Grade 11F Science Related Reading/Biology
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Biological Basis of
Inheritance
Biology Gr11F
Read the passage below. Then answer the questions that follow.
Mutation
Mutation is defined as an alteration in the nucleotide sequence of DNA
(deoxyribonucleic acid) of an organism. The structure of DNA consists of
nitrogenous bases (purines and pyrimidines), glucose and phosphate. Purine
consists of adenine (A) and guanine (G) bases, whereas pyrimidine bases are
cytosine(C) and thymine (T). The bases are arranged as codons (three bases)
that describe the coding of proteins for expression of cells. Mutation occurs
when the sequence of nitrogenous base is changed, either due to copying
errors during the replication or an exposure to mutagens such as chemicals,
pollution, cigarette smoke, viruses, radiation and sunlight. The outcome of
mutation is a change in the genetic message coded by the gene.
In general, mutation is classified into two types based on the cells that are
getting affected, namely, germ line and somatic. In the former case,
mutation occurs in the reproductive cells (egg and sperm), which is then
inherited from the parents to the offspring. This germ line mutation is
responsible for causing hereditary diseases and/or genetic disorders. Somatic
mutation, on the other hand, takes place in body cells like skin. It is not
inherited to the descendants.
Vocabulary
Development
Copy the key vocabulary
written in bold in the
space provided below.
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Reading Check
What is a mutagen?
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Table shows some examples of human diseases that are caused by inherited mutations .
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Grade 11F Science Related Reading/Biology
Depending upon the effect of mutation on the structure of DNA, mutation
can be further divided into several types namely substitution (exchanging
base between two DNA strands), insertion (extra base is inserted in a DNA
strand), deletion (loss of base pairs), inversion (reverse of base sequence)
and frame-shift (insertion and deletion of one or more base pairs). Let's
discuss in brief about single-base substitution or point mutation.
When a single base in the nucleotide sequence is replaced by another, then it
is known as point mutation. Point mutations also include insertion and/or
deletion of a single base in the DNA strand. Usually, they are caused due to
error in DNA replication. At times, it occurs after exposure to mutagens like
heat and radiation.
Looking closer
A gene has a nonsense
mutation. Will the
protein it produces be
longer, shorter, or the
same length as the
protein produced by the
normal gene?
(hint: what does a stop
codon do?)
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The effects of point mutation can vary depending upon the site of mutation
on the gene. If point mutation occurs in the coding sequence of DNA, then
the protein coded by the altered gene is changed.
Some other possible outcomes of point mutations include a failure of DNA
transcription (conversion of messenger RNA to proteins), alteration of
regulatory responses and other genetic changes. Hence, based on the
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Grade 11F Science Related Reading/Biology
functional changes of the gene, point mutations are divided into three types,
namely, nonsense (failure of expression), missense (base code for a different
protein) and silent (no significant change). The diseases like sickle cell
anemia and thalassaemia are caused due to missense mutation in the coding
sequence of a gene. With a missense mutation, the new nucleotide alters the
codon so as to produce an altered amino acid in the protein product.
In sickle-cell anemia, the replacement of A by T at the 17th nucleotide of
the gene for the beta chain of hemoglobin changes the codon GAG (for
glutamic acid) to GTG (which encodes valine). Thus the 6th amino acid in
the chain becomes valine instead of glutamic acid.
Although mutations that change in
protein sequences can be harmful
to an organism; on occasions, the
effect may be positive in a given
environment. In this case, the
mutation may enable the mutant
organism to withstand particular
environmental stresses better than
wild-type organisms, or reproduce
more quickly. In these cases a
mutation will tend to become more
common in a population through
natural selection. Nylonase is an
example of beneficial mutation in
bacteria. The nylonase bacteria can
eat short molecules of nylon
(nylon-6). The mutation in these
bacteria involves insertion of a
single nucleotide in the genetic
material. It is estimated that this
frameshift mutation might have
occurred in the 1940s when nylon
was invented. Nylonase can be
used in wastewater treatment
plants.
Critical Thinking
How can a positive
mutation lead to
evolution of a certain
population?
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SKILL: READING EFFECTIVELY
In the space provided, explain how the terms in each pair differ in
meaning.
1. Mutation, mutagen
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Grade 11F Science Related Reading/Biology
2. Point mutation, silent mutation
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Complete each statement by underlining the correct term in the
brackets.
1. If a mutation causes a sequence of nucleotides to change from
ACGAGA to ACGAGGA, the mutation is called a(n) [insertion /
deletion] mutation.
2. Mutations that change one or just a few nucleotides in a gene on a
chromosome are called [random / point] mutations.
3. If a point mutation is such that it causes a codon to specify a different
amino acid, the mutation is called a [missense / silent] mutation.
4.If a mutation causes a sequence of nucleotides to change from ACGAGA
to ACGGA, the mutation is called a(n) [insertion / deletion] mutation.
In the space provided, write the letter of the term or phrase that best
completes each statement.
_____ 1. Point mutations occur when
a. one nucleotide is replaced with a different nucleotide.
b. a gene’s location changes.
c. long segments of a gene are lost.
d. gametes are forming during meiosis.
_____ 2. Somatic mutations are
a. heritable.
b. not heritable.
c. in the germ cells.
d. Both (a) and (c)
_____ 3. Germ cell mutations are
a. not heritable.
b. heritable.
c. in the body cells.
d. Both (a) and (c)
_____ 4. If a deletion mutation eliminated all of the guanine bases from
the codon sequence GAT-CGC-CAA-TAG, the altered sequence
would read
a. ATC-TCA-ATA.
c. ACC-CAA-ATA.
b. ATC-CCA-ATA.
d. AAT-CCA-TAC.
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Grade 11F Science Related Reading/Biology
In the space provided, write T if the statement is true or F if it is false.
_____ 1. An insertion is the change of one nucleotide from one kind of
base to another.
_____ 2. A missense mutation has no effect on the resulting amino acid
sequence.
_____ 3. A frameshift mutation causes a change in the reading frame of a
DNA sequence.
_____ 4. A deletion can occur as a small change in the DNA or a larger
change in the chromosome.
SKILL: READING EFFECTIVELY
Read each question, and write your answer in the space provided.
1.How is a missense mutation different from a nonsense mutation? How
are they similar?
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2. Explain the difference between insertion and deletion mutations.
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3. List the possible causes of mutation.
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Grade 11F Science Related Reading/Biology
4. What is the difference between the gene coding for normal
hemoglobin and that coding for sickle-cell hemoglobin?
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5. Skin cancer can occur if the DNA in skin cells is mutated by ultraviolet radiation in sunlight. Can the mutation that causes skin cancer
be passed on to offspring? Explain your answer.
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6.Give two examples of recessive genetic disorders and one example of a
dominant genetic disorder.
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7. You read in a book that all mutations are bad. Do you agree? Explain.
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8. What are the different types of point mutation?
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9. Give two examples of missense point mutation
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