Giant Chromosomes

There are chromosomes which are extremely large compared to normal chromosomes. Such chromosomes, called giant chromosomes occur in some animal cells. Two types of giant chromosomes are known

1. Lamp Brush Chromosomes

These chromosomes occur in the oocytes (germcells in the ovary) of amphibians and in some insects. They are extremely large synapsed homologous chromosomes which can be seen in the diplotene stage of prophase-I in meiosis. They measure about 1500 to 2000 in length. A lampbrush chromosome consists of an axis from which paired loops extend in opposite directions, giving the appearance of a lamp brush. The axis consists of chromomeres (nucleosomes) and interchromomere regions. The loops consist of transcriptionally active DNA which can synthesize large amount of mRNA, necessary for the synthesis of yolk.

fig. 19.3 Lampbrush Chromosomes

2. Polytene Chromosomes

These are giant chromosomes found in the salivary gland cells of the fruitfly Drosophila. They are many times larger than the normal chromosomes reaching a length of 2000 and are visible even under a compound microscope. The polytene chromosomes appear to contain five long and one short arm radiating from a central point called chromocentre. It is formed by the fusion of centromeres of all the eight chromosomes found in the cell. Of the 6 arms, the short arm represents the fused IV chromosome and the longest represents the fused sex chromosomes. These arms contain numerous chromonemata resulting from repeated replication of DNA, without separation into daughter chromosomes. The arms show characteristic dark bands and light bands. The dark bands are euchromatic regions. Some of the dark bands temporarily swell up and form enlargements called chromosomal puffs or Balbiani rings. These regions contain actively transcribing DNA involved in the synthesis of RNA types.

fig. 19.5 Polytene Chromosomes

Homologous Chromosomes

In the diploid number, chromosomes are always found in pairs. In each pair there are two identical chromosomes, called homologous chromosomes. These chromosomes are identical in size, appearance and position of the centromere. Apart from this, the two homologous chromosomes are genetically identical, carrying the same number of genes, controlling the same characters. One member of each pair is called paternal chromosome since it comes from the male parent and the other member is called maternal chromosome since it comes from the female parent. Thus, for example, in the human cells there are 23 pairs of homologous chromosomes (2n=46) of which 23 represent paternal chromosomes and the other 23 represent maternal chromosomes.

Genome

Genome is the term used to describe the sum total of all the genes in one haploid set of chromosomes. A diploid cell hence has two genomes, a triploid cell has three genomes and so on.