Evolution

Chromosomes are structures made of tightly coiled DNA and proteins. Human body cells have 46 chromosomes (23 pairs), while sex cells have 23 chromosomes.

Genetic variation within a population is essential for evolution. Variation comes from mutations, sexual reproduction, and gene flow between populations.

Selective breeding is when humans choose organisms with desired traits to reproduce. Dog breeds, crop varieties, and livestock have been developed through thousands of years of selective breeding.

The cell membrane is a thin barrier that surrounds every cell and controls what moves in and out. It lets in useful materials like nutrients and oxygen while keeping out harmful substances, a feature called selective permeability.