All the organisms in the world are characterized with their own unique features. Features of individual organisms pass to the next generation and evolve in them. This natural principle is applied to all organisms in the world. This is why we notice rice plant grows from rice seed, mangoes from their seeds; jute does also this thing in the same way. This way the characteristics of species are maintained in offspring through generations. Heredity is the passing of traits from father and mother to their offspring through generations. In depth discussions and researches concerning heredity are done in the special branch of biology called genetics. Work: Identify the similar and dissimilar traits from parents and place them in a chart with a nice presentation. Components carrying (heredity materials) behavioural features to the offspring from generation to generation The features of parents are transferred to their offspring through heredity materials. These are chromosomes, genes, DNA and RNA. These are discussed below: Chromosome: The main heredity material is chromosome. It is spread in the nucleoplasm of a nucleus as a thread like structure named chromatin. The scientist Strasburger in 1875 first discovered chromosome. In different species the number of diploid set of chromosome (two sets chromosome, one set is from father and the another set is from mother) may be 2 to 1600. In length a chromosome may be form 3.5- 30.00 micron (1micron=1/100mm) and in width 0.2 to 2.0 micron. The function of chromosomes is to carry genes (which control the characteristics of organisms) to the offspring from parents. Colours of human eyes, nature of hair, compositions of skin etc. continue intact through the flow of heredity carried by chromosomes. This is why chromosomes are designated as the physical basis of heredity. DNA The main component of chromosome is deoxyribonucleic acid. It is usually a double stranded spiral structure of polynucleotides. A strand is complementary to the other. In it there are five carbon sugars, nitrogen bases (adenine, guanine, cytosine, thymine) inorganic phosphate. These three components collectively are called a nucleotide. DNA is a stable substance in a chromosome. American scientist Watson and British scientist Crick in 1953 described first the double helical structure of DNA and for this contribution both of them won Nobel Prize. Nitrogen bases are of two types, such as, purines and a pyrimidine. Adenine (A), guanine (G) are purines and cytosine (C) and thymine (T) are pyrimidines. Adenine of a strand bonds with a thymine of another strand by two hydrogen bonds, and guanine of a helix connects with a cytosine of another helix by three hydrogen bonds. This bond is always developed in between a purine and pyrimidine. So a strand of DNA is complementary to another strand but not just as the same. A complete twist in a helix is 34A0 long and in a complete twist, there are ten nucleotides. So the length between the two adjacent nucleotides is 3.4A0. Two strands ofpolynucleotides in DNA are positioned antiparallelly. It mostly looks like some rungs in a twisted ladder. Bases are connected flatly and horizontally in the position from the main axis. External two strands or two axis of DNA are composed of consecutive arrangement of sugar and phosphate, and internal nitrogen bases are plainly aligned. In fact, DNA is a precise thread but in prokaryotic cell DNA is usually circular and the surface area would be some microns to centimeters. It is composed of thousands of nucleotides and the diameter of the twisted helical structure in everywhere is 20A0. DNA is the main component and chemical carrier of heredity. DNA is the true structure and carrier of the behavioural characters of organisms and it directly carries the characters of parents to their offspring from generation to generation.
Posted on: Tue, 24 Sep 2013 03:52:52 +0000
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