Edexcel IGCSE Biology 复习笔记 3.2.2 Protein Synthesis

• Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain
• Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
• Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
• Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
• Each RNA polynucleotide strand is made up of alternating ribose sugars and phosphate groups linked together, with the nitrogenous bases of each nucleotide projecting out sideways from the single-stranded RNA molecule
• An example of an RNA molecule is messenger RNA (mRNA), which is the transcript copy of a gene that encodes a specific polypeptide. Two other examples are transfer RNA (tRNA) and ribosomal RNA (rRNA)

Messenger RNA (mRNA) provides a good example of the structure of RNA

• 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 (protein) 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)
• The mRNA molecule 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) 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 (protein)

An overview of protein synthesis

The triplet code of DNA (carried by mRNA) is read by the ribosome and amino acids are attached together in a specific sequence to form the protein

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