Mutations

A mutation is

a change in the structure of a gene, or
a change in the structure of a chromosome, or
a change in the number of chromosomes

Mutations

are rare events
occur at random
are usually harmful for cells and are the major cause of cancer
are an important source of variation, especially when they occur in in the production of gametes as the offspring may carry or display the mutation
may give the affected organism an advantage

The rate of mutation can be artificially increased by chemicals or irradiation- these are called mutagenic agents e.g UV light, mustard gas and nuclear radiation

Gene Mutations
These are alterations of a DNA base type or sequence of bases of a gene

As a result the sequence of amino acids in a protein is altered. The altered protein is unable to carry out it's normal function.

There are 4 types of gene mutation

(a) SUBTITUTION
one base pair is replaced by another
AGA GTC TAC becomes AGT GTC TAC

(b) INVERSION

two or more bases exchange position

AGA GTC TAC becomes AAG GTC TAC

In both these mutations only one triplet codon is affected therefore only one amino acid in the protein is altered, the resulting protein is slightly changed and the protein function is slightly altered.

Sickle Cell Anaemia

A substitution in a gene which codes for an amino acid which produces haemoglobin S
Haemoglobin S is a very inefficient carrier of oxygen- sufferers have a severe shortage of oxygen
red blood cells tend to collapse into a sickle (c) shape
Sickle cells stick together and cause blockages and organ damage

(c) DELETION
one or more bases lost
AGA GTC TAC becomes AGG TCT AC

(d) INSERTION

one or more bases added

AGA GTC TAC becomes ATG AGT CTA C

In both these mutations there is a frame shift in the triplet code and many triplet codes are affected therefore many amino acids are altered. The resulting protein is changed and the protein is non-functional.

Cystic Fibrosis

a deletion in a gene which codes for a membrane protein
epithelial cells produce thick mucus secretions
this causes blockage of airways and glands

Deletion and insertion cause severe effects as the gene code suffers major change.

These mutations are lethal as sufferers have reduced life expectancy.

Chromosome Mutations

There are 2 types of chromosome mutation:

Changes in structure of one chromosome

(a) INVERSION

a section breaks free, rotates and rejoins

ABCDEFGH becomes ABCFEDGH

(b) DUPLICATION

a segment from the homologous partner becomes inserted and a set of genes is repeated

ABCDEFGH becomes ABCDEFDEFGH

Inversion and Duplication lead to minor changes in the sequence of genes. They are usually harmful but may produce beneficial

(c) DELETION

a section breaks free and is lost

ABCDEFGH becomes ABCGH

(d) TRANSLOCATION

a section breaks free from one chromosome and becomes attached to another non-homologous chromosome

CHROMOSOME 1 ABCDEFGH

CHROMOSOME 2- PQRSTUV

becomes FGHPQRSTUV

Deletion and Translocation lead to major changes in the number of genes. The gametes are either non-viable or the offspring severely affected.

Changes in numbers of chromosomes

This is caused by spindle failure at mitosis or meiosis. Chromosomes fail to separate and this is called non-disjunction. This may happen to only one pair of chromosomes or to a whole set.

Down's Syndrome

caused by non-disjunction of chromosome pair 21
individuals are mentally disabled and have reduced life expectancy
the karyotype shows 47 chromosomes
mostly due to higher number of mutations in older egg mother cells
its frequency increases with the age of the mother

egg mother cell --> non-disjunction at meiosis --> 2 ovums- one 22 pairs of chromosomes, one with 24 (containing extra chromosome 21)

24 pair chromosome egg fuses with 23 chromosome pair sperm= abnormal zygote

Polyploid

increase in the number of sets of chromosomes by 3 times or more
common in plants but very rare in animals
occurs by complete non-disjunction at meiosis or mitosis
allows sterile hybrids to be converted into fertile polyploids

Polyploidy plants

Important in crop production
are larger with increased size of seeds and fruit
show increased vigour and disease resistance e.g. tomato, coffee, apples
can be seedless e.g. banana

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A summary of mutations