DNA & RNA

DNA, RNA, translation, transcription

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differences between DNA & RNA

  • DNA has bases GCAT, RNA has bases GCAU.
  • DNA has two nucleotide strands, RNA has one nucleotide strand.
  • DNA has deoxyribose sugar, RNA has ribose sugar.
  • DNA has more nucleotides in chain than RNA.
  • DNA is helical, RNA is either straight or clover shaped.
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Differences between mRNA & tRNA

  • mRNA has no hydrogen bonds but tRNA does.
  • mRNA has a bigger nucleotide chain than tRNA.
  • mRNA has a straight chain. tRNA is clover shaped.
  • mRNA doesn't have a binding site, tRNA has a binding site for an amino acid.
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Transcription

  • DNA unwinds at the point where a gene is to be used.
  • Hydrogen bonds break so the strands of DNA seperate, exposing the bases.
  • RNA nucleotides complementary base pair with the sense strand.
  • A single chain of RNA nucleotides which are complementary to the DNA has been produced.
  • RNA polymerase joins the ribose-phosphate backbone to form a molecule of RNA.
  • The molecule created is mRNA.
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post transcriptional processing of mRNA

  • The molecule of mRNA that has been produced is edited before it leaves the nucleus.
  • non-coding intron sequences are removed.
  • The coding sequences are then spliced back together.
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Translation (1)

  • mRNA travels from the nucleus to the cytoplasm.
  • mRNA binds to a ribosome.
  • two mRNA codons are exposed on the ribosome.
  • A tRNA carrying a specific amino acid binds to the mRNA.
  • this is due to complementary base pairing.
  • A 2nd tRNA-amino acid complex binds to the 2nd codon on the mRNA.
  • Condensation reaction takes place between the two amino acids, forming a peptide bond.
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Translation (2)

  • Ribosome moves along the mRNA to the next codon.
  • 1st tRNA goes back to the cytoplasm to pick up another amino acid.
  • The process continues until a stop codon is reached.
  • The polypeptide terminates here.
  • The polypeptide is released into the cytoplasm or the lumen of the E.R.
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The genetic code ...

  • Is universal - the same for all organisms.
  • Is degenerate - There are at least two possible codons for many amino acids.
  • Is non-overlapping - Each base is part of one codon only.
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Gene mutation

  • A change in the base sequence carried by DNA.
  • Mutagenic agents e.g uv radiation, x rays and cigarette smoke trigger mutations.
  • Alterations to DNA may have major effects on the shape and function of a protein.
  • A mutation may cause no change (because the code is degenerate).
  • A change to one triplet ( when substitution takes place).
  • A change to several triplets (addiition/deletion causing a frameshift).
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