Download ch. 16 Molecular Basis of Inheritance

Molecular
Basis of
Inheritance
•DNA Structure
•DNA Replication
Evidence that DNA is the Hereditary
Material of Life?
• Griffith-Avery Experiment—DNA can
transform bacteria
• Hershey-Chase Experiment—Viral DNA
can program cells
• Chargaff—Analysis of DNA composition
Griffith-Avery Experiment:
Transformation of Bacteria
Controls
Hershey-Chase Experiment:
DNA, the Hereditary Material in Viruses
Chargaff:The Composition of DNA
• Base composition of DNA varies
between species
• Regularity in ratios of nucleotide bases
Adenine = 30.9%
Thymine = 29.4%
Guanine = 19.9%
Cytosine = 19.8%
• Chargaff’s Rules: A = T and G = C
Scientists in the Race for the Double
Helix
• Linus Pauling
• James Watson
• Francis Crick
• Rosalind Franklin
Structure of DNA
Purines
Adenine
Pyrimidines
Thymine
Guanine
Cytosine
Structure of DNA
• DNA is a double
helix with a uniform
width
• Purine and
pyrimidine bases
stacked
• Purine + Pyrimidine:
width consistent
with X-ray data,
• base ratios
consistent with
Chargaff’s rules:
A = T and G  C
Structure of DNA is related to 2 primary functions:
1. Copy itself exactly for new cells that are created
2. Store and use information to direct cell activities
Complementary (Anti-Parallel)
Strands of DNA
• If one strand is known, the other strand
can be determined
3’ A = T 5’
C
G
G
T
A
T
C
C
5’
G
C
C
=A
=T
=A
G
G
3’
DNA Replication: Semi-Conservative
Model
• 2 strands of the
parental DNA
separate, and
each functions as
a template for the
synthesis of a new
complementary
strand
DNA Replication
• This replication process assures that
daughter cells will carry the same genetic
information as each other and as the
parent cell.
Each daughter DNA has one old strand of DNA
and one new strand of DNA
DNA Replication
• Replication occurs simultaneously at
many sites (replication bubbles) on a
double helix
Allows DNA replication to occur in a shorter
period of time
Elongating a New
Strand
• DNA polymerases
can only attach
nucleotides to the 3’
end of a growing
daughter strand
• Thus, replication
always proceeds in
the 5’ to 3’ direction
Priming DNA
Synthesis
• Primase adds
RNA primer to
strand
• DNA polymerase
adds DNA
nucleotides to
strand
• Another DNA
polymerase
replaces RNA
with DNA
DNA Replication
• Within the replication bubbles, one daughter strand
is made continuously (leading strand) while the
other daughter strand must be made in short pieces
(lagging strand) which are then joined together by
DNA ligase
These short pieces of DNA are called Okazaki
fragments
Overall Direction of Replication-5’ to 3’
DNA Replication
Checking for Errors
• 1/1,000,000,000 chance of an error in DNA
replication
– Can lead to mutations
• DNA polymerases have a “proofreading” role
– Can only add nucleotide to a growing strand if the
previous nucleotide is correctly paired to its
complementary base
• If mistake happens, DNA polymerase
backtracks, removes the incorrect nucleotide,
and replaces it with the correct base
Mismatch Repair
• DNA polymerase proofreads each nucleotide
and catches mistakes
• The polymerase removes mistake and replaces
with correct nucleotide
Excision Repair
• Nuclease
• Used by skin cells
when repairing
genetic damage
caused by UV rays
of sunlight
End-Replication
Problems
End-Replication
Problems
Telomerase—
enzyme lengthens
telomeres (ends of
nucleotide
sequences)