Anatomy & Physiology

Anatomy & Physiology Study Guide complementary bases. These bases are Adenosine, Guanine, Thymine, and Cytosine. The pairs are Adenosine with Thymine or Uracil (when RNA transcribed) and Cytosine with Guanine. 3.3 Cell Life Cycle Life begins with a single cell; at maturity, the human body has roughly 75 trillion cells. This process of multiplication involves the reproduction of cells through cell division. The division of a single cell produces a pair of daughter cells , each half the size of the original. Before dividing, each of the daughter cells will grow to the size of the original cell. The life span of a cell varies from hours to decades, depending on the type of cell and the stresses involved. The death of a cell is called apoptosis. Because a cell does not live nearly as long as a person, cell populations must be maintained over time by cell division. For cell division to be successful, the genetic material in the nucleus must be duplicated accurately, and one copy must be distributed to each daughter cell. The duplication of the cell’s genetic material is called DNA replication, and nuclear division is called mitosis. Mitosis occurs during the division of somatic cells. The production of sex cells involves a different process called meiosis. Mitosis Mitosis separates the chromosomes of a cell into two identical nuclei. There are four stages of mitosis: prophase, metaphase, anaphase, and telophase. • Stage 1 Prophase: Prophase begins when the chromosomes coil so tightly that they become visible as individual structures under a light microscope. Two copies of each chromosome now exist. Each copy, called a chromatid, is physically connected to its duplicate copy at a single point called the centromere. As the chromosomes appear, the nucleoli disappear. Around this time, the two pairs of centrioles move toward opposite poles of the nucleus. • Stage 2 Metaphase: Metaphase begins as the chromatids move to a narrow central zone called the metaphase plate. Metaphase ends when all the chromatids are aligned in the plane of the metaphase plate. • Stage 3 Anaphase: Anaphase begins when the centromere of each chromatid pair splits and the chromatids separate. The two daughter chromosomes are pulled toward opposite ends of the cell. • Stage 4 Telophase: During telophase, each new cell prepares to return to the interphase state. The nuclear membranes re-form, the nuclei enlarge, and the chromosomes gradually uncoil. Once the chromosomes have relaxed, the nucleoli reappear, and the nuclei resemble those of interphase cells. This stage marks the end of mitosis.

©2018

Achieve Page 31

of 368