CHANGE IN CHROMOSOMAL STRUCTURE AND NUMB

Physical and chemical disturbance and errors occur during meiosis. They damage chromosome or alter their number in a cell. There can be folloyn chromosomal alterations:

Point mutation

The change of sequence of nucleotides is called point mutation. It is cau. by replacement, addition or deletion of the nucleotides. Mutations are always, random. There are following reasons of point mutation:

  1. Sometimes, spontaneous base pairing error occurs during replication substitutes one base pair for another.
  2. Certain environmental factors like electromagnetic radiations and rn, chemical mutagens change mutation rates.

Effects of mutations

Some mutations are unnoticed. Some are beneficial. But mutations disrupt structure of proteins. The evolutions of these proteins occur as result of evolution: of million of years. Therefore, the results of mutations are always negative.

Change in Chromosome Number

In this case, number of chromosome in an affected person can be increase decreased. Following types of chromosomal abnormalities are produced du( change in chromosomal number.

(a) Aneuploidy

The random abnormal number of chromosomes in the animals is called

aneuploidy. Aneuploidy is mostly caused by nondisjunction. The members

pair of homologous chromosomes do not move apart properly dui

nondisjunction. Therefore, the sister chromatids fail to separate at meiosis II. In these cases, one gamete receives the same type of chromosome and another receives no copy. The other chromosomes are usually distributed normally. If these abnormal gametes unite with a normal one, the offspring will have an Abnormal chromosome number It causes aneuploidy.

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pes of aneuploidy

ere are following types of aneuploidy:

1 Monosomic: (2n 1): If a chromosome is missing the aneuploidy is monosomic. Therefore, the cell has 2n — 1 chromosomes.

2 Trisomic (2n + 1): If the chromosome. is present in triplicate in the fertilized egg the aneuploid cell is called trisomic. The cell has a total of 2n + 1 chromosome.

  1. Tertrasomics: If the chromosome is present in tetrad form in the fertilized egg the aneuploid cell is called tertrasomics for that chromosome. The cell has a total of 2n + 2 chromosomes.
  2. Nullisomics: If a set of chromosome is missing the aneuploidy is nullisomics. Therefore, the cell has 2n — 0 chromosomes.

Affects of aneuploidy

A euploidy is produced in sperm or egg. Later it is transmitted to the zygote. Mi osis transmits the aneuploidy to all embryonic cells. Such organism may not su ive. If the organism survives, it becomes abnormal. Nondisjunction can also oc ur during mitosis. The early aneuploid condition is passed to a large number of ells by mitosis. It has greater effect on the organism.

(a Polyploidy

A ondition in which the organisms have more than two complete sets of ch omosomes is called polyploidy. The natural occurrence of polyploids in the ani al kingdom is extremely rare. But polyploidy can be induced experimentally in ertain animals, such as frogs and rabbits. There are following types of polyploidy:

  1. Triploidy (3n): They have an extra haploid set of chromosome in each pair. It may be produced by the fertilization of an abnormal diploid egg with sperm produced by nondisjunction.
  2. Tetraploidy (4n): They have increase of one diploid set of chromosome in each pair. Sometimes, there is the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent’ mitosis will then produce a 4n embryo.
  3. Mosaic polyploidy: It is more common than complete polyploid animals. In this case, the animals have patches of polyploidy cells.

Comparison of aneuploidy and polyploidy


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,Polyploids are more normal in appearance than aneuploids. One extra (or missing) chromosome disrupts genetic balance in aneuploids.

CHANGE IN CHROMOSOME STRUCTURE

Sometimes, there can be breakage of a chromosome. It can produce variety arrangements. These arrangements affect the genes of that chromosome. There are following types of chromosomal structural changes:

  1. Deletion: The loss of part of chromosome is called deletion. Fragments without centromeres are usually lost during cell division. Therefore, the chromosome will miss certain genes.
  2. Duplication: In this case, the fragment joins to the homologous chromosome with a duplication of genes.
  3. Inversion: In this case, the fragment reattach to the original chromosome but in reverse orientation.
  4. Translocation: In this case the fragment may join a non-homologous

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C uses of changes in chromosomal structures

1 The major source of deletions and duplications is the error during crossing over. Non-sister chromatids sometimes break at different places. As a result one partner gives up more genes than it receives. The products of such a crossover are one chromosome with a deletion and one chromosome with duplication.

2. Some environmental agents also cause changes in structure of chromosomes. These agents are ionizing radiations and certain chemicals. A ects of change in chromosomal structures

S me organisms inherit a homozygous deletion. It is usually lethal. Most genes

a     vital for the existence of organism. Duplications and translocation also
p •duce deleterious effects. In mutual translocation and in inversions, the

b lance of gene is not abnormal. All genes are present in their doses. But in ersions and translocation can alter phenotype.

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