Lecture Notes-19 Biology 1004

North Arkansas College

Topic: Mitosis

1. We usually think of reproduction in terms of multicellular organisms.
2. Reproduction does occur in unicellular organisms - binary fission (asexual process).
3. All new cells, regardless of type, come from pre-existing cells - Cell Theory (Robert Cooke).
4. Cell division results in reproduction:

1. Multicellular organisms - the individual organism gets larger.
2. Unicellular organisms - the population gets larger.

5. Cell division and reproduction has 3 functions:

1. Renewal of cells to replace damaged or worm out cells.
2. To control surface to volume ration (cells must remain small).
3. Cells are totipotent (contain a complete complement of inherited traits) - Division and reproduction ensure that each of the offspring cells will receive equal amounts of inherited (genetic) material.

6. The genetic material resides in the chromosomes.
7. In non-dividing cells, the chromosomes are not visible and exist as long, thin threads of DNA in genetic material called chromatin.
8. During the later stages of Interphase, the chromosomes duplicate, shorten and are more readily stained and visible.
9. Somatic cells (regular body cells) contain 2 of each chromosome (a pair) and are diploid (2N).
10. Sex cells or gametes (sperm & egg) contain 1 copy of each chromosome, and they are said to be haploid (1N).
11. Cell division in eukaryotes is called karyokinesis and/or cytokinesis.
12. Humans have 23 pairs or 46 chromosomes. A potato has 44 chromosomes, primates have 46 chromosomes, and some ferns have 1200 or >.
13. Cell division in eukaryotes that is asexual (somatic cells) occurs by the process of mitosis.
14. Cellular division in eukaryotes that is sexual occurs by the process of meiosis.

1. This is the period between rounds of cell division.
2. Most of the life cycle is spent in Interphase.
3. In this phase, all of the things needed for growth and reproduction are synthesized.
4. Most of DNA & protein needed are synthesized during Interphase.
5. Three (3) stages of the cell cycle are observed during Interphase.

1. G₁ Stage - The first Gap Stage is the period between the end of mitosis and the start of Synthesis Stage (S₁).
2. S₁ Stage - The Synthesis Stage is the period of DNA synthesis. Other chromosomal components such as proteins are also synthesized.
3. G₂ Stage - The second Gap Stage is the stage between the Synthesis Stage and the start of the next round of mitosis. Protein synthesis occurs. This is the final step in preparation for mitosis. Therefore, the G₂ stage marks the beginning of mitosis.

6. During mitosis, the spindle fibers form at the poles of cells. Centrosomes (barrel shaped object) - The main microtubule synthesizing component which makes the spindle fibers. Each of the centrosomes contains a pair of centrioles which move to poles of cell during early stages of mitosis. Plant cells have no centrioles.

1. The nuclear membrane disintegrates.
2. Nucleolus disappears.
3. Spindle fibers develop.
4. Centrosomes migrate to opposite poles, and asters form at the poles.
5. Kinetochore fibers develop on the side of each centromere of chromosome.

1. In early Metaphase (pro-metaphase), the kinetochore fibers attach to spindle fibers.
2. Chromosomes move back and forth and align themselves in center of cell (along the Metaphase plate).

1. Centromeres divide and chromosome pairs separate. Each chromosome contains a centromere of its own (sister chromatids). Chromatid - a single chromosome.
2. Chromosomes move along spindle toward opposite poles of cell.
3. As chromosomes move toward opposite poles, kinetochore fibers disintegrate.
4. Spindle fibers lengthen placing maximum distance between poles of cell.
5. Dynenin is the protein causing chromosomes to move toward poles.

1. Spindle fibers disappear.
2. New nuclear membrane develops around chromosomes at opposite poles of cell.
3. Each daughter cell has equal amounts of inherited information. Eventually, cells undergo cytokinesis (split into 2) as a cleavage furrow is formed in center of dividing cell.

Mitosis - 1 diploid cells (2N) makes 2 diploid cells.

1. Rigid cell wall prevent a cleavage furrow from forming. Instead, a cell plate is formed which divides the 2 daughter cells.
2. Plants have no centrosomes, centrioles or asters. However, they do form spindle fibers and undergo mitosis.

1. Sexual process of reproduction - Occurs only in sex cell (gametes in form of sperm & egg).
2. The sex cells are haploid (1N) or 23 chromosomes.
3. This allows pooling of genetic information from each parent for variety (permits adaptation).
4. It also prevents the increase in the total genetic material with each succeeding generation.

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