The average eukaryotic cell is about 100 µm (100 x 10^-6 m. = 10^-4 m.) long.

You have 200 trillion (2 x 10^14) somatic cells (p. 230).

Total length of your cells lined up end-to-end: (2 x 10^14) x (10^-4 m.) = 2 x 10^10 m. = 2 x 10^7 km.

The earth's circumference is 40 million m. (40 x 10^6 m. = 4 x 10^7 m. = 4 x 10^4 km.

2 x 10^7 km. / 4 x 10^4 km. = 500: your cells can wrap around the earth 500 times. Updated Apr 01, 2011 by Peter Chen

Prokaryotes such as Escherichia coli reproduce by a process called binary fission.

DNA replication begins at the origin of replication.

One copy of the origin moves toward the other end of the cell. Replication is complete, yielding two chromosomes. The plasma membrane grows inward, forming a fission furrow. The furrow deepens to produce two genetically identical "daughter cells".

The chromatin condenses into visible chromosomes during cell division in a process called mitosis.

Before cell division, each chromosome has 1 chromatid, composed of a DNA double helix.

After DNA duplication, each chromosome has 2 sister chromatids, attached by a centromere.

The chromatids separate during cell division.

Gametes (sex cells) have 1 set (n) of chromosomes (haploid) and are produced by meiosis.

Somatic (non-reproductive) cells have 2 sets (2n) of chromosomes (diploid) and reproduce asexually by mitosis.

In humans, the haploid number is 23, and diploid number is 46.

The mitotic (M) phase alternates with interphase, which is composed of 2 growth phases (G[1] and G[2]) and a S (DNA synthesis) phase.

In the M phase, mitosis divides the nucleus.

Then cytokinesis divides the cytoplasm, producing 2 genetically identical "daughter" cells.

Exercise:

A nuclear envelope bounds the nucleus.

The nucleus contains one or more nucleoli, where ribosomes are synthesized.

Chromosomes have been duplicated, but are not yet visible under the light microscope.

In animal cells, centrosomes are also replicated.

The chromatin condense into discrete chromosomes observable with a light microscope.

The nucleoli disappear.

The replicated centrosomes move apart, toward opposite "poles" of the cell.

The mitotic spindle begins to be formed from microtubules, which radiate from the centrosomes and appear as star-like asters.

The nuclear envelope fragments.

The spindles continue radiating from centrosomes, which are at opposite "poles" of the cell.

Each of the 2 chromatids of a chromosomes has a kinetochore, which attach the chromatid to the spindle microtubules at the centromere.

The chromosomes convene on the metaphase (equatorial) plate.

For each chromosome, the kinetochores of the sister chromatids are attached to spindle microtubules coming from opposite poles.

The 2 sister chromatids are transported by spindle microtubules toward opposite ends of the cell,

The non-kinetochore microtubules lengthen.

Each chromatid is now a full-fledged chromosome.

Two daughter nuclei begin to form.

Nuclear envelopes and nucleoli reform.

The chromosomes become less condensed.

Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

In animal cells, a cleavage furrow forms, pinching the plasma membrane.

Then a contractile ring of microfilaments splits the cell.

Animal mitosis movie:

In plant cells, a cell plate grows between the "daughter" cells, making a new plasma membrane.

Then new cell walls made of cellulose are formed along the new membrane.