Translation in Protein Synthesis (DP IB Biology)

Synthesis of Polypeptides

• Translation involves taking the genetic code from the mRNA and synthesising a polypeptide

  • A polypeptide is a sequence of amino acids covalently bonded together

  • The order of the amino acids is based on the information stored in the genetic code of the mRNA

• This stage of protein synthesis occurs in the cytoplasm of the cell

• The mRNA template comes from the process of transcription, and so translation always takes place following these events

  • After transcription the mRNA moves out of the nuclear pore and diffuses into the cytoplasm towards the ribosome for translation

Roles of RNA & Ribosomes in Translation

• After leaving the nucleus, the mRNA molecule attaches to a ribosome

• A ribosome is a complex structure that is made of a large and small subunit

  • Ribosomes are themselves made of proteins and RNA (called ribosomal RNA or rRNA)

• There are binding sites on the subunits for the various other molecules involved in translation

  • The mRNA binds to the small subunit

  • Two tRNA molecules are able to bind to the large subunit simultaneously 

mRNA in the ribosome diagram

A ribosome is built of large and small subunits, ribosomal RNA and an area on the surface that catalyses the formation of peptide bonds in a newly-synthesised protein

• Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA

• In the cytoplasm, there are free molecules of tRNA (transfer RNA)

• 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

tRNA and mRNA before translation diagram

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

Codons & Anticodons

• Codons of three bases on mRNA correspond to one amino acid in a polypeptide

  • A triplet is a sequence of three DNA bases that codes for a specific amino acid

  • A codon is a sequence of three mRNA bases that codes for a specific amino acid

  • A codon is transcribed from the triplet and is complementary to it

• An anticodon is a sequence of three transfer RNA (tRNA) bases that are complementary to a codon

  • The transfer RNA carries the appropriate amino acid to the ribosome

  • The amino acid can then be condensed onto the growing polypeptide chain

• Certain codons carry the command to stop translation when the polypeptide chain is complete. These are called stop codons

Structure of tRNA diagram

The anticodon is positioned at the bottom of the tRNA molecule and consists of three exposed RNA bases 

mRNA and tRNA binding diagram

Complementary base pairing occurs between the mRNA and the corresponding tRNA molecule, resulting in the correct sequence of amino acids being synthesised into the polypeptide

Analogy: Think of transcription and translation as being like converting between languages

• Each language has its alphabet, just as nucleic acids and proteins have their monomers

• Transcription is like converting text from English to French

  • The same characters are used, but there are slight differences

  • French uses the same alphabet as English but employs occasionally accented characters like â, é, or ç

  • DNA and RNA employ largely the same monomers but with slight differences in the two pentose sugars and U replacing T.

• Translation is like converting text from a Western language to a language that uses a different alphabet, like Japanese

  • A completely different set of characters is used

  • The sequence of characters is unrecognisable from the original

  • If we could see them, a chain of amino acids would look nothing like a chain of nucleotides

Transcription and Translation Can be Likened to the Conversion Between Languages Table