Cell Division Mitosis

Think About This All living organisms age…… What changes are occurring when organisms live?

In the case of multicellular organisms, as organisms age, they grow more and more cells. When they begin dying, they lose more and more cells. The process whereby cells divide is called cell division. It is important to remember that cells do not grow infinitely larger in size as organisms grow but they grow larger in number.

Limits of Cell Growth Organisms are limited in size by their surface area to volume ratio. If cells are too large, they would have too difficult of a time transporting materials in and out of the cell.

Why do cells divide? Reproduction Growth Repair

From One Cell to Trillions of Cells

The Cell’s Genome The cell’s entire hereditary endowment of DNA is called its genome. A cell must replicate and distribute an identical copy of its genome to each one of its daughter cells. A typical human cell has about 3 meters of DNA- a length about 300,000 times greater than the cell’s diameter.

Chromosomes Each chromosome is made out of tightly packed chromatin fiber. Chromatin is made out of complexes called nucleosomes which are DNA strands coiled around histone proteins. Humans have 23 pairs or 46 chromosomes in each somatic cell (all cells excluding gametes).

Chromosome Structure Each chromosome is composed of a centromere and chromatids. After DNA replication, there are two sister chromatids. Sister chromatids are pulled apart during mitosis, one going into each daughter cell.

DNA core of histone One nucleosome Fig. 8.3, p. 129

one chromosome (unduplicated) one chromatid two sister chromatids one chromatid one chromosome (duplicated) Fig. 8.2, p. 128

Chromosomes