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.
As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins . This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. The properties of a protein, which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed.
The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the four-letter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—the genetic code—is not obvious from the DNA structure, and it took over a decade after the discovery of the double helix before it was worked out. This code in detail in the course of elaborating the process, known as gene expression, through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.
Transcription is the process by which DNA is copied (transcribed) to mRNA, which carries the information needed for protein synthesis. Transcription takes place in two broad steps. First, pre-messenger RNA is formed, with the involvement of RNA polymerase enzymes. The process relies on Watson-Crick base pairing, and the resultant single strand of RNA is the reverse-complement of the original DNA sequence. The pre-messenger RNA is then "edited" to produce the desired mRNA molecule in a process called RNA splicing. The mechanism of transcription has parallels in that of DNA replication. As with DNA replication, partial unwinding of the double helix must occur before transcription can take place, and it is the RNA polymerase enzymes that catalyze this process.
Unlike DNA replication, in which both strands are copied, only one...