Transcription – DNA –> RNA

RNA is much the same as DNA, except for a few points:

  1. The sugar is ribose rather than deoxyribose – deoxyribose has one fewer OH group – on C2:
  2. The DNA base Thymine is replaced with Uracil (same but without methyl group):
  3. RNA is single stranded rather than double stranded like DNA. Instead, it folds into well defined structures (rather than combining two seperate strands that can be broken apart by denaturing).

There are several different types of RNA:

  • mRNA – Messenger RNA – template for protein synthesis.
  • rRNA – Ribosomal RNA – major component of ribosomes.
  • tRNA – Transfer RNA – carries activated amino acids to ribosomes.
  • snRNA – participates in RNA splicing.
  • miRNA – binds to mRNA and inhibits translation.
  • siRNA – Small Interfering RNA – binds to mRNA and promotes degradation.

– The Process of Transcription

A strand of RNA is produced from a strand of DNA – much the same as during DNA replication but in this case it is catalysed by RNA polymerase using rNTPs (ribonucleotide triphosphates). No primer is required. The synthesis occurs in the same direction as for DNA replication (5′->3′) and pyrophosphates are still released when the ribonucleotide triphosphates bind to the backbone.

  • ~17 base pairs of DNA duplex uncovered at a time as the DNA is trancribed in RNA. Of those ~17 base pairs, only 9 are paired with RNA at any one time.
  • The transcription ‘bubble’ moves down the DNA strand 3′–>5′ at a rate of ~50 bases/sec until it reaches a termination sequence.
  • In prokaryotes, transcription AND translation occur at the same time.

– The Control of Transcription

The interactions between RNA polymerase and its promoter can be enhanced by activators or blocked by repressors.

A good example is the lac operon in prokaryotes – in Eukaryotes this is much more complex and may require chromatin remodelling to allow access to genes for transcription.

The Lac Operon controls expression of genes related and involved in the metabolism of lactose. A regulatory gene leads to the production of a repressor protein, which (in the absense of lactose) will bind to the operator gene, blocking expression of the later genes. When lactose appears, this disables the repressor protein, changing it’s active site so that it can no longer bind to the Operator gene. This allows expression of the genes further along the strand.

The above diagram shows the events when (a) no lactose is present, and (b) when lactose is present. The diagram below shows what occurs in the Tryptophan operon – you’ll see it is very similar.

– RNA Splicing

Splicing removes non-coding RNA sections from the newly synthesised strand. I mentioned non-coding DNA previously as DNA that has a purely structural role and does not code for any proteins etc. When it is copied into RNA during translation it has no further use and so is removed by splicing.

  • A non-coding segment is called an INTRON (for intragenic regions). These sites start with GU and end with AG.
  • A coding segment is called an EXON (for regions that will be expressed).

By removing non-coding segments, several proteins can be synthesised by just one gene.

Incorrect splicing is a high risk though, and up to 15% of all genetic diseases have been caused by errors and mutations during splicing.

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