DNA

DNA is double stranded and consists of two long chains of nucleotides. Each nucleotide is made up of a phosphate, sugar, and a base. The two strands are wound around each other to form a double helix. The four types of bases are Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). The bases pair according to the base pairing rule: A pairs with T, and C pairs with G.

Humans have 46 chromosomes, which contain the entire genome. We inherit half of these chromosomes from each parent, resulting in 23 pairs of homologous chromosomes. Gametes (sex cells) are haploid and contain 23 chromosomes each. During fertilization, the egg and sperm fuse to form a zygote, which is diploid and contains the normal number of chromosomes (46).

A gene is a section of DNA that contains a specific sequence of bases, coding for a chain of amino acids that folds into a protein, determining a characteristic. Alleles are alternative forms of a gene; individuals may have different versions of the same gene, differing by a few bases. Each individual inherits two alleles for each gene, one from each parent, found on homologous chromosome pairs.

Mutations are permanent changes to the sequence of bases in an individual's genome. They can be classified as:

1. Silent - no effect
2. Deleterious - negative effect
3. Lethal - deadly
4. Advantageous - positive effect

Mutations can occur in germ cells (affecting all cells of the resulting organism) or somatic cells (affecting only a portion of the individual).

The base pairing rules are crucial for the structure and function of DNA. They dictate that Adenine (A) pairs with Thymine (T) and Cytosine (C) pairs with Guanine (G). This specific pairing allows for the stability of the DNA double helix and ensures accurate replication and transcription of genetic information, which is essential for coding characteristics.