Eukaryotes - Nuclear Chromatin
Description:
Interphase nucleus has a loose network of nucleoprotein fibres called chromatin.
During cell division the nuclear chromatin undergo condensation by coiling and dehydration into a number of rods. These are called chromosomes.
Chromatins contain the genetic material DNA and also some basic proteins called histone proteins.
Chromatins also contain some non-histone proteins and RNA.
A human cell has about 2 meters long DNA thread which is distributed among 23 pairs of chromsomes.
Structure
Every chromosome has a primary constriction called the centromere.
At the sides of the centromere there are disc shaped structures called the kinetochores.
A chromosome consists of two identical and spirally coiled thread called sister chromatids. The sister chromatids are joined at the centromere.
Based on the position of the centromere chromosomes can be −
Metacentric − Centromere is at the middle forming two equal arms of the chromosome.
Sub-metacentric − Centromere slightly away from the middle resulting into one shorter arm and one longer arm of the chromosome.
Acrocentric − Centromere is situated close to the end of the chromosome resulting into one extremely short arm and one very long arm.
Telocentric − The centromere is at the terminus of the choromosome.
Satellite chromosome − Some chromosomes contain secondary constrictions at constant locations. They give the appearance of a small fragment which is termed as satellite.