Transcript Mutations - Point Pleasant Beach School District / Overview

Mutations
In molecular biology and
genetics, mutations are
changes in the DNA
sequence of a cell's
genome.
http://learn.genetics.utah.edu/co
ntent/variation/
http://www.genetichealth.com/g101_changes_in_dna.shtml

Everyone acquires some changes to their DNA during the course of their
lives. These changes occur in a number of ways.

Sometimes there are simple copying errors that are introduced when
DNA replicates itself - every time a cell divides, all of its DNA is
duplicated so that the each of the two resulting cells have a full set of
DNA.

Other changes are introduced as a result of DNA damage through
environmental agents including sunlight, cigarette smoke, and radiation.

Any physical or chemical agent that changes the genetic material,
increasing the rate of mutations is called a MUTAGEN.

Some mutagens can also be carcinogens – cancer causing agents, or
teratogens – cause birth defects in fetus..

Our cells have built in mechanisms that catch and repair most of the
changes that occur during DNA replication or from environmental
damage. As we age, however, our DNA repair does not work as effectively
and we accumulate changes in our DNA.

Some of these changes occur in cells of
the body (“somatic” cells) — such as in
skin cells as a result of sun exposure —
but are not passed on to children.

But other errors can occur in the DNA
of cells that produce the eggs and sperm.
These are called germline mutations
and can be passed from parent to child.

If a child inherits a germline mutation
from their parents, every cell in their
body will have this error in their DNA.

Germline mutations are what cause
diseases to run in families, and are
responsible for hereditary diseases.
So, what’s the deal?

A gene is essentially a sentence made up
of the bases A, T, G, and C that describes
how to make a protein.

Any changes to those instructions can
alter the gene's meaning and change the
protein that is made, or how or when a
cell makes that protein.
How does it work?

There are many different ways to alter a
gene, just as there are many different ways
to introduce typos into a sentence. In the
following examples of some types of
mutations, we use the sentence to
represent the sample gene:
THE FAT CAT ATE THE RAT
(this is how the code should read)
Here’s a handy table listing all the types of mutations:
http://www.uvm.edu/~cgep/Education/Mutations.html
Point mutation: a change in a single
nucleotide
Example 1:
 A SUBSTITUTION mutation
 occurs where one nucleotide base is
replaced by another.
◦ example
 THE FAT CAT ATE THE RAT
 THE FAT HAT ATE THE RAT
Point mutations: types

Base (A,T,C,G) substitutions can lead to
“Missense” or “Nonsense” mutations:
Missense: a change in DNA sequence that
changes the codon to a different amino acid.
This can alter the protein enough to render it
nonfunctional.
Not all missense mutations are deleterious,
some changes can have no effect.
Because of the ambiguity of missense
mutations, it is often difficult to interpret the
consequences.
Nonsense: a change in the
genetic code that results in the
coding for a stop codon rather
than an amino acid.
The shortened protein is
generally non-function or its
function is impeded.
Point mutations can sometimes be “SILENT”.
How does this change the protein that this
gene codes for?
More often than not,
“third-base mutations”
are silent. Why?
Another example:
Amino acid: leucine
GAA
GAG
GAT
GAC
Sickle Cell Anemia

Is an inherited blood disorder than is caused by
a substitution point mutation in the hemoglobin
gene.
This is an
example of a
____?____
mutation.
Look at the effect of
abnormally-made hemoglobin
in red blood cells:
Point mutation: a change in a single
nucleotide in a gene

Examples 2 & 3:
◦ Insertion example
 THE FAT CAT ATE THE RAT (original gene)
 THE FAT CAT LAT ETH ERA T
◦ Deletion example
 THE FAT CAT ATE THE RAT (original gene)
 THE FAT ATA TET HER AT
These substitutions are known as “Frameshift
Mutations” because they shift the reading frame of
the genetic message (triplets) so that the protein
may not be able to perform its function.
Usually more serious than a substitution mutation. Why?
Follow this summary:
Remember: not all mutations are bad!

A harmful mutation is a mutation that
decreases the fitness of the organism.

Sometimes, mutations can change a gene
form in a nonharmful, or even beneficial,
way.

“Polymorphisms”: slight variations in a
gene that make us different, ex: eye colors,
blood types, etc. Are these good or bad?

A mutation may enable the mutant
organism to withstand particular
environmental stresses better than nonmutant organisms, or reproduce more
quickly. In these cases a mutation will tend
to become more common in a population
through natural selection. This is how
populations EVOLVE over time.
Many mutations are neutral

A neutral mutation has no harmful or
beneficial effect on the organism.

Many of these mutations occur in the
noncoding DNA describes components of
an organism's DNA sequences that do not
encode for protein sequences.

More than 98% of the human genome does
not encode protein sequences. We often
call this “junk DNA” (extra).
Chromosome Mutations

A chromosome mutation is an unpredictable change that
occurs in a chromosome.

These changes are most often brought on by problems
that occur during meiosis (cell division that makes
gametes) or by mutagens (chemicals, radiation, etc.).

Chromosome mutations can result in changes in the
number of chromosomes in a cell or changes in the
structure of a chromosome.

Usually MUCH more serious than a gene mutation. Why?
Changes in chromosome number or structure
(a) A deletion removes a
chromosomal segment.
A B C D E
F G H
A B C D E
F G H
Deletion
A B C E
F G H
(b) A duplication repeats a
segment.
(c) An inversion reverses a
segment within a chromosome.
d) A translocation moves a
A B C D E
segment from one chromosome
to another, nonhomologous one.
•
•
In a reciprocal translocation,
the most common type,
nonhomologous chromosomes
exchange fragments.
Nonreciprocal translocations
also occur, in which a
chromosome transfers a
fragment without receiving a
fragment in return.
A B C D E
F G H
Duplication
Inversion
A B C B C D E
A D C B E
F G H
M N O C D E
F G H
F G H
F G H
Reciprocal
translocation
M N O P Q
R
A B P
Q
R
15
Test your knowledge

Changes in Chromosomal Structure
(animation)

http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter24/chang
es_in_chromosome_structure.html

The Consequence of Inversion (animation)

http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/
free/0072437316/120082/bio33.swf::The%20Consequence
%20of%20Inversion