Transcription (AQA A Level Biology): Revision Note

Transcription

• The process of protein synthesis occurs in two stages:

  • Transcription – DNA is transcribed, and an mRNA molecule (messenger RNA) is produced

  • Translation – mRNA is translated, and an amino acid sequence is produced

The process of transcription

• Transcription occurs in the nucleus of the cell

• A section of the DNA molecule unwinds; this section contains the gene from which a particular polypeptide (protein) will be produced

• Unwinding occurs due to the breaking of hydrogen bonds between the complementary base pairs; the DNA is said to be 'unzipped'

  • This reaction is catalysed by the enzyme helicase, as in DNA replication

• The gene to be transcribed is now exposed

• A complementary copy of the code from the gene is made by creating a molecule of mRNA

  • Free activated RNA nucleotides pair up (via hydrogen bonds) with their complementary DNA bases on the ‘unzipped’ DNA molecule; this DNA strand is called the template strand

    • The strand of the DNA molecule that is not transcribed is called the non-template strand or the non-transcribed strand

    • The base sequence of the non-transcribed strand will be the same as the base sequence of the mRNA transcript, but with uracil replacing thymine

  • The sugar-phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar-phosphate backbone of the mRNA molecule

• When the gene has been transcribed, the mRNA molecule is complete, the hydrogen bonds between the mRNA and DNA strands break, and the DNA molecule re-forms into its double helix strcuture

• The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

The role of RNA polymerase

• RNA polymerase moves along the template strand in the 3' to 5' direction

  • This means that the mRNA molecule grows in the 5' to 3' direction

• Because the mRNA is formed by complementary pairing with the DNA template strand, the mRNA molecule contains the same sequence of nucleotides as the DNA coding strand (although the mRNA will contain uracil instead of thymine)

The template strand of the DNA molecule is the one that is transcribed