Re: how does a chloroplast copy itself?during mitosis?

Becky, Chloroplasts ( http://pfo.smcdsb.edu.on.ca/courses/andrews/MYPAGE7.HTM or http://falcon.cc.ukans.edu/~jbrown/ chloroplast.html)are semi-independent organelles. They contain their own DNA in the form of a circular plasmid which encodes some - but not all - of the molecules needed for chloroplast function.. It is theorized that this is the DNA from the free-living photosynthetic prokaryote that became an endosymbiont and then an organelle. This theory, The Endosymbiotic Theory, is widely accepted as a plausible explanation for the evolution of organelles. It was proposed and championed by Lynn Margulis (See: http://www.dc.peachnet.edu/~pgore/students/w96/ joshbond/symb.htm, and http://www.geo.umass.edu/faculty/margulis.html)

Chloroplasts are not independent now, although there are cases where chloroplasts live outside of a plant cell. Some seaslugs can take in chloroplasts from the algae they eat, and they remain functional for a while but do not divide.

The chloroplast DNA is not enough for a chloroplast to live freely. Some proteins required by chloroplasts are synthesized from nuclear DNA and imported into the chloroplast. Isolated chloroplasts do not typically replicate and survive for limited time when removed from the plant cytosol. It is the nucleus that "cues" the chloroplast to divide.

Sometimes mitosis (http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/ cells3.html) is defined as only nuclear division, but cell division includes nuclear division and cytokinesis. Cytokinesis is the splitting of the cytoplasm and allocation into the new cells of the cytoplasm and organelles including plastids (a generic term for chloroplasts and modified chloroplasts, see: http://www.fi.edu/qa97/ biology/cells/cell4.html). So chloroplasts are distributed into the two daughter cells at cell division.

Chloroplasts do multiply however. Think of what would happen if they didn't. A plant cell that contained a number of chloroplasts would divide them among daughter cells, cutting the number by half. After several divisions, the chloroplasts would be "diluted" out of the cells. So, they must replicate. I mentioned nuclear division and cytokinesis, but there are other periods in a cell's life cycle (http:// arnica.csustan.edu/boty1050/Mitosis/mitosis.htm or http:// www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells2.html). The complete cell division cycle is broken down into phases. The period between cell divisions is termed interphase, which is further broken down into stages. During interphase the S (synthesis) stage is when DNA is replicated. Prior to the S stage is the G1 (Gap #1) stage. During G1 is when organelles like chloroplasts are usually reproduced.