primordial ancestor that arose between 3.5 billion and 4 bil-lion years ago. Biologist Richard Dawkins describes life asa river of DNA that runs through time, connecting allorganisms past and present.That all organisms have a common genetic systemmeans that the study of one organism’s genes reveals princi-ples that apply to other organisms. Investigations of howbacterial DNA is copied (replicated), for example, providesinformation that applies to the replication of human DNA.It also means that genes will function in foreign cells, whichmakes genetic engineering possible.Unfortunately,thiscommon genetic system is also the basis for diseases such asAIDS (acquired immune deficiency syndrome), in whichviral genes are able to function—sometimes with alarmingefficiency—in human cells.This chapter explores cell reproduction and how geneticinformation is transmitted to new cells. In prokaryoticcells, cell division is relatively simple because a prokaryotic16•The Diversity of Life•Basic Cell Types: Structures andEvolutionary Relationships•Cell ReproductionProkaryotic Cell ReproductionEukaryotic Cell ReproductionThe Cell Cycle and Mitosis•Sexual Reproduction and GeneticVariationMeiosisConsequences of meiosisMeiosis in the Life Cycle of Plantsand AnimalsChromosomes and CellularReproduction2This is Chapter 2 Opener photo legend.(Art Wolfe/Photo Researchers.)
Chromosomes and Cellular Reproduction 17tion and are the bases ofsimilarities and differencesbetween parents and progeny.Basic Cell Types: Structure andEvolutionary RelationshipsBiologists traditionally classify all living organisms intotwomajor groups, the prokaryotesand the eukaryotes.Aprokaryoteis a unicellular organism with a relativelysimple cell structure (FIGURE2.1). A eukaryotehas a com-partmentalized cell structure divided by intracellular mem-branes; eukaryotes may be unicellular or multicellular.◗cell usually possesses only a single chromosome.Ineukaryotic cells, multiple chromosomes must be copied anddistributed to each of the new cells. Cell division in eukary-otes takes place through mitosis and meiosis, processes thatserve as the foundation for much of genetics; so it is essentialto understand them well.Grasping mitosis and meiosis requires more than sim-ply memorizing the sequences of events that take place ineach stage, although these events are important. The key isto understand how genetic information is apportioned dur-ing cell reproduction through a dynamic interplay of DNAsynthesis, chromosome movement, and cell division. Theseprocesses bring about the transmission of genetic informa-2.1Prokaryotic andeukaryotic cells differ instructure. (Left to right: T.J.Beveridge/Visuals Unlimited;W. Baumeister/SciencePhoto/Library/PhotoResearchers; BiophotoAssociates/Photo Researchers;G. Murti/Phototake.)◗
18Chapter 2Research indicates that dividing life into two majorgroups,the prokaryotes and eukaryotes,is incorrect.