Chapter 3.12 - Protein synthesis: Transcription

Question 1

What are the sites of protein synthesis?

Answer 1

Ribosomes free floating in the cytoplasm or ribosomes on the rough endoplasmic reticulum

Question 2

What is transcription?

Answer 2

The process of copying section of DNA base sequence to produce smaller molecules of mRNA, which can be transported out of the nucleus via nuclear pores to the site of protein synthesis

Question 3

Why is transcription required in protein synthesis?

Answer 3

DNA molecules are too large to leave the nucleus to supply the coding information needed

Question 4

What are the steps in transcription?

Answer 4

1. DNA helicase binds to the DNA and breaks the hydrogen bonds between the complementary bases, unwinding a region of the gene
2. The antisense strand acts as the template strand
3. Free floating RNA nucleotides are attracted to their complementary bases and pair accordingly, the thymine base in RNA nucleotides is replaced with uracil so uracil binds to adenine on the DNA template strand
4. Phosphodiester bonds form between RNA nucleotides and are catalysed by the enzyme RNA polymerase
5. This process ends when RNA polymerase reaches a stop codon
6. A complementary mRNA strand is formed, carrying the same base sequence as the DNA sense strand
7. The mRNA is released then detaches from the DNA template and leaves through a nuclear pore, the DNA double helix reforms

Question 5

What is a sense strand?

Answer 5

The strand of DNA that runs 5’ to 3’ and contains the genetic code for a protein

Question 6

What is an antisense strand?

Answer 6

The strand of DNA that runs 3’ to 5’ and is complementary to the sense strands, it acts as a template strand during transcription

Question 7

Why does the antisense strand act as the template strand rather than the sense strand?

Answer 7

The antisense strand acts as the template strand so that the complementary RNA strand formed carries the same base sequence as the sense strand