GENE MUTATIONS

Recall that with a GENE mutation, only one gene on the chromosome is affected. However, many genes are crucial for producing key enzymes and for controlling key cellular processes.

WARNING !!!We always use mRNA CODONS to look up what AMINO ACIDS that codon represents. You never look up DNA codons in a Genetic Code table! If you see the letter “T” for thymine, you know that a DNA codon is being represented. Having said all that, you will see a couple of Google images I use in this PPT will state the codon in the DNA form, just ignore.

Two Major CategoriesA) Point Mutation B) Frameshift Mutation

In a POINT mutation, one incorrect base replaces an existing base in the gene. This only affects that one CODON.

Healthy Gene

Mutant Gene

In a FRAMESHIFT mutation, a base is taken out OR an extra base is added in. Each CODON from that point is shifted out of whack.

POINT MUTATIONSThere are three different types of POINT mutations:1. SILENT2. MISSENSE3. NONSENSEIn a SILENT point mutation, one base is substituted for another. But there is no effect on the sequencing of amino acids. Because of the “DEGENERACY” of the genetic code. Often, 3-4 different codons may still code for the same amino acid. For example the Codon “GAA” codes for the amino acid “Glutamic Acid”, but the codon “GAG” also codes for the amino acid “Glutamic Acid”. So there is absolutely no affect on the resulting protein being synthesized.

MISSENSE Point MutationIn a MISSENSE point mutation, one base is substituted for another. But this time, the codon has changed in a way that no longer makes it code for the original Amino Acid. For example the codon “GAA” codes for “Glutamic Acid”. But when the last base is changed to a “C”, the new codon “GAC” now codes for the Amino Acid “Aspartic Acid”. Although this only affect one AA in the protein, sometimes the protein will not take its proper shape and will not properly function.

NONSENSE Point MutationIn a NONSENSE point mutation, again one base is substituted for another. But the change to the codon, changes it from a normal coding codon to a “STOP” codon. Rather than another tRNA coming in and dropping off another amino acid, a “Release Factor” comes into the Ribosome and causes “Termination” to translation. So the polypeptide is cut off a that point. It is impossible for the polypeptide that formed to take on the proper shape and function of the intended protein. So this will always be big trouble for the body.

FRAMESHIFT Gene MutationsThere are two different types of Frameshift Mutations:A) DeletionB) AdditionIn a DELETION frameshift mutation, a single base is deleted, but every single codon from that point on is affected. So the resulting polypeptide is completely messed up. Perhaps as much as 90% of the amino acids end up being in the wrong spot, not to mention that a “Stop Codon” could easily show up.

Frameshift – Addition Mutation

Different cause, but same results. All the following codons are thrown off.

PRACTICE QUIZ:1. Specifically name the type of Gene Mutation Shown Below:

GENE MUTATION

Healthy Gene

ANSWER : MISSENSE POINT MUTATION

Question #2 – Specifically identify the gene mutation shown here

Gene Mutation

Healthy Gene

ANSWER : DELETION FRAMESHIFT MUTATION

Question #3 Specifically Identify the following Gene Mutation

Healthy Gene

Mutant Gene

ANSWER: SILENT POINT MUTATION

Question #4-Specifically Identify this GENE Mutation

Healthy Gene

Mutant Gene

ANSWER : NONSENSE POINT MUTATION