Bio ch 26 Molecular genetics

• Genetic code


  Base sequence of DNA determines the amino acid sequence in a polypeptide

  
  

  T changes to U

  
  

  Universal

  
  

  花名： Triplet code
• Translation


  The mRNA attaches to a ribosome.

  
  

  A specific amino acid is carried to the ribosome by a transfer RNA (tRNA) molecule. The anticodon on tRNA binds to the first (start) codon on the mRNA as they are complementary to each other.

  
  

  Another tRNA molecule carrying another amino acid binds to the next codon.

  
  

  The two amino acids are then joined by a peptide bond.

  
  

  The ribosome moves along the mRNA to add amino acids to the growing polypeptide chain, one at a time.

  
  

  Finally, the ribosome reaches a stop codon. The polypeptide formed is released from the ribosome.
• From polypeptides to proteins

  Most proteins

  made at ribosomes on the rough ER

  are secreted outside the cell

  (e.g. as digestive enzymes, hormones and antibodies)

  
  

  or embedded in the cell membrane (e.g. as carrier proteins and channel proteins).

  
  

  Proteins

  made at free-floating ribosomes

  remain in the cytoplasm.

  They are usually used by the cell itself.

  These proteins may serve as enzymes

  to catalyse reactions that occur inside the cell.

  Enzymes that catalyse the reactions in glycolysis (a stage of respiration) are examples of these proteins.
• substitution and inversion (gene)

  A substitution or an inversion of base(s)

  usually leads to a change of one triplet code only.

  
  

  Depending on the base(s) changed, there are three possible consequences:

  
  

  If the new code specifies thesame amino acid

  (e.g. a change of AAA into AAG on the template strand of DNA, both specify amino acid phenylalanine),

  the mutation hasno effecton the protein produced.

  
  

  If the new code specifies adifferent amino acid,

  the polypeptide produced will have one different amino acid.

  If the amino acid is important in determining the shape of the protein,

  the change willalter the shape of the protein

  and the resulting protein will becomenon-functional.

  
  

  If the new code specifies astop signal

  (e.g. a change of ATG into ATC on the template strand of DNA),

  the production of the polypeptidewill be stopped prematurely.

  The resulting protein is probablynon-functional.
• Deletion and insertion (gene)


  A deletion or an insertion of base(s)

  often has a dramatic effect on the protein produced.

  
  

  Deleting or inserting base(s) that is not a multiple of three

  will shift the reading frame

  (the grouping of bases).

  Thus, the whole amino acid sequence after the point of mutation is altered.

  The resulting protein is usually non-functional.
• Chromosome mutation

  i) Deletion Duplication Inversion Translocation

  
  

  ii) chromosome numb (Down syndrome)
• causes of mutations

  chemicals, radiation

  Blablabla