Biology: Life and Systems - Genetics and DNA Structure

DNA (Deoxyribonucleic Acid) is the molecule that carries the genetic instructions for life. It is structured as a double helix, which resembles a twisted ladder.

The basic building block of DNA is the nucleotide. Each nucleotide consists of three parts: a phosphate group ($PO_4^{3-}$), a deoxyribose sugar ($C_5H_{10}O_4$), and a nitrogenous base.

There are four types of nitrogenous bases: Adenine ($A$), Thymine ($T$), Cytosine ($C$), and Guanine ($G$).

Complementary Base Pairing Rule: In the DNA double helix, Adenine always pairs with Thymine ($A-T$) via two hydrogen bonds, and Cytosine always pairs with Guanine ($C-G$) via three hydrogen bonds.

Genes are specific sequences of DNA that code for proteins. Humans have approximately $20,000$ to $25,000$ genes packaged into $23$ pairs of chromosomes ($46$ in total).

Alleles are different versions of the same gene, represented by letters (e.g., $B$ for dominant and $b$ for recessive).

Genotype refers to the genetic makeup (e.g., $Bb$), while Phenotype refers to the physical expression of the trait (e.g., Brown eyes).