Download Lesson 1.1: Mutation

Biology - Student Reader & Workbook
Unit 4, Evolution - Chapter 1: Mutation
LESSON 1.1: MUTATION ..............................................................................................2
Lesson Objectives ....................................................................................................................... 2
Lesson Vocabulary ..................................................................................................................... 2
Lesson Introduction .................................................................................................................... 2
Causes of Mutation ..................................................................................................................... 4
Types of Mutations ..................................................................................................................... 6
Chromosomal Alterations..................................................................................................................... 7
Point Mutations .................................................................................................................................... 9
Frameshift Mutations ......................................................................................................................... 10
Effects of Mutations.................................................................................................................. 11
Harmful Mutations .................................................................................................................... 12
Beneficial Mutations ................................................................................................................. 13
Graphic Organizer: Spontaneous and Induced Mutations ........................................................ 15
Graphic Organizer: Harmful and Beneficial Mutations ........................................................... 16
Graphic Organizer: Germline and Somatic Mutations ............................................................. 16
Graphic Organizer: Chromosomal Alterations, Point Mutations, and Frameshift Mutations .. 17
Lesson Summary....................................................................................................................... 18
Lesson Review Questions ......................................................................................................... 18
Recall.................................................................................................................................................. 18
Apply................................................................................................................................................... 18
Think Critically .................................................................................................................................. 18
Points to Consider ..................................................................................................................... 18
Multimedia Links ...................................................................................................................... 19
1
Lesson 1.1: Mutation
Lesson Objectives
•
•
•
Identify causes of mutation.
Compare and contrast types of mutations.
Explain how mutations may affect the organisms in which they occur.
Lesson Vocabulary
Please read the words below. Decide how well you know each of the words and indicate your
knowledge level by marking the appropriate column.
Word
3
Can
Define/
Use It
2
Heard It/
Seen It
1
Don’t
Know It
Definition
Mutation
Mutagen
Chromosomal
alteration
Point mutation
Frameshift
mutation
Genetic disorder
Somatic mutation
Germline mutation
Harmful mutation
Beneficial mutation
Allele
Lesson Introduction
From the previous lesson you learned about how DNA codes for proteins, which are ultimately
responsible for all of your physical structure. There’s so much variation in organisms because
each gene on a chromosome may have two or more alleles, different forms of that gene. For
example, purple and white alleles for pea flowers. If a change occurs in the DNA or RNA
sequence this can lead to a change in the protein. This is a way of introducing new alleles –
2
perhaps a new flower color! A change in the sequence of bases in DNA or RNA is called a
mutation. Does the word mutation make you think of science fiction and bug-eyed monsters?
Think again. Everyone has mutations. In fact, most people have dozens or even hundreds of
mutations in their DNA. Mutations are essential for evolution to occur. They are the ultimate
source of all new genetic material in a species. Although most mutations have no effect on the
organisms in which they occur, some mutations are beneficial. Even harmful mutations rarely
cause drastic changes in organisms.
Before Reading:
Pick an animal and four colors. Then fill in the missing words below and answer the following
questions.
Once upon a time there was a group of ___________________. These __________________
(animals)
(animals)
usually were __________________ , __________________ , __________________. A
(color 1)
(color 2)
(color 3)
__________________ __________________ and a __________________
(color 1)
(animal)
(color 2)
__________________ were going to have a baby __________________. When the DNA
(animal)
(animal)
from the two __________________ was combining and dividing there was a mutation
(animals)
in the allele for color. The baby __________________ ended up __________________!
(animal)
(color 4)
1. Explain what happened to the parents of the baby. They want to know what an allele is and
how that mutation happened.
___________________________________________________________________________
___________________________________________________________________________
2. How might this be a harmful mutation?
___________________________________________________________________________
3. How might this be a beneficial mutation?
___________________________________________________________________________
3
Causes of Mutation
Mutations have many possible causes. Some mutations seem to happen spontaneously without
any outside influence. These are called spontaneous mutations. They occur when mistakes are
made during DNA replication or transcription. For example, the wrong nitrogen base may be
inserted or a nitrogen base may be skipped while creating a complementary DNA strand or
mRNA strand. This is similar to the mistakes you may make in spelling or math. If you
accidentally change the order of the letters or numbers, you end up with something completely
different. Mutations may also occur during mitosis and meiosis. As the cells divide and grow a
mistake happens. Spontaneous mutations happen when a mistake is made during normal cell
functions.
Other mutations are caused by environmental factors. These are known as induced mutations
because it is caused by something outside of the cell or organism. These environmental factors
can cause a change in the DNA in several different ways. One method is by changing the
structure of a nucleotide, making a mistake during replication more likely to occur. Anything
in the environment that can cause a mutation is known as a mutagen. Examples of mutagens
are pictured in Figure 1.1
Figure 1.1: Types of mutagens include radiation, chemicals, and infectious agents.
4
Examples of Mutagens
Radiation
UV Radiation
X Rays
both natural sunlight
and tanning bulbs
medical, dental, airport
security screening
Chemicals
Barbecuing
creates mutagenic
chemicals in
Cigarette Smoke
Benzoyl Peroxide
contains dozens of
mutagenic
common ingredient
in acne products
Nitrite and Nitrate
Preservatives
in hot dogs and other
processed meats
Infectious Agents
Helicobacter pylori
bacteria spread through
contaminated food
Human Papillomavirus
(HPV)
sexually transmitted virus
5
Reading Check:
1. What is a mutagen? ___________________________________________________________
2. Can you name other examples of mutagens for the categories shown in Figure 7.11?
______________________________________________________________________________
______________________________________________________________________________
3. As our awareness of potential mutagens increases, what are some examples of actions we take
to help protect ourselves from mutagens?
______________________________________________________________________________
______________________________________________________________________________
4. We typically have negative associations with the word “mutation”. However, most mutations
have no effect at all. How do you think this is possible? Recall the previous chapter on protein
synthesis and how DNA is transcribed into the form of mRNA and translated into protein to help
answer the question.
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
Types of Mutations
There are a variety of types of mutations. Two major categories of mutations are germline
mutations and somatic mutations.
•
•
Germline mutations occur in gametes (sex cells). These mutations are especially
significant because they can be transmitted to offspring and every cell in the offspring
will have the mutation.
Somatic mutations occur in any of the cells of the body besides gametes. These
mutations may have little effect on the organism because they are confined to just one
cell and its daughter cells. Somatic mutations cannot be passed on to offspring.
Mutations also differ in the way that the genetic material is changed. Mutations may change
the structure of a chromosome or just change a single nucleotide.
6
Reading Check:
1. An example of this would be mutations in a skin cell causing skin cancer. (circle one)
germline mutations
somatic mutations
2. A mistake occurring during meiosis would lead to what type of mutation? (circle one)
germline mutations
somatic mutations
3. Why can’t somatic mutations be passed on to offspring? ______________________________
______________________________________________________________________________
Before Reading:
RECALL: Which type of mutation usually causes the greatest amount of harm, a chromosomal
mutation or a gene mutation? Why? ________________________________________________
______________________________________________________________________________
Chromosomal Alterations
Chromosomal alterations are mutations that change chromosome structure. They occur when a
section of a chromosome breaks off and rejoins incorrectly or does not rejoin at all. Possible
ways these mutations can occur are illustrated in Figure 1.2
7
Figure 1.2: Chromosomal alterations are major changes in the genetic material.
Chromosomal Alterations
Think Aloud:
When do you think a chromosomal alteration is most likely to occur?
______________________________________________________________________________
Chromosomal alterations such as duplication, deletions and translocations are most likely to
occur when chromosomes cross-over during meiosis. Deletions can also be due to a loss of a
chromosome fragment during cell division. Chromosomal alterations are very serious. They
often result in the death of the organism in which they occur. If the organism survives, it may
be affected in multiple ways. An example of a human chromosomal alteration is the mutation
that causes Down Syndrome. You may recall that Down Syndrome can occur when
8
nondisjunction causes someone to have an extra chromosome 21, but it can also be caused by a
duplication or translocation mutation. Down Syndrome leads to developmental delays and
other abnormalities.
Reading Check:
Use Figure 1.2 to help you identify the following chromosomal alterations:
1. A section of a chromosome is flipped and inserted into the same chromosome.
_______________________
2. A section of a chromosome is repeated. _______________________
3. Two nonhomologous chromosomes exchange segments. __________________
4. A section of a chromosome is missing. ________________
5. One segment of a chromosome gets transferred to a nonhomologous chromosome.
___________________
Point Mutations
A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually
less serious than a chromosomal alteration. An example of a point mutation is a mutation that
changes the codon UUU to the codon UCU. Sometimes point mutations occur in noncoding
regions of DNA, so they may not have an impact on the organism. Point mutations that do
occur in coding regions can be silent, missense, or nonsense mutations, as shown in Table 1.1.
Table 7.1: The effects of point mutations depend on how they change the genetic code.
Point Mutations and Their Effects
Type
Description
Silent
Mutated codon codes for
the same amino acid.
Missense
Mutated codon codes for a
different amino acid.
Nonsense
Mutated codon is a
premature stop codon.
Example
CAA (glutamine) →
CAG (glutamine)
CAA (glutamine) →
CCA (proline)
CAA (glutamine) →
UAA (stop)
9
Effect
none
variable
usually
serious
Word Work
One meaning of the root “sense” is meaning.
The prefix “mis-” means incorrect or wrong. A gene with a missense mutation can still be read,
but the meaning is wrong because an amino acid has been changed.
The prefix “non-” means absence of something. A gene with a nonsense mutation has no
meaning because it stops protein production early.
Reading Check:
1. How is a point mutation different from a chromosomal mutation? _____________________
______________________________________________________________________________
2. When is a point mutation likely to occur? __________________________________________
3. Explain why the effect of a nonsense mutation is usually serious while the effect of a missense
mutation is variable. ____________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Frameshift Mutations
A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the
reading frame of the base sequence. Deletions remove nucleotides, and insertions add
nucleotides. Consider the following sequence of bases in RNA:
AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine
Now, assume an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after
the start codon AUG. How has the amino acid sequence been affected?
AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine
(insertion)
Even though the rest of the sequence is unchanged, this insertion changes the reading frame
and thus all of the codons that follow it. As this example shows, a frameshift mutation can
dramatically change how the codons in mRNA are read. This can have a drastic effect on the
protein product. Point mutations and frameshift mutations occur during DNA replication or
transcription into mRNA.
10
Reading Check:
1. Explain why the term “frameshift” mutation is appropriate for how the mutation causes a
change in the protein. ___________________________________________________________
_____________________________________________________________________________
2. Compare the effects of a frameshift mutation to a point mutation. _______________________
______________________________________________________________________________
______________________________________________________________________________
Effects of Mutations
The majority of mutations have neither negative nor positive effects on the organism in which
they occur. These mutations are called neutral mutations. Examples include silent point
mutations. They are neutral because they do not change the amino acids in the proteins they
encode. This can happen if it occurs in a noncoding area of DNA. Another way a mutation can
have no effect is if it causes a change in a codon that still allows it to code for the same amino
acid.
Many other mutations have no effect on the organism because they are repaired before protein
synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA. One way
DNA can be repaired is illustrated in Figure 1.3. If a cell’s DNA is permanently damaged and
cannot be repaired, the cell is likely to be prevented from dividing. Many organisms have ways
of getting rid of mutated somatic cells.
11
Figure 1.3: This flow chart shows one way that damaged DNA is repaired in E. coli bacteria.
DNA Repair Pathway
1. Repair proteins
(Uvr-A, Uvr-B, UvrC) cut out damaged
nucleotides.
2. Enzymes (DNA
polymerase I and
DNA ligase) add
normal nucleotides in
their place.
Harmful Mutations
Imagine making a random change in a complicated machine such as a car engine. The chance
that the random change would improve the functioning of the car is very small. The change is
far more likely to result in a car that does not run well or perhaps does not run at all. By the
same token, any random change in DNA is likely to result in a protein that does not function
normally or may not function at all. Such mutations are likely to be harmful. Harmful
mutations may cause genetic disorders or cancer.
•
A genetic disorder is a disease caused by a mutation in one or a few genes. A human
example is cystic fibrosis. A mutation in a single gene causes the body to produce thick,
sticky mucus that clogs the lungs and blocks ducts in digestive organs.
•
Cancer is a disease in which cells grow out of control and form abnormal masses of
cells. It is generally caused by mutations in genes that regulate the cell cycle. Because
of the mutations, cells with damaged DNA are allowed to divide without limits. Cancer
genes can be inherited.
12
Beneficial Mutations
Some mutations have a positive effect on the organism in which they occur. They are called
beneficial mutations. They lead to new versions of proteins that help organisms adapt to
changes in their environment. There are several well-known examples of beneficial mutations.
Here are just two:
•
Mutations in many bacteria that allow them to survive in the presence of antibiotic
drugs. The mutations lead to antibiotic-resistant strains of bacteria.
•
A unique mutation is found in people in a small town in Italy. The mutation protects
them from atherosclerosis, which is the dangerous buildup of fatty materials in blood
vessels. The individual in which the mutation first appeared has even been identified.
Mutations are important for adding variation to the gene pool. A gene pool is the complete set
of unique alleles in a population or species. It is all of the alternative forms of a gene in the
entire group of an organism. Thinking back to our examples of alleles, the gene pool for a
flower species contains every possible color for that species. There are hundreds of colors of
roses. That is a large gene pool! Variation is important for the health of a population because
this enables to population to be more likely to survive if there is a change in the environment.
Let’s look at bacteria as an example. If an antibiotic drug were introduced to a population of
bacteria that were all the same and had no method of resisting the antibiotic, the entire
population of bacteria would die. However, if there is variation in the bacteria population, there
may be a few that have a method of fighting the antibiotic. Those bacteria would survive,
reproduce, and pass on the ability to fight the antibiotic to their offspring. This would maintain
the population and this new population would be considered antibiotic-resistant.
Organisms with beneficial variations make them better suited to live in a certain environment
than others. This makes them more likely to survive and therefore be able to reproduce and
pass on those traits to future generation. This is called natural selection. Those with less
favorable traits resulting from mutations may be selected against and removed from the
population. The more favorable mutations that cause beneficial or advantageous traits tend to
accumulate in that population, resulting, over time, in a change in that population. This is
known as evolution.
13
Reading Check:
1. What are three ways a mutation can end up having no effect on the organism?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
2. While it’s fun to imagine mutations leading to the creation of people with superpowers like the
X-Men, mutations are much more likely to be harmful than beneficial. Why is that?
______________________________________________________________________________
______________________________________________________________________________
3. Why is it important to have a diverse gene pool?
______________________________________________________________________________
______________________________________________________________________________
4. Explain how beneficial mutations can lead to evolution.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
14
Graphic Organizer: Spontaneous and Induced Mutations
Complete the graphic organizer below by comparing the cause of spontaneous mutation versus
the cause of induced mutation.
induced
mutation
spontaneous
mutation
caused by
a change in the
DNA sequence or
order
15
caused by
Graphic Organizer: Harmful and Beneficial Mutations
Complete the graphic organizer below by describing the results of harmful mutations and
beneficial mutations.
harmful mutations
result in
beneficial mutations
result in
Graphic Organizer: Germline and Somatic Mutations
Complete the graphic organizer below by comparing which cells germline and somatic
mutations occur in and what they can or cannot do.
germline
mutation
somatic
mutation
occurs in
occurs in
can be
cannot be
16
Graphic Organizer: Chromosomal Alterations, Point Mutations, and Frameshift Mutations
Complete the graphic organizer below by listing the different types of chromosomal
alterations, point mutations, and frameshift mutations, comparing their effects, and stating
when each type of mutation is likely to occur.
Chromosomal
Alteration
types
___________
Point Mutation
types
___________
___________
types
___________
___________
___________
___________
___________
___________
___________
Frameshift
Mutation
effect
effect
effect
occurs during
occurs during
occurs during
Putting It All Together: Think/Pair/Share
Oh no! You’ve been told that your prize-winning show hamster has an induced, harmful,
germline, missense point mutation! How would you explain to your parents what this means and
why its days as a potential prize breeder are over?
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
17
Lesson Summary
•
•
•
Mutations are caused by environmental factors known as mutagens. Types of mutagens
include radiation, chemicals, and infectious agents.
Germline mutations occur in gametes. Somatic mutations occur in other body cells.
Chromosomal alterations are mutations that change chromosome structure. Point
mutations change a single nucleotide. Frameshift mutations are additions or deletions of
nucleotides that cause a shift in the reading frame.
Mutations are essential for evolution to occur because they increase genetic variation
and the potential for individuals to differ. The majority of mutations are neutral in their
effects on the organisms in which they occur. Beneficial mutations may become more
common through natural selection. Harmful mutations may cause genetic disorders or
cancer.
Lesson Review Questions
Recall
1.
2.
3.
4.
5.
Define mutation and mutagen.
List three examples of mutagens.
Identify three types of chromosomal alterations.
Distinguish among silent, missense, and nonsense point mutations.
What is a frameshift mutation? What causes this type of mutation?
Apply Concepts
6.
Assume that a point mutation changes the codon AUU to AUC. Why is this a neutral
mutation?
7.
Look at the mutation shown below. The base A was inserted following the start codon
AUG. Describe how this mutation affects the encoded amino acid sequence?
AUG-GUC-CCU-AAA → [mutation] → AUG-AGU-CCC-UAA-A
Think Critically
8.
Compare and contrast germline mutations and somatic mutations.
9.
Why are mutations essential for evolution to occur?
Points to Consider
Sometimes even drastic mutations do not affect the proteins produced by a particular type of
cell. The reason? The genes affected by the mutations are not normally used to make proteins
in that type of cell. In all cells, some genes are turned off, while other genes are turned on.
•
•
How do cells control which genes are turned on and used to make proteins?
Can you think of a mechanism that might prevent transcription of a gene?
18
Multimedia Links
For a video about mutagens, go the link below.
http://www.youtube.com/watch?v=0wrNxCGKCws&feature=related
Go this link for a video about chromosomal alterations:
http://www.youtube.com/watch?v=OrXRSqa_3lU&feature=related
You can watch an animation about nonsense mutations at this link:
http://www.biostudio.com/d_%20Nonsense%20Suppression%20I%20Nonsense%20Mut
ation.htm
You can watch a video about cystic fibrosis and other genetic disorders at this link:
http://www.youtube.com/watch?v=8s4he3wLgkM&feature=PlayList&p=397710758E9B
CB24&playnext_from=PL&playnext=1&index=17
You can learn more about hereditary cancer by watching the video at the link below.
http://www.youtube.com/watch?v=LWk5FplsKwM
19