Lecture 33 Part I: Mutations
Ed. 280-281; 322-323.
Ed. 286; 328-330.
Ed. 298; 344-346
Part II: Gene Regulation in Bacteria
PART I: Mutations
We learned earlier that mutations can affect phenotype.
For instance, a mutant fly had white eyes
instead of the normal, wild-type red eyes.
Later work showed exactly how that occurred.
Mutations are changes, or uncorrected errors, in the DNA sequence.
Normally, DNA polymerase is
very accurate during DNA replication, because of its proof-reading function.
occasionally, it can make a mistake.
(Mutations can also arise from harmful agents in the
environment that damage DNA).
Once these are in place, they are passed down every time the
DNA is replicated.
An error in the DNA sequence of a gene can lead to an error in the amino acid
sequence of a protein.
This often makes it non-functional.
There are 3 classes of mutation: Point Mutations (single base changes), Insertions or Deletions
(where one or more bases is added or left out), and Large-scale Chromosomal Changes.
discuss each in turn.
Another name for a point mutation is a substitution
This is a change in one
base in DNA; that is, a substitution of one base for another.
There are three classes of substitution
The first is a silent mutation
This is where substitution changes a codon to another that
codes for the same amino acid.
This can occur because the Genetic Code is redundant, and more
than one codon can code for the same amino acid.
For instance, if a point mutation occurred in the
codon CCA, and it were accidentally mutated to CCG, it would still code for proline.
kind of substitution mutation is a missense mutation
In these cases, a point mutation changes a
codon so that it codes for a different amino acid.
A famous example is in the beta-globin gene.
This codes for one form of globin, a protein in red blood cells that’s important in carrying oxygen to
A missense mutation can occur to change one codon to a different codon that codes for
a different amino acid.
The globin protein that’s made with the wrong amino acid doesn’t work as
It causes red blood cells to have a distinctive abnormal sickle-like shape, and results in sickle
This is an example of a genetic disease, because it results from a mutation that’s
passed from one generation to the next.
The third kind of substitution mutation is a nonsense
In this case, a single nucleotide change creates a new stop codon.
This causes premature
termination during translation; that is, translation stops part-way through the normal coding
sequence, resulting in a shorter protein.
These are usually non-functional.
The second class of mutation is insertion or deletion