Genes, DNA, RNA, and Protein Synthesis Review

A gene is a discrete unit of inheritance that programs the sequence of a polypeptide.
Genes consist of DNA (deoxyribonucleic acid), which is typically inherited from an organism's parents.
DNA provides directions for its own replication and programs a cell's activities by directing the synthesis of protein.

DNA does not directly build proteins; it requires RNA (ribonucleic acid) as an intermediary.
In the cell's nucleus, DNA is transcribed into RNA.
In the cytoplasm, RNA is translated into proteins.

DNA and RNA are composed of monomers called nucleotides.
Nucleotides have three parts: a 5-carbon sugar, a phosphate group, and a nitrogenous base.

Sugars: Ribose in RNA, deoxyribose in DNA.
DNA Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).
RNA Nitrogenous Bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G) (Uracil replaces Thymine).

A polynucleotide forms from nucleotide monomers when the phosphate of one nucleotide bonds to the sugar of the next via dehydration reactions.
This process produces a repeating sugar-phosphate backbone with protruding nitrogenous bases.
RNA is usually a single polynucleotide strand.
DNA is always a double helix, with two polynucleotide strands wrapped around each other.
DNA Base Pairing: Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G) via hydrogen bonds.
RNA Base Pairing: Adenine pairs with uracil (A-U), and cytosine pairs with guanine (C-G).

Most people stop producing the enzyme lactase in early childhood, leading to lactose intolerance.
Lactose tolerance represents a recent mutation and a survival advantage for human cultures with available milk and dairy products.
Researchers identified new mutations in East African ethnic groups that permanently keep the lactase gene turned on, which differ from European mutations.