Molecular Basis of Inheritance 📝

1. Structure of DNA and RNA

• DNA Structure:

• Composition: Deoxyribonucleic acid(DNA) consist of deoxyribose sugar, phosphate group and nitrogenous bases adenine(A), thymine(T), cytosine (C) and guanine(G).

• Double helix: DNA forms a double-stranded helix structure connected by hydrogen bonds between nucleotide bases. -Base pairing: A pairs with T, while C pairs with G, following the Chargaff’s rule.

• Types of RNA:

• Messenger RNA(mRNA): Carries genetic information from DNA to ribosomes for proteins synthesis.

• Transfer RNA(tRNA): Brings specific amino acids to the ribosome based on the mRNA codon during protein synthesis.

• Ribosomal RNA(rRNA): Essential components of ribosomes, catalyzing peptide bond formation during translation.

2. DNA Replication

• The process by which DNA duplicates itself before cell division.

• Key steps:

  • Initiation: Begins at specific origin sites of DNA.
  • Elongation: DNA polymerases enzyme add complementary nucleotides to the growing strands, following the base pairing rules.
  • Termination: Reaches specific termination sequences, signalling the end of replication.

3. Transcription

• The process of synthesizing RNA molecules using DNA as the template.

• Key steps:

• Initiation: RNA polymerase enzyme binds to the promoter region of DNA.

• Elongation: RNA polymerase synthesizes mRNA following the base pairing rules, reading the DNA template from the 5’ (promoter) end towards the 3’ end (terminator).

• Termination: Transcription stops at specific termination signals.

4. Translation

• The process of synthesizing proteins based on the information carried by mRNA.

• Key steps:

  • Initiation: Ribosome binds to mRNA at the start codon (usually AUG).
  • Elongation: tRNA, with specific amino acids, bind to the mRNA codon, facilitated by the ribosome. Peptide bonds form between successive amino acids.
  • Termination: Reaches a stop codon, signalling the end of translation.

5. Gene Expression

• Regulation of gene expression allows cells to control when and where proteins are made.

• Mechanisms:

• Transcriptional: Control gene expression at the level of transcription.

• Translational: Control gene expression by regulating the initiation and elongation steps of translation.

• Post-translational: Control gene expression by modifying proteins after their synthesis.

6. Mutations

• Changes in the DNA sequence, can be caused by errors in DNA replication or exposure to mutagens.

• Types:

• Substitutions: Replacement of one nucleotide with another.

• Insertions: Insertion of additional nucleotide(s) into the sequence.

• Deletions: Removal of one nucleotide(s) from the sequence.

• Frameshift: Insertion or deletion that alters the reading frame, leading to incorrect amino acid sequence.

7. Genetic Code

• The set of rules that determines how nucleotide triplets (codons) in mRNA correspond to specific amino acids during translation.

• Properties:

• 64 codons: Three stop codons (UAA, UAG, and UGA) signals the end of translation, while the remaining 61 codons specify amino acids.

• Degenerate: Multiple codons can code for the same amino acids.

• Wobble hypothesis: Some tRNA contain modified nucleotides that can recognize multiple codons.

8. DNA Repair Mechanisms

• Cellular mechanisms that identify and repair damages in DNA.

• Types:

  • Base excision repair: Repairs damages like oxidized or methylated bases.
  • Nucleotide excision repair: Removes damaged nucleotides along with surrounding nucleotides.
  • Mismatch repair: Corrects errors during DNA replication.

9. Recombination

• Process of exchanging genetic material between homologous chromosomes during meiosis.

• Types: -Crossing-over: Exchange of genetic material between non-sister chromatids. -Gene conversion: Non-reciprocal transfer of genetic material from one chromosome to its homologue.

10. Transposable Elements

• Mobile DNA sequences that can insert and excise themselves within the genome.

• Types:

  • Class I (retrotransposons): Transcribe RNA before inserting a copy into the DNA genome.
  • Class II (DNA transposons): Directly insert itself into the DNA genome without an RNA intermediary.

11. Biotechnology and Genetic Engineering

• Applications of genetic engineering and recombinant DNA technology.

• Techniques:

• Gel electrophoresis: Separates nucleic acids or protein fragments by size.

• Recombinant DNA technology: Combines DNA from different organism, creating recombinant molecules

• Gene cloning: Isolates and amplifies specific genes for study or application.

12. Human Genetics

• Study of genes and inheritance in human.

• Key topics:

  • Mendelian inheritance: Inheritance of traits following Mendelian principles.
  • Linkage analysis: Studying how genes located close together on a chromosome are inherited together.
  • Genetic counseling: Providing information and support to individuals and families affected by genetic disorders.

13. Evolution at the Molecular Level

• Use molecular data to understand evolutionary relationships and processes.

• Techniques: Comparative genomics: Compares the DNA sequences of different species.

• Molecular clocks: Utilizes the rate of mutation accumulation in DNA to estimate the divergence time between species.

• Analysis of genetic variation: Studies the genetic variation within populations.

14. Phylogenetics

• Study of evolutionary relationships among species.

• Key concepts:

• Construction of phylogenetic trees based on molecular data

• Analysis of branching patterns and divergence times

• Inferring evolutionary history and common ancestor.

References:

• NCERT Biology Class 11: Chapter 6 - Molecular Basis of Inheritance
• NCERT Biology Class 11: Chapter 12 - Biotechnology: Principles and Processes
• NCERT Biology Class 11: Chapter 9 - Strategies for Enhancement in Food Production
• NCERT Biology Class 12: Chapter 3 – Molecular Basis of Inheritance
• NCERT Biology Class 12: Chapter 10 – Microbes in Human Welfare
• NCERT Biology Class 12: Chapter 13 - Organisms and Populations