Biological Sciences 100 Laboratory 7 2 I. MITOSIS The cell theory states that " all cells come from pre-existing cells".Single-celled organisms - such as bacteria and protists - divide to give rise to two individuals. In multicellular organisms, cell division is necessary for growth and for replacement of dead or dying cells. Prokaryotic cells reproduce primarily by binary fission. The cell divides in two, with each daughter cell receiving a complete set of genetic material. In eukaryotes, cell division is more complex. Eukaryotic DNA is complexed with protein and organized into chromosomes. Cell division in eukaryotes consists of two processes - mitosis, or nuclear division, and cytokinesis, or cytoplasmic division. Mitosis is a complex process, ensuring the production of two nuclei with identical sets of chromosomes. A. The Cell Cycle Dividing cells pass through a regular sequence of events called the cell cycle. Mitosis occupies only a very small portion of the cycle. Each dividing cell spends most of its time in interphase. Interphase is a very active phase in the cell cycle and can be divided into three phases. Cytoplasmic growth and organelle replication takes place during the G1period. During the S period, the DNA is duplicated in preparation for mitosis. During G2, structures directly involved in mitosis, e.g. microtubules, are synthesized.B. The Chromosome During interphase, each chromosome is extended as a very thin strand within the nucleus. In this form, individual chromosomes are not distinguishable and the mass of chromosomes is called chromatin. During the Sphaseof interphase, the DNA replicates itself, providing two identical copies of the genetic information of the cell. During early mitosis, the chromatin condenses (shortens and thickens) and each replicated chromosome (duplicated chromosome)can be visualized as two identical sister chromatids, attached together at the centromere. Label these features in the figure below, an electron micrograph of a condensed human chromosome.