Concept 13: The flow of information from genes to gene products

Describe transcription initiation and the cellular components involved

• Transcription is when DNA is copied into mRNA

• In initiation, RNA polymerase (or the protein initiator complex) recognises the promoter in DNA, where transcription begins. Prokaryotic sigma factors and Eukaryotic transcription factors are both proteins that bind to both the template DNA and RNA polymerase to join them together

• RNA polymerase (or the protein initiator complex) unwinds the DNA to initiate RNA synthesis.

• Transcription can be regulated by epigenetic modifications that activate or inhibit transcription and thereby gene expression.

Describe transcription elongation and the cellular components involved

• After RNA polymerase (or the protein initiator complex) recognises promoters and initiates transcription, RNA polymerase synthesises a complementary RNA strand that is a copy of the DNA template strand in a 5’ prime to 3’ prime direction.

• Simultaneously, RNA continues unwinding DNA.

• This ensures DNA’s genetic information is stored in mRNA.

Describe transcription termination and the cellular components involved

• A termination sequence on the DNA sequences indicates the end of a gene.

• This leads RNA polymerase to release from DNA, restoring it’s double strands, whilst the mRNA is released.

• RNA does not need a primer because it transcribes nucleotides in a 5' prime to 3' prime direction

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Recognise the key differences between eukaryotic and prokaryotic cell transcription

• Transcription occurs in different places for eukaryotic and prokaryotic cells.

• Eukaryotes: occurs in the nucleus

• Prokaryotes: occurs at the cytoplasm

• The different cells also indicate whether pre-mRNA undergoes processing.

  
  

• Eukaryotes: pre-mRNA undergoes processing at the end of the molecules, with a 5’ prime end cap and a 3’ prime end tail being attached to bind the mRNA to ribosome for translation and transporting the mRNA out of the nucleus, respectively. These biochemical alterations also prevent the molecule from degrading.

• Splicing occurs after this attachment where a protein-RNA complex searchers for markers at the ends of introns to remove them from mRNA and join exons. Incorrect splicing can lead to disease.

  
  

• Prokaryotes: processing is not needed because there are no introns that need to be removed; transcription and transcription occurs simultaneously.

• Eukaryotes also have 3 RNA polymerases (RNA Polymerase I, II, III) whilst prokaryotes only have one. RNA Polymerase II is responsible for mRNA transcription.

• The binding of eukaryotic transcription factors to RNA polymerase II is also dependent on whether the transcription factors can phosphorylate RNA polymerase II’s carboxyl terminal domain

Identify an open reading frame

• Open reading frames are the coding regions of DNA

• They start with AUG, which codes for methionine, continues coding for other codons and end with a stop codon (UAA, UAG or UGA) that terminates translation.

• The genetic code that constructs open reading frames is almost universal

• Sense codons = the translated codons.

• There are three possible reading frames: this depends on whether the coding sequence is read from the 1st, 2nd or third nucleotide because they will form different combinations of codons.

• However, only one reading frame usually codes for the functional protein, even though they all produce a different polypeptide.

Understand the significance of degeneracy (declining) in the genetic code

• Degeneracy refers to the redundancy in the genetic code, where multiple codons can specify the same amino acid.

• This means that silent mutations can occur and don’t alter protein function.

Describe translation initiation and the cellular components involved

• Translation is the second step of the central dogma and can be regulated by small RNA inhibitors.

• Translation occurs at the ribosomes, which consist of a large and small subunit.

• The small ribosome binds to the 5’ prime end of mRNA, then scans for the start codon before assembling the initiation complex with tRNA carrying methionine and the large ribosomal subunit for translation elongation.

• If the wrong codon is read as the start codon, this is likely to produce a non-functional protein.

Describe translation elongation and the cellular components involved

• Translation elongation involves both ribosome subunits, mRNA and tRNA.

• The ribosome has three sites: E,P, A

• tRNA enters the A site, recognises mRNA’s 3’ prime end codon and forms an amino acid. This tRNA molecule moves to the P-site and when another tRNA molecule enters the A site to undergo complementary binding, produces another amino acid.

• These amino acids are joined with peptide bonds using energy from ATP hydrolysis and is catalysed by rRNA.

• This continues, with the uncharged DNA (that releases the amino acid) leaving the ribosome at the E site, and the tRNA-ribosome complex moving along mRNA to produce a polypeptide.

Describe translation termination and the cellular components involved

• Stop codons indicate when translation terminates.

• Since there is no tRNA that is able to recognise a stop codon, a release factor protein recognises it at the A site, does not form a peptide bond and is able to release the polypeptide chain.

• The ribosomal subunits and release factors dissociate from mRNA to initiate translation for a new protein.

Recognise the key differences between prokaryotes and eukaryote translation

• Initiation is different for both cells

• Prokaryotes: The small ribosomal subunit binds to the Shine-Dalgarno sequence before scanning for the AUG start codon

• Eukaryotes: The small ribosomal unit recognises mRNA’s 5’ prime cap before scanning for the AUG start codon.