Eukaryotic chromosomes are circular in structure. Eukaryotic chromosomes are composed of DNA and RNA. Here we discuss what is Eukaryotic chromosomes basic unit.
Eukaryotic chromosomes
DNA is the genetic material, and it exists with protein in the form of chromosomes in eukaryotic cells. During most of the life of a cell, chromosomes are in a highly dispersed state called chromatin. During these times, units of inheritance called genes (Gr. genos, race)may actively participate in the formation of protein. When a cell is dividing, however, chromosomes exist in a highly folded and condensed state that allows them to be distributed between new cells being produced. The structure of these chromosomes will be described in more detail in the discussion of cell division that follows.
Chromatin consists of DNA and histone proteins. This association of DNA and protein helps with the complex jobs of
packing DNA into chromosomes and regulating DNA activity.There are five different histone proteins. Some of these proteins form a core particle. DNA wraps in a coil around the proteins, a combination called a nucleosome .The fifth histone, sometimes called the linker protein, is not needed to form the nucleosome but may help anchor the DNA to the core and
promote the winding of the chain of nucleosomes into a cylinder.
Further folding and the addition of protective proteins result in
the formation of chromosomes during mitosis and meiosis.Not all chromatin is equally active. Some human genes, for
example, are active only after adolescence. In other cases, entire chromosomes may not function in particular cells. Inactive portions
of chromosomes produce dark banding patterns with certain staining procedure and thus are called heterochromatic regions, whereas active portions of chromosomes are called euchromatic regions.
Sex chromosomes and autosomes
In the early 1900s, attention turned to the cell to find a chromosomal explanation for the determination of maleness or female-ness. Some of the evidence for chromosomal basis for sex determination came from work with the insect Protenor. One darkly staining chromosome of Protenor, called the X chromosome, is represented differently in males and females. All somatic (body)cells of males have one X chromosome (XO), and all somatic cells of females have two X chromosomes (XX).
Autosome:
Similarly, half of all sperm contain a single X, and half contain no X, while all female gametes contain a single X. This pattern suggests that fertilization involving an X-bearing sperm will result in a female offspring and that fertilization involving a sperm with no X chromosome will result in a male offspring.
Sex chromosome:
Chromosomes that are represented differently in females than in males and function in sex determination are sex chromosomes. Chromosomes that are alike and not involved in determining sex are autosomes (Gr. autus, self soma, body).
The system of sex determination described for Protenor is called the X-O system. It is the simplest system for determining sex because it involves only one kind of chromosome. Many other animals (e.g., humans and fruit flies) have an X-Y system of sex determination.
In the X-Y system, males and females have an equal number of chromosomes, but the male is usually XY, and the female is XX. (In birds, the sex chromosomes are designated Z and W, and the female is ZW.) This mode of sex determination also results in approximately equal numbers of male and female offspring:
SpermX YEgg X XX XY1 female : 1 maleN
Equal no of chromosomes
NUMBER OF CHROMOSOMES
Even though the number of chromosomes is constant within a species, chromosome number varies greatly among species.
Chromosomes are present in sets, with the number in a set being characteristic of each kind of animal and expressed as “N.”N identifies the number of different kinds of chromosomes.
Most animals have two sets,or 2 Nchromosomes. This is the diploid (Gr. di, two eoides, doubled) condition.Some animals have only one set, or N chromosomes (like gametes) and are haploid
(Gr. hapl, single) (e.g., male honeybees and some rotifers).Very few animals (e.g., brine shrimp, snout beetles, some flatworms, and some sow bugs) have more than the diploid number of chromosomes, a condition called polyploidy (Gr. polys,more). The upset in numbers of sex chromosomes apparently in-
terferes with reproductive success. Asexual reproduction often accompanies polyploidy.
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