Protein Synthesis

Question 1

Describe the journey of a protein after synthesis.

Answer 1

1) Synthesised in ribosomes on the rER.
2) They then pass into the cisternae and are packaged into transport vesicles.
3) Vesicles containing the newly synthesised proteins move towards the Golgi apparatus via the the transport function of the cytoskeleton (microtubules).
4) The vesicles fuse with the cis face of the Golgi apparatus and the proteins enter. The proteins are structurally modified before leaving the Golgi apparatus in secretory vesicles from its trans face.
5) Secretory vesicles carry proteins that are to be released from the cell. The Secretory vesicles move towards and fuse with the cell-surface membrane, releasing their contents by exocytosis. Some vesicles form lysosomes which contain enzymes for use in the cell.

Question 2

Describe the process of transcription.

Answer 2

• DNA helicase breaks H-bonds between complementary base pairs on opposite DNA nucleotides on the two strands, ‘unzipping’ the double helix.
• Free RNA bases move into place on the template (antisense - 3’ to 5’) strand as it is a complementary copy of the sense strand. H-bonds form between complementary base pairs between the new RNA nucleotides and the DNA nucleotides on the template strand.
• RNA polymerase moves along the free RNA nucleotides, catalysing the formation of phosphodiester bonds between adjacent RNA nucleotides by condensation.
• Transcription stops at the end of the gene, and the completed molecule is called mRNA. It is identical to the sense strand except for Uracil replacing Thymine.
• mRNA is small enough to leave the nucleus so leaves through a nuclear pore. It can now take part in translation.
• The DNA double helix reforms.

Question 3

Describe the process of translation.

Answer 3

• The mRNA binds to a specific site on the small subunit of a ribosome. The ribosome holds the mRNA molecule.
• There are free tRNA molecules present in the cytoplasm.
• tRNA molecules have a specific triplet of unpaired bases at one end (the anticodon) and a region where a specific amino acid can attach at the other.
• The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome.
• The tRNA molecule with a complementary anticodon to the mRNA molecule’s start codon pairs with the mRNA start codon.
• The second tRNA molecule binds in the same way, this time to the second mRNA codon.
• rRNA in the ribosome’s subunit catalyse the formation of peptide bonds between the first two adjacent amino acids. Once they are bonded together the first tRNA molecule moves away, leaving its AA behind.
• This process of tRNA molecules binding to the mRNA strand one by one continues until a stop codon is reached. Simultaneously AAs are peptide bonded together to form a polypeptide. Both processes stop once a stop codon is reached.
• The polypeptide can now move away from the ribosome.