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TRANSCRIPTION IN PROKARYOTES, SANDHYA A 191822016 - Coggle Diagram
TRANSCRIPTION IN PROKARYOTES
DEFINITION
Transcription is the process through which a DNA sequence is enzymaticaly copied by an RNA polymerase to produce a complementary RNA.
SYNTHESIS
The RNA is processed into messenger RNA [mRNA], which is then used for synthesis of a protein.
RNA is synthesized from a DNA template.
The RNA thus synthesized is called messenger RNA (mRNA), because it carries a genetic message from the DNA to the protein-synthesizing machinery of the cell.
TEMPLATE USED
The RNA is synthesized from a single template [strand] of a DNA molecule.
The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit.
The DNA strand which is used in RNA synthesis is called template strand; because it provides the template for ordering the sequence of nucleotides in an RNA transcript.
A transcription unit codes the sequence that is translated into protein.
The DNA strand which does not take part in DNA synthesis is called coding strand, because, its nucleotide sequence is the same as that of the newly created RNA transcript.
ENZYME INVOLVED
The enzyme that carries out transcription is called RNA polymerase, and it consists of four kinds of polypeptides, designated α, β, β’ and σ, which are bound together into a complex called a holoenzyme.
GENETIC INFORMATION
In this process, the genetic information coded in DNA is copied into a molecule of RNA. The genetic information is transcribed or copied, from DNA to RNA.
FIRST STEP
The expression of a gene consists of two major steps, viz., transcription and translation. Thus transcription is the first step in the process of gene regulation or protein synthesis.
DIRECTION OF SYNTHESIS
As in DNA replication, RNA is synthesized in the 5′ —> 3′ direction. The DNA template strand is read 3′ —> 5′ by RNA polymerase and the new RNA strand is synthesized in the 5’—> 3′ direction.
RNA polymerase binds to the 3′ end of a gene (promoter) on the DNA template strand and travels toward the 5′ end.
MECHANISM
INITIATION
RNA polymerase (RNAP) binds to one of several specificity factors, to form a holoenzyme.
At this stage, the DNA is double-stranded (“closed”). This holoenzyme/wound-DNA structure is referred to as the closed complex.
The DNA is unwound and becomes single-stranded (“open”) in the vicinity of the initiation site (defined as + 1).
The RNA polymerase transcribes the DNA, but produces about 10 abortive (short, nonproductive) transcripts which are unable to leave the RNA polymerase because the exit channel is blocked by the cr-factor.
The a-factor eventually dissociates from the holoenzyme, and elongation proceeds.
Uridine-5′- triphosphate (UTP) and cytidine-5′-triphosphate (CTP) (pyrimidine nucleoside triphosphates) are dis-favoured at the initiation site.
ELONGATION
In the prokaryotes, the elongation starts with the “abortive initiation cycle”
This continues to occur until the σ factor rearranges, which results in the transcription elongation complex (which gives a 35 bp moving footprint).
During this cycle RNA Polymerase will synthesize mRNA fragments 2-12 nucleotides long.
TERMINATION
Rho-independent termination
It is also known as intrinsic transcription termination. It involves terminator sequences within the RNA that signal the RNA polymerase to stop.
Rho-dependent termination
In the “Rho-dependent” type of termination, a protein factor called “Rho” [P factor] is used to stop RNA synthesis at specific sites.
SANDHYA A 191822016