Question

Chromosomes are the physical basis of appearance. Explain in 5 points

Answer 1

The earlier geneticists had a conception of heredity quite different from the modern idea based on chromosomes. Early in the eighteenth century the idea of performation prevailed. It was supposed that the future human body was already present in a minia­ture form as the ‘homonculus’ (also spelt homunculus) either in the spermatozoa or in the ovum and then in the embryo.

Answer 2

Chromosome appearance (i.e. morphology) changes with different phases of cell division. Especially the metaphase stage of mitotic division is most suitable for the study of chromosome morphology. The size of chromosomes, the position of centromeres, the presence of secondary constrictions and above all the shape of the chromosomes are revealed through individual’s karyotype.

Chromosomes were first described by Strausburger in 1875 and the term was first used by Waldeyer in 1888. These are small elongated rod-shaped bodies, clearly visible during the stages of cell- division. Their number can be counted with relative ease only during mitotic metaphase, under powerful microscope. Chroma means colour and soma means body.

The name chromosome therefore signifies the affinity of these bodies toward basic dyes. On application of the dyes, these bodies stain very deeply while the cytoplasm remains relatively unstained. Studies subsequent to Mendel have clearly shown that the chromosome is composed of thin chromatin threads called chromanemata. The chromanemata undergo coiling and supercoiling during early stage of Meiosis and so became progressively thicker to be readily visible under a microscope.

In general, each species has a definite and constant somatic and gametic chromosome number. Somatic chromosome number is the number of chromosomes found in the somatic cell of a species, which is represented by 2n. Generally, somatic cells contain two equal sets or copies of each chromosome except sex-chromosomes.

.