Synthesis of RNA
- This process of protein synthesis occurs in two stages:
- Transcription – DNA is transcribed and an mRNA molecule is produced
- mRNA is a single stranded RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm
- mRNA production requires the enzyme RNA polymerase
- Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced
- Transcription – DNA is transcribed and an mRNA molecule is produced
The process of transcription
- This stage of protein synthesis occurs in the nucleus of the cell
- Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)
- This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)
- A complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
- Free RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule
- 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 (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break and the double-stranded DNA molecule re-forms
- The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope
- This is where the term messenger comes from - the mRNA is despatched, carrying a message, to another part of the cell
- DNA can't make this journey; it's too big to fit through the pores in the nuclear envelope
Transcription in the nucleus diagram
DNA is transcribed and an mRNA molecule is produced
Examiner Tip
Be careful – DNA polymerase is the enzyme involved in DNA replication; RNA polymerase is the enzyme involved in transcription – don’t get these confused.