of a chromosome. The most widely Centromere Short arm Long arm Centromere Equal arm Figure . Types of chromosomes based on position of centromere Genetics * Cytosine (C) links Guanine (G) with three hydrogen bonds( C ≡ G) This is called complementary base pairing . Figure .
Structure of DNA . Hydrogen bonds between the nitrogenous bases make the DNA molecule stable. . Each turn of the double helix is A° ( .
nm). There are ten base pairs in a complete turn. . The nucleotides in a helix are joined together by phosphodiester bonds.
Figure . Nucleotides in a DNA More to Know Chargaff rule of DNA base pairing Erwin Chargaff states that in DNA, the proportion of adenine is always equal to that of thymine. and the proportion of guanine always equal to that of cytosine. accepted model of DNA is the double helical structure of James Watson and Francis Crick .
They proposed the three-dimensional model of DNA on the basis of X-ray diffraction studies of DNA obtained by Rosalind Franklin and Maurice Wilkins. In appreciation of their discoveries on the molecular structure of nucleic acids Watson, Crick and Wilkins were awarded Nobel prize for Medicine in . Chemical Composition of DNA molecule DNA is a large molecule consisting of millions of nucleotides. Hence, it is also called a polynucleotide .
Each nucleotide consists of three components. . A sugar molecules – Deoxyribose sugar. .
A nitrogenous base. There are two types of nitrogenous bases in DNA. They are (a) Purines (Adenine and Guanine) (b) Pyrimidines (Cytosine and Thymine) . A phosphate group Nucleoside and Nucleotide Nucleoside = Nitrogen base + Sugar Nucleotide = Nucleoside + Phosphate The nucleotides are formed according to the purines and pyrimidines present in them.
. . Watson and Crick model of DNA . DNA molecule consists of two polynucleotide chains.
. These chains form a double helix structure with two strands which run anti-parallel to one another. . Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.