CH 16: The Molecular Basis CH 16: The Molecular Basis of Inheritanceof Inheritance

Francis Crick

James Watson

Major Concepts

• The discovery of DNA as the The discovery of DNA as the genetic materialgenetic material

• How DNA is replicated and How DNA is replicated and repairedrepaired

To assist in your note taking, To assist in your note taking, key key conceptsconcepts have been underlined and have been underlined and vocabvocab terms are in orange terms are in orange

Life’s Operating Instructions

DNA is an example of a nucleic acid

Of all the macromolecules, nucleic acids are the only ones that can replicate themselves

DNA is the chemical language that is responsible

for biochemical, anatomical, physiological, & even some behavioral traits

DNA DNA isis the Genetic Material… the Genetic Material…But How Do We Know???But How Do We Know???

The Debate: DNA vs. ProteinIn the 1940’s….

Proteins Nucleic Acids: DNA

•A very wide variety was known

•Little was known about them

•They were known to be very specific

•They seemed too simple to account for the great diversity of life

Building a Case for Building a Case for DNA: DNA: DNA Can DNA Can

Transform BacteriaTransform BacteriaFrederick Griffith’s Experiment, 1928Transformation: The uptake & assimilation of external DNA by a bacterial cell that results in a change in genotype and phenotype Concl: Living R

bacteria were transformed into S bacteria by a heritable substance from the dead S cells

virulent harmless

Quick Think

Describe Griffith’s experiment & the conclusion that he reached

What was the change in phenotype that the transformed Streptococcus pnumoniae underwent?

Follow-up to Griffith’s Experiment

Griffith new that something from the heat-killed S bacteria was transforming the harmless R strain, but he didn’t know what…

Oswald Avery was determined to pick up where Griffith left off….

How would YOU design an experiment to figure out which part of the pathogenic bacteria was transforming the harmless bacteria???

Avery performed Griffith’s experiments, but treated the non-pathogenic bacteria with isolated molecules from the heat-killed pathogenic strain

Lysed cells

DNA was the only macromolecule to successfully transform the R strain

When Avery concluded that DNA was the transforming agent, few took

interest! Most were hung up on the idea that protein was the genetic material…..plus not much was known about DNA at that point……

Building a Case for Building a Case for DNA:DNA:

Viral DNA Can Program Viral DNA Can Program CellsCellsBackground Info

•A virus must infect a cell in order to reproduce

•A Bacteriophage ( AKA phage) is a virus that infects bacterial cells

•The T2 virus infects the bacteria E. coli •The T2 virus is mademostly of DNA & protein

Viral DNA injected into cell

Building a Case for Building a Case for DNA:DNA:

Viral DNA Can Program Viral DNA Can Program CellsCellsIn 1952,

the research duo Alfred Hershey & Martha Chase wanted to figure out whether it was viral protein or its DNA that caused a cell to become a virus factory

Building a Case for DNA:Building a Case for DNA:Chargaff’sChargaff’s

Investigations, 1947Investigations, 1947

Each nucleotide of DNA has 3 parts-

•Chargaff found diversity in the DNA molecules of different organisms: Various organisms had different percentages of the 4 bases•Chargaff’s Rule: In

any given organism, the % of A in DNA = the % of T. In the same way, % G = % C

nucleotide

Quick Think

Human DNA is composed of 30% thymine. What would be the percent

composition for the other three bases??

Building a Case for DNA:Building a Case for DNA:Additional EvidenceAdditional Evidence

Prior to cell division,eukaryotic cells make exactcopies of their DNAand then the DNA isdistributed evenlyamong the daughter cells.

DNA as the Genetic DNA as the Genetic Material:Material:

How Does Form Fit How Does Form Fit FunctionFunction??By the early 1950’s, the race was on to find the 3-D

structure of DNAIn 1950 Maurice Wilkins & Rosalind Franklin were hard at work using X-ray diffractiontechnique to figure out DNA structureThey planned to use mathematical equations

derived from the diffraction images to determine the 3-D structure of DNA.

DNA as the Genetic DNA as the Genetic Material:Material:

How Does Form Fit How Does Form Fit Function?Function?Watson & Crick used Franklin’s x-ray diffraction

images to deduce the following:

•DNA is a double helix with a sugar-phosphate backbone on the outside of the “ladder” and relatively hydrophobic bases on the inside; the bases are the rungs of the ladder.

•Base-pairing rules: A pairs with T and C pairs with G. In other words, purines always bond with pyrimidines.

•Watson & Crick’s model suggested the basic mechanism for DNA replication

Quick Think

How did Watson & Crick’s model explain the basis for Chargaff’s rule?