Protein Synthesis (OCR GCSE Biology A (Gateway))

How Proteins are Made

Higher Tier Only

DNA Codes for Proteins

• DNA contains a genetic code that contains the information needed to make all the proteins needed by an organism

• A section of DNA that codes for a particular protein is called a gene

  • There are many different genes within the DNA molecule coding for thousands of proteins

• Proteins are made of chains of molecules called amino acids

  • The amount, type and order of amino acids in the chain is variable

  • This results in many types of proteins that differ in shape

  • The shape of the protein helps to define its function

Proteins are coded for by genes in DNA

The Triplet Code

• It is the particular order of bases in DNA that dictates which amino acids will be used to build the protein

• A DNA sequence can be thought of as a collection of groups of three bases

  • Each three base grouping is called a triplet code

  • The triplet code contains all the information to code for a single specific amino acid

• The order of bases (triplet codes) controls which type of amino acids will later be joined together to make a protein

Protein synthesis is a multi-step process

• Proteins cannot be made directly from the DNA gene sequence

• Therefore, protein synthesis occurs in two stages called transcription and translation

  • In eukaryotes transcription takes place in the nucleus, translation in the cytoplasm 

• During transcription an intermediary molecule called mRNA (messenger RNA) is produced

  • The mRNA contains a copy (transcript) of the information stored in the gene

  •  mRNA is similar to DNA but much shorter and it is only single-stranded

  • To make the gene accessible, the two DNA strands in the double helix need to be unwound and separated (un-zipped)

  • mRNA can then be made using the DNA as a template to copy from

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

• In eukaryotes the newly produced mRNA molecule moves out of the nucleus into the cytoplasm

• During translation the mRNA is translated (decoded) to determine which amino acids are needed

  • Amino acids that match the triplet codes on the mRNA are joined together in the correct order

  • A long chain of amino acids is produced which forms into a protein