• Created by: Jenny Le
  • Created on: 15-04-14 21:02


RNA has a phosphodiester linked sugar phosphate backbone

The sugar in RNA is ribose

RNA has 5' to 3' polarity

RNA is single stranded however, it may fold back on itself and form double stranded regions. Also some viruses have a dsRNA genome


  • A, U, G, C
  • No thymine
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RNA species

  • Referred to by function and size
    • nt
    • S, Svedberg (sedimentation)
  • Synthesised by transcribing a complementary RNA molecule to a DNA strand (template strand)
    • cf DNA replication
    • RNA polymerases
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End products of gene expression

  • 'Stable RNA' (long T1/2)
  • Many involved in protein synthesis
    • ncRNA (non-coding)
    • rRNA
    • tRNA
    • snRNA
    • snoRNA
    • scRNA
    • telomerase RNA
    • Xist RNA
    • Vault RNA
    • Ribozymes
    • miRNA
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Intermediate between gene and product

  • Short T1/2
  • 1 minute - bacterial
  • 1-6 hours eukaryote
    • mRNA (processing, maturation)
    • hnRNA - information carrier, translated to give a polypeptide

RNA polymerases

  • do not require a primer
  • 5' to 3'
  • prokaryotic
    • single RNA polymerase type - core enzyme + O (delta) factor
  • eukaryotic
    • three nuclear RNA polymerases
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Eukaryotic RNA polymerases

  • RNA polymerase I
    • larger rRNA precursor molecule
  • RNA polymerase II
    • hnRNA/mRNA, snRNA
  • RNA polymerase III
    • 5S rRNA, tRNA, snRNA, snoRNA
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rRNA: ribosomal RNA

  • 80% of total cellular RNA
  • components of ribosomes
  • eukaryotes: RNA polymerases I and III
  • need one copy of each rRNA molecule per ribosome
  • transcribe as a single unit

Transcribed and non-transcribed spacer DNA

  • Eukaryotes, each rDNA cluster = nucleolar organising region
  • Nucleolus, loops of DNA corresponding to the rDNA genes
  • rRNA: a multipcopy gene family:
    • H.sapiens
    • 280 copies of the rDNA
    • 2000 copies of 5SrRNA gene
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tRNA: transfer RNA

  • 15% of total cellular RNA
  • Adapter molecules between the nucleotide sequence of the mRNA molecule and the amino acid.
  • Eukaryotes RNA polymerase III
  • Another multicopy gene family 
    • H. sapiens has 1300 copies tRNA genes
  • small molecule (74-95 nt)
  • number of different tRNA genes, each in multiple copies
  • each tRNA has the same basic structure
  • linear ssRNA molecule
  • folded and formation of complementary bases (AU, GC)
  • tRNA 'cloverleaf'
  • tRNA tertiary structure is L shaped
  • acceptor and anticodon arms are at opposite ends of the molecule
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mRNA: messenger RNA

  • In bacteria, the mRNA molecules are a direct copy of the gene
  • In eukaryotes, the original transcript (hnRNA) is much longer than the mRNA
  • Chemical modification to the ends
  • Removal of introns
    • hnRNA (nuclear) up to 20 knt
    • mRNA (cytoplasm) 2 knt
  • In rare cases, alterations to the sequences
    • mRNA editing
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Modifications/Processing (eukaryotes)

5' cap

  • addition of G to 5' end and then is methylated
  • function: interaction of mRNA with the small ribosomal subunit


  • 3' poly A tail
  • up to 250 A's
  • added by a poly(A) polymerase
  • function: increase half life
    • but not all mRNA is polyA+
    • histone mRNA is never polyA+
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Intron Splicing

Genes in eukaryotes may be split

Exons: coding sequences

Introns: interruptions (non-coding sequences)

Primary transcript starts upstream of the first codon

  • continues past last stop codon
  • continues through the introns

Introns have to be removed before translation

  • splicing by spliceosomes, in the nucleus
  • only once this has happened does the mature mRNA leave the nucleus
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  • can undergo transcription and translation simultaneously and this takes place in the nucleoid.


  • separation of transcripton and translation by the nuclear membrane
  • processing of the primary hnRNA transcript

Prokaryotes and Eukaryotes:

  • general process of transcription is the same (DNA template, enzyme reaction)
  • promoters may differ
  • RNA polymerases differ
  • processing
  • function of the transcript
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Initiation of Transcription


  • catch the RNA polymerase
  • initiation starts at the correct point

Attachment point for RNA polymerase:


  • short nucleotide sequence
  • recognised by RNA polymerase

Template strand = RNA molecule

Sense strand

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Summary Facts

Central Dogma of Molecular Biology


The flow of information within a cell (DNA--RNA--protein)

  • Eukaryotic genes may have introns, but need not; prokaryotic genes do not.
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