Download gene mutation

Gene Mutations
• A gene mutation is a change in the sequence
of bases (A, G, C, and T) within any given gene.
– A change in bases in DNA means that mRNA will
be changed, too!
– If mRNA is changed, the final protein will be
affected.
Gene Mutations
Causes of mutations:
• Errors in DNA replication
– Sometimes, new DNA strands contain mismatched
pairs of bases (eg: G with A, instead of G with C).
– Very rare – DNA polymerase “proof-reads” its
work by checking the original DNA strand.
– Around 1 mistake per every billion bases copied.
Gene Mutations
Causes cont’d:
• Mutagens – anything (chemicals, energy,
viruses, bacteria, etc.) that can induce a gene
mutation.
– Includes radiation (UV, X-rays, radioactive
elements), organic chemicals (some pesticides,
compounds in cigarette smoke).
– Rate of mutation from these is low because DNA
repair enzymes constantly check and repair DNA.
Gene Mutations
Causes cont’d:
• Mutagens cont’d
– Radiation and chemicals can damage genes and
make them unreadable.
– Repair enzymes can also replace a damaged or
missing base incorrectly (eg: replace a missing G
with a T).
Gene Mutations
Gene Mutations
Causes cont’d
• Transposons – DNA sequences that can
“jump” from one chromosome to another, or
to other spots on the same chromosome
(hence why they’re called “jumping genes”)
– Can jump into the middle of another gene,
thereby disrupting it.
Transposons
Gene Mutations
Types of Mutations:
• Point mutations – occur when one nucleotide,
carrying a certain base (A, G, C, or T) is
replaced by another carrying a different base.
– Results vary greatly!
– If the new codon still creates the same amino
acid, there is no change – this is a silent mutation
(eg: UAC is mutated to UAU – both code for
tyrosine).
Gene Mutations
Types of Mutations cont’d:
• Point mutations cont’d
– If the mutation causes a new stop codon to form,
the protein could end up too short and become
dysfunctional (eg: if UAC is mutated into UAG).
This is called a nonsense mutation.
Gene Mutations
Types of Mutations cont’d
• Point mutations cont’d
– If the mutation causes a different amino acid to
end up in the protein, it will affect the shape and
functionality of that protein. This is called a
missense mutation.
Missense Mutation
Summary of Point Mutations
Gene Mutations
Types of mutations cont’d:
• Frameshift Mutations – occur when one or
more nucleotides (and their bases) are
inserted or deleted from DNA.
– This changes the ordering of the codons, causing
the ribosome to use the wrong amino acids to put
the protein together.
– It changes the “reading frame” of the mRNA.
Frameshift Mutations
Genetics of Cancer
• All cells follow a life cycle called the cell cycle
– they grow, replicate their components,
divide, and eventually die.
• In cancer, cells divide uncontrollably, invade
nearby tissues, and disrupt the normal
functioning of the body.
• This is often the result of specific mutations.
Genetics of Cancer
• Normal cells stop dividing when they come
into contact with neighboring cells.
• Tumor cells do not stop dividing at all.
– Comes from damage to or mutations in 2 kinds of
genes:
– Oncogenes usually create proteins that move the
cell cycle along (like stepping on the gas pedal).
• If mutated, they can override anything stopping the cell
cycle.
Genetics of Cancer
• Tumor cells… cont’d
– Tumor suppressor gene usually make proteins
that slow or stop the cell cycle (like stepping on
the brakes) and also cause cell death (apoptosis)
• Mutations can damage these genes and cause them to
no longer create the proper proteins.
– If either of these kinds of genes are damaged, the
cell cycle is thrown off balance causing tumor cells
that may never die on their own.
Genetics of Cancer
• As cells divide more and more, risk of further
mutations increases.
• If these mutations allow certain enzymes to
be created, the tumor cells can leave their
original place and travel throughout the body
– Called metastasis (creates a malignant tumor)