INTRODUCTION All new cells come from previously existing cells. New cells are formed by the process of cell division, which involves both division of the cell’s nucleus (called mitosis) and division of the cytoplasm (called cytokinesis). Dividing cells experience both of these events along with a period of time between divisions called interphase. During interphase, the nucleus appears normal and the cell is performing its usual cellular functions. Also, the cell is increasing all of its components, including such organelles as the mitochondria, ribosomes, and centrioles if present. DNA replication (making an exact copy of the DNA) occurs toward the end of interphase. Thereafter, the chromosomes which contain DNA are duplicated and contain two chromatids held together at the centromere. These chromatids are called sister chromatids. During mitosis, the new nuclei receive the same number of chromosomes as the parental nucleus. When cytokinesis occurs, two daughter cells are produced. In multicellular organisms, mitosis permits growth and repair of tissues. In eukaryotic, unicellular organisms, mitosis is a form of asexual reproduction. Sexually reproducing organisms utilize another form of nuclear division called meiosis. In animals, meiosis is a part of gametogenesis or the production of gametes (sex cells). The gametes are sperm in the male and eggs in the female. As a result of meiosis, the daughter cells have half the number of chromosomes as the parental cell. Because crossing-over of genetic materials takes place and the chromosomes appear in various combinations in the daughter cells, meiosis contributes to the recombination of genetic material and therefore to variation among sexually reproducing organisms.
THE CELL CYCLE As stated in the introduction, the period of time between actual cell divisions is called interphase. Early investigators saw little visible activity between cell divisions and so they referred to this time as a resting stage. However, when it was discovered that DNA replication and chromosome duplication occurred during interphase, the concept of the cell cycle was proposed. The cell cycle can be broken down into four distinct stages.
State what happens in each stage on the line provided:
G1 The cell increases in size
S DNA replication occurs
G2 The cell begins to duplicate its organelles
M Cell growth an protein production stop, and the cell actually divides into two separate cells.
The time required for the entire cell cycle depends on the organism and the cell involved. The typical time for animal cells is 18-24 hours. The majority of this time is spent in interphase with anywhere from 1 to a maximum of 2 hours spent in actual mitosis.
MITOSIS Mitosis is nuclear division that results in two new nuclei, each having the same number of chromosomes as the original nucleus. The parent cell is the cell that divides and the daughter cells are the cells that result from the division. They are all genetically identical. When cell division is about to begin, chromatin starts to condense and form visible rod-like sister chromatids held together at the centromere. The illustration represents a chromosome as it appears just before nuclear division occurs. Label the sister chromatids, centromere and the kinetochore.
Figure 9-1. Chromosome Structure.
1. Sister Chromatids
2. Centromere
3. Kinetochore
Spindle The spindle is a structure that is formed and causes the orderly distribution of chromosomes to the daughter cells. A spindle has fibers that stretch between the two ends of the cell. Spindle fibers are bundles of microtubules, which are protein cylinders. The centrosome is the main microtubule-organizing center of the cell. It divides before mitosis so that each pole of the spindle has a centrosome. In animal cells, each centrosome contains two centrioles. Most plant cells lack centrioles which suggest that centrioles are not absolutely critical for spindle formation
