Chromosomes
Chromosomes (Gk. Chroma-colour, some-body) are filamentous bodies present in the nucleus. They are visible only during cell division.
Chromosomes have the ability of self replication and they play an important role in heredity, variation, mutation and evolution of species.
Chromosomes which are morphologically and genetically identical are called homologous chromosomes.
Number : All the individuals of a species usually have same number of chromosomes. Presence of whole sets of chromosomes is called euploidy. It includes haploid (one set of chromosomes, n), diploid (two set of chromosomes, 2n), triploid (3n), tetraploid (4n) etc. For example in human beings each somatic or body cell contains two sets of chromosomes (diploid, 2n=46) and gametes contain one set of chromosomes (haploid, n=23). Out of 23 pairs of chromosomes 22 pairs are autosomes and one pair is sex chromosome.
Size : The size of chromosome varies from species to species. Each metaphase chromosome varies from 0.1 to 33µm in length and 0.2 to 2µm in thickness.
Shape : The shape varies according to the stages of cell division. In interphase, chromosomes are in the form of chromatin network while in metaphase they show maximum condensation and appear short and thick.
Structure : A metaphasic chromosome has two identical halves called sister chromatids. Each chromatid is in turn made up of sub-chromatids called chromonemata (singular-chromonema). The chromatids lie side by side and are held together at one point called the centromere. The centromere is also called the primary constriction. During cell division the spindle fibres are attached at the centromere. Besides the primary constriction, additional narrow areas called secondary constrictions are present. Some of the secondary constrictions are called nucleolar organizers because they are necessary for formation of nucleolus. The part of the chromosome beyond the nucleolar organizer is short, spherical and is called satellite. The tip of the chromosome is called telomere. It has a unique property in that it prevents the ends of the chromosomes from sticking together but attaches to the nuclear envelope. The surface of a chromosome bears number of small swellings called chromosomes.
 |
| Structure of Chromosome |
Types of Chromosomes :
According to the positions of the centromere, chromosomes are classified into following types -
1. Metacentric : If the centromere is situated in the middle of the chromosome it is called metacentric chromosome. The two arms of the chromosomes are nearly equal. It appears 'V' shaped during anaphase.
2. Sub-metacentric : If the centromere is situated some distance away from the middle it is called sub-metacentric chromosome. One arm of the chromosome is shorter than the other. It appears 'L' shaped in anaphase.
3. Acrocentric : If the centromere is situated near the end of the chromosome. One arm of the chromosome is very short while other is long. It appears 'J' shaped in anaphase.
4. Telocentric : If the centromere is situated at the tip of the chromosome it is called telocentric chromosome. It shows only one arm and appears rod shaped.
 |
| Types of Chromosomes |
Chromosomes are responsible for inheritance of characters. They regulate the protein synthesis and thus help in cell division and cell growth. They control cell metabolism by directing the synthesis of enzymes. Sex chromosomes are essential for determination of sex. Chromosomes replicate to produce exact copies of themselves which are passed on to daughter cells.
Structure of X and Y chromosomes :
X chromosomes is longer than Y chromosome.
X chromosome contains large amount of euchromatin and small amount of heterochromatin. The euchromatin contains large amount of DNA or genes. Y chromosomes contains small amount of euchromatin and large amount of heterochromatin. It has less DNA hence genetically less active.
X and Y chromosomes show homologous and non-homologous regions. Homologous regions show similar genes while non-homologous regions show dissimilar genes. Crossing over occurs only in homologous regions.
 |
| Sex Chromosomes |
Non-homologous region of X chromosome contains more genes than that of non-homologous region of Y chromosome.
Non-homologous region of X chromosome contains X-linked genes while non-homologous region of Y chromosomes contains Y-linked genes.