Cells Grow and Divide

The most remarkable feature of cells and entire organisms is their ability to reproduce. Biological reproduction, combined with continuing evolutionary selection for a highly functional body plan, is why today's horseshoe crabs look much as they did 300 million years ago, a time span during which entire mountain ranges have risen or fallen. The Teton Mountains in Wyoming, now about 14,000 feet high and still growing, did not exist a mere 10 million years ago. Yet horseshoe crabs, with a life span of about 19 years, have faithfully reproduced their ancient selves more than half a million times during that period. The common impression that biological structure is transient and geological structure is stable is the exact opposite of the truth. Despite the limited duration of our individual lives, reproduction gives us a potential for immortality that a mountain or a rock does not have.

The simplest type of reproduction entails the division of a "parent" cell into two "daughter" cells. This occurs as part of the cell cycle, a series of events that prepares a cell to divide followed by the actual division process, called mitosis. The eukaryotic cell cycle commonly is represented as four stages (Figure 1-17). The chromosomes and the DNA they carry are copied during the S (synthesis) phase. The replicated chromosomes separate during the M (mitotic) phase, with each daughter cell getting a copy of each chromosome during cell division. The M and S phases are separated by two gap stages, the G1 phase and G2 phase, during which mRNAs and proteins are made. In single-celled organisms, both daughter cells

▲ FIGURE 1-17 During growth, eukaryotic cells continually progress through the four stages of the cell cycle, generating new daughter cells. In most proliferating cells, the four phases of the cell cycle proceed successively, taking from 10-20 hours depending on cell type and developmental state. During interphase, which consists of the G1, S, and G2 phases, the cell roughly doubles its mass. Replication of DNA during S leaves the cell with four copies of each type of chromosome. In the mitotic (M) phase, the chromosomes are evenly partitioned to two daughter cells, and the cytoplasm divides roughly in half in most cases. Under certain conditions such as starvation or when a tissue has reached its final size, cells will stop cycling and remain in a waiting state called G0. Most cells in G0 can reenter the cycle if conditions change.

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

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