2.6.1 Transcription

• 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

DNA is transcribed and an mRNA molecule is produced

Sense and anti-sense strands

• 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 (with the substitution of Thymine with Uracil)
• The strand of the DNA molecule that carries the genetic code is called the sense strand
• The opposite DNA strand is called the antisense strand
• To get an RNA transcript of the sense strand, the antisense strand is the one that is transcribed to form the mRNA molecule
  • This mRNA molecule will later be translated into an amino acid chain

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

Analogy: Think of transcription and translation as being like converting between languages

• Each language has its alphabet, just as nucleic acids and proteins have their monomers
• Transcription is like converting text from English to French
  • The same characters are used, but there are slight differences
  • French uses the same alphabet as English but employs occasional accented characters like â, é, or ç
  • DNA and RNA employ largely the same monomers, but with the slight differences of the two pentose sugars and of U replacing T.

• Translation is like converting text from a western language to a language that uses a different alphabet, like Japanese
  • A completely different set of characters is used
  • The sequence of characters is unrecognisable from the original
  • If we could see them, a chain of amino acids would look nothing like a chain of nucleotides

Transcription and Translation Can be Likened to Conversion Between Languages Table