Protein Synthesis (OCR Gateway GCSE Biology)

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