2.1.3 Nucleotides and Nucleic acids - Protein Synthesis

Question 1

Explain transcription

Answer 1

1. The section of DNA that contains the gene unwinds and unzips under the control of a DNA helicase, beginning at a start codon. This involves the breaking of hydrogen bonds between the bases.
2. Free RNA nucleotides (A-U, C-G) will base pair with complementary bases exposed on the antisense strand when the DNA unzips.
3. Phosphodiester bonds are formed between the RNA nucleotides by the enzymes RNA polymerase.
4. Transcription stops at the end of the gene and the completed short strand of RNA is called messenger RNA (mRNA).
5. The mRNA then detaches from the DNA template and leaves the nucleus through a nuclear pore. The DNA double helix reforms.
6. This mRNA molecule then travels to a ribosome in the cell cytoplasm for the next step in protein synthesis.

Question 2

DNA to Protein steps

Answer 2

DNA –> RNA –> polypeptide chain of amino acids (protein)

Question 3

Why does transcription need to occur?

Answer 3

DNA molecules (double helix) are too large to leave the nucleus through the nuclear envelope therefore they need to be transcribed to RNA in order to supply the coding information needed to determine the protein’s amino acid sequence.

Question 4

What is a sense strand?

Answer 4

Runs from 5’ to 3’. Only one of the two strands of DNA contains the code for the protein to be synthesised.

Question 5

What is an antisense strand?

Answer 5

Runs from 3’ to 5’. It acts as a template strand during transcription, so that the complementary RNA strand formed carries the same base sequence as the sense strand.

Question 6

Where does translation in eukaryotic cells occur?

Answer 6

In the ribosomes. The ribosomes have two subunits, one large and one small.

Question 7

What is rRNA important for?

Answer 7

rRNA is important in maintaining the structural stability of the protein synthesis sequence and plays a biochemical role in catalysing the reaction.

Question 8

Quick summary of translation (2-3 marks)

Answer 8

After leaving the nucleus, the mRNA binds to a specific site on the small subunit of a ribosome. The ribosome holds mRNA in position while it is decoded, or translated, into a sequence of amino acids. (this process is known as translation).

Question 9

Explain transcription (5-6 marks)

Answer 9

1. The mRNA binds to the small subunit of the ribosome at its start codon (UAG)
2. A tRNA with the complementary anticodon (UAC) binds to the mRNA start codon. (This tRNA carries the amino acid)
3. Another tRNA with the anticodon UGC and carrying the corresponding amino acid, 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 is transferred to the amino acid on the second tRNA by the formation of a peptide bond. This is catalysed by the enzyme peptidyl transferase, which is an rRNA component of the ribosome.
5. The ribosome then moves along the mRNA, releasing the first tRNA. The second tRNA becomes the first.
6. Stages 3-5 are repeated, with another amino acid added to the chain each time. The process keeps repeating until the ribosome reaches the end of the mRNA at a stop codon and the polypeptide is released.