Nucleic acids and protein synthesis

Requirements of a Genetic Material

• Must have:

  • Ability to store information → instructions to control cell behaviour.

  • Ability to copy itself accurately → ensures no loss of information during cell division.

• Early assumption (before 1940s): proteins carried genetic info (thought too complex for DNA).

• 1940s–50s: Experiments proved DNA is the genetic molecule.

Structure of DNA and RNA

• DNA → Deoxyribonucleic acid.

• RNA → Ribonucleic acid.

• Both are nucleic acids (originally found in nucleus).

• Both are polymers (polynucleotides) → built from nucleotides (monomers).

Nucleotides components

• Nitrogen-containing base

  • DNA: A (adenine), G (guanine), T (thymine), C (cytosine).

  • RNA: A, G, C, U (uracil replaces T).

  • Purines (2 rings): A, G.

  • Pyrimidines (1 ring): T, C, U.

• Pentose sugar

  • Ribose → RNA.

  • Deoxyribose → DNA (one oxygen atom less).

• Phosphate group → gives acidic nature.

ATP

• A nucleotide, not part of DNA/RNA.

• Structure: adenine + ribose + phosphate groups.

• Forms:

  • AMP (adenosine monophosphate).

  • ADP (adenosine diphosphate).

  • ATP (adenosine triphosphate).

• ⚠ Don’t confuse:

  • Adenine (base) vs. Adenosine (adenine + sugar).

  • Thymine (base) vs. Thiamine (vitamin).

Polynucleotides

• Nucleotides join via condensation → phosphodiester bonds (phosphate + sugar).

• Structure: sugar-phosphate backbone with bases projecting sideways.

• Key terms:

  • Dinucleotide → two nucleotides joined by phosphodiester bond.

  • Phosphodiester bond → joins nucleotides, two ester bonds (one on each side).

Structure of DNA

• Discovered by Watson & Crick (1953).

• Features:

  • Two anti-parallel polynucleotide chains.

  • Twisted into double helix.

  • Held by hydrogen bonds between bases.

  • Complementary base pairing:

    • A pairs with T (2 hydrogen bonds).

    • G pairs with C (3 hydrogen bonds).

  • Purine always pairs with pyrimidine → width constant (3 rings).

  • One complete turn = 10 base pairs.

  • Base ratio rule (Chargaff):

    • %A ≈ %T, %G ≈ %C.

• Information storage: sequence of bases = coded message.

• Replication: possible by unzipping → each strand acts as template.

Structure of RNA

• Single polynucleotide strand.

• Types:

  • mRNA → messenger, carries code to ribosome.

  • tRNA → transfer, carries amino acids.

  • rRNA → ribosomal, structural component of ribosomes.

DNA Replication

• Occurs during S phase of cell cycle.

• Steps:

  1. DNA unzips (hydrogen bonds broken).

  2. DNA polymerase attaches to each strand, adds complementary nucleotides (5′ → 3′ direction).

     • Leading strand → continuous replication.

     • Lagging strand → discontinuous replication → short Okazaki fragments.

  3. DNA ligase joins nucleotides and Okazaki fragments with phosphodiester bonds.

• Semi-conservative replication → each new DNA has one original strand + one new strand.

The Genetic Code

• Sequence of bases = code for sequence of amino acids in proteins.

• Features:

  • Triplet code (3 bases = 1 amino acid).

  • Universal (same in all organisms).

  • Punctuated → start (e.g., TAC for methionine) & stop codons.

  • Redundant/degenerate → amino acids have more than one triplet.

Protein Synthesis

Stage 1: Transcription (in nucleus)

• DNA → mRNA.

• Enzyme: RNA polymerase.

• Only template strand copied.

• Base pairing: A → U, G → C, T → A, C → G.

• Primary transcript → modified by RNA processing.

  • Introns removed, exons joined (splicing).

  • Alternative splicing → different proteins from same gene.

• mRNA leaves nucleus via nuclear pore.

Stage 2: Translation (at ribosomes)

• mRNA → polypeptide.

• tRNA:

  • Carries amino acid at one end.

  • Has anticodon (3 bases complementary to mRNA codon).

• Steps:

  1. tRNA binds to codon on mRNA.

  2. Ribosome holds two tRNAs side by side → peptide bond forms.

  3. Ribosome moves along mRNA, process repeats.

  4. Stops at stop codon → polypeptide released and folded.

Gene Mutations

• Mutation = random change in DNA base sequence.

• Caused by copying errors, radiation, carcinogens (mutagens).

• Types:

  1. Substitution → one base replaced.

     • May change amino acid (missense), no change (silent), or stop codon (nonsense).

     • Example: Sickle cell anaemia (Glu → Val substitution in haemoglobin β-chain).

  2. Deletion → base removed.

  3. Insertion → base added.

  • Deletion/insertion → frame-shift mutations → big effect on protein.