Chapter 9
Molecules of Heredity
Chapter 9 Outline
• 9.1 How Did Scientists Discover That Genes Are
Made of DNA?
• 9.2 What Is the Structure of DNA?
• 9.3 How Does DNA Encode Information?
• 9.4 How Does DNA Replication Ensure Genetic
Constancy During Cell Division?
• 9.5 How Do Mutations Occur?
9.1 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
9.2 DNA Structure
•
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
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 sugarphosphate 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
9.3 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
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
– 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
9.4 DNA Replication
•
•
•
•
All cells come from pre-existing cells
Cells reproduce by dividing in half
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)
9.5 How Do Mutations Occur?
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
Types of Mutations
•
Insertion mutation one or more nucleotide
pairs are inserted into
the DNA double helix
Types of Mutations
•
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
Types of Mutations
•
Translocation - chunk of DNA (often
very large) is removed from one
chromosome and attached to another