Biology 5 DNA

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1) RNA polymerase attches to the DNA double helix at the beginning of the gene

2) The hydrogen bonds between the two DNA strands in the gene break up, seperating the two strands

3) One of the strands is then used as a template to make mRNA

4) The RNA polymerase lines up with free RNA nucleotides along side the template strand- specific base pairing means that the mRNA molecule will end up being complementary copy of the DNA (except T is replaced with U)

5) Once the RNA nucleodies have paired up with their specific bases on the DNA strand they're joined toether forming a mRNA molecule

6) The hydrogen bonds between the uncoiled strands of the DNA reform once the RNA polymerase has passed by and the strands coil back into a double helix

7) When RNA polymerase reaches a stop signal it stops making mRNA and detaches from the DNA

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Translation

1) The mRNA attaches itself to a ribosome and the transfer RNA molecule carry amino acids to the ribosome

2) A tRNA molecule, with an anticodon which is complementary to the first codon on the mRNA, attach itself to the mRNA by specific base pairing

3) A second tRNA molecule attaches itself to the next codon on the mRNA in the same way

4) The two amino acids attched to the tRNA are joined by a peptide bind and the first tRNA molecule moves away leaving the amino acid

5) This process continues until there is a stop signal

6) The polypeptide chain created the moves away from the ribosome

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Regulation of transcription and translation

Transcriptional factors

  • Transcriptional factors move from the cytoplasm to the nucleus 
  • In the nucleus they bind to specific DNA sites near the start of their target gene
  • They control the rate of transcription
  • Activators increase the rate of transcrition and repressors decease the rate

Oestrogen

  • Oestrogen is a hormone which affects trascription by binding to the transcriptional factor called an oestrogen receptor
  • It moves to into the nucleus where it binds to a specific DNA sites near that start of the target gene
  • Its a repressor and an activator- depends on the cell type

siRNA 

  • In the cytoplasm siRNA + proteins bind to the target mRNA
  • The proteins cut up the mRNA- so translation cant occur
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Gene mutation

Mutations- passed on if in gametes but not if in body cells

Subsitution of bases

Non-sense mutations

  • Base changes results in premature stop codon-Full protein isnt formed
  • Protein cant function properly

Mis-sense mutations

  • Base changes results in different amino acid being coded for
  • Significance depends on the role of the amino acid- bonding could be affected

A silent mutation

  • Different base but still codes for the same amino acid due to degenerate nature

Deletion of bases

  • Amino acid sequence is different- frame shift. Less signifant if at the end of the gene.
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