TRANSCRIPTION AND SPLIT GENE

TRANSCRIPTION

Formation of RNA over  DNA template is coled transcription.  Out of two strand of DNA only one strand participates in transcription and called "Antisense strand" or "Template strand".

If both strands act as a template during transcription they would code for RNA molecule with different sequence and if they code for protein the sequence of aminoacid in these protein would be different and another reason that if the two RNA molecule produced they would be complementary to each other and form a ds RNA which prevent translation of RNA.

A gene is defined as the functional unit of inheritance. It is difficulty to literally define a gene in terms of DNA sequence, because  the DNA sequence coding for tRNA or rRNA molecule is also define a gene  (But information of protein is present on the DNA segment which code mRNA).

• The segment of DNA which contains signal for the synthesis  of one polypeptide is known as "Cistron".
• RNA polymerase enzyme is involved in transcription. In eukaryotes there are three types of RNA polymerases.
• RNA polymerase-l For 28s RNA, 18s RNA, 5.8 rRNA synthesis.
• RNA polymerase-Il for hn-RNA synthesis.(Precursor of mRNA).
• RNA polymerase-III for t-RNA, 5s rRNA, SnRNA synthesis.
• Prokaryotes have only one type of RNA polymerase which synthesizes all types of RNAs.
• RNA polymerase (Core enzyme) of E. Coli has five polypeptide chains  β,  β', α, α and ω. polypeptide chain is also known as a σ factor (sigma factor).

Core enzyme + Sigma factor = RNA polymerase.

(β,  β', α, α and ω)     (σ)

A transcription unit in DNA is defined primarily by the regions in the DNA:-

1. A promoter.

2. The structural gene

3. Terminator

Following steps are present in transcription

1. INITIATION :

• DNA has a "Promoter site" where RNA polymerase binds and a "Terminator site" Where transcription stops.
• Sigma factor (σ) recognises the promoter site of DNA.
• With the help of sigma factor RNA polymerase enzyme attached to a specific site of DNA calked “Promoter site".
• In prokaryotes before the 10 N2,  base from "starting point" a sequence of 6 base pairs (TATAAT)  is present on DNA, which is called "Pribnow box".
• In eukaryotes before the 20 N2,  base from "Starting point" a sequence of 7 base pairs  (TATAAAA) or (TATATA) is present on DNA which is called "TATA box" or "Hogness box".
• At start point RNA polymerase enzyme breaks H-bonds between two DNA strands and separates them.
• One of them strand takes part in transcription. Transcription proceeds in 5'-3' direction.
• Ribonucleoside triphosphate come to lie opposite complementary nitrogen bases of anti sense strand.
• These Ribonucleotides present in the form of triphosphate ATP, GTP, UTP and CTP. When they are used in transcription, pyrophosphatase hydrolyse two phosphates from each activated nucleotide. This  releases energy. 
• This energy  is used in the process of transcription.

2. ELONGATION : 

• RNA polymerase enzyme establishes phosphodiester bond between adjacent ribonucleotides.
• Sigma factor separates and RNA polymerase moves along the anti sense strand till it reaches terminator site.

3. TERMINATION :

• When RNA polymerase enzyme reaches at terminator site, it separates from DNA templet.
• In most cases RNA polymerase enzyme can recognise the "Terminator site" and stop the synthesis of RNA chain, but in prokaryotes, it recognises the terminator site with the help of Rho factor  ( ρ factor).
• Rho (ρ) factor is a specific protein which helps RNA polymerase enzyme to recognise the terminator site.

fig : Process of Transcription in Prokaryotes (Bacteria)

SPLIT GENE

Discovered by Phillip A. sharp and  Richard J. Roberts. They were awarded Nobel Prize in 1993. Gene which contains non functional part along with functional part is known as Split gene. Non functional part is called intron and functional part is called exon. 

By transcription split gene produces RNA which contains coding and non coding sequence and called hn RNA (Hetero genous nuclear RNA). This hn RNA is unstable. Now 7 methyl guanosine is added to its 5' end, and a cap like structure is formed. It is called capping and 200-300 nucleotides of adenylic acid are added to its 3' end, which is called poly 'A' tail. Now it becomes stable. By the process of RNA splicing hn-RNA produces functional m-RNA that is exonic RNA. In RNA splicing non coding parts is removed with the help of spliceosome enzyme and coding part join together with the help of RNA ligase.

Some specific proteins are also helpful in RNA- splicing called 'Small nuclear ribonucleoprotein' or 'SnRNP or 'Snurps'. These SnRNP proteins combine with some other proteins and SnRNA to form spliceosome complex. This spliceosome complex uses energy of ATP to cut the RNA, releases the non-coding part and joins the coding-part to produce functional RNA. Non coding part of hn RNA. remained inside the nucleus and not translated in to protein. Only coding part moves from nucleus to cytoplasm and gets translated into protein.

• Mostly Eukaryotic genes are example of split gene, but gene which forms histone and interferon protein are non split gene.
• Mostly prokaryotic genes are example of non split gene.
• In eukaryotes after transcription splicing process also occured.
• The split gene represent an ancient (primitive) feature of gene.
• Presence of intron is a primitive character.
• The splicing process represent the dominance of RNA world.

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