Gene Mutations Bio Factsheet

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B io Factsheet
September 2001 Number 94
Gene Mutations
By studying this Factsheet you should gain a knowledge of:
Remember : ­ DNA and RNA are polymers of nucleotides, designated by
· the nature and causes of gene mutation;
· the effects of gene mutations; the letters of their nitrogenous bases, A (adenine), T (thymine), U (uracil),
· gene mutation as a source of variation which may be of selective C (cytosine) and G (guanine). The bases are arranged into threes
value. making up the codons which are the units of the genetic code. Particular
codons cause the insertion of specific amino acids into the polypeptides
This Factsheet assumes that the student has a basic knowledge of the nature so that if the codons are altered, the wrong amino acid may be inserted.
of DNA, RNA, replication, the genetic code and protein synthesis. These Also remember that A is complementary to T in DNA and to U in RNA.
were covered in Factsheet 22, Protein synthesis I ­ nucleic acids and Factsheet G is complementary to C in both RNA and DNA. Complementary means
49, Protein synthesis II ­ mechanisms. that they will combine together by hydrogen bonding.
The nature of gene mutation In sickle cell anaemia the mutation affects the amino acid sequence of part
A mutation is a sudden inheritable change in the genetic material. Gene of the -globin protein chain of the haemoglobin (Fig 2). This results in the
mutation is sometimes called point mutation because the mutation may formation of abnormal haemoglobin-S which in conditions of low oxygen
only involve change of one nucleotide (base) in the codon sequence of the tension causes the red blood cells to collapse into sickle shapes.
gene. The replication of DNA and transcription of messenger RNA
(mRNA) is not 100% efficient with the result that occasional errors Fig 2. The mutation which results in Sickle Cell Anaemia
occur. These errors may cause mutations. The mutation may be due to
the insertion (addition) or deletion of a nucleotide, the substitution of one Amino acid sequence Normal
nucleotide for another or inversion of two nucleotides (Fig 1). of -globin chain
of haemoglobin: proline glutamic acid glutamic acid
Transcription Translation
Fig 1. Types of point mutation. code code code
for for for
Original DNA AGT CCC AAA CCA Codons on mRNA: CCA GAA GAA
Corresponding mRNA UCA GGG UUU GGU
Insertion of A as fourth nucleotide Bases on DNA: GGT CTT CTT
New DNA AGT ACC CAA ACC A
Altered mRNA codons UCA UGG GUU UGG U Amino acid sequence Sickle Cell
of -globin chain
Transcription Translation
Deletion of the third nucleotide, T of haemoglobin: proline valine glutamic acid
New DNA AGC CCA AAC CA codes codes codes
for for for
Altered mRNA codons UCG GGU UUG GU Codons on mRNA: CCA GUA GAA
Substitution of fourth nucleotide with T.
New DNA AGT TCC AAA CCA
Bases on DNA: GGT C AT CTT
Altered mRNA codons UCA AGG UUU GGU
Inversion of second and third nucleotides The substitution of A for T gives valine instead of glutamic acid
New DNA AGC TCC AAA CCA
In sickle cell anaemia, the base T marked (in the normal diagram) with an
Altered mRNA codons UCG AGG UUU GGU arrow is substituted with a base A. This means that the resultant codon on
the mRNA is GUA instead of GAA and so the amino acid valine is inserted
into the protein in place of glutamic acid. This results in the formation of
Altering the base sequences in the codons of the gene may alter the haemoglobin-S. This type of gene mutation is called a `missense' mutation
sequence in which amino acids are assembled into the polypeptide made and only changes one amino acid in the polypeptide chain.
by that gene. The altered polypeptide may then have an altered effect or
function which in turn may alter one or more characteristics of the Sometimes a base alteration in a codon does not alter the amino acid sequence
organism. The resulting mutation is harmful, e.g. sickle cell anaemia, of the polypeptide made by the gene. This is called a `samesense' mutation.
or it may confer evolutionary or survival benefit e.g. in industrial It occurs because the genetic code is degenerate. Degenerate means that
melanism which occurs in many butterflies and moths. If the mutated some amino acids can be coded for by several mRNA codons which vary in
allele is situated on a sex chromosome then the mutated condition will be their third base. For instance, serine is coded for by UCU, UCC, UCA and
sex-linked, as in haemophilia and red-green colour blindness. In these UCG. If the mutation changed the third base in UCA to make UCU, the
conditions the mutated allele is on the X-chromosome. new codon will still insert serine into the polypeptide.
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Gene Mutations Bio Factsheet
A `nonsense' mutation changes an amino-acid-specifying codon into a Natural selection plays a part when heterozygous sickle cell trait individuals
chain-terminating codon. For instance, if UCA, which codes for serine, is live in areas where malaria is prevalent. The malarial parasite lives in normal
substituted, making UGA, then this is a codon which halts synthesis of the red cells and feeds on haemoglobin A. It cannot feed successfully on
polypeptide. haemoglobin-S and finds difficulty in entering sickled red cells.…read more

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Gene Mutations Bio Factsheet
Other examples in nature of gene mutations which have produced beneficial 2. The drawings below show the appearance of normal human red cells
variations to the mutated organism are: and red cells from an individual suffering from sickle cell anaemia. The
· the development of antibiotic resistance in certain strains of bacteria; condition is caused by a mutant co-dominant allele.…read more

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Gene Mutations Bio Factsheet
Answers 3. (a) (i) recessive;
1.…read more

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