Transcript Transposons_&_DNA_Mutations

GENETICS
Genetics is the study of the transmission of things
from one generation to the next
Genetic characteristics of a population can change
over time
– “Evolution”
These things can be
– Traits / characteristics
– Chromosomes
– Genes
GENETIC VARIATION
Genetic variation is produced in two ways
– through Mutation
Heritable changes in DNA sequence
– Through Gene transfer
Acquiring genes from another member of our species
MUTATION
Inheritable change in DNA sequence
Relatively rare
Generally occur during DNA replication or repair
– May also occur in response to mobile DNA
elements
Transposons and viruses
May affect gene expression
SPONTANEOUS MUTATIONS
Occur without effects of outside agents
–
Radiation, chemical mutagens, etc.
Various types
1. Base substitutions
One or more base pairs changed
2. Insertions and deletions
Sometimes caused by transposable elements
– Jumping genes
Insertion of one or more bases
Deletion of one or more bases
MUTATIONS
POINT MUTATION
One base pair altered
Substitution can change the entire
reading frame of the DNA causing
1. Silent mutation – no result because same Amino
Acid is produced (lucky there was no change)
2. Missense mutation
Wrong AA produced which messes up the protein (Sickle Cell
Anemia)
3. Nonsense mutation
Stop command results, so proteins don’t even get made
properly
Substitution
Insertion Mutations
MUTATIONS
Insertions & Deletion Mutations
“Frameshift mutations”
Insertion or deletion of base pair(s)
– e.g., GGA  GAGA (gly  glu)
Generally alter reading frame
– “Frameshift”
– All downstream amino acids altered
Protein function generally affected
– Typically “knockout” mutants
Insertion / deletion &
genes that jump
“Transposons” = “Jumping Genes”
Transposons are DNA segments spontaneously entering
or exiting chromosomes
Transposition into a gene constitutes a large insertion
– Gene is generally inactivated
Transposition out of a gene may restore gene function
TRANSPOSABLE ELEMENTS
First discovered by Barbara McClintock in the 1940s
Worked with maize (corn)
Kernel color varied as DNA sequences jumped in and
jumped out, messing with the pigment genes
 no pigment
 partial pigment
 normal pigment
TRANSPOSABLE ELEMENTS
Discovered them in 1940s, but
nobody paid attention
Ideas finally accepted in 1970s
Nobel prize in 1983
– She was 81 years old!!
Some of Barbara’s historically
significant research plots were
destroyed by nutcases protesting
genetic engineering
– None of the plants were engineered
INDUCED MUTATIONS
Some mutations occur spontaneously
– Rare
Certain chemicals or radiation can cause
mutations
– We call these “Mutagens”
– Greatly increase the frequency of mutations
e.g., 1,000X or more
INDUCED MUTATIONS
Chemical Mutagens
Chemicals
Radiation
– Ultraviolet
– Ionizing radiation
(X-rays, gamma rays)
Breaks in single strands
Breaks in double strands
May result in deletions,
insertions
DNA REPAIR
Mutations are rare
– Many errors are automatically corrected by DNA
Polymerase’s spell checking abilities
Mutations in genes for DNA repair enzymes are
particularly problematic
– Increase mutations
NORMAL DNA REPAIR
Proofreading is always happening
– DNA Polymerase detects error during synthesis
– Backs up, excises wrong base, and continues
– Relatively accurate
Mismatch repair is completed
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Recognizes mismatch if DNA polymerase misses it
Cuts out incorrect segment
Fills in gap correctly
DNA ligase joins the segments
Relatively accurate