Download MTC19: transcription and gene expression 02/10/07

MTC19: TRANSCRIPTION AND GENE EXPRESSION
02/10/07
LEARNING OUTCOMES
Describe the role of the various parts of a gene in transcription
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A gene in transcription can be defined as a segment of DNA extending from the site of initiation to the
site of termination
Genes consist of exons (sequences to be subsequently translated into proteins) separated by introns,
which can contain other control regions or even other genes to allow more complex control networks
Gene promoters define both the starting point and the direction of transcription
Promoter regions are found immediately upstream from and include the transcription initiation sites
and consist of ‘modules’ of simple base sequences which are recognised by RNA polymerase
A common gene promoter is the Goldberg-Hogness TATA box, which is commonly found roughly 10bp
upstream from the transcription initiation site
Describe the basic mechanism of transcription
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Transcription is catalysed by RNA polymerase (II for mRNA) which uses one strand of DNA as a
template called the antisense strand
RNA polymerases do not need a primer; however, they do need a gene promoter (control region)
within the DNA to identify the starting point (and direction) for transcription
The transcript is synthesised using A/C/G/UTP molecules as sources of ribonucleotide monomers
When RNA polymerase reaches and recognises a transcription terminator site it detaches from the
DNA leaving the primary transcript hnRNA ready for processing
Describe the post-transcriptional processing of RNA
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RNA processing involves the following:
o Addition of a 5’ methylguanosine cap (an unusual variation of the G base which is methylated
at the 7th position)
o snURPs identify the polyadenylation signal which is often AUAA / ATAA and then cleaves the
transcript roughly 20 nucleotides downstream from it
o polyA polymerase adds a sequence of 70 – 200 A bases at the 3’ end (a poly-A ‘tail’) which
contributes to mRNA stability and is termed polyadenylation;
o snRNPs recognise and remove unwanted introns to produce the final mature mRNA ready for
export and translation
The mature mRNA contains both 5’ and 3’ untranslated regions (UTR)
At the splicing stage of processing, one gene can produce multiple distinct mRNAs depending on how
it is spliced