Organization of Genetic Material into Chromosomes

Abstract

The genetic information of most living organisms is stored in deoxyribonucleic acid (DNA). DNA usually exists as a double helix, with two strands of opposite polarity, held together by hydrogen bonds between the complementary bases. Adenine is paired with thymine with double hydrogen bonds, whereas guanine is paired with cytosine with triple hydrogen bonds. The specific base sequence and their complementarity make it a unique feature for each organism to store and transmit genetic information. Ribonucleic acid (RNA) usually exists as a single-stranded molecule containing uracil instead of thymine. The DNA molecules present in the bacterial chromosomes are circular and contain about 50 negatively supercoiled domains. Each eukaryotic chromosome contains one giant molecule of DNA packaged into 10 nm ellipsoidal beads called nucleosomes. The 10 nm fibers are further coiled into chromatin fibers about 30 nm in diameter, which are organized into domains by scaffolds comprising nonhistone chromosomal proteins. The chromosomes are maximally condensed during metaphase of meiosis and mitosis. The centromeres, which are spindle fiber attachment regions, and telomeres, which are the ends of chromosomes, have unique structures that facilitate their respective functions.