Nucleic Acids

Bacteriophages

Viruses that infect bacteria

Chargaff - 1947

DNA composition varies among species:

• likely candidate for heredity

• Adenine / cytosine / guanine / thymine

Chargaff’s rule → A=T, C=G in composition

DNA characteristics

Polymer of nucleotides

Nucleotide has:

• a nitrogenous base

• A sugar

• A phosphate group

Wilkins and Franklin

X-Ray crystallography → pattern of diffraction

Watson and Crick

Double helix model

Concluded that DNA has 2 antiparallel sugar-phosphate backbones, 3’ and 5’

DNA double helix

Held together by H-bonds between the bases

• A-T → 2 H bonds

• G-C → 3 H bonds

Arthur Kornberg - 1955

Discovered how DNA is synthesized

4 components are required

• Nucleotides (A,C,T,G)

• DNA template

• DNA polymerase

• Mg2+ → optimizes DNA polymerase activity

Replication

Each strand acts as a template for building a new strand

Base pairing rules

Parent unwinds:

• two daughters built

• Base pairing rules

DNA replication

Semi-conservative

• each daughter has 1 parent strand

Begins at sites of origins:

• eukaryotes have hundreds / thousands

Replication sequences: First Steps

• initiator proteins

• Helicase unwinds helix

• Binding proteins stabilize template strands

• Primase “primes” strands → with RNA

• Elongation by DNA polymerase III → add nucleotides 3’ end only

• DNA polymerase I replaces primer RNA

• Ligase “glues” together

Leading strands

Synthesize a complementary strand continuously

Moving toward the replication fork

Made as a series of segments:

• called Okazaki fragments

• Joined by DNA ligase

Moves away from replication fork

Primers

DNA polymerases can’t initiate synthesis → they only add nucleotides to 3’ end

Initiated by RNA or DNA primer → short nucleotide strand

Leading strand → only 1 primer needed

Landing strand → each Okazaki fragment must be primed

Leading vs Lagging strands

Leading:

• primase adds RNA primer once

• DNA Poly III builds continuously toward fork

• Poly I replaces primer

• Ligase binds it to other segment (initial section)

Lagging:

• primase adds short primer sequence

• DNA polymerase III adds nucleotides to 3’ end until it reaches next primer (Okazaki segment)

• DNA polymerase I replaces primer nucleotides with DNA

• Ligase bonds segments together

Proof-reading / mismatch repair

Enzymes cut out and replace damaged stretches of DNA

Thymine dimers

Caused by exposure to UV light

2 adjacent thymine residues become covalently linked

Eukaryotes

Chromosome ends get shorter with replication

Nucleotide sequences called telomeres postpone the erosion at ends → repeater non-coding sequence

Telomerase

Adds nucleotides (telomeres) to the ends of the DNA → prevents loss of true genes with replication

Gametes and stem cells

Telomerase → catalyzes the lengthening of telomeres in germ cells

Copying DNA

Polymerase chain reaction → asked called PCR

A method of making many copies of a piece of DNA → millions

Rapid process