MUTATIONS
Term
Chrom-some; somaHomo-logKaryo-
Meaning
Example
A mutation is a permanent change in the DNA sequence of a gene. Sometimes mutations in
DNA can cause changes in the way a cell behaves. This is because genes contain the instructions
necessary for a cell to work. If some of the instructions to the cell are wrong, then the cell may
not know what it is supposed to do!
THE EFFECT OF MUTATIONS
It is not correct to assume that all mutations are harmful. There are many documented cases
where a mutation has helped an organism survive. These beneficial mutations occur mostly in
viruses and bacteria, but sometimes occur in multicellular organisms as well. Sometimes a
mutation may be neutral; that is, it causes a change in trait, but does not help or hinder the
organism. Additionally, a mutation may occur that does not change the trait at all. These are
known as silent mutations.
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TWO MAIN TYPES OF MUTATIONS
1. Gene mutations- affect the DNA strand itself
2. Chromosomal mutations- affect or involve the chromosomes
GENE MUTATIONS
HOW DO MUTATIONS OCCUR?
There are two ways in which DNA can become mutated:
1. Mutations can be acquired. This happens when environmental agents damage DNA, or
when mistakes occur when a cell copies its DNA prior to cell division.
2. Mutations can be inherited. This means that if a parent has a mutation in his or her DNA,
then the mutation is passed on to his or her children.
ACQUIRED MUTATIONS
Acquired mutations occur in a body (somatic) cell after the organism has begun to develop
beyond the zygote (fertilized egg cell) stage. Thus, they are often referred to as somatic
mutations. Mutations like this will not be found in every cell of the organism, only those cells
that are daughter cells and successive progeny of the original mutant. In some cases, mutations
can trigger the onset of cancer. Mutations in DNA sequences generally occur through one of two
processes:
1. DNA damage from cancer-causing agents (mutagens): environmental agents such as
ultraviolet light (sunshine), nuclear radiation, certain chemicals, and viruses or
microorganisms.
2. Mistakes that occur when a cell copies its DNA in preparation for cell division.
DNA damage from environmental agents
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Modifying nucleotide bases
Ultraviolet light, nuclear radiation, and certain chemicals can damage DNA by altering
nucleotide bases so that they look like other nucleotide bases.
When the DNA strands are separated and
copied, the altered base will pair with an
incorrect base and cause a mutation. In the
example
below a
"modified"
G now pairs
with T,
instead of forming a normal pair with C.
Breaking the phosphate backbone
Environmental agents such as nuclear radiation can damage DNA by breaking the bonds
between oxygens (O) and phosphate groups (P).
Breaking the phosphate backbone of DNA within a gene creates a mutated form of the gene. It is
possible that the mutated gene will produce a protein that functions differently.
Cells with broken DNA will attempt to fix the broken ends by joining these free ends to other
pieces of DNA within the cell. This creates a type of mutation called "translocation." If a
translocation breakpoint occurs within or near a gene, that gene's function may be affected.
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SLOOZE WORM MUTAGENESIS
The Slooze worm is a creature that lives in the heavily polluted waters of Lake Asbestozene.
The Slooze worm's genome is one double-stranded DNA molecule composed of five nucleotide
base pairs. Each of the Slooze worm's eight cells contains one copy of the genome. (A cell
containing only one copy of its genome is called a haploid cell. If it had two copies, the cell
would be diploid.)
In order for the Slooze worm to grow from one cell into an eight-celled organism, it must divide
three times.
Cell Division 1
Before a cell divides, it must copy its DNA, in a process called "DNA
replication."
First, the double-stranded DNA must be separated into two singlestranded molecules.
Then the cell makes an exact copy of its DNA (red), using the
original DNA (black) as a template. To do this, the cell follows the
rules of base-pairing, which means that A nucleotides pair with Ts,
and G nucleotides pair with Cs.
The result is two copies of the same DNA molecule, each of which
goes to a new cell during cell division.
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Cell Division 2
Lake Asbestozene contains many harmful environmental agents called mutagens. Mutagens can
damage nucleotides, causing them to look different to the cell. As a result of their new look, the
damaged nucleotides pair with the wrong partner during DNA replication.
Before the second round of cell division, a mutagen from Lake Asbestozene damages the first G
nucleotide in one cell (purple G inside of orange star). The damage makes this G look more like
an A, so when the cell replicates its DNA, the damaged G pairs with a T nucleotide, instead of
with the usual C nucleotide.
After the double-stranded DNA is copied, each new DNA molecule goes to a new cell during
cell division. This results in one mutated cell and three normal cells.
Cell Division 3
No new DNA damage occurs, and the four cells divide to make eight new cells, creating a
mature Slooze worm.
1. How many of the eight cells will have mutations after the third cell division? Assume that the
purple G (in the orange star) still looks like an A nucleotide to the cell, and pairs with other
Ts. Note that the mismatched T still looks the same to the cell, and will pair correctly with
other A nucleotides.
2. How many cells in the eight-celled Slooze worm will have mutations if the DNA repair
enzymes change the G-T base pair back to a G-C base pair before the third cell division?
Assume that the G looks normal to the cell after repair occurs.
3. How many Slooze worm cells will be mutated if the DNA repair enzyme only corrects the GT mismatch to G-C after the third cell division?
4. How many cells in the Slooze worm will be mutated if the DNA repair enzyme changes the
G-T to an A-T before the third cell division? (The enzyme might do this if it treats A-T as the
correct nucleotide base pair and fixes the G.)
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Mistakes created during DNA duplication
Prior to cell division, each cell must duplicate its entire DNA sequence. This process is called
DNA replication.
Mutations result when a mistake occurs in base
pairing, which happens about once every
100,000,000 bases.
Actually, the number of mistakes that remain
incorporated into the DNA is even lower than this because cells contain special DNA repair
proteins that fix many of the mistakes in the DNA that are caused by mutagens. The repair
proteins see which nucleotides are paired incorrectly, and then change the wrong base to the right
one.
Types of mutations that can occur during DNA replication
During DNA replication, mistakes occur during the copying process. The diagram below shows
a normal, non-mutated replication event.
Substitution mutation. A substitution mutation occurs when one nitrogenous base is replaced
with a different one.
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Insertion mutation. An insertion mutation occurs when an extra nucleotide is added to the
sequence. This will change the “reading frame”, and likely result in a change in all amino acids
downstream from the mutation.
Deletion mutation. A deletion mutation occurs when a nucleotide is omitted from the sequence.
Like the insertion mutation, this will change the “reading frame”, and likely result in a change in
all amino acids downstream from the mutation.
INHERITED MUTATIONS
Inherited mutations are also called genetic mutations. This type of mutation occurs when a
mutated egg or sperm come together during fertilization. If an organism has an inherited
mutations, every cell in its body will have the mutation.
CHROMOSOME MUTATIONS
Chromosomal mutations only occur during meiosis. They involve the rearrangement of entire
blocks of genes, rather than just a few individual bases within a gene.
Deletion mutation. A break may occur at two points on the
chromosome and the middle piece of the chromosome falls
out. The two ends then rejoin to form a chromosome lacking
some genes. Alternatively, the end of a chromosome may
break off and be lost.
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Inversion mutation. An interior piece of chromosome
falls out, rotates 180°, then rejoins. There is no loss of
genetic material. The genes will be in reverse order.
Translocation. A group of genes moves to a different
chromosome. The two chromosomes are different types. This will
cause major problems during sperm or egg formation. Some
gametes will receive extra genes, while others will not receive
enough. This could be problematic if these gametes go through
fertilization.
Duplication. A segment is lost from one chromosome and is
added to another, similar chromosome. Some gametes will
receive double the genes while others will have none. This could
be problematic if these gametes go through fertilization.
ANEUPLOIDY
Aneuploidy is a type of chromosomal mutation in which an individual inherits more or fewer
chromosomes than normal. In some cases, this will result in miscarriage during pregnancy. In
other cases, the organism may live and survive. Down syndrome is an example of aneuploidy. In
Down syndrome, the individual inherits an chromosome 21. Thus, the individual has three of that
type of chromosome, giving that individual a total of 47 chromosomes. This type of
chromosomal mutation is most common with the sex chromosomes.
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COMMON ANEUPLOIDY CONDITIONS
Down syndrome- Extra chromosome 21- decreased cognitive ability, poor muscle tone, heart defects, certain facial
characteristics
Edwards syndrome- Extra chromosome 18- internal organ defects, high miscarriage rate
Patau syndrome- Extra chromosome 13- mental and motor abnormalities, polydactylism
Warkany syndrome- Extra chromosome 8- mental abnormalities, musculoskeletal and neural abnormalities
Cat-eye syndrome- Extra chromosome 22- eye has appearance of a cat eye
Turner syndrome- XO- female; typically sterile, ADHD and learning disabilities common
Klinefelter syndrome- XXY- male; small testicles, sterile, very common (1/500-1/1000), learning disabilities are
common, may have increased breast tissue. XXYY is a variation of this disorder, with more severe mental and
learning disabilities. XXXXY is another variation.
Triple X syndrome- XXX- female; can be infertile, behavioral and learning disabilities are common; typically,
these females are asymptomatic and go undiagnosed. XXXX and XXXXX are more severe variations.
XYY syndrome- male; common (1/1000), usually asymptomatic and undiagnosed, higher incidence of behavioral
and learning disorders
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