CHROMATIN : Chromatin means colored
Material. The material of the nucleus stained by the basic dyes was given the name chromatin by Flemming . Interphase nucleus has a loose and indistinct network of nucleoprotein fibres called chromatin. In simple word we can say that chromatin are loose thread like structure.CHROMATIN contains DNA and some basic proteins called histones, some non histone proteins & also RNA.
During cell division chromatin fibres gets condensed and highly coiled to form short thick rod like structure called chromosomes.
•STRUCTURE OF CHROMOSOME: A chromosome consist of two identical halves the chromatids held together at one point called primary constriction or centromere on the sides of which disc shaped structure called kinetochores are present.
•Some chromosome may have an additional non staining constriction near their ends called secondary constriction ,Part of chromosome beyond secondary constriction is called satellite & these chromosome are termed as sat chromosomes or marker chromosomes because secondary constrictions are always constant in their position and can be used as markers.
•Certain secondary constrictions are the site for the formation of nucleoli & are known as nucleolar organizer & these chromosomes as nucleolar chromosomes.
•A chromatid contain a single fine chromatin fibre , the chromonema ( Double helix DNA)
•A single human cell has approximately two metre long thread of DNA distributed among its 46 chromosomes.
TYPES OF CHROMOSOMES
Metacentric chromosomes—- In which the centromere is near middle and two arms are equal.
Submetacentric chromosome—-Centromere is slightly away from the middle point & hence one arm is slightly short 7 other slightly long.
Acrocentric chromosomes—- Centromere is near the end and hence one arm is very short and other is very long.
Telocentric chromosomes—Centromere is at the end and the arms are on one side only.
EUCHROMATIN: After cell division most of the fibres become uncoiled extended & scattered that stain lightly called euchromatin or true chromatin.
HETEROCHROMATIN : After cell division some chromatin fibre remain coiled & compact which stain deeply it lie close to the nuclear lamina.
NUCLEOSOMES : Chromatin fibre as a chain of similar subunits called nucleosomes .Nucleosomes have core particles ( an octomer of 8 histone molecules) . Each nucleosome is connected to the next by DNA linker. Nucleosome and linker together called chromatosomes.
NUCLEOLUS: It is dense rounded dark stained organelle without limiting membrane It formed at specific sites called nuleolar organizers present on certain chromosomes. This help in protein synthesis and having precursor RNA for ribosome synthesis.