Chapter 16—The Molecular

Basis of Inheritance

I. DNA as the Genetic Material

What was the transforming agent?

Transformation of Bacteria—Griffith Experiment (1928)

Bacteriophages

The Hershey-Chase Experiment (1952)

Conclusion?

DNA, not protein, functions as the genetic material for viruses

Structure of a DNA strand

Structure of a DNA strand

The double helix

Watson, Crick, Franklin, Wilkins, Pauling (1953)

Base Pairing in DNA

Base Pairing in DNA—Chargaff

Chargaff’s Rules—A & T and G & C

are found in equal quantities in DNA

samples (1947)

A pairs with T (2 hydrogen bonds)

G pairs with C (3 hydrogen bonds)

II. DNA Replication & Repair

• Simplified DNA Replication

Basic idea—Watson & Crick 1954

Replication of DNA

base pairing allowseach strand to serveas a pattern for anew strand

Three Models of DNA Replication

Needed to be verified through experimentation

Meselson-Stahl Experiment—(1958)

Meselson-Stahl Experiment—1958

Meselson-Stahl Experiment—1958

� labeled nucleotides of “parent” DNA strands with heavy nitrogen = 15N� labeled new nucleotides with lighter isotope = 14N� replicated strands were found to be half 15N & half 14N

Semiconservative Replication supported

Origins of Replication in Eukaryotes

more than a dozen enzymes & other proteins participate in DNA replication

Helicase—opens DNA helix enabling replication

DNA Strands are Antiparallel

Sugar-Phosphate backbones run in opposite directions

5´ End contains a phosphate group

3´ End contains a hydroxyl (OH) group

Elongating a DNA Strand

DNA Polymerase� Adds nucleotides only to 3´ end (elongation always 5´ → 3´ direction)� Nucleoside-P-P-P links to sugar-P backbone� Losing 2-P provides energy for bonding

Can get energy from:ATP → AMPTTP → TMPGTP → GMPCTP → CMP

Synthesis of Leading & Lagging Strands

Leading strandcontinuous synthesis

(5‘ → 3‘ direction)

Lagging stranddiscontinuous synthesis

Okazaki fragments (still 5‘ → 3‘ direction)

joined by ligase“spot welder”

Priming DNA Synthesis with RNA

DNA polymerasecan only extend anexisting DNAmolecule (Cannotstart a new one)

� short RNA primeris built first onparent DNA strandby primase

� RNA primer laterremoved by another DNApolymerase

Replication Fork

Summary of DNA Replication

DNA Replication Enzymes

Editing & Proofreading DNAAt 1000 bases/second,lots of typos!

Hundreds of DNA repair enzymes exist

Nucleotide Excision Repair

� Nucleases excise

mismatched bases� DNA polymerase fills the

gap� Ligase seals the nick

(reduces error ratefrom 1 in 10,000 to 1in 100 million bases)

The End-Replication Problem

Ends of chromosomesare eroded with eachreplication

Telomeresexpendable, non-codingsequences at ends of DNA

� short sequence of bases

� repeated 100 to 1,000 times

� TTAGGG in humans

� Protects genes from being eroded by replication

� Telomerase enzyme—catalyzes lengthening of

telomeres

“Central Dogma”—Flow of genetic

information within a cell