Mutations Worksheet
Mutation
The human genome is made of 3 billion base pairs broken up into 23 segments called chromosomes. In each of your somatic cells
(except gametes) you have two sets of each chromosome for a total of 46! This is called diploid since there are two sets. In this case,
each chromosome is in a homologous pair, as each chromosome has the exact same genes. As you are only human, mistakes can be
made in your DNA. These mistakes are called mutations. A mutation is any change in the original DNA structure, whether it is on the
nucleotide level (base changes), the gene level (entire genes are erroneous), or the chromosome level (duplication or deletion of entire
chromosomes). Depending on the location and the magnitude, a mutation may lead to no change (neutral) or catastrophic changes
(death).
1. How is your DNA organized?
2. How many chromosomes do humans have in their somatic cells?
3. What are homologous chromosomes?
4. How many sets of chromosomes are in somatic cells? What is this called?
5. What is a mutation?
Nucleotide Mutations
There are two types of mutations concerning the nitrogenous bases in the DNA. If the mutation occurs in the intron (the part of DNA
that doesn’t code for a protein, nothing happens. If the mutation occurs in the exon, the results usually change the protein coded for.
The first type of nucleotide mutation is called a point mutation and occurs when one base (A, C, T, G) is accidentally changed into a
different base. Depending upon on the codon, it may or may not change the amino acid. If the amino acid does change, it can change
how the protein is folded, essentially changing the shape. If the shape changes, it no longer works in the way nature intended. Two
examples of this type of change leading to a disease are sickle cell anemia and cystic fibrosis. In sickle cell anemia, the protein
hemoglobin is changed from a circular shape to a sickle or half moon shape. This leads to the smallest blood vessels called capillaries
getting clogged, resulting in decreased oxygen to tissues. Cystic fibrosis changes the protein in mucus-producing cells causing more
mucus to be produced and changing the mucus from a watery consistency to a super-thick sticky goo. This type of mucus will
eventually kill the patient usually before the age of 30. The second type of nucleotide mutations is called a frameshift mutation and this
occurs when one or more bases are inserted or deleted. This shifts the entire reading of the codons after the mutation and alters the
protein greatly. Many diseases result from this including
Huntington Disease (HD), Tay-Sachs, Fragile X Syndrome, and
Muscular Dystrophy (MD). In Huntington Disease, an insertion
of CAG in the exon causes the brain cells to start dying rapidly,
eventually killing the victim. Unfortunately, the person may have
already had children, unknowingly passing the killer trait down.
Tay-Sachs results from an insertion of four bases that render a
lipid-eating enzyme defective. As a result, fat accumulates in
the brain cells and kills brain cells. Children born with this disease never live past age 5. Many extra GGG codons inserted on the X
chromosome results in Fragile X Syndrome, affecting mostly boys. Boys with this disease have mental retardation. Muscular Dystrophy
is another disease as a result of an insertion, this time of the bases CTG. The protein altered here is dystrophin and it leads to
weakening muscles. Although there are many types of MD, the most common one occurs on the X chromosome, so mostly boys are
affected. The lack of muscle results in early death.
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What are introns and exons?
What are the two types of nucleotide mutations? How are they caused?
Which of these is more devastating and why?
What diseases result from point mutations?
What diseases result from frameshift mutations? Which ones are a result of insertions and which are a result of deletions?
Which diseases primarily affect boys? Why?
Chromosome Numbers Mutations
When gametes (sex cells such as sperm or eggs) are being formed, a process called meiosis splits the chromosome number into half,
making it a haploid cell with only one set. This cell division can result in mutations of whole chromosome numbers. If a mutated sperm
or egg becomes fertilized, the organism may have chromosomal numbers mutations discovered by mapping them with a karyotype.
Human karyotypes show 22 homologous pairs of non-sex chromosomes numbered 1-22. They are arranged in length and shape. The
last set of chromosomes, number 23, is the sex chromosomes. Females have two X (XX) and they are homologous. Males have one X
and one Y which are not homologous. Most mistakes in chromosomal numbers will result in miscarriage where the embryo
spontaneously aborts itself before 20 weeks gestation. Only a few mutations of chromosome numbers will allow the baby to be born,
albeit most will have physical and/or mental impairments that greatly diminish their life span. One such chromosomal number
abnormality is trisomy, where the individual has three of the same chromosome instead of the usual two. Down’s syndrome is a trisomy
of chromosome 21. Children with trisomy 21 have mental retardation that limits their
intellect and shortens their life span to about 50. Trisomy 13 is called Patau Syndrome
and children born with this have many physical defects and severe mental retardation.
Their average life span is 10 years. The last of the trisomies that affect the non-sex
chromosomes is trisomy 18 or Edwards Syndrome. The physical defects of these
individuals is so severe that few babies born with this life past age 1. Chromosome
mutations can occur on sex cells, too. Girls born with only one X have a disorder called
Turner’s syndrome. This type of monosomy affects the girls’ physical development. The lack of an X causes impairments in physical
features as well as sex organs. Most Turner Syndrome girls live a full life. A second sex chromosome-related disorder is called Triple X
Syndrome where girls are born with trisomy X. These individuals are some learning disabilities, and a few have physical impairments of
muscle tone. Two sex-chromosome number mutations affect males, as well. The first one is called Klinefelter’s Syndrome in which
males have an extra X giving them XXY. This trisomy decreases the amount of testosterone a boy has, which results in female-like
structures and sterility. Boys with an extra Y have a disorder called Jacob’s syndrome which makes them mostly average. They are
generally taller, have excess acne, and are slightly more aggressive.
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When do mutations in chromosome numbers take place?
How many chromosomes do humans have in their gametes?
How many sets of chromosomes are in gametes? What is this called?
What are the two types of mutations that can result from incorrect meiosis?
How are chromosome abnormalities discovered?
How are chromosomes numbered?
What chromosomes determine males versus females?
What is a trisomy?
What is a monosomy?
What conditions result from trisomy? Make sure to discuss the specific trisomy.
What conditions result from monosomy? Make sure to discuss the specific trisomy.
Gene Mutations
During meiosis, chromosomes often come in contact with each other and can break apart. For the most part, the body corrects these
breakages and puts them back together. Sometimes, mutations occur. Since each chromosome contains many genes, if these genes
are missing or on incorrect chromosomes, diseases occur. Four specific types of mutations occur with chromosome breakage. The first
type is a deletion in which a gene or genes are deleted and lost. A deletion on chromosome 4 results in Wolf-Hirschhorn syndrome,
causing severe physical defects and mental retardation. If this occurs on chromosome 5, a syndrome called Cri-du-chat or cat’s cry
syndrome occurs. Children with this syndrome cry like a kitten at birth (hence the name) and suffer from physical and mental
disabilities. If a deletion occurs on chromosome 15, mental retardation and possible extreme obesity occurs. This is called Prader-Willi
syndrome. Finally, if deletion occurs on chromosome 22, 22q11 occurs and a variety of physical and mental disabilities result. The
second type of chromosome mutation is duplication also known
as
insertion. In this case extra genes are placed on the
chromosome and genes are duplicated. A disease of
duplication is Charcot-Marie-Tooth disease in which tendons,
ligaments, and muscles of the hand and foot are destroyed. A
third type of mutation that is a result of chromosome breakage
is
inversion where genes are reattached to the correct
chromosome but upside-down. Norrie disease is an inversion of
genes on the X chromosome and causes blindness, oftentimes deafness, and sometimes mental disabilities. Since the mutation occurs
on the X chromosome, males are more likely to suffer from the disease than females. The last type of gene mutations on chromosomes
is called a translocation, in which a broken chromosome gets attached to a completely different chromosome. This type of mutation
seems to occur more than the others and can cause many common diseases including infertility, certain cancers, and schizophrenia.
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When do chromosomes break apart?
What are the four types of gene mutations?
Which diseases are results of deletion?
Which disease is a result of duplication?
Which disease is a result of inversion?
Which diseases are results of translocation?
Which mutation is the most common?
Why is Norrie disease more common in males than females?
ANSWERS
1. DNA is organized into 23 segments called chromosomes
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46 chromosomes in somatic cells
Homologous chromosomes are chromosomes that have the same genes. In addition, they look exactly alike.
There are 2 sets. This is called diploid.
A mutation is any change or alteration in the original DNA structure.
6. Introns are segments of DNA that don’t code for anything while exons are segments of DNA that do code for
proteins so they are also considered genes.
7. Two types of nucleotide mutations are point mutations and frameshift mutations. Point mutations are caused when
one base gets changed or mutated into a different base. A frameshift mutation is when a base is deleted or added,
shifting the codon sequence after the mutation.
8. Frameshift mutations are more devastating because it causes more amino acid changes and can even change one
into a stop codon thereby shortening the protein significantly.
9. Point mutations cause sickle cell anemia and cystic fibrosis.
10. Frameshift mutations cause Huntington’s disease, Tay-Sachs disease, Fragile X syndrome, and Muscular
dystrophy. All four are a result of insertions.
11. Fragile X syndrome and Muscular Dystrophy primarily affect boys since both mutations occur on the X
chromosome. Boys only have one X chromosome, so a defect on that becomes a disease. Girls have two X
chromosomes, so a defect on one X gets overridden by the “good” X.
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Mutations in chromosome numbers take place during meiosis.
Gametes have 23 chromosomes.
Gametes have 1 set of chromosomes – called haploid
Monosomy and trisomy are two types of mutations that result from incorrect meiosis.
Doing a karyotype of an individual’s chromosomes uncovers abnormalities.
Chromosomes are numbered according to length and shape. The non-sex chromosomes are numbers 1-22 while
the two sex chromosomes (X or Y) are number 23.
Males have XY chromosomes while females have XX chromosomes. The Y determines maleness.
Trisomy is having three of one type of chromosome.
Monosomy is having only one of a particular chromosome.
Down’s syndrome is trisomy 21. Individuals with this syndrome suffer from mental retardation and a shortened life
span. Patau syndrome is trisomy 13. Individuals with this suffer from physical and mental defects and live to about
10. Trisomy 18 is Edward syndrome and individuals with this have severe physical and mental defects and rarely
live past age 1. Triple X syndrome is a trisomy X and females born with this may have learning disabilities and
physical impairments. Kleinfelter’s syndrome is when boys have XXY resulting in female-like features and sterility.
Jacob’s syndrome is when boys have XYY resulting in extra testosterone, increased height, excess acne, and more
aggression.
Turner syndrome is the only monosomy disorder in which girls are missing an X chromosome. Girls with Turner
syndrome have physical impairments and are sterile.
23. Chromosomes break apart during meiosis.
24. Four types of gene mutations are deletion, inversion, duplication, and translocation.
25. Deletions of gene cause Wolf-Hirschhorn syndrome, Cri-du-chat or cat’s cry syndrome, Prader-Willi syndrome, and
22q11 syndrome.
26. Only Charcot-Marie-Tooth is a disease of duplication.
27. Norrie disease is from inversion of genes.
28. Many diseases including schizophrenia, sterility, and some cancers.
29. Translocation is the most common gene mutation.
30. Norrie disease is more common in males since it occurs on the X chromosome and males only have one making
them more susceptible since they don’t have a “good” X to override any problems.