DNA — The Blueprint of Life
DNA (Deoxyribonucleic Acid) is a molecule that carries the genetic instructions for the development, functioning, growth and reproduction of all living organisms.
Structure of DNA
DNA is shaped like a double helix — two strands twisted around each other, like a spiral ladder.
- The sides of the ladder are made of alternating sugar (deoxyribose) and phosphate groups
- The rungs are pairs of nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
- Base-pairing rules: A pairs with T; G pairs with C
Genes and Chromosomes
- A gene is a segment of DNA that codes for a specific protein or trait
- Genes are located on chromosomes
- Humans have 46 chromosomes (23 pairs) in every body cell
- Sex chromosomes: females have XX, males have XY
Mendel's Laws of Inheritance
Law of Segregation: Each individual has two alleles for each trait; they separate during gamete formation.
Law of Independent Assortment: Genes for different traits are inherited independently of each other.
Dominant and Recessive Alleles
If you inherit a dominant allele (A), it is expressed. If you inherit two recessive alleles (aa), the recessive trait appears.
Example: Tall (TT or Tt) vs Short (tt) in plants.