Chemically genes can be DNA or RNA but in most life forms DNA is the genetic material. The genetic substances not only controls the inheritance of traits or characters from one generation to the next, but is also able to express its effect through the formation and functioning of traits.
The basic concept of heredity is the ability of cells to use the coded information in their genetic materials to produce particular proteins, which there by determine the behavior of the cell. In fact proteins are the tools of heredity. It is important to note that a gene can be responsible for the synthesis of a polypeptide or for the synthesis of a tRNA or rRNA. All these three functions together determine the characters of the organism. The relationship between the genes and the characters can be summarized as: Genome to phenome concept.
It is evident that cells use RNA to make proteins. Hence gene expression involves two steps: transcription (RNA making) and translation (Protein making). In the first step (transcription) the coded message present in the DNA as the nucleotide sequence is passed on to the mDNA (messenger RNA). In the second step of translation, the coded message in mRNA is translated into the language
of polypeptides. The mRNA simply acts as a messenger recruited by DNA to carry the genetic information to the site of protein synthesis, i.e, and the ribosomcs. Initially most of the information about gene expression were gathered from prokaryote (E.coli) and gradually with developed techniques information about eukaryotic gene expression were obtained from Yeast, Arabidopsis thaliana and other eukaryotes. Central Dogma
The flow of genetic informations from DNA through DNA to protein and the transmission of characters through replication of DNA are known as central dogma in molecular biology. (Fig.8.9)
This central dogma holds good for most of the cellular genes. But some viruses like Tobacco Mosaic Virus have DNA instead of DNA as the genetic material. Further two other information path ways have been discovered. These are: (1) DNA dependant DNA synthesis or DNA replication and (2) DNA dependant DNA synthesis or reverse transcription.
The first step in the genetic information path way or the central dogma is the transcription. In this step the genetic information from DNA is passed to RNA through enzymatic synthesis of a RNA molecule on a DNA template. Generally we mean the synthesis of mDNA as transcription but the synthesis of rDNA and tRNA is also transcription. For a particular DNA (either mDNA or rDNA or tDNA) in a defined sequence of DNA acting as gene only one of the two strands of the defined DNA sequence act as the template strand. This template strand is known as template / sense / plus (+) strand. On the template strand RNA is synthesized basing on the principle of base complementarity except for Uracil (U) in place of Thymine (T).
The other strand in that particular DNA segment is known as coding / nonsense / minus (-) strand for that particular RNA. It may be noted that for any other DNA for which the gene is read in reverse direction this template strand becomes coding strand and the coding strand becomes the template strand.
The synthesized DNA is a copy of the coding strand except for the presence of Uracil in place of Thymine of coding DNA strand. Two types of genes are transcribed; (1) RNA genes and (2) structural genes. The RNA genes transcribe tRNA and rRN’A and the structural genes are transcribed into mDNA which are translated into polypeptides. The segment of DNA that takes part in transcription is known as transcription unit.
This unit has three components; (l) the promoter sequence, (2) the structural genes or the RNA genes and (3) the terminator sequence. The eukaryotic transcription may require other sequences like enhancer, silencer etc. The promoter sequence precedes the structural gene on the
coding strand, i.c, towards the 5′-cnd of structural gene or upstream side of structural gene (which is .3′-end of template strand)
The promoter is the sequence to which the transcribing enzyme DNA polymerase (DNA Pol) binds. This sequence is known as TATA box or Pribnow box in prokaryotes and TATA or Ilogncss box in eukaryotes.
The enzyme that carrics out transcription is known as DNA dependant RNA polymerase or simply RNA polymerase (RNA pol). Like the DNA pol RNA pol catalyzes polymerization of ribonucleotides basing on complementarity of the template strand.
The new ribonucleotides arc added to the open 3′-end so that the RNA is always synthesized in 5′-end to 3′-end. The only difference is Uracil incorporated to growing RNA instead of Thymine. The RNA pol does not require a primer unlike DNA polymerase.
RNA Pol. is purified and its structure and function is known in detail. It consists of five polypeptide chains: 2 chains of a-polypeptides, one chain each of (3, P2 and 5. Thus the enzyme can be represented as o^P^. The association of o factor is not very firm and it dissociates from the core enzyme, ie, oc2P, P2 – The core enzyme associated with a factor is known as holoenzyme. The core enzyme alone is capable of polymerization of ribonucleotides but sigma factor is required to initiate the RNA transcription at the correct site. The core enzyme has two ribonucleotides binding sites: (1) Initiation site and (2) Elongation site.