Unit Three. The Continuity of Life
8.2. Eukaryotes Have a Complex Cell Cycle
The evolution of the eukaryotes introduced several additional factors into the process of cell division. Eukaryotic cells are much larger than prokaryotic cells, and they contain much more DNA. Eukaryotic DNA is contained in a number of linear chromosomes, whose organization is much more complex than that of the single, circular DNA molecules in prokaryotes. A eukaryotic chromosome is a single, long DNA molecule wound tightly around proteins, called histones, into a compact shape.
Cell division in eukaryotes is more complex than in prokaryotes, both because eukaryotes contain far more DNA and because it is packaged differently. The cells of eukaryotic organisms either undergo mitosis or meiosis to divide up the DNA. Mitosis is the mechanism of cell division that occurs in an organism’s nonreproductive cells, or somatic cells. An alternate process, called meiosis, divides the DNA in cells that participate in sexual reproduction, or germ cells. Meiosis results in the production of gametes, such as sperm and eggs, and is discussed in chapter 9.
The events that prepare the eukaryotic cell for division and the division process itself constitute a complex cell cycle. The Key Biological Process illustration below walks you through the phases of the cell cycle:
Interphase. This is the first phase of the cell cycle, step 1 in the figure below, and is usually considered a resting phase, but the cell is far from resting. Interphase is itself made up of three phases:
G1 phase. This “first gap” phase is the cell’s primary growth phase. For most organisms, this phase occupies the major portion of the cell’s life span.
S phase. In this “synthesis” phase, the DNA replicates, producing two copies of each chromosome.
G2 phase. Cell division preparation continues in the “second gap” phase with the replication of mitochondria, chromosome condensation, and the synthesis of microtubules.
M phase. In mitosis, a microtubular apparatus binds to the chromosomes and moves them apart, shown in steps 2-5.
C phase. In cytokinesis, the cytoplasm divides, creating two daughter cells, shown in step 6.
Human cells growing in culture typically have a 22-hour cell cycle. Most cell types take about 80 minutes in this 22 hours to complete cell division: prophase— 23 minutes, metaphase—29 minutes, anaphase—10 minutes, telophase—14 minutes, and cytokinesis—4 minutes. The proportion of the cell cycle spent in any one phase of mitosis varies considerably in different tissues.
Key Learning Outcome 8.2. Eukaryotic cells divide by separating duplicate copies of their chromosomes into daughter cells.