Biology 212 General Genetics Fall 2006
Lecture 1 "DNA"
• Class Roster
• Handout: Course syllabus
• Review of syllabus: content
Main topics: Classical (Mendelian) Genetics, Molecular Genetics, Human
Genetics, Population Genetics
Genetics: The study of genes, the individual units of heredity
Genes: Segments of DNA that contain the information for a protein or RNA product
Molecular genetics vs. classical genetics
• Molecular genetics examines the chemical nature of genes.
o The molecule DNA contains information for our traits.
• Classical or Mendelian genetics is based on the experiments of Gregor Mendel
(1860's) with the garden pea
o Developed concept that heredity is determined by discrete units
o Traits can be dominant or recessive
Human genetics: Examined through pedigree analysis (genetic family trees), karyotype
(chromosome) analysis, human genome project, DNA testing
Population genetics: Inheritance in groups of interbreeding organisms. Relevant for
conservation biology, forensics.
• Review of syllabus course requirements
Reading: Text Chapter 1 pp. 2-9
1. DNA is the genetic material
a. Transformation experiments
b. Phage experiments
2. Chemical structure of DNA
3. DNA structure suits its function
1. DNA is the molecule of heredity: Experimental evidence
1869 Friedrich Miescher
o discovered DNA (deoxyribonucleic acid)
o DNA is abundant in cell nuclei
o DNA is a weak acid
Late nineteenth century
o Male and female reproductive cells fuse in fertilization
o Discovery of chromosomes
o Chromosomes are closely associated with DNA
1928 Frederick Griffith
o Demonstrated bacterial transformation; bacteria can be converted from a non-
virulent to a virulent strain by injection of a substance.
S=smooth strain, causes pneumonia in mouse
R=rough strain, non-virulent
Heat-killed S cells + living R cells à inject into mouse; causes pneumonia
1944 Oswald Avery, Colin Macleod, Maclyn McCarty
o Showed that the transforming substance was DNA.
DNA from S cells + R cells à produce S cells
Therefore the substance responsible for genetic transformation is DNA.
DNA is the genetic material.
1952 Alfred Hershey, Martha Chase
o Demonstrated that transmission of information from one generation to another
relies on DNA.
o DNA or RNA (genetic material)
o Protein coat and tail fibers
• Parasites that require host cell to reproduce
Fig. 1. 4 The Hershey-Chase Experiment
• T2 virus is a bacteriophage, virus that infects bacteria
• Can "tag" DNA or protein parts of T2 virus
• Tag DNA of virus with radioactive phosphorus
• Infect E. coli bacteria with tagged virus particles
• Allow reproduction of virus and lysis of bacteria
• The bacteria release progeny viruses tagged on their DNA.
• When viral proteins were similarly tagged, the released progeny viruses do not
have tagged proteins.
2. The chemical structure of DNA
Could the molecular structure of DNA help answer
• How genetic material is duplicated?
• How DNA controls traits?
• What happens when a mutation occurs on the DNA?
1953 James Watson, Francis Crick
o Developed a 3D-structural model of DNA structure, the "DNA double helix".
Critical features of the model:
• DNA is made of nucleotides
o Nitrogenous bases (Fig. 6.2)
• Nucleotides are linked together in long chains (Fig. 6.4)
• Sugars and phosphates form the backbone
• Bases occur in the interior
• Bases form hydrogen bonds
o A pairs with T
o C pairs with G
• The bases that pair are said to be complementary
• The order of the bases on DNA comprises the information
• Two strands of DNA are antiparallel à the chains point in opposite directions
• The two paired strands coil around each other in a double helix
3. Knowledge of the structure of DNA helps understand its function
• Each strand provides information for synthesis of a complementary strand.
• DNA contains information as a linear code: the order of the bases.
• Changes in the DNA (mutations) can occur from errors in copying.
• Process of copying the DNA. Each parental DNA strand directs the synthesis of a
new partner strand.
• DNA synthesis carried out by enzyme, DNA polymerase
5' A T G C C G T A 3'
3' T A C G G C A T 5'
daughter DNAs 5' A T G C C G T A 3' 5' A T G C C G T A 3'
3' T A C G G C A T 5' 3' T A C G G C A T 5'