VARIATION IN CHROMOSOME NUMBER

AND STRUCTURE

WHAT IS A CHROMOSOME? In the nucleus of each cell, the DNA

molecule is packaged into thread-like structures called chromosomes.

Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Each chromosome has a constriction point called the centromere into two sections, or “arms”.

CHROMOSOMAL MUTATION…. An unpredictable change that occur in a

chromosome. These changes are most often brought by

problems that occur during cell division or by mutagens like chemicals, radiations, etc.

Chromosome mutations can result in change in the number of chromosomes in a cell or changes in the structure of a chromosome.

VARIATION IN CHROMOSOME STRUCTUREThere are four common types of chromosomal mutations involving changes in chromosome structure: 1. deletion 2. duplication 3. inversion 4. translocation

Deletion and duplication involve change in the amount of DNA on a

chromosome

Inversion involve a change in the orientation of a chromosomal

segment

Translocation involve a change in the location of a chromosomal

segment

Deletion

It is a chromosomal mutation in which a part of a chromosome is missing or deleted.

Since, a segment of chromosome is missing so deletion mutations cannot be revert to the wild type.

ACENTRIC CHROMOSOME: If the deletion involves the loss of the centromere

cont…..

Some human disorders are caused by deletions of chromosome segments.

Cri-du-chat is one of the human disorder caused by a heterozygous deletion.

A segmental deletion in chromosome number 5 causes this syndrome.

Children with this syndrome are severely- mentally retarded, many physical abnormalities, cry with a sound like the mew of a cat. About 1 out of 50,000 live births has a cri-du-chat syndrome.

cont….

Another example of chromosomal deletion is Prader-Willi syndrome.

Results due to deletion of long arm of chromosome number 15.

Sucking reflex of the effected infants are weak which results in poor growth.

But, at the age of around 5 to 6, children with this syndrome become compulsive eaters, producing obesity and related health problems. Reason still unknown.

About 1 in 25,000 suffers from this syndrome, predominantly males.

Cri-du-chat syndrome

Prader-Willi syndrome

Duplication

A duplication is a chromosomal mutation that results when a segment of DNA breaks off and attaches onto the homologous chromosome.

There are different forms of chromosomal duplications or different tandem configuration

1. tandem 2. reverse tandem 3. terminal tandem

When the order of genes in the duplicated segment is the opposite of the order of the original, the mutation is a reverse tandem duplication.

If the duplicated segments are arranged in the tandem at the end of a chromosome , the mutation is then called terminal tandem duplication.

Duplications can cause phenotypic variation that might at first appear to be caused by a simple gene mutation. The Bar-eye phenotype in Drosophila is a classic example. Instead of the normal oval-eye shape, Bar-eyed flies have narrow, slitlike eyes.

Inversion It is a chromosomal mutation that results when a segment of a

chromosome is excised and then reintegrated at an orientation 180 degree from the original orientation.

There are two types of inversions: 1. Paracentric inversion 2. Pericentric inversion

Typically genetic material is not lost when and inversion takes place.

If the centromere is not part of the rearranged chromosome segment, it is a paracentric inversion.

If the centromere is part of the inverted segment, it is described as a pericentric inversion.

Translocation If a segment of chromosome breaks off and attaches to a non-

homologous chromosome then the situation is called translocation.

Translocation can be of three different types: 1. Nonreciprocal intrachromosomal translocation : when a chromosome segment changes position within the the same. chromosome. 2. Nonreciprocal interchromosomal translocation : when a chromosome segment is transferred from one chromosome to another and it was a one way transfer. 3. Reciprocal interchromosomal translocation : when a chromosome exchange occurs between both the chromosomes.

Robertsonian translocation

One common type of translocation involves breaks at the extreme ends of the short arms of two nonhomologous acrocentric chromosomes. These small segments are lost, and the larger segments fuse at their centromeric region. This type of translocation produces a new, large submetacentric or metacentric chromosome, often called a Robertsonian translocation.

One such translocation accounts for cases in which Down syndrome is familial (inherited).

Other types of chromosome structural aberrations…..

Isochromosomes :

An isochromosome is a chromosome with two identical arms. Instead of one long (q) arm and one short (p) arm, an isochromosome has two long arms or two short arms. As a result, these abnormal chromosomes have an extra copy of some genes and are missing copies of other genes.

Cont…..

Dicentric chromosomes : Unlike normal chromosomes, which have a single constriction point (centromere), a dicentric chromosome contains two centromeres. Dicentric chromosomes result from the abnormal fusion of two chromosome pieces, each of which includes a centromere. These structures are unstable and often involve a loss of some genetic material.

Ring chromosomes: Ring chromosomes usually occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure. The ring may or may not include the chromosome’s constriction point (centromere). In many cases, genetic material near the ends of the chromosome is lost.

Cont…..

VARIATION IN CHROMOSOME NUMBER

Numerical aberration are mainly of two types, (1) Euploidy, (2) Aneuploidy, which are again further divided into-

Aneuploidy

Chromosomal mutation which results in the variation of individual chromosome number are examples of aneuploidy.

Changes in chromosome number can occur in both diploid and haploid organisms.

The nondisjunction* of one or more chromosome during meiosis I or meiosis II typically is responsible for generating gametes with abnormal numbers of chromosomes.

…Nondisjunction*

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division

Nullisomy

A Nullisomic cell involves a loss of one homologous chromosome pair- the cell is 2N-2.

Monosomy

loss of a single chromosome : (2n - 1)

Human monosomies are conceived but none survive embryonic and fetal development. With the exception of Turner syndrome (45, X), these are NOT tolerated in animals.

Trisomy It involves a single extra

chromosome: 2N+1

In general, the effects of trisomy parallel monosomy, but the addition of an extra chromosome produces somewhat more variable individuals.

Tetrasomy

A tetrasomic cell involves an extra pair of chromosome, i.e. there are four copies of one particular chromosome:

the cell is 2N+2

Aneuploid conditions in human

Down syndrome

Patau syndrome

Edward syndrome

Turner syndrome

Euploidy

Euploid organisms have varying numbers of complete haploid chromosomal sets.

MONOPLOIDY and POLYPLOIDY involve variation from the normal state in the number of complete sets of chromosomes.

Monoploidy and Polyploidy are lethal in most animal species, but are less consequential in plants.

Monoploidy

A monoploid individual has only one set of chromosomes instead of the usual two sets.

Some male wasps, ants and bees, for example, are monoploid because they develop from unfertilized eggs.

Cells of a monoploid individual are very useful for producing mutants, because there is only one dose of each of the genes. Thus, mutants can be isolated.

Polyploidy Polyploidy is the chromosomal constitution of a cell or an

organism that has three or more sets of chromosomes. Almost all animals and plants probably have some

polyploidy tissues. For example, the endosperm of plants is triploid, the liver of mammals and perhaps other vertebrates is polyploidy.

Plants that are completely polyploidy include wheat, which is hexaploid (6N), and the strawberry, which is octaploid (8N). Some animal species, such as the North American sucker (a freshwater fish), salmon, and some salamanders are polyploid.

Two types of polyploidy are encountered in plants: 1. Autopolyploidy 2. Allopolyploidy

Autopolyploidy

In this, all the sets of chromosomes originate in the same species. The condition probably results from a defect in meiosis that leads to diploid or triploid gamets.

The cultivated banana is an example of triploid autopolyploid plant.

In general,the development of “seed-less” fruits such as grapes and watermelons relies on odd-number polyploidy.

Allopolyplody

In this, the sets of chromosomes involved come from different, though usually related, species.

This situation can arise if two different species interbreed to produce an organism with one haploid set of each parents chromosomes and then both chromosome sets double.

A classic example of allopolyploidy resulted from crosses made between cabbages and radishes by Karpechenko in 1928.