Chromosomes

CHROMOSOMES

The chromosomes are the nuclear components of special individuality, organization, and function.It is capable of self-reproduction and also plays the great role in heredity, mutation, variation and evolutionary of species.

HISTORICAL

Karl Nagil (1842) observed rod-like chromosomes in the nuclei of plants cells.E.Russow (1872) first serious attempt to describe chromosomes.A.Schneider (1873)published a most important paper dealing with the relation between chromosomes and stages of cell division.E.Strasburger (1875)discovered thread like structure appearing after cell division.Walter Flemming (1878)introduced the chromatin and describe the thread like material of nucleus that became intensely coloured after staining.

W.Roux(1883)suspected the involvement of chromosomes in the inheritance.Benden and Bovery (1887)reported a number of chromosomes for each species.The present name chromosome (Gr.,chrom=colour,soma=body)was coined by W.Waldeyer (1888)to dark stain bodies of nucleus.W.S.Sutton and T.Boveri in 1902 suggested chromosomes is the physical structures that acted as messengers of heredity.Sutton (1902)observed the chromosomes pair in synopsis made up of one maternal and one paternal member.He trusts that chromosomes,act in this way may be the physical basis for Mendelian laws of heredity.He credits the originator theory of the chromosomal basis for heredity.He is credited as the originator of the theory of the chromosomal basis for heredity.Thomas Morgan and Hermann Multer in the early 1900s established the cytological basis for laws of heredity.Working with Drosophila chromosomes,which located 2000 genetic factors on the four chromosomes of fruit fly in 1922.In 1914,Robert Feulgen demonstrated contain DNA.In 1942,using cytochemical procedures ,Bracket demonstrated the chromosomes contain DNA.The term ‘nucleosome’was contained by P.Outdet.

CHROMOSOME NUMBER

The chromosomes are constant for a particular species in number.Therefore,these are of great importance in the determination of the phylogeny and taxonomy of species.The set of the chromosomes of the genetic cells as sperms and ova called gametic,reduced or haploid sets of chromosomes.The genome is also called the haploid set of the chromosomes.Diploid cells are the somatic cell of organisms contain two haploid set or genomes.The chromosomes that union of the haploid male and female gametes and the sexual reproduction.The suffix “_ploid”refers to chromosome “sets”.The prefix indicates the degree of the ploidy.

The number of chromosomes in each somatic cell is the same for all numbers of a given species.Ascaris megalocephaly univalent has only two chromosomes in the somatic cell(i.e.,2n=2).Aulacantha is found in radiolarian protozoan that is diploid number approximately 1600 chromosomes.Lastly while ‘normally signifies the gametic or haploid chromosome number ‘2n’ is the somatic or diploid chromosome number in an individual.In polyploid individuals,however, it becomes necessary to establish an ancestral primitive number,which is represented as ‘x’ and is called the base number.For example,in wheat Triticum aestiveum 2n=42 ; n=21 and x=7,showing that common wheat is a hexaploid(2n=6x).

Autosomes and Sex Chromosomes

A diploid cell,there are two of each kind of chromosomes (these termed homologous chromosomes),except for the sex chromosomes.One sex has two of the same kind of sex chromosomes and the other has one of each kind.A chromosome number which was established by Tijo and Levan in 1956.The human female has 44 non-sex chromosomes,termed autosomes and one pair of homomorphic (morphologically similar)sex chromosomes given the destination XX.The human male has 44 autosomes and one Y chromosome.

MORPHOLOGY

Size

The size of the chromosome is normally measured at mitotic metaphase and may be as short as 0.25um in fungi and birds,or as long as 30um in some plants such as Trillium.Most metaphase chromosomes fall within a range of 3um in fruitfly(Drosophila),to 5um in man and 8um in maize.The organism with less number of chromosome contains comparatively large sized chromosomes than the chromosomes of organisms having many chromosomes.

Monocotyledons plants contain large-sized chromosomes than the dicotyledon plants.The plants have generally a large sized chromosome than the animals.The chromosomes in a cell are never alike in size,some are exceptionally large and others are too small.The lampbrush chromosomes of certain vertebrate oocytes and polytene chromosomes of certain insects are largest chromosomes.

Shape

The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division.In the resting phase or interphase stage of the cell ,the chromosomes occur in the form of thin,coiled,elastic and contractile,thread like stainable structures,the chromatin threads.In the metaphase and the anaphase,the chromosomes contain a clear zone called centromere or kinetochore with their length.The centromere where its divide the chromosomes into two parts each part called chromosome arm.The position of centromere varies from chromosome to chromosome and provides different shapes the latter which is as follows:

1.Telocentric.The rod-like chromosomes having the centromere on the proximal end called telocentric chromosomes.

2.Acrocentric.The acrocentric chromosomes are also rod-like in shape but these have the centromere at one end and thus giving a very short arm and an exceptionally long arm.The locusts have the acrocentric chromosomes.

3.Submetacentric.The submetacentric chromosomes are J- or L-shaped.In these way, centromere occurs near the centre or at the medium portion of the chromosomes and thus forming two unequal arms.

4.Metacentric.The submetacentric chromosomes are V-shaped and in these chromosomes the centromere occurs in the centre and forming two equal arms.The amphibians have metacentric chromosomes.

Structure

The structure of the chromosomes during various places of the cell cycle,cell biologists introduced many terms for their various components.Before familiar with the following to understand more clearly the structures of chromosomes:

1.Chromatid.At mitotic metaphase, the chromosome consists of two symmetrical structures called chromatids.Each chromatid contains a single DNA molecule.Both chromatids are attached to each other only by the centromere and become separated at the beginning of anaphase,the sister chromatid of a chromosome migrate to the opposite poles.

2.Chromonema(ta).During mitotic prophase, the chromosomal material becomes visible as thin filaments,called chromonemata (a termed coined by Vejdovsky in 1912).A chromonema represents a chromatid in the early stages of condensation.The chromonemata from the gene –bearing portions of the chromosomes.

Chromosomes have more than one chromonemata which are embedded in the achromatic and amorphous substance called the matrix.It is enclosed in a sheath or pellicle .Both matrix and pellicle are nongenetic materials and appear only at metaphase when the nucleolus disappears.It is believed that nucleolar material and matrix are interchangeable ,i.e.,when chromosomal matrix disappears,the nucleolus appears,the occurrence,however, have questioned the occurrence of pellicle and matrix in them.

3.Chromomeres.The chromomeres are bed like accumulations of chromatin material that are sometimes visible along interphase chromosomes.Chromomeres become especially clear in the polytene chromosomes,where they become aligned side by side,constituting the chromosome beads.The regions of tightly folded DNA and have great interest for the cell biologists are chromosomes.It is believed to the units of genetic function in chromosomes.Most geneticists considered these chromomeres as genes, i.e.,the units of heredity.

4.Centromere and kinetochore.The centromere consists of small granules or spherules.The chromonema remains connected with the spherules of the centromere.The centromere is a term preferred by the geneticists.It lies with a thinner segment of chromosomes,the primary constriction.The centromeres are found to contain specific DNA sequences with special proteins bound them forming a disc-shaped structure called kinetochore.It is a term which preferred the cytologist.The kinetochore appeared a plate or cup like the disc,0.20 to 0.25 nm,in diameter upon primary constriction of the centromere.The main function of kinetochore is that its provide a centre an assembly for microtubules.The dicentric chromosomes lead to fragmentation so two centromeres to migrate the opposite poles.

5.Telomere.According to the Greek word the terms Telomere indicated as telo=for and meros=part.The extremity of the chromosome has a polarity so it prevents other chromosomal segments to fused it.Those chromosomal ends are known as telomeres.If the chromosome breaks then the broken ends can fuse each other because of lack of telomeres.

6.Secondary constriction.The primary constriction or the centromeres possess secondary constriction at any point of the chromosomes.It's constant in their position and extent,this construction is useful in identifying particular chromosomes in the set.Secondary construction can also be distinguished from primary constriction due to the chromosomes bends only at the position of centromere during anaphase.

7.Nucleolar organizers.It is a certain secondary constriction which contains genes coding for 5.8S, 18S and 28S ribosomal RNA that induces the formation of nucleoli.The RNAs genes transcribed very actively because of the secondary constriction may arise.Hence it interferes with chromosomal condensation.In human beings, the nucleolar organizers are located in the secondary constrictions of chromosomes 13,14,15,21 and 22 that are acrocentric and having satellites.

8.Satellite.The chromosomes sometimes bear round elongated or knob like appendages called as satellites.It remains connected with the rest of the chromosome by a thin chromatin filament.The chromosomes with the satellite are designated as the sat chromosomes.The satellite has a shape and size which remain constant.The chromosomal satellites are a morphological entity and should not be confused satellite DNAs that are highly repeated DNA sequence.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.