CIE A Level Biology复习笔记6.2.3 Constructing Polypeptides

• 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 complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)
• Free activated 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

The transcription stage of protein synthesis – DNA is transcribed and an mRNA molecule is produced

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) and 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 triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (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 – this 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

The translation stage of protein synthesis – tRNA molecules bind with their specific amino acids

The translation stage of protein synthesis – an amino acid chain is formed