Nuclues has DNA molecule in it. It is packaged into thread like structure known as chromosomes. Each and every chromosome is made up of DNA tightly coiled many times around proteins which is known as histones. Histones supports the structure. Chromosome cannot be visible even under the microscope if the cells are not dividing.
DNA consist of 2 long polynucleotide which is composed of 4 types of nucleotide units. Nucleotides are composed of 5 carbon sugar attached to the phosphate group and a nitrogen containing base. The base might be Adenine,Cytosine,Guanine,or Thymine. Nucleotides are linked together by covalent bond through the sugar and phosphate. The 3d structure of DNA is the double helix. It is formed from the 2 polynucleotide chains. Both chains are held together by hydrogen bonding. Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The discovery of the structure of the DNA double helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section, and we shall examine them in more detail in subsequent chapters.
DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base—A, C, T, or G—can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. Organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages.