The Chromosomes are thread-like and rod bodies located in the nucleus of a cell.
The chromosomes houses include the genes that are mainly responsible for the transmission of characters from parents to the offspring.
Chromosomes can be observed with a microscope only at the time of cell division (mitosis or meiosis). They appear at the starting of cell division as long slender threads. As cell division steps forward, they shorten (or condense) and thicken. After a short time, each and every chromosome is seen to be made up of the two threads termed as chromatids, held altogether at the centromere.
Number of Chromosomes:
All organisms of similar species encompass the similar number of chromosomes in all body cell. The number of chromosomes in a body cell is recognized as the diploid number is twice the number of chromosomes in a gamete termed as the haploid number. The diploid number is symbolized by 2n whereas the haploid number is symbolized by n. A few illustrations of chromosome number in body cells are: fruit fly (drosophila 8 (4 pairs), housefly 12 (6 pairs), garden pea 14 (7 pairs), domestic fowl 18 (9 pairs), cat 38 (19 pairs), man 46 (23 pairs) and dog 52 (26 pairs).
In human organisms, there are naturally 46 chromosomes which take place as 23 pairs (23 x 2 = 46). Each and every chromosomes of a pair occupies the similar position and is similar in size and shape to the other. Each pair is thus termed as homologous chromosomes and only separate at the time of meiosis. Chromosomes are numbered in multiples of 23 while is symbolized as n; that is, 23 = n. Thus, the 46 chromosomes are symbolized as 2n, that is, 2 x n = 2 x 23 = 46. As 46 chromosomes are twice n, it is termed as the diploid number of chromosomes whereas n (23) is termed as the haploid number.
Structure of Deoxyribonucleic Acid (DNA):
DNA is found in the chromosomes positioned in the nucleus of cells. It stores the organism's hereditary traits or features and directs the daily metabolic actions of each cell in the organism. The chromosome is build up of protein and deoxyribonucleic acid (DNA). The DNA molecule comprises of two helical chains coiled around one other to make a double helix. The deoxyribose nucleic acid is a very big molecule made up of repeating units termed as nucleotides.
Each nucleotide is build up of:
a) Deoxyribose, that is, a sugar molecule phosphate.
b) An organic nitrogen compound that might be guanine, adenine, thymine and cytosine.
The replicating units or chains are set in the form of a double helix and are held altogether by hydrogen bonds. The sides of the helix are made by phosphate and sugar. The step in the ladder is made by the organic nitrogen compounds in a definite manner. That is, the two chains are held altogether by hydrogen bonds among the nitrogenous bases. Guanine for all time pairs with cytosine and adenine with thymine.
The two chains are as well termed to as complementary strands of DNA as one is the exact opposite of the other. Guanine is the opposite of cytosine and adenine is the opposite of thymine. Therefore, when guanine is present on a complementary DNA strand, it is cytosine which will occupy the similar position on other strand and bind with it. Likewise adenine will bind by thymine.
Replication of DNA
DNA can replicate itself at the time of cell division and is solitary material which is passed on from parents to offspring. A DNA strand is held altogether through weak hydrogen bonds among the complementary base pairs. Such bonds can break simply, causing the DNA to separate or 'unzip' into two half-strands. Free nucleotides present in the nuclear material then organize themselves all along each half-strand. Hydrogen bond form among the complementary bases, resultant in the two similar DNA strands.
Role of Chromosome in the transmission of Hereditary Characters:
1) A gene is positioned in chromosome that is responsible for the expression of a character therefore genes are accountable for the transmission of hereditary traits from parents to offspring.
2) Chromosomes in a body cell are organized in pairs. In each and every pair, the chromosomes are precisely similar in size and shape and they carry the genes responsible for the transmission of the similar feature organized in the similar order all along their length.
3) The genes associating to the similar character like tallness in pea plant (T) and shortness (t) occupy similar places or loci on the chromosomes. The pairs of chromosomes which are similar are termed as homologous chromosomes.
4) The genes on a homologous chromosome whether individual will be heterozygous or homozygous for certain characters, example: chromosomes associating to the similar character are similar that is, T and T for tallness, the individuals are stated to be homozygous for that character however if they are dissimilar, that is, T for tallness and t for shortness, the genes are stated to be allelic or alleles and the individual is stated to be heterozygous for that character.
Methods of Transmission of Hereditary Characteristics by Chromosomes:
1) The chromosomes bring the genes which pass into the gamete all through meiosis.
2) Throughout the first phase of meiosis, the homologous chromosomes split into two daughter cells.
3) In the second phase of meiosis, the two chromatids in every chromosomes separate. This procedure brings about the segregation of genes. Each gamete thus can just include one gene for one of a pair of contrasting trait.
4) At the time of fertilization, the gametes fuse altogether. The zygote obtains two genes for the similar character - one from a chromosome in the egg from female parent and another from one chromosome in the sperm from the male parent.
5) If the two genes are similar, the offspring is homozygous however if they are different, the offspring is a hybrid (that is, heterozygous) with respect to that character.
6) Gene in a chromosome dictates the development of a protein, generally an enzyme that influences the formation of cell products which bring about the expression of a character example: skin color and so on.
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