Transcript SBI 4U Genetics 5

Genetics 5:
DNA Mutations
Regulation of Gene Expression
Single Gene Mutations
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Changes in the DNA sequence of only one gene
Point Mutation: just one change of one base pair.
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If there is a deletion or insertion of a base pair, it
can cause frameshift mutation.
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May not have a huge effect on the coding sequence.
Reading frame of the entire gene can shift one base
pair over.
Mutations can be referred to as either silent
mutations (no altered amino acid sequence) or a
missense mutation (altered amino acid
sequence that still makes a protein) or a
nonsense mutation (mutation that shortens a
protein by prematurely adding a stop codon)
Chromosome Mutations
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When you get a change to the entire
chromosome and it may involve
numerous genes.
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Deletion
Duplication
Inversion
Translocation
Causes of Mutations
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Spontaneous mutations happen naturally in
the cell
Some segments of DNA can actually jump
from area to area within the genome. They
are called transposons.
Mutations can also happen due to an induced
factor (mutagen).
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Chemical mutagens: a molecule that enters the
cell’s DNA and causes substitution or frameshift
changes. EG. Gasoline fumes, nitrites and
compounds found in cigarette smoke
Physical mutagens: physically change the DNA
structure. EG xrays, UV rays.
Regulation of Gene Expression
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Some genes are always active because
they are needed for the survival of the
cell. These are called constitutive
genes or housekeeping genes.
Other genes are turned on or off at
different times depending on when they
are needed by the cell.
Prokaryotes: lac operon
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The operon is a cluster of genes grouped
together under the control of one promoter.
The genes that help the bacterium E.coli
digest lactose are found in the lac operon.
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It has a regulatory region that contains the
promotor for transcription,
regulation sites,
an operator (DNA sequence where proteins can
bind onto that can stop the transcription process),
and a catabolic activator protein (CAP) binding
site.
When there is no lactose, the lac repressor
protein binds onto the operator and prevents
RNA polymerase from binding to the promoter
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In order to start the lac operon, lactose must
be present.
When lactose is present, it binds onto the
repressor protein and changes the shape of it
so it can no longer bind onto the operator.
Then the CAP activator can bind on and
controls how fast the transcription works.
The lac operon is known as an inducible
operon because transcription is induced when
lactose is present.
The trp operon
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The operon contains 5 different genes that
help code for enzymes required to make the
amino acid tryptophan.
Regulatory region contains a promotor and an
operator region
Normally, we need tryptophan, so we will
transcribe the gene and there is no repressor.
When we have enough tryptophan, we need
to stop the transcription, and the amino acid
itself will bind onto the repressor protein.
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This time the repressor protein will change shape
so that it does fit into the operator and stop the
transcription.
lac vs trp Operons
Regulation of Gene
Expression in Eukaryotes
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Pre-transcriptional Control: areas not
needed for transcription are wound too
tightly in chromatin to be copied.
Transcriptional Control: each gene has
its own promoter and controls
transcription with the help of activator
proteins, transcription factors,
enhancers and RNA polymerase
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Post-transcriptional control: modifications can
be made to the mRNA so that the 5’ cap and
the 3’ poly-A tail are not added (and do not
leave the nucleus)
Translational control: micro RNA and small
interfering RNA can associate with protein
complexes and turn off gene expression.
Post-translational control: many polypeptides
are not active right away. Some may need a
few amino acids to be cleaved off, or adding a
covalent bond in places.
Questions:
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Page 266 # 8, 10, 12, 13.
Page 272 #3 - 7, 13