MOLECULAR BASIS OF INHERITANCE|PART-V|TRANSCRIPTION

#biostripsmedia# #pratheeshpallath#

In this session Bio Strips Media takes through the important topics of Molecular Basis of Inheritance. It is one of the most important and scoring topics in Plus Two Biology Exam and NEET Exam. 

 

This lecture covers:

·         Central dogma of molecular biology

·         Transcription in prokaryotes

·         Transcription in eukaryotes

·         mRNA processing

 

The content being discussed in this video will be helpful for those appearing for the Plus Two Biology Exam and NEET Biology Exam.

 

The information in this video is very useful to them and it helps those NEET aspirants to score maximum marks in Biology.

 

Central dogma of Molecular  biology

Crick (1958) Proposed the central dogma of molecular biology. Central dogma is the unidirectional flow of information from DNA to RNA and from RNA to polypeptide. H. Remind and Baltimore (1970) Introduced the concept of reverse central Dogma, ie, formation of DNA from RNA. it is also called Teminism and of occurs in retroviruses. Many tumour viruses contain RNA as genetic material and replicate by first synthesizing a complimentary DNA. this process  is called reverse transcription. It is carried out by an RNA dependant DNA polymerase called reverse transcriptase.             

Transcription

The process of transferring information stored in DNA to RNA through the synthesis of mRNA over the template of DNA is called transcription. Transcription Occurs in the nucleus during G1 and G2 phases of cell cycle. A transcription unit in DNA has three regions:

·        A promoter

·       Structural gene and

·       A terminator

The process is catalysed by DNA-dependent RNA-polymerase, which catalyses the polymerization of nucleotides only in 5' - 3'’ direction. 

The DNA strand with 3' - 5'’ polarity is called template strand while the other strand with 5' - 3'’ polarity is called coding strand.

The promoter refers to a particular sequence of DNA located towards the 5'’end of the coding strand, where the RNA polymerase becomes bound for transcription.

The terminator is a sequence of DNA located towards the 3' end of the coding strand, where the process of transcription stops. 

Only one of the strands of the DNA act as the template for RNA synthesis for the following reasons:

If the both strands code for RNA, two different RNA molecules and two different proteins would be formed. 

Since the two RNA molecules produced would be complementary to each other, they would wind together to form a double stranded RNA without carrying out translation.

Transcription in prokaryotes

In bacteria transcription takes place in three steps which are initiation, elongation and termination.

In prokaryotes the structural genes are polycistronic and continuous.

In prokaryotes there is a single DNA-dependent RNA-polymerase that catalyses the transcription of all the three types of RNA (mRNA, tRNA, rRNA).

RNA polymerase binds to the promoter and initiates the process along with certain initiation factors (sigma factor). The sigma factor Recognises the promoter region while the remaining core enzymes take part in the transcription.

The initiation also facilitates opening of the helix and continues elongation.

It uses ribonucleoside triphosphate for polymerization on a DNA template following complementarity of bases.

Once the RNA polymerase reaches the terminator the RNA polymerase falls off and the nascent RNA separates; it is called termination of transcription and is facilitated by certain termination factors (rho factor).

In prokaryotes the mRNA synthesized does not require any processing to become active and both transcription and translation occur in the same cytosol; translation can start much before the mRNA is fully transcribed, ie, transcription and translation can be coupled.  

TRANSCRIPTION IN EUKARYOTES

In eukaryotes the structural genes are monocistronic and split.

They have coding sequence called exons that form part of mRNA and non-coding sequences called introns, that do not form part of the mRNA and are removed during splicing.

In eukaryotes there are at least three different RNA polymerases in the nucleus, which function as follows:

·       RNA polymerase I transcribes rRNAs

·       RNA polymerase II transcribes mRNAs (hnRNA) and small nuclear RNAs.

·       RNA polymerase III transcribes tRNAs and small ribosomal RNAs.

The primary transcript contains both exons and introns and it is subjected to a process called splicing, where the introns are removed and exons are joined in a definite order to form mRNA.

The hnRNA undergoes two additional processes called capping and tailing.

In capping methyl guanosine tri-phosphate is added to the 5' end of hnRNA.

In tailing adenylate residues (about 200-300) are added at the 3' end of hnRNA.

The fully processed hnRNA is called mRNA and is released from the nucleus into the cytoplasm.

This processing helps in keeping mRNA away from the degrading action of nucleases present in nucleus.