• 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 a, é, or ?
  • DNA and RNA employ largely the same monomers, but with the slight difference 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