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Chromosomes
2013-02-21, 10:30 PM

CHROMOSOMES

                      (Chroma-colour, soma-body)

                Karl Nageli (1842) observed deeply stained thread like structures in the nucleus later Waldayer (1888) named them as chromosomes .

Sutton and Boveri (1902) proposed chromosome theory which states that these are the vehicles of heredity.

                Chromosomes involved in controlling and regulating the activities of a cell which transfers the genetic traits from one generation to next generation, hence referred as physical basis of heredity.

Ø  Morphology

Structure, size, shape, number of chromosomes differ from species to species in both plants and animals but remains constant in each species and maintain specifity and stability.

§  Size:  1µm-50µm in length, 0.2 µm -2 µm in  diameter

§  In plants (30 µm)-Trillium they are  bigger than the animals

§  Somatic cell: 2n (diploid)

§  Gametes: n (haploid)

§  Shape: V,L,J& I

 

Ø  Classification

Based on the number of centromeres, they are of five types as follows

·         Acentric

·         Monocentric

·         Dicentric

·         Holocentric

·          Polycentric)

Monocentric chromosomes are further classified on the basis of the position of the centromeres. They are of four types as follows:

·         Metacentric(V Shape): centromeres present in middle point, both arms are equal in length during anaphase

·         Submetacentric(L Shape): away from the middle point and arms are unequal, during anaphase

·         Acrocentric (J Shape): towards one side, short and long arms during anaphase.

·         Telocentric (I Shape): end of the arm, during anaphase.

Ø  Number

In plants and animals , no of chromosomes is represented by diploid condition.

·         Vegetative/somatic cells: diploid cells (2n)

·         Gametes/reproductive: haploid cells (n)

Examples: Plants

o   Pisum sativum-14

o   Zea mays-16

o   Oryza sativa-20

o   Nicotiana tobacum-24

o   Human beings-46

o   Ophioglossum-1260 (highest)

 

·         Karyotype: single set of chromosomes.

·         Idiogram: diagrammatic representation of single set of chromosomes.

·         Gene: a unit of heredity which transfer from parent to offspring

·         Genome: genes constituting a single set of chromosomes.

·         Alleles: pair of genes.

·         Somatic: any cell of an organism other than reproductive cell.

·         Gamete: reproduction cell which unite during sexual reproduction to form a new cell  zygote.

Ø  Structure

The body of the chromosomes is divided into three parts as follows:

·         Pellicle: Outermost covering

·         Matrix: Ground substance

·         Chromonemata: Chromatids

A chromosome appears clearly during metaphase of cell division.

Chromatin material transforms into chromosomes and becomes thick and stout.


Ø  Primary constriction

·         Metaphase chromosomes show a colorless constriction (centromere/kinetochore/kinomere).

·         Centromeres help in the attachment of spindle fibres.

·         On both sides of centromeres, they are long cylindrical parts known as arms which  splits into 2 parts known as chromatids are of two types

§  Paranemic coil: loose coil, separated easily

§  Plectonemic coil: tight coil, cannot separated

·         Each arm is bounded by a transparent coat known as pellicle. Inside this, matrix is present.

·         In this tightly coiled DNA extends from one end of arm to the other.

·         The ends of chromatid arms exhibit polarity known as telomeres.

·         Due to the polarity, chromosomes do not fuse with one another and maintains as individual.

·         During prophase, chromosomes appear as beads with swellings and constrictions alternating with one another known as centromeres, if they are bigger in size they are known as knobs.

 

Ø  Secondary constriction

The part of the chromosome arm present near to the nuclear organization appears like a sphere/satellite/trebent. Such chromosomes are known as SAT chromosomes (Sine-Acid-Thymonucleonic) .these is present without DNA.Such chromosomes have secondary constrictions from which nucleoli are formed known as nuclear organizers.

§  SAT: Two chromosomes are present in each diploid nucleus which plays a vital role in the formation of nucleolus.

Ø  Chemical nature

·         It is composed of genetic material associated with histone proteins, hence they are chemically called as nucleo-proteins.

·         When chromosomes are stained with acetocarmine/acetoorcein, some parts will appear darker; those areas are called as heterochromatin (inactive genes). If they are  lighter, those areas are Euchromatin(active genes)

 

Ø  Types

·         Autosomes: These are more in number which determines the somatic activities of cell like growth, development etc..,

·         Allosomes/Sex chromosomes: Determines the sexual characters of organisms in human beings which are identified as X, Y chromosomes.

 

Ø  Function

·         Helps in reproduction process by the duplication of the genetic material to pass on to the next generation.

·         Acts as controllers of heredity.

 

 

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