Chromosomes: Location, Shape and Size | Genetics

In this article we will discuss about:- 1. Background of Chromosomes 2. Location of Chromosomes 3. Number 4. Shape 5. Size 6. Chemistry 7. Chemical Components 8. Physiology.

Historical Background of Chromosomes:

i) The term chromosome (Gr. chromo, color; soma, body), when used by Waldeyer in 1888, represented the darkly stained individualized bodies located in the nucleus.

ii) Walter S. Sutton, 1900, after the redis­covery of Mendel’s laws, described the parallelism between the segregation of chromosomes during meiosis and the transmission of hereditary factors or determiners (genes) during gametogenesis. On this basis “chromosome theory of heredity” was propounded.

iii) Flemming (1979) introduced the term chromatin for the deeply staining mate­rial of nuclei, and (1882) mitosis for the process of division.

Location of Chromosomes:

Chromosomes are universally present in the nuclei of living organisms. Most of the chromosomes in a cell are called auto­somes. In addition there is a pair of sex- chromosomes or heterosomes which carry the genes for determination of sex.

Number of Chromosomes:

In general, the chromosome set or Karyotype is constant for the somatic cells of the individual and for all individuals of the species, though nume­rous exceptions to both of these statements exist.

The lowest conceivable haploid number is, of course, one, which occurs in the horse nematode Parascaris equorum (= Ascaris megalocephala), but this is a compound chro­mosome which divides into as many as 190 chromosomes in somatic cells. The highest haploid number is 510 in the fern Ophiglossum petiolatum and approximately 800 in protozoa (Aulocanthe).

Shape of Chromosomes:

Chromo­somes are classified into four types by their shape in metaphase or in anaphase, which, in turn, is determined by the position of the centromere:

i) Telocentric chromosomes:

Rod-like and have a centromere situated on the proxi­mal end.

ii) Acrocentric chromosomes:

Rod-like and have a small, or even imperceptible, arm.

iii) Submetacentric chromosomes:

Chromosomes having unequal arms are thus ‘L’ shaped.

iv) Metacentric chromosomes:

Having equal or almost equal arms and thus are ‘V’-shaped.

Size of Chromosomes:

The length of chromosomes varies from 1 µ (some fungi) to 30 µ (Trillium). This excludes the salivary gland chromosomes of Diptera, which may be 2 mm long size of the chromosomes size may vary greatly in closely related genera.

For example, the chromosomes of Trillium are hundred times the size of the chromosomes of Madeola, a closely related genus. The fly chironomous Thumi thumi has 27% more DNA than Columba livia (Pigeon).

Chemistry of Chromosomes:

The structural units of each chromosome are DNA and protein. The substance composing chro­mosomes is known as chromatin—a nucleoprotein, composed of nucleic acids and pro­tein. There are two types of nucleic acids, DNA and RNA.

The proteins are also of two types:

i) Low molecular weight proteins: histones or basic proteins;

ii) High molecular weight proteins or non-­histones or acidic protein. The acidic pro­teins have enzymatic activities. The most important acidic proteins are DNA- polymerase, RNA-polymerase, DNA- pyrophosphorylase, nucleoside triphos­phatase.

Orientation of Chemical Components of Chromosomes:

1. Does the chromosome contain a single strand or multiple strand of DNA?

When the chromatin fibre is treated with trypsin, a protein digesting enzyme, a central axis remains. This axis is sensitive to treat­ment with DNA-ase and stains with uranyl accetate. So, the chromatin fibre consists of a DNA double helix to which is attached protein.

2. Is the DNA present as a continuous strand from one end of the chromosome to the other, or is it interrupted by linkers?

On the basis of data on DNA content of chromatids, it has been estimated that the largest human chromosome would contain a DNA helix 7.3 cms long, and the smallest chromosome a DNA molecule 1.4 cms long. By using auto-radiographic methods Sasaki and Norman (1966) have demonstrated DNA molecules 1 to 2.2 cms long in human cells.

Treatment of DNA filaments by protease failed to break up the filaments. The authors, therefore, con­cluded that if any linkers are present, they are resistant to protease. So, the DNA is present as a continuous thread in each chromatid.

On the other hand it has been shown that chro­mosomal DNA may consist of a series of seg­ments of replicons. Also, under certain conditions, the DNA may break into segments 100µ in length. In spite of this, however, there is no definite evidence of non- DNA linkers between DNA sub-units. Indications are that the mitotic chromosome is formed by coiling and folding of a single DNA molecule associated with protein.

3. How is the protein associated with the DNA molecule?

There are different interpretations as to how the protein is associated with DNA:

a) According to one interpretation, in the case of protamines the small groove of the DNA molecule is occupied by the polypeptide chains. Histones occupy both grooves of the DNA molecule.

b) The histone molecules are also repre­sented as bridges across the turns of the DNA supercoils.

c) According to Dupraw the DNA molecule is held in the supercoiled configura­tion by histones and other non-acidic linked proteins. These has been visualized as being wedge-shaped molecules with two binding sites each.

Physiology of Chromosomes:

Chromosomes are formed of strands of DNA molecules which contain information for the development of different characteristics and performance of various metabolic activities of the cells. The coordination of various functions is brought about through the formation of enzymes, which are complex protein molecules.

The information for the synthesis of these protein molecules is contained in the DNA molecules in the sequence of nitrogen bases. The sequence of the three nitrogen bases codes for one molecule of a particular amino acid, it constitutes a triplet codon. DNA of a particular cistron transcribes its m-RNA by their specific anticodons.

The amino acids are thus linked together forming polypeptide chain, which is finally changed into a functional protein either singly or by the associations of several polypeptide chains.

Conclusion:

Direct support for the idea that a chromosome contains only one DNA molecule has been obtained by Petes and Fangman (1972). It is evident that none of the data provides good evidence for multistrandedness. Protamines should occur in associa­tion with the chromosomes in the sperm of some animals, instead of the histone found in other nuclei, may be related to the smaller size of the protamine molecule allowing the chro­mosomes to become more compact.