7 Cell Division and the Cell Cycle
Learn how cells grow, copy their DNA, and divide through the cell cycle, mitosis, and meiosis.
The cell cycle at a glance
The cell cycle is the sequence of growth, DNA replication, and division through which a cell produces new cells. In eukaryotes, it consists of and M phase. M phase includes nuclear division— or —and usually , which divides the cytoplasm.
: growth and DNA replication
A cell spends most of its life in , growing and preparing for division. has three stages:
G₁ (first gap): The cell grows, performs its normal functions, and makes proteins and other cell components. Some cells leave the cycle and enter a resting or specialized state called G₀.
S (synthesis): The cell replicates its DNA. Each chromosome is copied, creating two identical sister chromatids joined at a centromere.
G₂ (second gap): The cell grows further and prepares materials needed for division.
DNA replication happens in S phase, not during . After , the cell enters M phase.
Cell-cycle checkpoints and control
Cell-cycle checkpoints can delay progression when conditions are unsuitable or when errors are detected. Cyclins and cyclin-dependent kinases (CDKs) help regulate transitions between stages.
G₁ checkpoint: Checks cell size, growth signals, and DNA integrity. A cell may pause, attempt repair, or enter G₀.
G₂ checkpoint: Checks that DNA replication is complete and the DNA is sufficiently intact before .
M (spindle) checkpoint: Checks that chromosomes are properly attached to spindle fibers before sister chromatids separate. This helps reduce the risk of daughter cells receiving unequal chromosome sets.
Takeaway: Checkpoints help ensure that division proceeds only when the cell and its chromosomes are adequately prepared.
and
separates the copied chromosomes into two nuclei. Its stages describe how chromosomes condense, attach to spindle fibers, align, separate, and become enclosed in new nuclei:
Prophase: Chromosomes condense and become visible; the mitotic spindle begins to form.
Prometaphase: The nuclear envelope breaks down, and spindle microtubules attach to chromosome kinetochores.
Metaphase: Chromosomes align near the cell’s middle.
Anaphase: Sister chromatids separate and move toward opposite poles. Once separated, each chromatid is considered a chromosome.
Telophase: Chromosomes reach the poles, begin to decondense, and new nuclear envelopes form.
usually occurs as finishes. In animal cells, a contractile ring creates a cleavage furrow that pinches the cell in two. In plant cells, a cell plate forms between the new nuclei and develops into a partition. The usual result is two daughter cells.
and genetic variation
produces haploid cells for sexual reproduction. DNA is replicated once before begins, followed by two divisions, I and II.
In I, homologous chromosomes—the corresponding chromosomes inherited from each parent—pair during prophase I. They may exchange DNA segments through . Their orientation at the cell’s middle is independent, creating different chromosome combinations. Homologous chromosomes separate in anaphase I.
In II, sister chromatids separate, much as they do in . There is no additional DNA replication between the two meiotic divisions. The usual outcome is four haploid cells, each with one chromosome from each homologous pair. For example, a human diploid cell with 46 chromosomes produces cells with 23 chromosomes.
and the independent assortment of homologous chromosomes create genetic variation. The cells produced depend on the organism: in animals, produces cells that develop into gametes; in plants and some other organisms, it produces spores.
Takeaway: has two divisions after one DNA replication: homologous chromosomes separate first, then sister chromatids.