Click4Biology

Transcription

7.3.1 The direction of transcription on the DNA molecule.

7.3.2 The Sense and Antisense strands of DNA.

7.3.3 Transcription in prokaryotes.

7.3.4 Post transcriptional modification in eukaryotes.

As with DNA replication, the free nucleotides in the transcription are initially nucleoside triphosphates. The breaking away of the two phosphates under the influence of the RNA polymerase provides the free energy for the construction of the covalent phosphodiester bonds between nucleotides.

In the diagrams the nucleoside triphosphates are just drawn as nucleotides. The main phases of translation are imaged although remember that prokaryotes do not have intron regions of the eukaryotes. The inclusion of promoter regions and terminator regions is a simplification of prokaryote gene expression.

Lac-Operon theory
In 1965 Francoise Jacob, jacques Monod and Andre Lwoff received the Nobel Prize for their discoveries of the mechanism which controls prokaryotic gene expression. The implications for our understanding of gene expression and developmental biology are wide ranging and even today are still opening up new fields of understanding in biology. If every you thought a scientist is a boring individual wearing white coats tucked away in a laboratory, then you should read the works of Jacob and Monod. These war time freedom fighters in the French Resistance not only produced beautiful theories but also crafted elegant literature prized as much for its philosophy as its biology.

J. Monod, Chance and Necessity (New York: Alfred A. Knopf, 1971)

7.3.1 Transcription direction on the DNA molecule.

mRNA is composed of nucleotides based on Ribose sugar and four bases Uracil, , Adenine, Guanine and Cytosine.
The 5' end of the nucleotide is added to the 3' of the already existing mRNA chain.
This is a key point to learn in the interpretation, drawing and understanding of transcription.

7.3.2 Sense and anti sense strands of DNA.

The sense strand has the same base sequence as the transcribed mRNA except that the base thymine is replaced by the base uracil.
The anti-sense strand acts as the template for the transcription of mRNA.
The RNA nucleotides are polymerised along the sugar phosphate backbone by RNA polymerase.

7.3.3 Transcription in prokaryotes.

Initiation:

The Promoter region allows the binding of RNA polymerase. The RNA polymerase is then able to:

Find the anti-sense strand.
Find the start for transcription.
Know the direction of transcription.

The hydrogen bonds between the bases of the DNA helix are opened up by DNA helicase.
The bases of the anti-sense strand ('3 to 5' for DNA) are exposed progressively.
RNA nucleotides complementary base pair with the anti-sense nucleotide bases.
The free nucleotides (nucleoside triphosphates) are based on RNA. The sugar is the pentose ribose and there are four different nitrogen bases.
The nucleotides are adenine, guanine, cytosine and uracil.

Elongation:

The RNA polymerase forms covalent bonds between the nucleotides.
Free energy is released from the oxidation of the nucleoside triphosphates to form the bond.
The bonds are formed by joining the 5' of the free nucleotide to the 3' end of the nucleotide already part of the mRNA chain.
The RNA polymerase works along the nucleotides completing the pentose-phosphate backbone.

The mRNA builds up with the RNA polymerase moving along the anti-sense strand joining the nucleotides.
As with the other biochemical processes considered in the syllabus there are additional factor involved in transcription. These are not required for the examination.

Termination:

The RNA polymerase reaches the terminator and the RNA polymerase stops.
The mRNA is complete

Various factors result in the RNA polymerase being released and will return to catalyze another mRNA.
The mRNA itself is released from the antis-sense strand.
The mRNA strand in prokaryotes can be use straight away unlike the eukaryotic mRNA which requires further modification (see below).

7.3.4 Post transcriptional modification in eukaryotes.

Pre-mRNA has been produced through transcription of the anti-sense strand as described for prokaryotic transcription.

(a) The non coding introns are spliced out of the mRNA.

The introns are broken down in the nucleus.

(b) The remaining mRNA is called mature mRNA and is exported from the nucleus to the cytoplasm for translation into the polypeptide.