General Knowledge on Chromosomes – Notes
Chromosome GK Notes for your competitive examinations for government jobs such as IAS, UPSC, Railway, Banking, SSC, CGL etc…
Chromosomes are small but thick, rod like darkly stainable structures. These are seen in the nuclei of dividing cells only at metaphase and anaphase stage of cell division. Whereas in non-dividing cells they are seen as a network of thread-like structures called chromatin reticulum.
Chromosomes were first discovered Hofmeister in 1848. He noticed them in a rapidly dividing cells of Tradescantia in 1888 gave the name chromosomes in this nuclear filaments.
The number of chromosomes always remain constant in the cells of each species. Depending upon the number of chromosomes, the cells are said to be haploid (n) or diploid (2n).
A cell said to be haploid when all the types of chromosomes found in it are present in a similar set. Similarly, an organism is said to be haploid when its body cells are haploid. On the other hand, a diploid cell contains two sets of all the types of chromosomes found in it or can say that the chromosomes are present in pairs. The two chromosomes of one pair of exactly similar in the shape, size structure are bear genes controlling same characters on the same loci. Such chromosomes are called homologous chromosomes. Out of each pair of homologous chromosomes, one is shared the female parent (Maternal chromosomes) and other shared by male pare (Paternal chromosomes).
|Sl No.||Specis||Chromosome Number (2n)|
|1||Ascaris||2 = 1 pair|
|2||Drosophila||8 = 4 pairs|
|3||Frog||26 = 13 pairs|
|4||Lion, Tiger, Domestic cat||38 = 19 pairs|
|5||Rat||42 = 21 pairs|
|6||Rabit||44 = 22 pairs|
|7||Man||46 = 23 pairsq|
|8||Gorila||48 = 24 pairs|
|9||Fowl, Dog||78 = 39 pairs|
|10||Mouse||40 = 20 pairs|
|11||Maize||20 = 10 pairs|
|12||Onion||16 = 8 pairs|
|13||Garden pea||14 = 7 pairs|
|14||Crayfish||200 = 100 pairs|
|15||Ophioglossum (Fern-plant)||1262 = 631 pars|
The diploid number of chromosome of some species are shown above the table. On screen this table this is clear that the diploid number is always paired. Moreover, it is very clear that the characters of an individual do not depend upon the number of chromosomes e.g. Fowl and dog are totally different but the number of chromosomes is the same in both of them. Similarly the complexity of an individual is also free of the number of chromosomes e,g, protozoans are the most simple creatures but they possess numerous chromosomes. The characters and complexity of an individual rather depend upon the types of genes present on the chromosomes.
Structure of Chromosomes
Chromosomes are highly condensed chromatin fibres formed at the prophase stage of cell division. The structure of chromosome is best studied either during metaphase or anaphase of cell division. During metaphase each chromosome consists of two exactly similar folds called chromatids. The two chromatids are joined together at a point called centromere. Parts of chromatids on the two sides of centromere are called arms. In ach chromatids is present a highly coiled thread of DNA and proteins called chromonema. The chromonema bears darkly stainable granules called chromomeres. The chromonema is called chromomeres. The chromonema is surrounded by matrix which is covered by a covering called pellicle.
Some chromosome possess a small fragment or knob like structure called satellite. Such chromosomes are known as SAT chromosomes. The terminal regions on either side of the chromosomes are known as telomeres. The chromosomes consist of about 40% DNA and about 60% histones (a particular type of protein).
The histone and DNA form a kind of complex in which the DNA strands wind around a core of eight histone molecules. Each such complex is called nucleosome. A single human chromosome may have about one million nucleosomes.
Nucleic acid : Chemical analysis of chromosomes shows two types of nucleic acids DNA and RNA. They were first discovered by Frederick Miescher. Nucleic acids have complex chemical structures composed of pentose sugars., nitrogenous bases and phosphoric acid.
The DNA (Deoxyribonucleic acid) is a very large molecule (Macromolecule). It is composed of two strands, coiled around a common axis in a double helix. The double helical structure of DNA molecule was shown in 1953 by James. D. Watson and Francis H.C Crick.
They inferred that in each DNA molecule there are two long and parallel strands or chains helically coiled around the same axis giving an appearance of spiral staircase.
The DNA molecules contain four nitrogenous bases. There are
In these nitrogenous bases, guanine is complementary to cytosine and thymine to adenine. The DNA and RNA differ from each other as follows
|Pentose sugar is deoxyribose.||Pentose sugar is robose|
|Mostly double stranded||Mostly single stranded|
|Nitrogen bases present||In RNA in place of thymine, Uracil is present|
|Concerned with genetic information.||Concerned with syntheses of proteins|
Genes are the specific parts of a chromosome composed of DNA. They determine the hereditary characters. They are responsible for the transmission of characters from one generation to another. They are located in chromosomes and occupy a fixed position in the chromosomes. They are arranged in a single linear order in chromosome. Scientists have identified around 30,000 genes in human chromosomes.
Function of chromosome :
Chromosomes form the only link between the two generations.
Sex chromosomes (XX and XY) determine the sex of individuals.
Through the process of crossing over chromosomes induce variations.
Chromosomes undergo mutations (suddenly changes) and contribute towards evolution a life.
They control cell metabolism
They control the synthesis of structural proteins which help in cell division, cell repair and cell growth.
Types of Chromosomes
The position of centromere varies is different chromosomes. The two arms of a chromatid may be equal or unequal depending upon the position of centromere. Based upon the position of centromere the chromosomes are categorized into four types :
Metacentric : The centromere is present in the centre and the two arms of the chromatid are equal. During anaphase such chromosomes appear “V“shaped.
Submetacentric : Centromere is slightly away from the centre so that one arm is slightly larger than the other. Such chromosomes are “L” shaped during anaphase.
Acrocentric : The centromere is present near one end of the chromatid so that one arm is larger than the other. Such chromosomes look “J” shaped during anaphase
Telocentric : The centromere is present at one end of the chromosome so that it looks “I” or rod shaped during anaphase.
Sex Chromosomes and autosomes.
The chromosomes which determine the sex (maleness or femaleness) of an individual in sexually reproducing organisms are called sex chromosomes or allosomes or idiosomes. For example, in human beings an individual is female if its cells contain XX chromosomes and male if its cells contain XY chromosomes.
All the chromosomes other than sex chromosomes are always found in pairs of homologous chromosomes in diploid cells and are known as autosomes. For example, in human cells 22 pairs of chromosome are autosomes. They determine the characters other than sex.
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