227The Molecular Basis of Inheritance.ppt)€¦ · Chapter 16—The Molecular Basis of Inheritance....

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