Transcript What Causes Mutations?

11.6: DNA Mutation
Describe the types of mutations that can affect genes.
Explain what can cause a mutation.
A sickle ….
Sickle Cell Disease
Sickle cell disease is an
example of a disorder caused
by a genetic mutation.
In sickle cell disease, the
hemoglobin in red blood cells
tends to bind together when
oxygen levels are low. The
hemoglobin crystals deform
the red blood cells into sickle,
or crescent, shapes.
The sickle-shaped cells
clog tiny blood vessels,
dangerously blocking the
normal flow of blood.
Blood capillaries
burst…other painful
symptoms result…
The molecular basis for the disease lies in the
difference of only one DNA nucleotide out of a
438-base sequence.
Sickle cell
protein
How Mutations Affect Genes
A mutation is any change in the nucleotide sequence
of DNA. Mutations can involve large regions of a
chromosome or just a single nucleotide pair, as in the
sickle cell allele described previously.
How can small changes such as altering a single
nucleotide in the DNA sequence cause such big
changes in the phenotype?
Mutations within a gene can be divided into 2 general
categories:
1. Base substitutions
2. Base insertions or deletions.
A base substitution is the
replacement of one base or
nucleotide with another.
Occasionally, a base substitution
causes no change to a protein, but
sometimes it results in a change
that affects the function of a
protein, sometimes drastically (as
in the base substitution leading to
sickle cell disease).
Mutation that does not cause a
change in protein structure is
called a "silent mutation."
Other changes of a single
nucleotide may result in an mRNA
that codes for an amino acid whose
chemical properties are similar
enough to the original amino acid
that there is little effect on the
overall function of the protein.
Base Insertion or Deletion Mutation
Insertion or deletion of one or more nucleotides in a gene is usually
more disastrous than the effects of a base substitution.
Because mRNA is read as a series of triplets, adding or subtracting
nucleotides may alter the triplet groupings of the genetic message.
Therefore, all the nucleotides that are "downstream" of the mutation will
be regrouped into different codons.
These new codons code for new amino acids. The result will be a
different, and probably nonworking, protein.
What Causes Mutations?
Mutations may occur when
errors are made during
DNA replication, or when
errors are made during
chromosome crossovers
in meiosis.
What Causes Mutations?
Physical or chemical agents that cause
mutations are called mutagens . The
most common physical mutagen is high-energy
radiation, such as X-rays and ultraviolet light.
What Causes Mutations?
One type of chemical mutagen consists of chemicals
that are similar to normal DNA bases but cause
incorrect base-pairing when incorporated into DNA.
Most Mutations are Deadly!
Mutations happen.
They happen with great regularity.
Almost all mutations are lethal……to the individual.
Especially is it involves a “frameshift”.
Remember that an insertion or deletion causes the
frame shift.
A silent mutation is not lethal because it is a point
mutation and it does not change the amino acid that is
coded for.
Some rare mutations may not be
harmful!
Although mutations are often harmful, they can alter a protein
in a way that may be beneficial in certain environments.
For example, a genetic mutation is responsible for the dark
color seen in some females of the tiger swallowtail butterfly
species, Papilio glaucus. The mutation may be advantageous
when the environment includes a related species, the
poisonous black swallowtail Battus philenor. Predators that
avoid eating the black swallowtail may also avoid eating its
mimic, the dark form of P. glaucus.
Papilio glaucus fitness is thus increased!
Helpful Mutations:
Fitness
Papilio glaucus
Mutant female
Battus philenor
Papilio glaucus
(Bitter-tasting)
Normal female
If a mutation is present in an organism's gametes, it can be
passed on to its offspring.
According evolution theory, mutations are the “engine” for
the genetic diversity in the living organisms.
30 New Mutations per Lifetime
As scientists learn to read the instructions in our genes, they are discovering that much of our
DNA is riddled with errors.
Fortunately, most of these errors are harmless. Considering the difficulties involved—the 6 feet
of DNA in a human cell consists of 6 billion subunits, or base pairs, coiled and tightly packed
into 23 pairs of chromosomes, all of which must be duplicated every time a cell divides—our
general state of health is something of a miracle.
We each inherit hundreds of genetic mutations from our parents, as they did from their
forebears. In addition, the DNA in our own cells undergoes an estimated 30 new mutations
during our lifetime, either through mistakes during DNA copying or cell division or, more often,
because of damage from the environment.
Bits of our DNA may be deleted, inserted, broken, or substituted. Most mutations affect only the
parts of DNA that do not contain instructions for making a gene, so we need not worry about
them. Problems arise only when an error in DNA alters a message that tells certain cells to
manufacture a certain protein. Such messages are spelled out in varying sequences of the four
chemical bases that make up DNA: adenine (A), thymine (T), guanine (G), and cytosine (C).
To stay alive and functioning, the human body requires a daily crop of billions of fresh protein
molecules—about 40,000 different kinds of proteins that must be supplied in the right quantities,
at the right times, and in the right places. We need hemoglobin to carry oxygen through the
bloodstream, antibodies to fight foreign substances, hormones to deal with stress,
neurotransmitters to evoke movements, emotions, and thought, and many other proteins to give
structure to organs or speed up chemical reactions.