Biology NUCLEIC ACIDS (RNA)

Transcription

`->` Formation of RNA over DNA template is called transcription. Out of two strand of DNA only one strand participates in transcription and called "Antisense strand".

`->` If both strands act as a template during transcription they would code for ENA molecule with different sequence and If they code for proteins the sequence of aminoacid in these protein would be different and another reason that if the two HNA 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 difficult to literally define a gene in terms of DNA sequence, because the DNA sequence coding for tENA or rENA molecule is also define a gene (But information of protein is present on the DNA segment which code mRNA. So generally it is
reffered for it)

`->` The functional unit of DNA involved in transcription is "Cistron " .
`->` RNA polymerase enzyme is involved in transcription. In eukaryotes there are three types of RNA polymerases.
# RNA polymerase-I for 28s rHNA, 18s rENA, 5.8s rENA synthesis.

# RNA polymerase-ll for m-HNA synthesis.

# RNA polymerase-Ill for t-rmA, 5s rENA, SnRNA synthesis.

`->` In eukaryotes RNA polymerase enzyme is made up of `10-15` polypeptide chains.

`->` Prokaryotes have only one type of ENA polymerase which synthesizes all types of HNAs.

`->` RNA polymerase of E. Coli has five polypeptide chains `beta , beta', alpha ., alpha`. and `sigma`.

`->` `sigma` polypeptide chain is also known as a factor (sigma factor).
`->` Core enzyne + Sigma factor `=>` RNA Polymerase
(beta , beta' , alpha , alpha ) (sigma)
A transcription unit in DNA is defined primarily by the in three gigons in the DNA :-

(i) A promoter, (ii) The structural gene (iii) A terminator

Following steps are present in transcription -

(1) Initiation :-

`->` DNA has a "Promoter site or initiation site" where transcription begins and a "Terminator site" where transcription stops .

`->` Sigma factor (a) recognises the promoter site of DNA.

`->` With the help of sigma factor RNA polymerase enzyme attached to a specific site of DNA called "Promoter site" .

`->` In prokaryotes before the `10 N_2` 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 N_2` base from "Starting point" a sequence of `7` base pairs (TATAAAA) or (TATATAT) is present on DNA which is called "TATA box or Hogness box"

`->` At promoter site 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 release ebergy.

`->` This energy is used in the process of transcription.

(2) Elongation :-

`->` RNA polymerase enzyme establishes phosphodiester bond between adjacent ribonucleotides.

`->` Sigma factor separates and core enzyme 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 tem.plet

`->` At terminator site on DNA, `N_2` bases are present in palindromic sequence.

`->` In most cases RNA polymerase enzyme can reco[Jnise the 'Terminator site' and stop the synthesis RNA chain, but in prokaryotes, it recognises the terminator site with the help of Rho factor (p factor).

`->` Rho ( `rho` ) factor is a specific protein which helps RNA polymerase enzyme to recognise the terminator site.

Split Gene : -

Discovered by sharp and Roberts. They awarded by 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 a RNA which contains coding and non coding sequence and called hn RNA (Hetero genous nuclear RNA). This hn RNA is unstable. Now 7 methyl guanonsine is added to its 5' end, and a cap like structure is formed. It is called capping and 200 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 mm-RNA that is exonic RNA. In RNA splicing non coding parts removed with the help of ribonuclease 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 codingpart 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 translated into protein.

Mostly Eukaryotic genes are example of split gene, but gene which forms histone and interferon protein are non split gene. It contains only and only exonic part.

Mostly prokaryotic genes are example of non split gene.

� In euckaryotes after transcription splicing process also occurecl.

� 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.