The instructions for building and maintaining an organism are encoded in deoxyribonucleic acid, or DNA, a double-helix molecule that stores genetic information in sequences of four nucleotide bases: adenine, thymine, cytosine, and guanine. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of these nitrogenous bases. The two strands of the helix are held together by complementary base pairing, where adenine pairs with thymine through two hydrogen bonds and cytosine pairs with guanine through three, ensuring that the genetic information can be accurately copied and passed on.
DNA does not work alone. Ribonucleic acid, or RNA, is a single-stranded nucleic acid that uses uracil in place of thymine and comes in several forms: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), all of which collaborate during protein synthesis. The flow of genetic information follows a pattern often summarized as DNA to RNA to protein. In transcription, RNA polymerase binds to a gene's promoter region in the nucleus and copies the DNA template strand into a complementary mRNA molecule. The mRNA then travels to a ribosome, where translation occurs: tRNA molecules, each carrying a specific amino acid, read the mRNA codons through their anticodons and link the amino acids together into a polypeptide chain.
A gene is a segment of DNA that codes for a specific protein or RNA molecule, and together the entire set of genes determines an organism's traits. Occasionally, errors called mutations occur in the DNA sequence. These can be point mutations, which change a single base, or insertions and deletions, which add or remove bases. Some mutations have little effect, while others cause significant changes, including diseases such as sickle cell anemia. Inheritance patterns of genes were first described by Gregor Mendel, whose two foundational laws still guide genetics today. The law of segregation states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele. The law of independent assortment states that genes for different traits are distributed to gametes independently of one another.
Punnett squares are simple diagrams that apply these laws to predict the genotypes and phenotypes of offspring, producing the classic 3:1 phenotypic ratio in a monohybrid cross between two heterozygous parents. In humans, traits such as ABO blood type are determined by multiple alleles of a single gene that code for different surface antigens, while the Rh factor adds another layer of inheritance that produces the positive or negative designation attached to each blood type.