Molecular Biology Essentials: DNA Structure, Replication, and Protein Synthesis

Posted on Feb 28, 2026 in Biology

1. DNA Structure

      • Double Helix: Twisted-ladder shape.

      • Nucleotide: Sugar (deoxyribose) + Phosphate + Nitrogenous base.

      • Bases: Purines (Adenine [A], Guanine [G]); Pyrimidines (Cytosine [C], Thymine [T]).

      • Pairing: A pairs with T (2 Hydrogen bonds), C pairs with G (3 Hydrogen bonds).

      • Chargaff’s Rule: A = T, and C = G.

      • Antiparallel Strands: Run in opposite directions (5’→3′ and 3’→5′).

Key Scientists

      • Griffith: Discovered the transforming factor.

      • Avery: Identified DNA as the transforming factor.

      • Chargaff: Established base pairing rules.

      • Hershey & Chase: Proved DNA is the genetic material.

      • Franklin & Wilkins: Produced X-ray diffraction images of DNA.

      • Watson & Crick: Developed the double helix model.

2. DNA Replication (Semiconservative)

Enzymes Involved

      • Helicase: Unzips the DNA double helix.

      • Topoisomerase: Relieves supercoiling ahead of the fork.

      • Primase: Adds the initial RNA primer.

      • DNA Polymerase III: Adds new nucleotides in the 5’→3′ direction.

      • DNA Polymerase I: Replaces the RNA primer with DNA nucleotides.

      • Ligase: Joins Okazaki fragments on the lagging strand.

      • Telomerase: Extends the ends (telomeres) of linear chromosomes.

Replication Steps

      1. Initiation: Helicase unwinds DNA, creating a replication fork.

      2. Primer Binding: Primase adds an RNA primer to start synthesis.

      3. Elongation: DNA Polymerase III adds nucleotides:

        • Leading Strand: Synthesis is continuous.

        • Lagging Strand: Synthesis is discontinuous, forming Okazaki fragments.

      4. Joining: Ligase seals the gaps between fragments.

      5. Termination: Replication concludes, often at telomeres.

3. RNA Structure & Types

      • Sugar: Ribose.

      • Bases: Adenine (A), Guanine (G), Cytosine (C), Uracil (U) replaces Thymine (T).

      • Structure: Typically single-stranded.

      • Types:

        • mRNA (messenger RNA): Carries the genetic code from DNA.

        • tRNA (transfer RNA): Brings specific amino acids; contains an anticodon.

        • rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

4. Protein Synthesis

Transcription (In the Nucleus): DNA → mRNA

      • RNA polymerase binds to the promoter region.

      • Elongation: RNA polymerase adds complementary RNA nucleotides.

      • Termination: Synthesis stops at a specific terminator sequence.

Translation (At the Ribosome): mRNA → Protein

      • An mRNA codon pairs specifically with a tRNA anticodon.

      • Ribosome Sites:

        • A site: Where the new tRNA carrying an amino acid enters.

        • P site: Holds the growing polypeptide chain.

        • E site: Where the empty tRNA exits.

      • Start Codon: AUG (codes for Methionine).

      • Stop Codons: UAA, UAG, UGA (signal termination).

Genetic Code Properties

      • Codon: A sequence of 3 nucleotides that specifies one amino acid.

      • The code is universal (used by most life) and redundant (multiple codons can code for the same amino acid).

5. Mutations

Gene Mutations (Point Mutations)

      • Point Mutation: A change in a single base pair.

        • Silent: No resulting amino acid change.

        • Missense: Results in a different amino acid.

        • Nonsense: Creates a premature stop codon.

      • Frameshift Mutation: Caused by insertion or deletion of bases, shifting the entire reading frame.

Chromosomal Mutations

      • Deletion: A segment of the chromosome is removed.

      • Duplication: A segment is repeated.

      • Inversion: A segment is reversed end-to-end.

      • Translocation: A segment moves to a non-homologous chromosome.

      • Nondisjunction: Failure of chromosomes to separate during meiosis, leading to abnormal chromosome numbers (e.g., aneuploidy).

Examples of Disorders

      • Down Syndrome: Trisomy 21 (three copies of chromosome 21).

      • Turner Syndrome: Monosomy X (XO).

      • Klinefelter Syndrome: XXY genotype.

      • Cri du Chat: Deletion on chromosome 5.

      • Progeria: Caused by a mutation in the LMNA gene.

      • Other Conditions: Chronic Myelogenous Leukemia (CML), Hemophilia A, and Charcot-Marie-Tooth disease result from various specific mutations.

6. Causes & Effects of Mutations

Causes (Mutagens)

      • Exposure to radiation (e.g., X-rays, gamma rays).

      • Exposure to certain chemicals.

      • Infection by certain viruses.

Effects

      • Harmful: Can lead to genetic diseases or disorders.

      • Beneficial: Can lead to adaptation, increased diversity, and evolution.

7. Key Techniques

      • Karyotyping: Technique used to visualize and analyze chromosomes to detect large-scale abnormalities.

Central Dogma Summary: DNA → RNA → Protein.

Replication Principle: Semiconservative replication ensures each new DNA molecule contains one old strand and one newly synthesized strand.

Mutation Impact: Mutations can be silent, harmful, or beneficial, affecting either individual genes or entire chromosomes.