3: Protein synthesis

Question 1

Where does protein synthesis occur?

Answer 1

In the cytoplasm at ribosomes

Question 2

What is transcription?

Answer 2

The process of copying and transporting base sequences of genes to the site of protein synthesis (a ribosome)

Question 3

What is the sense strand?

Answer 3

The strand of DNA that contains the code for the protein to be synthesised. Runs from 5’ to 3’

Question 4

What is the antisense strand?

Answer 4

The other strand of DNA that is a complementary copy of the sense strand and does not code for a protein. Runs from 3’ to 5’. Acts as a template strand during transcription, so that the complementary RNA strand formed carries the same base sequence as the sense strand

Question 5

Describe the process of transcription

Answer 5

1. The section of DNA that contains the gene unwinds and unzips (by DNA helicase), beginning at a start codon. Hydrogen bonds between bases are broken
2. Free RNA nucleotides will base pair with complimentary bases exposed on the antisense strand when the DNA unzips
3. Phosphodiester bonds are formed between RNA nucleotides by RNA polymerase
4. Transcription stops at the end of the gene and the completed short strand of RNA is messenger (m)RNA
5. The mRNA then detaches from the DNA template and leaves the nucleus through a nuclear pore
6. The DNA double helix reforms
7. The mRNA molecule travels to a ribosome for translation

Question 6

What is the role of ribosomal (r)RNA?

Answer 6

Maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction

Question 7

What is transfer (t)RNA?

Answer 7

Another form of RNA, necessary for translation of mRNA. Composed of a strand of RNA folded so that three bases (the anticodon) are at one end of the molecule. The anticodon will bind to a complementary codon on mRNA. tRNA molecules carry an amino acid corresponding to that codon. When tRNA anticodons bind to complementary codons along mRNA, amino acids are brought together in the correct sequence to form the primary structure of the protein coded for by the mRNA. This cannot happen all at once so amino acids are added one at a time and the polypeptide chain (protein) grows as this happens

Question 8

What do the ribosomes do?

Answer 8

Act as a binding site for mRNA and tRNA and catalyse the assembly of the protein

Question 9

Describe the full process of translation

Answer 9

1. mRNA binds to the small subunit of the ribosome as its start codon (AUG)
2. A tRNA with the complementary anticodon (UAC) binds to the mRNA start codon. This tRNA carries the amino acid methionine
3. Another tRNA with the anticodon UGC and carrying the corresponding amino acid, threonine, then binds to the next codon on the mRNA (ACG). A maximum of two tRNAs can be bound at the same time
4. The first amino acid, methionine, is transferred to the amino acid (threonine) on the second tRNA by the formation of a peptide bond. This is catalysed by peptidyl transferase (rRNA component of the ribosome)
5. The ribosome then moves along the mRNA, releasing the first tRNA. The second tRNA becomes the first
   Stages 3-5 are repeated, with another amino acid added to the chain each time, until the ribosome reaches a stop codon and the polypeptide is released