It has been Commonly observed that:
(1) In EuKaryotes, DNA the genetic material is largely restricked to the nucleus and Proteins are synthesized in association with ribosomes formed of RNA in the cytoplasm. Therefore DNA does not participate in protein synthesis directly.
(2) The amount of Protein in a cell is generally proportional to the amount of RNA but not to the amount of DNA.
(3) In EyKaryotes the cellular RNA is synthesized in the nucleus where DNA is found.
It has been suggested that an intermediate RNA molecule is involved in Protein synthesis. The genetic information stored in DNA is transferred to RNA during the initial stage of gene expression. The process by which RNA molecules are synthesized on a DNA tempelate (blue print or guide) is called “Transcription”.
This RNA is called mRNA (messenger RNA) or transcript, because it contains message which directs ribosomes to assemble amino acids in a order which is specific for a given kind or Protein.
EVIDENCE OF EXISTENCE OF mRNA:
Two important evidences suggest the existence of mRNA. These are:
(a) DNA-RNA complementarity: It has been show that the base pairs of RNA produced by various organisms were present in the same ratios as in their DNA from where these are synthesized i.e. from template DNA.
(b) DNA-RNA hybridization: Hall, Spiegelman and other denatured DNA by heating. They found that two strands of double helix separate. When the solution is cooled certain proportion of DNA strands rejoined and rewound. This suggest that complementary strands recognised each other and joined to perform double helix. In another experiment they added RNA to the solution of denatured DNA and solution of DNA. These existence of complementarity between DNA and RNA indicate that DNA act as template on which complementary RNA is made.
The process of transcription occurs on a DNA template in the synthesis of complementary single stranded RNA molecule. Most evidence suggest that only one of the two strands of DNA duplex is transcribed. The strand that is transcribed is called sense strand and the complementary strand is called antisense strand. Transcription may be divided into three stages.
(1) Template binding (2) Elongation (3) Termination
(1) TEMPLATE BINDING AND IMITIATION
It involves RNA polymerase and DNA molecule. The initial binding of an RNA polymerase enzyme to DNA molecule must occur at a specific position just in front of the gene to be transcribed. These attachments points are called Promoters.
Promoter: Promoter is a short nucleotide sequence that is recognized by an RNA polymerase enzyme as a point at which it can bind to DNA in order to begin transcription. Promoters occur just lipstream of genes and no where else.
(2) Elongation:
During this stage RNA polymerase enzyme migrates along the DNA molecule, melting (breaking base pairings) and unwinding double helix as it moves along. A structure called ‘open promoter complex’ is formed in the region of -10 box where breakage of base pairing and unwinding of DNA double helix occurs. After the formation of open promoter complex, the sigma factor dissociates and the holoenzyme is converted into core enzyme. At the same time first two ribonucleotide are base paired to the template polynucleoptide.
One elongation begins the synthesized portion of RNA molecule gradually dissociated from the template (guide) strand allowing the double helix to return to its original state. Thus only a limited region of DNA molecule is melted at any one time. The open region also called transcription bubble contains between 12 and 17 RNA-DNA base pairs. The transcription is longer than the gene.
(3) TERMINATION
The termination of transcription occur only as suitable positions shortly after the ends of genes. The termination does not involve specific sequence analogous to the promoter but it is brought by a more complex signal. The termination signals are complementary palindromes.
TRANSCRIPTION IN EUKARYOTES:
Transcription in Eukaryotes occurs by a process very similar to that in Prokaryotes. Most important differences are thet in eukaryotes initiation of transcription is more complicated and termination does not involve stem loop structures. There are three different kinds of RNA polymerases in eukaryotes each have their own promoters, so each enzyme transcribes only its own set of genes. For example RNA polymerase II cannot transcribe transfer RNA genes as these have promoters for RNA polymerase III.
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