Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 9
Molecules of Heredity
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 9 Outline
• 9.1 How Did Scientists Discover That Genes Are
Made of DNA? p. 150
• 9.2 What Is the Structure of DNA? p. 151
• 9.3 How Does DNA Encode Information? p. 157
• 9.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division? p. 157
• 9.5 How Do Mutations Occur? p. 158
Section 9.1 Outline
• 9.1 How Did Scientists Discover That
Genes Are Made of DNA?
– Transformed Bacteria Revealed the Link
Between Genes and DNA
Genes Are Made of DNA
•
Known since the late 1800s:
1. Heritable information is carried in discrete
units called genes
2. Genes are parts of structures called
chromosomes
3. Chromosomes are made of
deoxyribonucleic acid (DNA) and protein
Genes Are Made of DNA
• Transformed bacteria revealed the link
between genes and DNA
Genes Are Made of DNA
• F. Griffith worked with two strains of
Streptococcus pneumoniae bacteria
– S strain caused pneumonia when injected into
mice, killing them
– R strain did not cause pneumonia when injected
Genes Are Made of DNA
• Griffith made a sample of heat-killed S strain
and mixed it with R strain
– Injection of combination into mice caused
pneumonia and death
Genes Are Made of DNA
• Deductions from Griffith’s experiment
(1920s)
– Living safe bacteria (R strain) were changed by
something in the dead (but normally diseasecausing) S strain
– The living R strain bacteria were transformed
by genetic material released by the S strain
Genes Are Made of DNA
• Later findings by Avery, MacLeod, and
McCarty (1940s)
– The transforming molecule from the S strain
was DNA
Section 9.2 Outline
• 9.2 What Is the Structure of DNA?
– DNA Is Composed of Four Nucleotides
– DNA Is a Double Helix of Two Nucleotide
Strands
– Hydrogen Bonds Between Complementary
Bases Hold Two DNA Strands Together
DNA Is Composed of Four Nucleotides
•
DNA is made of chains of small subunits
called nucleotides
DNA Is Composed of Four Nucleotides
•
Each nucleotide has three components:
1. Phosphate group
2. Deoxyribose sugar
3. One of four nitrogen-containing bases
– Thymine
– Cytosine
– Adenine
– Guanine
DNA Is Composed of Four Nucleotides
•
In 1940, biochemist E. Chargaff
determined that:
– In a DNA molecule, amounts of
A = T, G = C
– “Chargaff’s Rule”
DNA Is a Double Helix
•
In the 1940s several other scientists
investigated the structure of DNA
DNA Is a Double Helix
•
Rosalind Franklin and Maurice Wilkins
studied DNA structure using X-ray
scattering
DNA Is a Double Helix
•
From X-ray diffraction patterns they
deduced that DNA
–
–
–
–
Is long and thin
Has a uniform diameter of 2 nanometers
Is helical, and is twisted like a corkscrew
Consists of repeating subunits
DNA Is a Double Helix
•
James Watson and Francis Crick
combined the X-ray data with bonding
theory to deduce DNA structure:
– DNA is made of two strands of nucleotides
– The deoxyribose and phosphate portions
make up the sugar-phosphate backbone
Hydrogen Bonds
• Nitrogen-containing bases protrude inward
from sugar-phosphate backbone
Hydrogen Bonds
• Hydrogen bonds hold certain nitrogenous
base pairs together
– A bonds with T, G bonds with C
– Bonding bases called complementary base
pairs
Hydrogen Bonds
• Ladder-like structure of the two DNA strands
are twisted into a double helix
Section 9.3 Outline
•
9.3 How Does DNA Encode Information?
How Does DNA Encode Information?
•
•
How can a molecule with only 4 simple parts
be the carrier of genetic information?
The key lies in the sequence, not number, of
subunits
How Does DNA Encode Information?
•
Within a DNA strand, the four types of bases
can be arranged in any linear order, and this
sequence is what encodes genetic
information
How Does DNA Encode Information?
•
The genetic code is analogous to languages,
where small sets of letters combine in various
ways to make up many different words
–
–
–
English has 26 letters
Hawaiian has 2 letters
The binary language of computers uses only two
“letters” (0 and 1, or “on” and “off”)
How Does DNA Encode Information?
•
The sequence of only four nucleotides can
produce many different combinations
– A 10 nucleotide sequence can code for
greater than 1 million different
combinations
Section 9.4 Outline
•
9.4 How Does DNA Replication Ensure
Genetic Constancy During Cell Division?
– Replication of DNA Is a Critical Event in a
Cell’s Life
– Replicated DNA Is Half New and Half Old
DNA Replication
•
•
All cells come from pre-existing cells
Cells reproduce by dividing in half
DNA Replication
•
•
Each of two daughter cells gets an exact
copy of parent cell’s genetic information
Duplication of the parent cell DNA is
called replication
DNA Replication
•
DNA replication begins when DNA
helicases separate the two strands
– Hydrogen bonds between bases are
broken
DNA Replication
•
A second strand of new DNA is
synthesized along each separated strand
by DNA polymerases, which position free
nucleotides across from complementary
nucleotides
DNA Replication
•
Base pairing is the foundation of DNA
replication
– An adenine on one strand pairs with a thymine
on the other strand; a cytosine pairs with
guanine
– If one strand reads ATG, the other reads TAC
DNA Replication
•
The two resulting DNA molecules have
one old parental strand and one new
strand (semiconservative replication)
Section 9.5 Outline
•
9.5 How Do Mutations Occur?
– Accurate Replication and Proofreading Produce
Almost Error-Free DNA
– Mistakes Do Happen
– Mutations Range from Changes in Single
Nucleotides to Movements of Large Pieces of
Chromosomes
– Mutations May Have Varying Effects on
Function
Replication And Proofreading
•
•
•
During replication, DNA polymerase
mismatches nucleotides once every
10,000 base pairs
DNA repair enzymes “proofread” each
new daughter strand, replacing
mismatched nucleotides
However…
Mistakes Do Happen
DNA is damaged in a number of ways
• Spontaneous chemical breakdown at
body temperature
• Certain chemicals (some components of
cigarette smoke)
Mistakes Do Happen
•
UV light from the sun causes DNA
damage
– DNA damage leads to uncontrollable
cell division and skin cancer
Types of Mutations
•
•
•
Point mutation - individual nucleotide in
the DNA sequence is changed
Insertion mutation - one or more
nucleotide pairs are inserted into the
DNA double helix
Deletion mutation - one or more
nucleotide pairs are removed from the
double helix
Types of Mutations
•
•
Inversion - piece of DNA is cut out of a
chromosome, turned around, and reinserted into the gap
Translocation - chunk of DNA (often
very large) is removed from one
chromosome and attached to another