1. Introduction

  • Inheritance is controlled by the transmission of genetic material from parents to offspring.
  • The molecular basis of inheritance lies in DNA (Deoxyribonucleic Acid), which carries genetic information.
  • Gene expression determines how genetic information is converted into functional proteins.
  • Mutations are changes in the DNA sequence that can lead to variations or genetic disorders.

2. Structure of DNA

  • DNA is a double-helical molecule discovered by Watson and Crick in 1953.
  • It consists of nucleotides, each containing a phosphate group, deoxyribose sugar, and nitrogenous base.
  • Four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
  • Complementary base pairing: A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds).
  • DNA strands are antiparallel (5' to 3' direction on one strand, 3' to 5' on the other).

3. DNA Replication

  • DNA replicates in a semi-conservative manner.
  • Enzyme DNA helicase unwinds the double helix.
  • DNA polymerase adds new complementary nucleotides.
  • Ligase joins Okazaki fragments on the lagging strand.
  • Ensures accurate transmission of genetic information to new cells.

4. Gene Expression: Transcription and Translation

(i) Transcription

  • Process of copying DNA into messenger RNA (mRNA).
  • Occurs in the nucleus of eukaryotic cells.
  • RNA polymerase synthesizes mRNA from the DNA template.
  • mRNA contains codons (triplet bases) that code for amino acids.

(ii) Translation

  • Process of converting mRNA sequence into a polypeptide (protein).
  • Occurs in the ribosome.
  • Transfer RNA (tRNA) carries amino acids to ribosomes.
  • Each tRNA has an anticodon that pairs with an mRNA codon.
  • Polypeptide chain is formed, which folds into a functional protein.

5. Mutations

  • Mutations are permanent changes in the DNA sequence.
  • Can occur due to errors in replication, exposure to mutagens (radiation, chemicals), or viral infections.
  • Types of mutations:
    • Point mutation: Change in a single nucleotide (e.g., sickle cell anemia).
    • Frameshift mutation: Insertion or deletion of nucleotides, altering the reading frame.
    • Chromosomal mutations: Large changes affecting multiple genes.
  • Mutations can be beneficial, neutral, or harmful.

6. Genetic Disorders Caused by Mutations

  • Sickle Cell Anemia: Caused by a single-point mutation in the HBB gene.
  • Cystic Fibrosis: Result of a deletion mutation in the CFTR gene.
  • Down Syndrome: Caused by trisomy 21 (extra chromosome 21).
  • Understanding mutations helps in genetic counseling and disease prevention.

7. Applications of Molecular Genetics

  • Used in genetic engineering to modify organisms.
  • Plays a role in DNA fingerprinting for forensic investigations.
  • Essential for gene therapy to treat genetic disorders.
  • Helps in biotechnology for drug production and agriculture.

8. Conclusion

  • The structure and function of DNA are essential for inheritance.
  • Gene expression controls how genetic information is used in cells.
  • Mutations lead to genetic diversity but can also cause diseases.
  • Advancements in genetics have applications in medicine, agriculture, and biotechnology.

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