Ch 15: Ribosomes and Protein Synthesis

protein synthesis

• decodes mRNA to produce a polypeptide

• polypeptides are formed when the amino group of one amino acid forms an amide (i.e. peptide) bond with the carboxyl group of another amino acid

• the reaction is catalyzed by ribosomes

• 5’-3' → N’-C’

translation process

1. initiation:

   1. mRNA attached to the smaller subunit of the ribosome

   2. AUG is the start codon - a tRNA with the appropriate anticodon attaches to small subunit, briefly binds to mRNA

   3. the large subunit of the ribosome then comes in

2. elongation:

   1. tRNAs move in with the appropriate amino acid, the amino acid chain grows using peptidyl transferase

3. termination

   1. STOP codon is reached

   2. the amino acid chain then is processed

      1. in eukaryotes, the amino acid chain moves into the endoplasmic reticulum to be further processed

         1. rough ER: early vesicles nubs: can take it anywhere in cell needed

molecular components of translation

1. transfer RNAs

2. ribosomes

3. messenger RNA

4. polypeptide

5. ATP

the structure and function of transfer RNA

• molecules of tRNA are not all identical

  • each carries a specific amino acid on one end

  • each has an anticodon on the other end

• a tRNA molecule

  • consists of a single RNA strand that is only about 80 nucleotides long

  • is roughly L-shaped

• aminoacyl-tRNA synthetase

  • enzyme that joins each amino acid to the correct tRNA

the ribosome

• responsible for translating the mRNA into protein

• consists of a large and small ribosomal subunit

  • assembly of the subunits on the mRNA forms tRNA binding sites

• during translation charged tRNAs enter the Acceptor site and the anticodon on the tRNA base pairs with the codon in the mRNA

• exposes first codon AUG, starts at 5’

• AND ATP!

ribosomal sites

• e-site (left): used tRNA exits

• p-site (center): growing polypeptide is housed (peptidyl)

• a-site (right): new tRNA comes in (aminoacyl)

the polypeptide

• produced through assembly of amino acids bonded together in a specific substance

  • interaction of one tRNA in P site with another tRNA in A site

    • directed by bonding of an mRNA codon to the anticodon of a tRNA

    • occurs in ribosomes found in cytoplasm of the cell

translation initiation

brings together mRNA, initiator tRNA, and 2 subunits of ribosome

general steps of translation initiation

1. mRNA binds to the ribosomal subunit

2. the START codon is located

3. the initiation tRNA binds to the START codon

4. energy is used to recruit and bind the large ribosomal subunit

translation elongation

amino acids are bonded together to build the polypeptide chain out of the P site

1. tRNA binds to exposed codon

2. new amino acid attached to polypeptide chain

3. ribosome shifts on codon over on the mRNA

translation termination

reached when the STOP codon is recognized in the mRNA

• no tRNA matches the STOP codon

general steps of termination

1. the STOP codon in the mRNA is reached and recognized

2. a release factor is recruited and binds to the STOP codon causing the hydrolysis of the polypeptide from the tRNA

3. this bonding and a bit of energy is utilized to cause the disassociation of the translation components

protein folding, modification, and targeting

• during and after translation amino acids may be chemically modified

• signal sequences at amino end direct protein to destination

  • signal-recognition particles (SRP) act as conductors

  • signal sequence removed

• chaperones help proteins fold properly

spontaneous mutations

occur randomly during DNA replication, recombination, or repair

induced mutations

mutagens are agents of mutation

• either random or induced

point mutations

• silent

• missense

• nonsense

silent mutation

has no effect on the protein sequence

missense mutation

results in an amino acid substitution

nonsense point mutations

substitutes a stop codon for an amino acid