Performance Benchmark L.12.A.4
Students know several causes and effects of somatic versus sex cell mutations. E/S
Mutations are simply changes in the DNA code. The first thing that a student should understand
when studying mutations is the difference between a somatic cell and a sex cell. A somatic cell
is a body cell that has a full complement of chromosomes while a sex cell (or germline cell) is a
cell involved in sexual reproduction with half the number of chromosomes. Therefore,
mutations can be separated into germline mutations, which can be passed on to offspring, or
somatic mutations, which only affect the individual.
Gene Level Mutations
1. Point Mutation (or substitution)
This type of mutation is where only a single nucleotide in a gene has been changed. The effect
can be quite dramatic. The result can be a nonsense triplet which will stop the protein’s
construction. This will result in truncated proteins. A missense triplet may result coding for a
different amino acid resulting in an incorrect protein. Lastly, silent mutations may occur coding
for the same amino acid which results in no change to the protein.
Figure 1. An example of a point mutation.
The DNA code is simply changed when an
adenine is replaced with a cytosine.
http://sps.k12.ar.us/massengale/chromosomes
%20&%20human%20genetics.htm
Point mutations hay happen spontaneously when mistakes are made during DNA replication.
The incidence of mutations can be increased by mutagens. A mutagen is a physical or chemical
agent that can be harmful to DNA. Examples of mutagens are radiation (UV rays) or chemical
(carcinogens).
Sickle Cell Anemia is an example of a point mutation. It is the most common inherited blood
disorder affecting 72,000 Americans (www.ncbi.nlm.nih.gov). It is caused by the point mutation
of a single amino acid, resulting in a valine being substituted for glutamine.
Figure 2. Normal Red
Blood Cells
http://publications.nigms.ni
h.gov/moleculestomeds/ph
armacology.html
Normal Hemoglobin
Val
His
Leu
Thr
Pro
Glu
Glu
Lys
Sickle Cell Anemia Hemoglobin
Val
His
Leu
Thr
Pro
Val
Glu
Lys
Figure 3. Sickle Cell Red
Blood Cells
http://publications.nigms.ni
h.gov/moleculestomeds/ph
armacology.html
2. Frameshift Mutations
A frameshift mutation results in the addition or deletion of nucleotides in the DNA sequence.
Three nucleotides (a triplet) are translated into an amino acid through the process of translation.
The addition or subtraction of nucleotides will shift the entire sequence and result in an entirely
different translation (protein).
Figure 4. This frameshift mutation
shows that if the sequencing is shifted
over only one nucleotide a completely
different amino acid sequence results.
http://ghr.nlm.nih.gov/handbook/illust
rations/frameshift
Chromosomal Level Mutations
Chromosomal mutations take place during cell division, either mitosis or meiosis. During cell
division, the chromatin material shortens and thickens into chromosomes. A chromosome
consists of two duplicate sister chromatids connected by a centromere. Each chromatid should
separate and travel to opposite ends of the cell to create two identical daughter cells. Sometimes,
mistakes happen in the separation of sister chromatids.
1. Reciprocal Translocation
Translocation is the transfer of a part of a chromosome with another nonhomologous
chromosome during cell division. Chromosomes are actually fairly fragile and some break
during cell division and the broken bits rejoin with neighboring chromosomes. If there is no loss
of genetic information, the translocation is usually harmless. However, it can have reproductive
results. A person who is a “carrier” of a translocation may have a higher incidence of
miscarriage or have a higher probability of having a child with birth defects.
Figure 5. The translocation of a piece of chromosome 20 and chromosome 4.
http://en.wikipedia.org/wiki/Image:Translocation-4-20.png
2. Chromosomal Deletion/Duplication
A chromosomal deletion is just what it sounds like. A segment of DNA is lost in the cell during
cell division. The lost piece broke off a chromosome and attached to the homologous
chromosome. The result in cell division is one daughter cell has duplicate DNA and the other
daughter cell is missing the same DNA sequence. The result can be detrimental. It can result in
an unbalanced number of chromosomes. The resulting karyotype may be 45 or 47 (instead of
46) resulting a variety of birth defects. In somatic cells, an addition/deletion may also result in
various types of cancers, especially leukemia.
Figure 6. Four types of genetic
rearrangements.
http://health.enotes.com/images/c
ancer/gec_01_img0056.jpg
3. Inversion mutation
In inversion mutation is where a piece of a chromosome breaks off and is reattached in reverse
order. Inversions don’t appear to be harmful in the individual as long as there is no loss or gain
of DNA. However, they are now a carrier and a resulting offspring has a small chance of
inheriting an unbalanced chromosome arrangement with missing or extra components. (See
Figure 6)
4. Nondisjunction/Polyploidy
The addition or loss of an entire chromosome is called nondisjunction. It results when two
chromosomes remain connected instead of separating during meiosis. The result is extra or
missing chromosomes. For example, an individual that inherits three copies of chromosome 21
(47 chromosomes) has a condition called Down’s syndrome. Most cases of nondisjunction
results in nonviability of an offspring. Nondisjunction can also result in the development of
cancers.
Figure 7. Nondisjunction during meiosis that
results in two gametes missing a chromosome
and two gametes with extra chromosomes.
http://wappingersschools.org/RCK/staff/teacherh
p/johnson/visualvocab/nondisjunction.gif
If an entire set of chromosomes fail to separate, a condition of polyploidy results. This is
commonly found in plant cells.
Figure 8. Polyploidy that results in a viable offsprin
with twice the number of chromosomes as the paren
http://www.bio.miami.edu/dana/104/104F02_15.htm
Somatic mutation vs. Gamete mutation
A somatic mutation is a mutation that takes place in any single cell of an organism except
gametes and is not inheritable. Some current studies have suggested that a lifetime of
accumulated mutations is why we age. Another result of harmful somatic mutations is cancer.
The mutation starts in a single cell and as this cell divides uncontrollably, a tumor develops.
A mutation in sex cells results in an inheritable mutation. This mutation starts in a gamete (ie.
egg, sperm, pollen, or ovule) and is passed into the resulting offspring. This genetic change, is
therefore, present in every cell of the organism. Some genetic defects cannot result in a viable
offspring. But if it does, a variety of genetic disorders can be expressed.
Performance Benchmark L.12.A.4
Students know several causes and effects of somatic versus sex cell mutations. E/S
Common misconceptions associated with this benchmark:
1. Students often have difficulty conceptualizing gene expression (via protein synthesis)
and that changes in the DNA code can be reflected in changes in gene expression.
Students have trouble seeing the big picture and following the pathway of DNA relationship
through to gene expression. It is a very abstract complicated process that involves many steps
and it’s easy for students to get lost in the details, so they give up. It is important that teachers
break down each step and convey its relevance before moving onto the next step. Students need
to have a good foundation in transcription, translations, and replication before mutations are
introduced. Otherwise, the idea of mutations would only compound their confusion.
2. Students may understand how DNA is replicated, transcribed, and translated, but they
still may not understand how a gene controls a trait.
Again, students have trouble seeing how one process plays into another. They may be able to
draw the diagrams and flow charts but when asked to describe the purpose of each step students
are often stumped. “So you may need to explain "up" or "down" how the parts relate to the
whole -- up, how the item under discussion fits into something bigger, and down, how the item is
made of smaller things. For example, if you are discussing genes, you should be prepared to go
"up" to chromosomes, genomes, traits, etc., and "down" to DNA, codons, nucleotides, and
bases.”
For more information about this misconception please visit this site:
http://cirtl.wceruw.org/diversityresources/resources/resource-book/overcomingmisconceptions.htm
3. Students think that all mutations are inheritable and have trouble differentiating
between somatic and germline mutations.
Students can tell you that all life forms are made up of cells and can even describe mitosis, but
when pressed some still have misconceptions about how we start as a single cell. This single cell
divides into two, then four, then eight, etc. The resulting ball of cells all have the identical set of
instructions but begin to differentiate and take on specific roles. Students have a hard time
understanding that a skin cell and a liver cell have the same identical set of instructions but use
only portions of the code specific to the cell’s job. Students also misunderstand that cells
continue to divide and grow even as adults and that mutations can occur in an adult skin cell that
results in skin cancer. Students are also confused as to how a genetic mutation is passed on to
offspring (L12A5).
For more information concerning misconceptions about genetic mutations please visit
(http://cirtl.wceruw.org/DiversityResources/resources/resourcebook/overcomingmisconceptions.htm )
Performance Benchmark L.12.A.4
Students know several causes and effects of somatic versus sex cell mutations. E/S
Sample Test Question
The information below represents a change in a portion of the base sequence in a DNA molecule.
ATCGAT
X-ray
1. This change can best be interpreted as
a. a point mutation
c. a framshift mutation
AACGAT
b. a translocation
d. an inversion mutation
2. In humans, a change similar to the one shown above have been responsible for a disorder
known as
a. Down’s syndrome
b. Polyploidy
c. Phagocytosis
d. Sickle-Cell Anemia
3. A gene mutation results from a change in the
a. nucleotides in a anticodon
b. chromosome number in a gamete
c. sequence of the nucleotides in DNA
d. chromosome number in a somatic cell
4. Changes in the genetic material in sex cells are mutations that
a. may be transmitted to the next generation
b. are always eliminated during meiosis
c. are always sex-linked
d. cannot affect the organism or its offspring
5. A mutation can be passed on to offspring if the mutation takes place in
a. a liver cell
b. an egg cell
c. a uterus cell
d. a skin cell
Performance Benchmark L.12.A.4
Students know several causes and effects of somatic versus sex cell mutations. E/S
Answers to Sample Test Questions
1. (a)
2. (d)
3. (c)
4. (a)
5. (b)
Performance Benchmark L.12.A.4
Students know several causes and effects of somatic versus sex cell mutations. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Animated sequences on the cause of cancer
This website has great animations that show students the cause of cancer from a
chromosome level down to the nucleotide level. It also has information on different
types of mutations with colorful, simple diagrams. The website does go into a lot of
detail about cancers but the “Genetic Change” section is quite appropriate for this
benchmark and addresses some of misconceptions associated with this benchmark.
To access the information and animated sequences visit:
http://www.cancerquest.org/index.cfm?page=53
2. Webquest activity on the use of technology in diagnosing disease
This webquest illustrates the use of technology in diagnosing an unknown disease. This
activity is appropriate for higher level students. “It will introduce students to the
concepts of bioinformatics, genetic diseases, and potential careers in science fields. This
webquest activity can be used in many different ways in your classroom. It is
recommended that students be divided up into groups of four, so that each student can
assume the role of one of the medical investigation team members. This encourages
cooperative learning in the student groups and also gives students a chance to work as a
team, much the way a real-world investigative team might do. Alternatively, the
webquest can be completed by students individually, investigating all four roles by
themselves. Either way, emphasis should be made on having the students communicate
their findings by creating a presentation using multimedia whenever possible. This
webquest is designed to take approximately one week. It would be possible to do a
shortened webquest with only the bioinformatics section, which should take one class
period.”
To access the information visit:
http://www.koshland-science-museum.org/teachers/webquest.jsp
3. Extensive information about Genetics
“DNA from the Beginning is organized around key concepts. The science behind each
concept is explained by: animation, image gallery, video interviews, problem/self
quizzes, biographies, and links.” This site has a wide range of topics grouped in three
categories: Classical Genetics, Molecules of Genetics, and Genetic Organization and
Control. A sampling of topics is below.
-Sex cells have one set of chromosomes; body cells have two
- All cells arise from pre-existing cells.
- Chromosomes carry genes.
- Genes get shuffled when chromosomes exchange pieces.
- One gene makes one protein.
- A gene is made of DNA.
- Mutations are changes in genetic information.
- Some types of mutations are automatically repaired.
- DNA is packaged in a chromosome.
- Different genes are active in different kinds of cells.
To access the information visit:
http://www.dnaftb.org/dnaftb/