(i) Telocentric:- When centromere is terminal or located at the tip of chromosome.

(ii) Acrocentric:- When the centromere is sub-terminal or located near the tip.

(iii) Metacentric :- When the centromere is located at mid of the chromosome.

(iv) Sub metacentric :- When the centromere located near centre or mid point of chromosome .

`->` The ratio of length of the long arm to the short arm of a chromosome is called arm ratio. Arm ratio is maximum in acrocenteric chromosome.

`->` Karyotype `=>` Karyotype is external morphology of all Chromosomes of a cell which is specific for each species of living organisms. Karyotype can be studied in metaphase of mitosis.

`->` Karyotype includes the number of chromosomes, relative size, position of centromere , lenght of the arms, secondary constrictions and banding patterns.

`->` Idiogram :-Diagrammatic representation of Karyotype. In idiogram chromosomes are arranged in decreasing order of size. Sex chromosomes are placed in last but in idiogram of Drosophila sex chromosomes are placed first. Idiogram is specific for every species.

(i) It suggests primitive or advanced features of an organism. If karyotyp(! shows a large size differences between the smallest and the largest chromosome of the set and having fewer melacentric chromosomes then it is called asymmetric karyotype, which is a relatively advance feature. Symmetric karyotype is primitive feature.

(ii) The karyotype of different species are compared and similarities in them represent the evolutionary relationships.

(iii) Karyotype is helpful in detection of chromosomal abberrations and polyploidy.

(iv) In research of medical genetics Forensic science cytogenetics and Anthropogenetics.

(Parts which appears in metaphase chromosome)

1. Pellicle - This is outermost, thin proteinaceous covering or sheath of chromosome.

2. Matrix-This is a liquid nongenetic achromatic ground substance of chron1osomc, which has different type of enzymes, rninerals, water, proteins.

3. Chromatid- At metaphase stage each chromosome is consistof two cylindrical structures- called chromatids. Both sister chromatids are joined together by a common centromere. A chrornosorne, may have single chromatid (in Anaphase or Telophase) or two chrornatid. (as in prophase rnetaphase)

`->` Each chromatid is consist of a single long thread of DNA associated with histone. Non histone proteins and RNA are also present .

`->` Sometimes bead like structure are seen on chromatid . which are called as chromomere.

4. Centromere/Kinetochore:- (Primary constriction)

`->` Each chromosome (at metaphase) is consist of two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called Centromere. At this point or cent rom ere two protein discs are present which is called Kinetochore .


`->` Kinetochores constitute the actual site of attachement of spindles to chromosomes during cell division .

`->` At the region of centromere the chromosorne is comparatively narrower than remaininq part of chromosomea and this part is essential for the functioning of chromosome thus it is termed as Primary constriction.

5. Secondary constriction: Besides prirrwry constrictions, other constriclion may also occurs on some chromosome, which are known as secondary constriction. These constriction are non staining and found at a constant location .

`->` Secondary constriction is also known as NOR (Nucleolar organizer region){13, 14,15,21.22 chromosomes in human)

part of chromosome remains after the NOE is known as chromosomes satellite/ Trabent.

`->` Chromosomes with satellite part are called as SAT chromosome (SAT Sine Acid Thymonucleinico)

7. Telomere : Chromosomes have polarity and polar ends of chromosomes are known as Telomeres .

`->` Telomere prevents fusion of one chromosomes to other chromosome. Telomere rich in Guanine base. `(5' -TTAGGG-3')`

`->` Enzyme Telomerase synthesize telomere part of chromosome, which is a Ribonucleoprotein.

Telomeres of chromosomes becomes shorter during ageing . rocess.

`->` Taken the distance between two consecutive base pairs as `0.34` nm `(0.34 xx 10^(-9) m)`, if the length of DNA double helix
in a typical mammalian cell is calculated (simply by multiplying the total number of bp with distance between two consecutive
bp, that is, `6.6 xx 10^(9) bp xx 0.34 xx 10^( 9)m/bp)`, it comes out to be approximately `2.2` metres. A length that is far greater
than the dimension of a typical nucleus (approximately `10^(-6)` m) .


`->` In eukaryotes, this organisation is much more complex. There is a set of positively chars}ed, basic proteins called
histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains .

`->` Histones are rich in the basic amino acid residues lysines and arginines. I-3otl1 the amino acid residues carry positive charges in their side chains .

`-> Histones are organised to form a unit of eight molecules called as histone octamer `[ H_(2)A, H_(2)B, H_(3), H_(4) xx 2 ]` The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome.

Nucleosome = Binding DNA + Octamer core `( H_(2)A, H_(2)B, H_(3), H_(4) xx 2)` + Linker DNA +`H_1` Histone

`->` A typical nucleosome contains `200` bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread-like stained (coloured) bodies seen in nucleus .

`->` The nucleosomes in chromatin are seen as 'beads-on-string' structure when viewed under electron microscope (EM).

`->` The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at rnetapha~e stas1e of cell division to form chromosomes.


`->` 6 Nuclesome units united (or super coiling) to forms Solenoid structure.

`-> ` `H_1` histone protein (sealing histone) joined the turns of binding DNA in nucleosome .
. 3

`->` Nucleosome unit have `1.75` or `1(3/4)` turns of binding DNA.