Chapter 9
DNA: The Molecule of Heredity
Human Genome Project
Genome = the entire set of genes possessed by an individual
Human Genome published – we know most of the sequence
Humane genome = about 30,000 different genes
We are still trying to decode genes
What does it mean when we say DNA is
the "molecule of heredity"?
DNA-the genetic information of all living organisms
Some viruses use RNA
Genetic information units are called genes
Gene-a length of DNA containing information to make a protein
What is the genetic material?
Popular early theory considered protein to be the genetic material
How do we know DNA is the genetic material?
The history portion of our lesson…
1870s-Friedrich Miescher – isolated nuclei, determined they
have an acidic phosphorous containing compound “Nuclein”
1880’s – 1890’s work done to separate P-containing compounds
from the proteins present. Because this was from the nucleus and
was acidic was also referred to as ‘nucleic acid’
Two forms:
DNA (deoxyribonucleic acid)
RNA (ribonucleic acid)
By 1930’s…
Determined structure was similar yet different between DNA/RNA
5-Carbon sugar in both
5 different N-containing bases
4 in RNA
3 + others in DNA
Sugars attached by a phosphate connection
Bacterial transformation
(Evidence for DNA v.s. Protein)
1928-Frederick Griffith – working with bacterium causing
pneumonia in mice
Transformation cont…
1944 Avery, MacLeod & McCarty - separated bacteria into parts
& showed that the single compound that had this ability was DNA
1952-Alfred Hershey and Martha Chase - DNA is the hereditary
molecule of bacteriophages
DNA Structure
Four nucleotide subunits
Sugar-phosphate backbone
Chargaff's rule: specific bases always bond together
A
T
C
G
Nucleotide Types
Purine –bases having two rings
A & G are purines
Pyrimidine - bases having only one ring
C & T are pyrimidines
Backbone of DNA
alternating sugar/phosphate backbone
Sugar – ribose or deoxyribose
Helical Structure
1952-Maurice Wilkins and Rosalind Franklin
X-Ray diffraction
Determined that DNA was twisted into a helix (corkscrew
Uniform diameter of 2 nanometers (2 billionths of a meter)
1953-James Watson and Francis Crick addressed the several
requirements that had to be met for a genetic material
Must be found in the nucleus
Must contain biologically useful information in a stable form
Must be reproduced & transmitted fully from cell to cell &
generation to generation
Must be able to express itself
(process now understood, but not then)
Must be capable of variation
Structure of DNA
Once decided that DNA is the genetic substance
Froth & fever of activity in England from 1950 – 1953 resulted in
basic understanding of DNA molecule - no simple task
Double Helix – two intertwining corkscrews
handrails of a spiral staircase…
Several important workers including
James Watson, Francis Crick,
Maurice Wilkins & Rosalind Franklin
DNA replication
Necessary for cell division
DNA replication requires energy and several enzymes
DNA helicase – unzips the helix, exposing the strands to be copied
DNA polymerase - makes new DNA along the exposed strand
DNA ligase – connects the sugar/phosphate backbones
Replication results…
Replication is semiconservative
Each new molecule is half new DNA half original strand
DNA Repair
DNA replication errors occur and are usually recognized
1 mistake per 10,000 base pairs reduced by repair enzymes to
1 mistake per billion base pairs
Several repair enzymes use complementary DNA strand to
repair damaged DNA
Mutations
Replication errors can result in mutations
DNA
DNA
DNA
DNA
DNA
Mutation Types
Types of changes in DNA molecules include:
• Nucleotide substitution
• Nucleotide deletion
• Nucleotide insertion
Unrepaired replication errors are a source of genetic variability
Mutations as a result of replication errors can have serious
health consequences – or may have no effect whatsoever
End of Chapter 9