Transcription
RNA
• Four bases; A, G, C, U (instead of thymine). Thymine has methyl group, uracil just has
hydrogen (C5)
• Uracil – pyrimidine
• Ribose instead of deoxyribose. Hydroxyl, so doesn’t form double-stranded structure
• Also has phosphate
• RNA is labile (short lived)
• Shorter structure. 1 to a few kilobases
Three Classes of RNA
• Messenger RNA (mRNA) – variable sizes
• Transfer RNA (tRNA) – 80-90 bases
• Ribosomal RNA (rRNA) – 3700, 1700, 120 bases)
• Other non-coding RNAs
• More coding potential = bigger protein
mRNA
• Genetic Messenger
• 1961 – Jacob and Monod proposed idea of mRNA. Looked at beta-galactosidase
synthesis
• Observed that synthesis of certain enzymes is rapidly turned on and off
• mRNA must be rapidly made and short lived
Evidence
• Within 10 mins of bacteriophage T2 infecting E. coli, 60% of the proteins synthesised
are viral. Have different ATGC composition, so new RNA coded for by virus as
composition becomes the same
• Not accompanied by net synthesis of RNA
• Minor RNA fraction new and pulse-chase experiments showed that mRNA was short
lived. rRNA and tRNA are stable
• Newly synthesised RNA had same base composition as phage DNA
• Hall and Spiegelman showed that it had the same base composition and the
sequence was the same as phage DNA.
mRNA is complementary to its DNA
template
• Hall and Spiegelman showed that
the T2 mRNA could hybridise to T2
DNA but not E.coli DNA
• mRNA is viral specific
The Central Dogma of Molecular Biology
• Crick 1958
• Gene expression = gene
transcription + translation
, • RNA polymerase responsible for transcription
• Ribosomes responsible for translation
Current concept of a gene
Sequence of bases on a DNA molecule that codes for:
• Sequence of amino acids in a polypeptide chain
• RNA molecule with a specific function
Products:
• Proteins
• Non-coding RNAs (ncRNAs):
1. tRNA and rRNA
2. Small inhibitory RNAs (siRNAs) and microRNAs (miRNAs) involved in
regulation of gene expression
3. Small nucleolar RNAs (snoRNAs) involved in rRNA folding and stability
Information flow
• Genes – discrete sections of DNA genome that code for protein/RNA
• RNA polymerase transcribe sections of the genome that encode 1 + genes
Transcription
• Not all DNA transcribed to RNA
• DNA transcribed in small units containing 1+ genes
• Monocistronic transcripts encode just one product i.e. contain one gene
• Polycistronic transcripts encode 1+ products i.e. contain more than one gene.
Ribosome binds to start of coding sequence and translate first section then
detaches. Another ribosome (or same one) attaches to second gene to translate it
etc…
• Eukaryotes don’t tend to have polycistronic transcripts. Prokaryotes can have both.
• Operons – polycistronic e.g. lactose operon
RNA Polymerase
• DNA-dependent
• First purified in 1960 by Stevens, Weiss, Hurwitz, Bresler and Diringer
• RNA polymerase (RNAP) requires a DNA template and nucleoside triphosphate
precursors
• RNAP doesn’t require a primer. Can bind and unwind DNA
• Must also know where to start and stop transcription
RNA synthesis by RNA polymerase
• Similar to DNA synthesis
• Incoming nucleoside triphosphate pairs with the base on the template strand
• Phosphodiester bond formed and pyrophosphate released
• RNA polymerase synthesises RNA in 5’ to 3’ direction
• Adds nucleotides onto 3’ end of chain
• RNA polymerase is processive – stays bound to DNA until finishes making the
product
RNA
• Four bases; A, G, C, U (instead of thymine). Thymine has methyl group, uracil just has
hydrogen (C5)
• Uracil – pyrimidine
• Ribose instead of deoxyribose. Hydroxyl, so doesn’t form double-stranded structure
• Also has phosphate
• RNA is labile (short lived)
• Shorter structure. 1 to a few kilobases
Three Classes of RNA
• Messenger RNA (mRNA) – variable sizes
• Transfer RNA (tRNA) – 80-90 bases
• Ribosomal RNA (rRNA) – 3700, 1700, 120 bases)
• Other non-coding RNAs
• More coding potential = bigger protein
mRNA
• Genetic Messenger
• 1961 – Jacob and Monod proposed idea of mRNA. Looked at beta-galactosidase
synthesis
• Observed that synthesis of certain enzymes is rapidly turned on and off
• mRNA must be rapidly made and short lived
Evidence
• Within 10 mins of bacteriophage T2 infecting E. coli, 60% of the proteins synthesised
are viral. Have different ATGC composition, so new RNA coded for by virus as
composition becomes the same
• Not accompanied by net synthesis of RNA
• Minor RNA fraction new and pulse-chase experiments showed that mRNA was short
lived. rRNA and tRNA are stable
• Newly synthesised RNA had same base composition as phage DNA
• Hall and Spiegelman showed that it had the same base composition and the
sequence was the same as phage DNA.
mRNA is complementary to its DNA
template
• Hall and Spiegelman showed that
the T2 mRNA could hybridise to T2
DNA but not E.coli DNA
• mRNA is viral specific
The Central Dogma of Molecular Biology
• Crick 1958
• Gene expression = gene
transcription + translation
, • RNA polymerase responsible for transcription
• Ribosomes responsible for translation
Current concept of a gene
Sequence of bases on a DNA molecule that codes for:
• Sequence of amino acids in a polypeptide chain
• RNA molecule with a specific function
Products:
• Proteins
• Non-coding RNAs (ncRNAs):
1. tRNA and rRNA
2. Small inhibitory RNAs (siRNAs) and microRNAs (miRNAs) involved in
regulation of gene expression
3. Small nucleolar RNAs (snoRNAs) involved in rRNA folding and stability
Information flow
• Genes – discrete sections of DNA genome that code for protein/RNA
• RNA polymerase transcribe sections of the genome that encode 1 + genes
Transcription
• Not all DNA transcribed to RNA
• DNA transcribed in small units containing 1+ genes
• Monocistronic transcripts encode just one product i.e. contain one gene
• Polycistronic transcripts encode 1+ products i.e. contain more than one gene.
Ribosome binds to start of coding sequence and translate first section then
detaches. Another ribosome (or same one) attaches to second gene to translate it
etc…
• Eukaryotes don’t tend to have polycistronic transcripts. Prokaryotes can have both.
• Operons – polycistronic e.g. lactose operon
RNA Polymerase
• DNA-dependent
• First purified in 1960 by Stevens, Weiss, Hurwitz, Bresler and Diringer
• RNA polymerase (RNAP) requires a DNA template and nucleoside triphosphate
precursors
• RNAP doesn’t require a primer. Can bind and unwind DNA
• Must also know where to start and stop transcription
RNA synthesis by RNA polymerase
• Similar to DNA synthesis
• Incoming nucleoside triphosphate pairs with the base on the template strand
• Phosphodiester bond formed and pyrophosphate released
• RNA polymerase synthesises RNA in 5’ to 3’ direction
• Adds nucleotides onto 3’ end of chain
• RNA polymerase is processive – stays bound to DNA until finishes making the
product
