Surgical Oncology

The cell cycle. The ability to recognize microscopically cells during both mitosis and DNA replication (by autoradiography after administering radiolabeled thymidine) led to the concept of the cell "cycle." In this concept new interphase cells undergo a period after leaving mitosis and before starting DNA synthesis called the first gap or G1. Another gap, G2, occurs after DNA replication and before the next mitotic prophase. After mitosis both new cells repeat this cycle. In rapidly dividing cells, G1 is a period in which cells accumulate the enzymes and nucleotides required for DNA replication, S is the period devoted primarily to DNA replication, G2 is a usually short period of preparation for mitosis, and M includes all phases of mitosis itself. In rapidly growing human tissues the cell cycle varies from 24 to 36 hours. The length of G1 depends on many factors and is usually the longest and most variable period; the length of S is largely a function of the genome size. G2 and mitosis together normally last only 2 to 3 hours. (Reproduced from Mescher A. Junqueira's Basic Histology: Text and Atlas. 12th ed. www.accessmedicine.com Copyright © The McGraw-Hill Companies, Inc. All rights reserved.)