Return

Eukaryotic cell (sexual reproduction)

Organisms have cells with nuclei and specialized organelles or specilized intracellular structures. Their genetic material is segregated into chromosomes.

The evidence for the existence of the eukaryotic cell about 1.4 billion years ago is based primarily on the size and wall structure of fossil cells. Many eukaryotic algae have spiny or complexly textured cell walls. Studying both flattened cells preserved in dense shale, a highly compacted rock, and three-dimentional cell remains preserved in chart, an uncompacted sedimentary rock consisting of finely crystal line quartz, the russian paleontologist B.F Timofeev and J. William Schopf of University of California at Los angeles have documented a change in the spectrum of size of algal cells about 1.4 billion years ago . Before this time, there were few cells with diameters larger than 10 micrometers and almost no cells reaching 60 micrometers across. Size below 20 micrometers typify prokaryotic cells today, and the first appearance in the fossil record of many larger cells is taken to signal the advent of the eukaryotic condition. Nearly all of the large, eukaryotic cells are assigned to the group of algae known as acritarchs. -Extinction- by Stanley, Steven M. ( Scientific American Library; 20) Copyright 1987 by Scientific American Books, Inc.