

Proposed that two DNA strands line up in the opposite direction of each other, in what is called antiparallel fashion; suggested that existing DNA strands could serve as a template for the production of new strands according to complementary base pairing

Chargaff’s Rules

A hydrogen binds to T, and C hydrogen bonds to G

Semiconservative replication

Each old DNA strand is copied to generate a new strand; each new chromosome is composed of one strand of old DNA and one strand of newly synthesized DNA

Conservative replication

The original chromosome is copied but remains unchanged; one chromosome is composed of old DNA and the other is composed of new DNA

Dispersive replication

Replication process generates two new chromosomes randomly mixed with old and new sections of DNA

Meselson-Stahl Experiment

Provided strong evidence that DNA replication is semi-conservative when future generation of E. Coli had both 14N and 15N content

dNTPs

deoxynucleotide triphosphates; building blocks of DNA

What direction is DNA synthesis?

5’ —> 3’

Prokaryotes and points of origin

Prokaryotes have one point of origin for DNA synthesis; theta structure

Eukaryotes and points of origin

Eukaryotes have multiple points of origin due to their linear structure

Helicase

Catalyzes the breaking of hydrogen bonds between base pairs and opening of DNA’s double helix

Single-stranded DNA binding proteins

Stabilizes single-stranded DNA by preventing base-pairs from pairing up again

Topoisomerase

-Breaks and rejoins the DNA double helix to relive twisting forces caused by the opening of the helix

Primase

Makes an RNA primer, or a short stretch of nucleic acid complementary to the DNA template, that provides a 3’ end for the DNA polymerase to work on (lagging strand)

DNA polymerase III

Elongation

Sliding clamp

Holds DNA polymerase in place during strand extension

DNA polymerase I

removes the RNA primer and replaces it with DNA

DNA ligase

Catalyzes the joining of Okazaki fragments into a continuous strand; seals nicks left behind by DNA polymerase I

Why do telomeres shorten during DNA replication?

When the last primer is removed, DNA synthesis cannot occur (because it needs a primer), so the place where the primer was remains unreplicated

Telomerase

Adds more repeating bases to the end of the lagging strand, catalyzing the synthesis of DNA from an RNA template that it carries with it

Result of telomerase

The lagging strand becomes slightly longer than it actually was

How does DNA polymerase III proofread?

It has an epsilon subunit that acts as an exonuclease that removes deoxyribonucleotides from DNA; goes in the 3’ —> 5’ direction

Exonuclease

An enzyme that removes nucleotides from the ends (exo) of a polynucleotide chain