IGCSE Biology / Chromosomes, genes and DNA
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Edexcel International GCSE Biology (4BI1), chapter 16: Chromosomes, genes and DNA, specification point 3.18B. Free lesson.
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Protein synthesis uses transcription to copy a gene into messenger RNA, then translation to build a protein from that message. The two stages happen in different places, and the order of DNA bases determines the order of amino acids in the protein.
Why messenger RNA is needed
A gene is a sequence of bases in DNA, which is kept in the nucleus. The ribosomes that build proteins are in the cytoplasm, so a message must carry the gene's instructions between them. DNA is too large to leave the nucleus through a pore in its membrane. Messenger RNA (mRNA) is a short, single-stranded copy that can pass through. The message allows the protein to be built in the cytoplasm without the DNA leaving the nucleus.
How transcription copies a gene
Transcription is the copying of a gene into mRNA. It happens in the nucleus. The DNA at the gene unwinds and its two strands separate when the hydrogen bonds between their bases break. One strand acts as a template, so RNA nucleotides pair with its exposed bases. In RNA, uracil pairs with adenine instead of thymine.
How translation builds a protein
Translation is the reading of the mRNA message to build a protein. It happens at a ribosome in the cytoplasm. The ribosome reads the mRNA in groups of three bases. Each group, called a codon, specifies one amino acid. Transfer RNA (tRNA) molecules bring amino acids to the ribosome. Each tRNA carries one specific amino acid and has an anticodon that pairs with the matching codon. The ribosome joins the amino acids in the order specified by the codons. When the ribosome reaches a stop codon, the amino acid chain is released and folds into a protein.
How base order determines protein function
The sequence of bases in a gene determines the sequence of codons in its mRNA. Those codons set the order of amino acids in the chain. The amino acid order determines how the chain folds, and its shape determines the protein's function. A gene does not directly make a protein. It is transcribed into mRNA in the nucleus, then the mRNA is translated at a ribosome in the cytoplasm.