Question

Describe the elongation process of transcription in bacteria?


class 12th CBSE Biology sample paper

Answer 1

There is a convention in defining the two strands of the DNA in the structural gene of a transcription unit.  Since the two strands have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction, that is, 5'→3', the strand that has the polarity 3'→5' acts as a template, and is also referred to as template strand. The other strand which has the polarity (5'→3') and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription.  Strangely, this strand (which does not code for anything) is referred to as coding strand.  All the reference point while defining a transcription unit is made with coding strand.

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In bacteria, there are three major types of RNAs: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).  All three RNAs are needed to synthesise a protein in a cell.  The mRNA provides the template, tRNA brings aminoacids and reads the genetic code, and rRNAs play structural and catalytic role during translation. There is single DNA-dependent RNA polymerase that catalyses transcription of all types of RNA in bacteria.

It uses nucleoside triphosphates as substrate and polymerises in a template depended fashion following the rule of complementarity.  It somehow also facilitates opening of the helix and continues elongation.  Only a short stretch of RNA remains bound to the enzyme. Once the polymerases reaches the terminator region, the nascent RNA falls off, so also the RNA polymerase.

In bacteria, since the mRNA does not require any processing to become active, and also since transcription and translation take place in the same compartment (there is no separation of cytosol and nucleus in bacteria), many times the translation can begin much before the mRNA is fully transcribed.
 

Answer 2

MECHANISM OF TRANSCRIPTION

The main enzyme taking part in transcription is called RNA polymerase.

There is single RNA polymerase in case of prokaryotes.

Eukaryotes have three types of polymerase

• i) RNA polymerase I
• ii) RNA polymerase II
• iii) RNA polymerase III

ELONGATION

• The RNA polymerase uses ribonucleoside triphosphate as substrates and polymerase in a template dependent manner following the rule of complementarity.

• RNA polymerase moves along the transcription unit and causes unwinding and opening the helix and continuous elongation of RNA.

• The ⌐ factor of the RNA polymerase separate on the initiation of RNA chain.

• The remaining part of RNA polymerase referred as core enzyme moves along the DNA template causing elongation of RNA chain at the rate of 30 nucleotides per seconds.

• Elongation takes place in 5’ – 3’ direction.