CHROMOSOMAL ORGANISATION

CHROMOSOMAL ORGANISATION

INTRODUCTION

Chromosomes (Chroma-colour, soma-body) are DNA complexes which constitute hereditary structures because they carry coded genetic information that can be replicated, transmitted and translated. Chromosomes constitute the physical basis of inheritance.

Chromosome Numbers

All animals have a characteristic number of chromosomes in their body-cells called the diploid (or 2n) number.

These occur as homologous pairs, one member of each pair having been acquired from the gamete of one of the two parents of the individual whose cells are being examined.

The gametes contain the haploid number (n) of chromosomes.

CHROMOSOME STRUCTURE

• For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.
• Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).
• As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.
• These duplicated chromosomes are called dyads.

• When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.
• While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bonafide chromosome with a full complement of genes.
• The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.
• The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.

Detailed structure of chromosome

Chromatid - At metaphase each chromosome consists of two symmetrical structure,the chromatids,each one of which contains a single DNA molecule.The chromatids are attached to each other only by centromere and become separated at the start of anaphase, when sister chromatids migrate to opposite poles.

Chromonema – During prophase and also interphase, the chromosomal material becomes visible as a very thin filament which are called chromonemata and which represents chromatid in early stages of condensation. It is a single linear DNA molecule with its associated proteins.

Centromere/Primary constriction – This is the region of chromosome that becomes attached to the mitotic spindle. The centromere lies within a thinner segment of the chromosome, the primary constriction. The regions flanking the centromere frequently contain highly repetitive DNA and stain more intensely with basic dyes (heterochromatin). Centromere contains specific DNA sequences with special protein bound to them forming a disc shaped structure called as kinetochore.

According to the location of the centromere on the chromosomes can be-

i) Metacentric- When two arms are equal, centromere is in the centre .

ii) Sub-metacentric-If centromere is near the centre.

iii) Acrocentric- If centromere is near the end.

iv) Telocentric- If the centomere is truly terminal.

Kinetochore – In thin electron microscopic section the kinetochore show a trilaminar structure, with a dense outer proteinaceous layer, a middle layer of low density and a dense inner layer tightly bound to the centromeric DNA. The function of the kinetochore is to provide a centre of assembly for microtubules.

Chromomere – These components are beads like accumulation of chromatin material that are sometimes visible along interphase chromosomes. Chromomeres are especially present in polytene chromosomes where they become aligned side by side, constituting the chromosome bands. These tightly folded regions of DNA are of great interest, as they may correspond to the units of genetic functions in the chromosome. At metaphase, chromomeres are not visible as chromosome is tightly coiled.

Telomere – This term refers to the tips of the chromosomes. Telomeres have special properties that when chromosomes are broken, the free end without telomeres fuse with other broken chromosomes; however they do not fuse with a normal telomere. Telomeres contain the ends of the long linear DNA molecule contain in each chromatid and therefore have an unusual DNA structure.

Secondary constrictions - A secondary constriction in the chromosome is a constriction other than the centromere. It is often associated with the nucleolus during interphase and may take part in the reorganization of the nucleolus at the end of cell division. Secondary constrictions are constant in their position and extents, and therefore useful in identifying particular chromosomes in a set. Secondary constrictions are distinguished from the primary constriction by the absence of marked angular deviation of the chromosomal segments during anaphase.

Nucleolar Organizers – These areas are certain secondary constriction that contains the gene coding for 18S and 28S ribosomal RNA and that induce the formation of nucleoli.

Satellites - Satellite is a rounded body separated from the rest of the chromosomes by a secondary constriction. The satellite and constriction are constant in shape and size for each particular chromosome.