Protein synthesis is the process by which cells build proteins based on the instructions carried in DNA. It involves two main steps: transcription and translation.

Transcription (DNA → mRNA)

• Where: In the nucleus (for eukaryotes).
• Process:
  • DNA is copied into messenger RNA (mRNA).
  • Only one strand of DNA acts as a template.
  • The base pairing rule is followed: A (adenine) pairs with U (uracil) in RNA (instead of T), C(cytosine) pairs with G(guanine).
• Result: A strand of mRNA that carries the code for a protein out of the nucleus to the ribosome.

Translation (mRNA → Protein)

• Where: In the ribosome (in cytoplasm).
• Key Players:
  • mRNA (the messenger)
  • Ribosome (the reader and builder)
  • tRNA (transfer RNA, the carrier of amino acids)
  • Amino acids (building blocks of proteins)

Codons and Reading Frame

• Codon: A group of 3 mRNA bases (nucleotides) that codes for one amino acid.
• The ribosome reads the mRNA 3 nucleotides at a time.(so the 5th amino acid in the sequence is coded by the 5th codon )
• Examples of codons:
  • AUG = Start codon (also codes for methionine)
  • UUU = Phenylalanine
  • GGC = Glycine
• There are specific codons for each amino acid, and the matching is universal across most living organisms.

(note that there’s no need to memorise this as a chart will be given for any question involving this concept)

Role of tRNA (Transfer RNA)

• Each tRNA molecule carries a specific amino acid.
• tRNA has an anticodon, a set of 3 bases that are complementary to the mRNA codon.
• Process:
  • The tRNA matches its anticodon to the codon on the mRNA.
  • The ribosome links the carried amino acid of tRNA to the growing protein chain.
  • The ribosome moves along the mRNA, reading codons and bringing the correct tRNAs one by one.

Building the Protein

• The ribosome joins amino acids together by forming peptide bonds.
• This chain of amino acids is called a polypeptide.
• The sequence in which amino acids are joined determines the primary structure of the protein.

How Protein Shape Depends on Sequence

• Protein structure levels:
  1. Primary structure: Sequence of amino acids.
  2. Secondary structure: Folding into alpha helices or beta sheets.
  3. Tertiary structure: Overall 3D shape due to side chain interactions.
  4. Quaternary structure (for some proteins): Arrangement of multiple polypeptide chains.
• The order of amino acids directly influences how the protein folds and its final shape.
• A protein’s shape determines its function (e.g., enzymes, hormones, antibodies).

Quick Summary Table

Step	Location	Main Idea
Transcription	Nucleus	DNA → mRNA
Translation	Ribosome (cytoplasm)	mRNA → Amino acid chain
Codon Reading	Ribosome reads 3 bases (codon) at a time
tRNA Role	Carries specific amino acid, matches codon
Protein Folding	Sequence of amino acids decides final 3D shape

Fun Facts

• The genetic code is universal: same codons code for the same amino acids in almost all organisms!
• One mistake in sequence (mutation) can change a protein’s shape and cause diseases (like sickle-cell anemia).

Written by Kasiban Parthipan