1 chromosomes

7/28/2019 1 Chromosomes

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CHROMOSOMESE straussberger (1875) discovered thread like structures during cell division and

called chromosomes (chroma=colour) due to their affinity to basic dye.

In eukaryotes, well organised nucleus contain definite number of chromosomes of

definite size and shape.

At leptotene stage of meiotic prophase chromosomes appear as headed structure

called chromomeres, size of chromomeres and interchromomeric regions are not

constant, so that every leptotene has its own particular pattern.

The DNA is though known to concentrate in the chromomeres, but it is believed to

be present in interchromomeric region also.

Chromosome number varies from 2n = 4 in Haplopappus gracilis (compositae) to 2n

= >1200 in some pteridophytes, Aulacantha, a radiolarian, 2n = 1600

n signifies gametic or haploid chromosome number

2n signifies somatic or diploid chromosome number

In polyploidsbasic chromosome number (x) Chromosome is normally measured in mitotic metaphase as short as 0.25 in fungi

and birds as long as 30 in some plants like Trillium, Drosophila 3, Humans 5,

Maize 8-12 .

Chromosome shape is usually observed at anaphase, when the position of

centromere/ primary constriction determines chromosome shape.

Centromere can be terminal, sub terminal or median

Number, size and shape of chromosome:


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http://dna-footprints.com/wp-content/uploads/2011/12/SisterChromatids.jpg


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Terminal Rod shape

Sub-terminal J shape

Median V shape

Sub median L shape

Corresponding to different position of centromere -

Acrocentric or Telocentric

Submetacentric

Metacentric

Secondary constriction can be observed in some chromosomes and forms a

satellite. It remains attached to rest of the body by a thread of chromatin.

Secondary constrictions are constant on their position and can be used as useful

markers. They can be distinguished from centromere because chromosome bends or

angular deviation only at the position of centromere, chromosomes having satellite

are called as SAT-Chromosomes.

The chromosome extremities or terminal regions on either side are called

telomeres.

Detailed study of chromosome morphology reveals a coiled filament throughout the

length of the chromosome. This filament is called chromonema (Vejdorslay, 1912).

The chromonemata form the gene-bearing portions of the chromosomes.

The chromonemata are embedded in the achromatic substance known as matrix.

Matrix is enclosed in a sheath or pellicle. Both matrix and pellicle are non-geneticmaterials and appear only at metaphase when the nucleolus disappears.

Morphology :


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Group of characteristics like chromosome number, size and shape of a chromosome

set up of a species is called as karyotype and is usually represented by a diagram called

as idiogram, where chromosomes of haploid set are ordered in a series of decreasing

size.

Karyotype are presumed to represent evolutionary relationships. Karyotypesuggests primitive or advanced features of an organism.

Asymmetric karyotype - karyotype showing large difference between smallest and

largest chromosome of the set and having fewer metacentric chromosomes. It is

considered to be a relatively advanced features when compared with symmetric

karyotype.

Levitzky (1931) suggested that in flowering plants there is a predominant trendtowards karyotype asymmetry.

When chromosomes stained with acetocarmine or feulgen (basic fuchin), the darkly

stained heterochromatic and lightly stained euchromatic regions can be differentiated.

Heterochromatin is found at chromomeres, chromocenters and knobs.

Chromomeres are regular feature of all prophase chromosomes, but their number,

size, distribution and arrangement are specific for a particular species at a particularstage of development.

It is believed that nucleolar material and matrix are interchangeable i.e. when

matrix disappears, nucleolus appears and vice versa.

Chromatid is a half chromosome & two chromatids being connected at centromere.

The chromonema is a sub-chromatid structure and there can be more than one

chromonemata at the centromere.

Karyotype :


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Human karyotype

Human male karyotype


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Chromocenters are heterochromatic regions of varying size which occurs near the

centromeres in proximal regions of chromosome arms.

Knobs are spherical heterochromatin bodies which may have a diameter equal to

there chromosome width but may reach the size having diameter which is several timesthe width of chromosomes.

Very distinct chromosome knobs can be observed in maize at pachytene stage. Knobs

are valuable chromosome markers for distinguishing chromosomes of related species

and races.

Constitutive heterochromatin - certain regions of chromosomes particularly those

proximal to centromeres are constant and are called constitutive chromatin.Facultative heterochromatin - other heterochromatin regions and represented by

whole sex chromosome which become heterochromatic only at certain stage.

Eg. female humans - one X chromosome is inactivated facultatively.

Plant - accessory chromosomes are heterochromatic.

Dioecious genera - Melandrium and Rumex - one or both sex chromosomes may

undergo partial or complete heterochromatization.

Y chromosome - heterochromatic having inactive genes in several dioecious plants and

animals.

Nucleosome model - R.D Kornberg and J.O Thomas (1974)

term nucleosome - P. Oudet (1975)

Folded fibre model - E.J dupraw (1965)


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Chromomeres in polytene chromosomes


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The BRM protein is associated with chromosome puffs and interbands of salivary

gland polytene chromosomes. (A) Distribution of BRM protein on wild-type

polytene chromosomes. Arrowhead indicates chromocenter. (B) The top panel

shows indirect immunofluorescence using an anti-BRM antibody (green), the

second panel is DAPI-stained DNA (blue), the third panel shows the merged images

and the bottom panel is a split image. Note that BRM protein is predominantly

found in the DAPI interbands. The distal region of chromosome arm 2L is shown.

Chromocenter

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