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Molecular Basis of Inheritance — Complete Lesson
?? DNA Structure (Watson-Crick Model, 1953)
DNA (Deoxyribonucleic Acid) is a double-stranded helix. Each strand is a polynucleotide chain consisting of:
- ?? Deoxyribose sugar
- ?? Phosphate group
- ?? Nitrogenous base (Adenine, Guanine, Cytosine, Thymine)
Chargaff's Rule (Base Pairing):
A pairs with T (2 hydrogen bonds)
G pairs with C (3 hydrogen bonds)
? %A = %T and %G = %C in any DNA
A pairs with T (2 hydrogen bonds)
G pairs with C (3 hydrogen bonds)
? %A = %T and %G = %C in any DNA
?? DNA Replication (Semi-Conservative)
Proposed by Watson and Crick; proved by Meselson and Stahl (1958). Each strand serves as a template for a new complementary strand. The result: 2 daughter DNA molecules, each with one old and one new strand.
?? Transcription (DNA ? mRNA)
The process of making mRNA from a DNA template. Key points:
- Template strand is read 3'?5'
- mRNA is synthesised 5'?3'
- Uracil (U) replaces Thymine (T) in RNA
- Enzyme: RNA Polymerase
?? Genetic Code
- A codon = 3 consecutive mRNA bases coding for 1 amino acid
- 64 codons total (4³); 61 code for amino acids, 3 are stop codons
- AUG = start codon (also codes for Methionine)
- UAA, UAG, UGA = stop codons
- Code is degenerate (multiple codons for same amino acid) and universal
Central Dogma (Francis Crick):
DNA ? (Transcription) ? mRNA ? (Translation) ? Protein
Reverse transcription possible in retroviruses (e.g., HIV).
DNA ? (Transcription) ? mRNA ? (Translation) ? Protein
Reverse transcription possible in retroviruses (e.g., HIV).
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