Ultrastructure

A chromatide is composed of a very fine filament the chromonema. Chromonema is a single double stranded DNA molecule, which extends from one end of the chromosome to the other and is greatly coiled. The DNA is associated with proteins. This – one stranded or unineme of the chromosome structure has replaced the earlier many stranded or polyneme view.

Chemical position. The eukaryotic chromosomes are composed of DNA, proteins (histones and acid or neutral nonhistoness, metal ions and enzymes (DNA polymerase, RNA polymerase, nucleoside triphosphatase).

Histones are low-molecular weight proteins rich in the basic amino acids lysine and arginine.

The DNA and histones are loosely bound together in 1:1 ratio form deoxyribonucleoproteins (DNP), also called chromatin. The histones prevent RNA transcription.

Nonhistone proteins are high–molecular weight proteins having amino acids tyrosine and tryptophan. They are believed to activate the genes to start RNA transcription.

4. Types of chromosomes. The chromosomes are classified on the basis of the position of centromere and number of the centromeres, and size of the arms.

There are 4 types of chromosomes:

Metacentric – the centromere is at the middle of the chromosome, and the arms are equal. The chromosomes looks V-shaped in anaphase.

Submetacentric. The centromere is near the centre of the chromosome, and the arms are slightly unequal. The chromosomes appears L-shaped in anaphase.

Acrocentric. The centromere is near one end of the chromosome, and one arm is very short, other arm is very long.

Telocentric. The centromere is at one end of the chromosome, and the arms are on one side only.

There are 4 types of chromosomes:

Monocentric with a single centomere. This is the most common type.

Dicentric with two centromeres, e.g. maize.

Polycentric with many centromeres, e.g. the roundworm Parascaris equorum.

Acentric without a centromere, also called holocentric as the entire surface functions as a centromere, very rare.

Number. The number of chromosomes varied from two, to a few hundred in different species. All individual of a species, however have the same number of chromosomes in all body cells. (but not in gametes). Man – 46; chimpanzee- 48, gorilla – 48, horse – 64, cow – 60; dog -78; cat-38; rat-42; rabbit 44; frog – 26.

Protists – Amoeba proteus – 250; garden pea – 14; maize – 20 rice – 24; wheat – 42; tomato-24; potato – 48; onion – 16; cabbage – 18.

Homologous chromosomes. There is always a pair of chromosomes of each kind.

A pair of chromosomes have similar genes, size, shape, position of centromere.

A gamete, sperm or egg, has a single set of chromosomes, it has one chromosome of each homologous pair. This chromosome number is called haploid number or reduced number. It is denoted by the symbol n or 1n. it is 23 in man.

The somatic, or body cells have double set of chromosomes, they have both the chromosomes of each homologous pair. This chromosomes number is termed the diploid number, or full number. It is indicated by the symbol 2n. It is 46 in man. Diploid number results from the union of sperm and ovum in sexual reproduction.

A cell may have more then two sets of chromosomes. Such cell is said to be polyploidy. Polyploidy is common in mosses, ferns and cultivated plants. A cell of banana is triploid (3n), of rice tetraploid (4n), wheat hexaploid (6n).

All of genes in one or haploid set of chromosomes are referred to as a genome.

Not all organisms are diploid. Many lower plants such as mosses, many algae and fungi are haploid. In honeybee, the female are diploid and the males are haploid (also called monoploid) because they developed parthenogonetically from unfertilized eggs.

Size. All the chromosomes of a species are not of the same size. The metaphase chromosomes usually vary from 0.1 to 33 mm in length and from 0.2 to 2 mm in thikness.

In diploid organisms, with separate sexes, a specific pair of chromosomes determine the sex of the individual. They are called sex chromosomes or heterosomes. All other chromosomes are termed autosomal chromosomes, or autosomes. The 2 members of each pair of homologous autosomes a similar in size, shape and carry genes which control the somatic traits and have no bearing in the sex.

Polytene chromosomes. These are found in the salivary gland cells of the fly larvae. However, they are also found in the cells of many other organs of the fly larvae, in certain cells of adult flies and in some cell of others insects. I plants, they occur in endosperm cells, antipodal cells of embryo sac and suspensor cells of embryo. The polytene chromosomes were first reported by E.B. Balbiani in 1881. They have numerous (512-1024 in fruit fly) chromonemes, they result from repeated replication of DNA without separation into daughter chromosomes, hence called polytene chromosomes.

At certain times, one or more bands of a polytene chromosomes temporarily swell up, forming enlargements termed puffs or bulbs. They large puffs are know as Balbiani rings. The puffs of Balbiani rings have uncoiled active DNA strands that transcribe RNAs (m RNA, rRNA, tRNA)

Karyotype – it is the chromosome complement of a cell or organism depicting the number, size and form of the chromosomes as seen in metaphase of mitosis.

The normal diploid (2n) chromosome number in human beings is 46. Each cell of our body contains 23 pairs of chromosomes. These include 22 pairs of autosomes and one pair of sex chromosomes.

- The autosomes are similar in males and females.

- The sex chromosomes are homomorphic (similar) in the female and are called XX. The sex chromosomes a heteromorphic (dissimilar in the male) and are known as XY. The Y chromosome is a much smaller than X chromosome.

The photographs or diagrams of chromosomes are artificially arranged in order of their size, except the sex chromosomes that a placed at the end. Such a pictorial representation of all the chromosomes of an organism is called an idiogram.

The human autosomes were numbered and distinguished morphologically into 7 groups, A to G, on basis of relative size and position of centromere by a conversion of excerpts held at Denver, Colorado (USA) in 1960. (Denver classification)

1. Group A. It includes the chromosomes 1-3. These are largest in size and have median centromere and equal arms (metacentric)

2. Group B. It includes the chromosomes 4 and 5. There are next largest in size and have submedian centromere and unequal arms (submetacentric)

3. Group C. It includes the chromosomes 6-12. These are medium-sized and have submedian centromere and unequal arms (submetacentric)

4. Group D. It includes the chromosomes 13-15. These are shorter than the chromosomes of group C, acrocentric. They are sat chromosomes as they bear satellites.

5. Group E. It includes the chromosomes 16-18. These are short-sized chromosomes. And have median or submedian centromere and equal or unequal arms (metacentric or submetaentric)

6. Group F. It includes the chromosomes 19-20. These are short - sized and have median centromere and equal arms (metacentric)

7. Group G. It includes the chromosomes 21-22. These are smallest in size and acrocentric. They are also called sat chromosomes as they carry satellites.

The X chromosome is placed in group C in view of its large size and submedian centromere.

The Y chromosome is placed in group G as it is very short and has terminal centromere.

Each chromosome pair has a distinct morphology as regards length of the arms and presents of satellites, size, shape, and position of the centromere. On this basis, it can be distinguished from other pairs of chromosomes.

Since 1969, special techniques have been developed to stain the human chromosomes with fluorescent dyes after various treatments.

In 1971 in Paris (France) cytologists on basis of special differential techniques with different treatments, the chromosomes show different banding patterns (alternated staining and sustaining regions). It is noted that each chromosome shows a unique banding pattern with a particular treatment, and its pattern remains constant with the same treatment. The banding patterns help identifying the various regions of individual chromosomes. Four kinds of banding pattern of chromosome are sow far known. These are named Q (Qui-nacrine), G (giemsa), R (revers of giemsa) and C (constitutive heterochromatin) patterns according to treatment given to the chromosomes.

Cytological studies of human chromosomes have shown a correlation between various congenital defects of individual and abnormalities in chromosome number and structure.

Location of genes:

13q – Retinoblastoma.

2q37 – BDE, BMDR – brachydactily

5q31-q33 – DFNA1, LFH – deafness

Xq 26.3 – ADFN, ALDS – albinism

5. Reproduction is the ability of living organisms to produce a new generation of living individuals similar to themselves. It is one of the fundamental characteristics of life.

Reproduction is aimed to increasing population and preparation of the species.

All organisms reproduce. Modes of reproduction vary in different organisms. However, all models have certain common basic features. These are:

1. Replication of DNA. This is a molecular basis of reproduction.

2. Cell division, only mitotic, or both mitotic and meiotic.

3. Formation of reproductive bodies or units.

4. Development of reproductive bodies into offspring.

There are 2 main types of reproduction: asexual and sexual.