Download DNA replicationRepair

Gene Expression &
DNA Replication
• DNA must be copied so
that the DNA is the
same in all cells
• Differences in gene
expression give rise to:
– different cells
– multicelluar organisms
DNA Replication
• DNA Synthesis during Sphase of Interphase (will
discuss w/ mitosis)
• H-bonds between 2 strands
are broken
• Two Strands separate
• Each strand is a template for
the other
DNA Replication
• The Watson-Crick model of DNA replication:
A
T
A
T
C
G
C
G
G
C
G
A
T
A
T
A
T
Parental
molecule
of DNA
A
C
C
Nucleotides
Both parental
strands serve
as templates
T
A
T
A
T
G
C
G
C
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
Two identical
daughter molecules
of DNA
DNA Replication
• https://www.youtube.c
om/watch?v=teV62zrm
2P0
• DNA replication review
DNA Replication
• Helicase breaks hydrogen bonds to separate
strands
• DNA Polymerase pairs new bases and creates
two new strands Semi-conservative
• DNA segments connected by ligase
When does the cell replicate DNA?
• Before cell division so that new cells have enough DNA to
divide
• Cells divide in embryogenesis, wound repair, cell
replacement
DNA Structure Review
• Covalent bonds hold a
strand together
• Hydrogen bonds hold
two strands together
– A pairs with T
– C pairs with G
• Each strand is used as a
template to replicate
DNA
DNA Replication
• If the DNA is not replicated correctly, mistakes
occur.
• Polymerase and other enzymes can correct
mistakes.
• If they are not corrected, they result in
mutation, a permanent change in DNA.
Two Types of Mutations
• Base Changes
– A change in the sequence of nucleotide bases
– Most are silent (do not change a protein)
• Chromosomal Abnormalities (macromutations)
– Large changes in whole regions of DNA
– Most are lethal
Base Mutations
• Incorrect base-pairings that are not fixed
– Mutagens increase the frequency
• Next replication includes these changes
• Source of new alleles (new variation)
– A few are beneficial (make a better protein)
– Most are silent (make no change in a protein)
– Some are harmful (most that change a protein)
Mutations
• Ask for examples of mutant human genes
• Bad mutations include ->
– sickle cell anemia, hemophilia, color blindness.
• Can you find a mutation in this picture?
Can you find two mutations?
(There are at least two.)
Mutations:
• blue eyes & white skin
(occurred long ago).
• some say …
DNA Repair
• DNA polymerases often
insert incorrect bases
– DNA mismatch
• Proofreading enzymes
correct mistakes
DNA Repair
• Enzymes recognize
damaged DNA
• Cut the backbone on
either side
• Damaged DNA is
removed
• DNA is corrected