Transcription in Protein Synthesis (SL IB Biology)

• 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

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

• In the transcription stage of protein synthesis, free RNA nucleotides pair up with the exposed bases on the DNA molecule but only with those bases on one strand of the DNA molecule
• The RNA will have a complementary base sequence to the DNA strand and will bind to the DNA using hydrogen bonds
• The adenine of the DNA is complementary to uracil on the new RNA strand, because a thymine RNA nucleotide does not exist

Complementary base pairing between the DNA and the RNA transcript table

• The strand of the DNA molecule that carries the genetic code is called the coding strand
• The opposite DNA strand is called the template strand
• To get an RNA transcript of the coding strand, the template strand is the one that is transcribed to form the mRNA molecule
  • This mRNA molecule will later be translated into an amino acid chain

DNA coding and template strand during transcription diagram

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

• DNA is a very stable molecule due to the hydrogen bonding between the DNA bases of the two strands and the strong phosphodiester bonds between adjacent nucleotides in each strand
  • This means that the genetic code is not prone to spontaneously breaking or changing
• This feature allows single DNA strands to act as reliable templates for transcription over several generations of cell replication
• In certain types of somatic cells that do not divide during their lifetimes, such as neurones and some types of muscle cells, the genetic sequence is conserved due to this stability and does not degrade over time

• There are approximately 20,000 protein-coding genes in the human genome
• Not every protein is needed in every cell
  • For example, the insulin protein is not needed in cardiac muscles of the heart
• As a result, our specialised cells have a way of switching certain genes off or on to match the requirements of the cell. This is called gene expression
  • Genes that are expressed are 'switched on' and undergo the process of transcription and translation
  • Genes that are not expressed are 'switched off' or silenced, and do not go through the process of transcription and/or translation
• There are various different mechanisms in the cell involved in controlling gene expression
• Transcription is the first stage of gene expression and so this is a key stage at which gene expression can be switched on or off