BIO 311C Lecture Notes - Lecture 16: 10 Nanometer, Pyrimidine, Matthew Meselson
Chapter 16: The Molecular Basis of Inheritance
1) Life’s Operating Instructions
a) In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of
deoxyribonucleic acid, or DNA
b) Hereditary information is encoded in DNA and reproduced in all cells of the body
i) This DNA program directs the development of biochemical, anatomical, physiological, and (to some
extent) behavioral traits
c) DNA is copied during DNA replication, and cells can repair their DNA
2) Concept 16.1: DNA is the genetic material
a) Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge
to biologists
b) The Search for the Genetic Material: Scientific Inquiry
i) When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of
chromosomes—DNA and protein—became candidates for the genetic material
ii) The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect
them
c) Evidence That DNA Can Transform Bacteria
i) The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928
ii) Griffith worked with two strains of a bacterium, one pathogenic and one harmless
iii) When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain,
some living cells became pathogenic
iv) He called this phenomenon transformation, now defined as a change in genotype and phenotype
due to assimilation of foreign DNA
v) In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming
substance was DNA
vi) Many biologists remained skeptical, mainly because little was known about DNA
d) Evidence That Viral DNA Can Program Cells
i) More evidence for DNA as the genetic material came from studies of viruses that infect bacteria
ii) Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research
(1) A virus is DNA (sometimes RNA) enclosed by a protective coat, often simply protein
iii) In 1952, Alfred Hershey and Martha Chase showed that DNA is the genetic material of a phage
known as T2
(1) They designed an experiment showing that only one of the two components of T2 (DNA or
protein) enters an E. coli cell during infection
(2) They concluded that the injected DNA of the phage provides the genetic information
e) Additional Evidence That DNA Is the Genetic Material
i) DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate
group
ii) The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
iii) In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next
iv) This evidence of diversity made DNA a more credible candidate for the genetic material
v) Two findings became known as Chargaff’s rules
vi) The base composition of DNA varies between species
(1) In any species the number of A and T bases are equal and the number of G and C bases are equal
vii) The basis for these rules was not understood until the discovery of the double helix
f) Building a Structural Model of DNA: Scientific Inquiry
i) After DNA was accepted as the genetic material, the challenge was to determine how its structure
accounts for its role in heredity
ii) Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study
molecular structure
iii) Franklin produced a picture of the DNA molecule using this technique
iv) Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical
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