DNA & protein synthesis

• describe the structure of DNA in prokaryotic cells

  short & circular

  • not wound around histones: supercoils itself
• describe the structure of DNA in eukaryotic cells

  linear: stored in chromosomes in the nucleus

  tightly wound around histones 


  DNA wrapped around histones form nucleosomes
• three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell.

  histones/ v no histones

  linear v circular

  long v short

  introns v prokaryotic DNA has no introns
• which organelles contains DNA like prokaryotic cells

  mitochondria & chloroplasts
• gene

  short section of DNA that codes for a polypeptide or functional RNA 
• allele

  alternative forms of the same gene
• locus:fixed position occupied by a gene
• The genetic code is described as being degenerate. What does this mean?

  more than one triplet can code for a particular amino acid.
• The genetic code is described as being non-overlapping. What does this mean?

  each base in a gene is part of only one triplet of bases - read as a discrete unit 
• The genetic code is described as being universal. What does this mean?

  the same triplet of bases code is the same in almost all organisms - evidence for evolution
• introns

  sections of DNA that doesnt code for amino acids
• extrons

  sections of DNA that does code for amino acids
• structure & function of mRNA

  single stranded 

  made in transcription

  • creates a copy of a gene in DNA and delivers it to the ribosome for translation
• structure & function of tRNA

  single stranded but cloverleaf shape (held by H bonds)

  transfers amino acids from cytoplasm to the ribosomes for translation

  • has an amino acid binding site & at the other end has an anticodon
• why does tRNA have an anticodon

  the anticodon are complementary to the mRNA codon
• whats the 2 stages of protein synthesis

  transcription & translation
• where does translation take place

  ribosomes in the cytoplasm - where protein synthesis occurs
• what is transcription

  DNA helicase breaks H bonds and exposes the bases

  mRNA nucleotides align with complementary base pairings and RNA polymerase bonds it together
• process of pre-mRNA to mRNA

  pre-mRNA is spliced by splicesome to remove the introns leaving the exons. 
• what is translation

  mRNA is attached to the ribosome at the start codon

  tRNA molecules anticodon which is complementary to the start codon will be held in place by the ribosome

  mRNA codon which matches to different tRNA anticodons are aligned through its complementary pairings

  amino acids at the top of the tRNA are joined by a peptide bond, requires ATP and enzymes

  • process carries on till stop codon and ribosome detaches