Constructing Polypeptides (Cambridge (CIE) AS Biology): Revision Note

Constructing polypeptides: transcription & translation

• A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)

• This process of protein synthesis occurs in two stages:

  • Transcription:

    • DNA is transcribed and an mRNA molecule is produced

  • Translation:

    • mRNA (messenger RNA) is translated and an amino acid sequence is produced

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)

  • The DNA triplets of bases are transcribed as mRNA codons

  • The single-stranded mRNA molecule can fit through the pores in the nuclear envelope, unlike double-stranded DNA

• 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

  • The double-stranded DNA molecule then re-forms

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

Translation

• This stage of protein synthesis occurs in the cytoplasm of the cell

• After leaving the nucleus, the mRNA molecule attaches to a ribosome

• In the cytoplasm, there are free molecules of tRNA (transfer RNA)

• These tRNA molecules have:

  • A triplet of unpaired bases at one end, known as the anticodon

  • A region where a specific amino acid can attach at the other

• There are at least 20 different tRNA molecules, each with a specific anticodon and specific amino acid binding site

• The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome

• The anticodon on each tRNA molecule pairs with a complementary codon on the mRNA molecule

• Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side

• A peptide bond is then formed between the two amino acids

• This process continues until a ‘stop’ codon on the mRNA molecule is reached 

  • The 'stop' codon acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete

• This amino acid chain then forms the final polypeptide