Chromosomes are thread-like structures in the nucleus of a cell that contain DNA, which is organized by proteins called histones. Humans typically have 46 chromosomes in 23 pairs, with 22 pairs of autosomes and one pair of sex chromosomes (XX for females and XY for males). Chromosomes carry genes that determine physical traits, and they are crucial for cell division and heredity
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When a prokaryotic cell reproduces, its circular chromosome replicates, and the cell divides in a process called binary fission (Figure 2.5). Replication usually begins at a specific place on the circular chromosome, called the origin of replication. The origins of the two newly replicated chromosomes move away from each other and toward opposite ends of the cell.
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Biologists have traditionally classified all living organisms into two major groups, the prokaryotes and the eukaryotes (Figure 2.1). A prokaryote is a unicellular organism with a relatively simple cell structure. **A eukaryote has a compartmentalized cell structure with components bounded by intracellular membranes; eukaryotes may be either unicellular or multicellular.