A mutation is a sudden change in chromosomes or the genes leasing to formation of new species or characteristics in an organism. Or a mutation is a sudden or spontaneous change in the structure or amount of an organisms genetic material which alters the way it controls development.
Types of gene mutations
(i) Gene mutations
These are sudden or spontaneous changes which occur in the square of base in a gene or DNA strand.
Likely changes in genes during mutations.
- Substitution. One or more nucleotides is replaced with another with a different organic base. This brought about sickle cell anaemia. Mutations which occur by change of only one nucleotide are called point mutations.
- Insertion. One or more nucleotides are added onto the original sequence of bases in a gene.
- Deletion. One or more nucleotides are lost from a gene.
- Inversion. Sequence of bases in a gene or DNA strand gets reversed i.e. a strand breaks into points and relates through on angle of 1800.
- Duplication. One or more bases are repeated in the original sequence of bases in a gene.
These mutations usually occur when the DNA is replicating or can be induced by mutagens.
(ii) Chromosome mutation
A sudden change in either chromosome structure or in number. Most chromosome mutations result from intertwining of chromosomes during meiosis during which they break and region after exchanging parts or even losing others. They are of 2 types.
Those due to structural change in chromosomes.
Those due to change in chromosome number
- Changes in the structure of the chromosomes.
A section of the chromosome breaks up from the middle and rotates through 1800 and rejoins on the same chromosome. The sequence of genes in this section is therefore reversed.
A section of one chromosome break up and is exchanged with another on a different chromosome to which it is not homologous.
A section of the chromosomes replicates. The sequence of genes in this section is therefore repeated.
A section of the chromosome is dropped. All the genes in that section are lost. It results in lethal characters.
- Mutations due to changes in chromosome number
An error during cell division where there is loss or gain of a whole chromosome. The failure of one homologous pair of chromosomes to separate during anaphase 1 of meiosis give rise to some gametes which lack an entire chromosome and some which have two copies of the same chromosome.
Zygotes containing an extra chromosome may develop while those lacking are chromosome may fail to develop. Individuals who develop possess some abnormalities as shown below.
(a) Down’s syndrome (Mongelism)
Caused by the presence of an extra chromosome in which one of the gametes contains an extra chromosome 21.
Mental retardation, stunted physical growth, reduced resistance to disease, cogenital heart abnormalities. Slit eye appearance, short thick necks.
Caused by an additional x charomosome (genetype xxy)
A sterile male with abnormal breast development, little facial hair, small tests body proportions generally female.
(c) Turner’s syndrome
Caused by deleted x chromosome. (genotype XO).
Female with poorly developed sexual characteristics, small in stature.
This is a condition where a cell has three or more times the haploid chromosome number. These arise when:-
- Diploid gametes fuse with haploid ones to produce triploids.
- It during mitosis, chromosomes replicate and fail to divide.
- Induced by chemical substances like colchine which inhibit spindle formation and the separation of chromosome during anaphase.
- Diploid gemmates self fertile to produce tetraploids.
Types of polyploidy
A change in chromosome number by addition or subtraction of whole chromosomes to the diploid set. It occurs when individual chromosomes rather than whole sets fail to separate during anaphase 1 i.e. from non disjunctions.
Types of aneuploidy
A change in the whole sets of chromosomes such that chromosome number of the nucleus, cell or organism is an exact multiple of the hapleid number so that all chromosomes are present in the diploid triploid.
Types of euploidy
(i) Monopoidy (n)
This is normal in some insects e.g. honey bees where males arise from unfertilized eggs and female fertilized eggs. Most monoploids tend not to survive unlexs they can reproduce vegetatively.
(ii) Diploidy (2n)
This is normal condition in most organisms.
(iii) Triploidy (3n)
Triploids tend to be interfile due to the fact that homologous chromosomes pairing is impossible and reproduce vegetatively e.g. bananas, apples etc
(iv) Tetraplosily (4n)
Tetraploids can reproduce amongst themselves because pairing of homologous chromosomes can occur.
Euploids can be referred to as autopolyploid if the genomes are alike having arisen from the same individual and are a result of somatic doubling of the chromosome number or allopolyploids if gametes are from different individuals.
Causes of mutations
- Increase in temperature
- High energy radiations e.g. j and b particles, x-rays, u.v light.
- Chemicals like calcimine, mustered gas, phenols, nicotine.
- Viruses i.e. they attach themselves to the genetic material of the cell thus altering its biochemical machinery.
Characteristics of mutations
- They arise spontaneously and affect specific genes and chromosomes.
- They are indirect by the environment.
- They are rare but persistent and can occur in any cell both somatic and gametic at any time.
- Most confer disadvantages to the organisms that inherit them hence useful mutations are rare.
- Can be reversed though the reversal is rare and so they are more less permanent changes.
- Don’t occur slowly/gradual but in a single abrupt step.
- Occur at random.
Role of mutations in evolutions
- Leads to the formation of variance which provides a basis for natural selection to occur thus leading to evolution of new species.
- They are random occurrences and are more likely to be harmful thus preventing the growth of organisms.
- May result in the development of beneficial characteristics e.g. resistance of malaria by carriers of sickle cell anaomia.
- They cause change in the organism such that it has a better chance to survive and reproduce.
- Occurrence of natural selection
- Can be lethal leading to extinction of some species.