Chromosomes and Cellular Reproduction

This is Chapter 2 Opener photo legend. (Art Wolfe/Photo Researchers.)

The Diversity of Life

Basic Cell Types: Structures and Evolutionary Relationships

Cell Reproduction

Prokaryotic Cell Reproduction Eukaryotic Cell Reproduction The Cell Cycle and Mitosis

Sexual Reproduction and Genetic Variation

Meiosis

Consequences of meiosis Meiosis in the Life Cycle of Plants and Animals

This is Chapter 2 Opener photo legend. (Art Wolfe/Photo Researchers.)

The Diversity of Life

More than by any other feature, life is characterized by diversity: 1.4 million species of plants, animals, and microorganisms have already been described, but this number vastly underestimates the total number of species on Earth. Consider the arthropods — insects, spiders, crustaceans, and related animals with hard exoskeletons. About 875,000 arthropods have been described by scientists worldwide. The results of recent studies, however, suggest that as many as 5 million to 30 million species of arthropods may be living in tropical rain forests alone. Furthermore, many species contain numerous genetically distinct populations, and each population contains genetically unique individuals.

Despite their tremendous diversity, living organisms have an important feature in common: all use the same genetic system. A complete set of genetic instructions for any organism is its genome, and all genomes are encoded in nucleic acids, either DNA or RNA. The coding system for genomic information also is common to all life — genetic instructions are in the same format and, with rare exceptions, the code words are identical. Likewise, the process by which genetic information is copied and decoded is remarkably similar for all forms of life. This universal genetic system is a consequence of the common origin of living organisms; all life on Earth evolved from the same primordial ancestor that arose between 3.5 billion and 4 billion years ago. Biologist Richard Dawkins describes life as a river of DNA that runs through time, connecting all organisms past and present.

That all organisms have a common genetic system means that the study of one organism's genes reveals principles that apply to other organisms. Investigations of how bacterial DNA is copied (replicated), for example, provides information that applies to the replication of human DNA. It also means that genes will function in foreign cells, which makes genetic engineering possible. Unfortunately, this common genetic system is also the basis for diseases such as AIDS (acquired immune deficiency syndrome), in which viral genes are able to function — sometimes with alarming efficiency—in human cells.

This chapter explores cell reproduction and how genetic information is transmitted to new cells. In prokaryotic cells, cell division is relatively simple because a prokaryotic cell usually possesses only a single chromosome. In eukaryotic cells, multiple chromosomes must be copied and distributed to each of the new cells. Cell division in eukary-otes takes place through mitosis and meiosis, processes that serve as the foundation for much of genetics; so it is essential to understand them well.

Grasping mitosis and meiosis requires more than simply memorizing the sequences of events that take place in each stage, although these events are important. The key is to understand how genetic information is apportioned during cell reproduction through a dynamic interplay of DNA synthesis, chromosome movement, and cell division. These processes bring about the transmission of genetic informa tion and are the bases of similarities and differences between parents and progeny.

Basic Cell Types: Structure and Evolutionary Relationships

Biologists traditionally classify all living organisms into two major groups, the prokaryotes and the eukaryotes. A prokaryote is a unicellular organism with a relatively simple cell structure ( FIGURE 2.1). A eukaryote has a compartmentalized cell structure divided by intracellular membranes; eukaryotes may be unicellular or multicellular.

Prukdr* urc

Prukdr* urc

Ch^rPïukiiry ulin mlli

[ukiiry ulin milt.

^i-l.!I:\fly tmiH, Pram 3 Lo 10 jjm RjelxlivHy EargK. irurrr IQ !□ lûDjim

■.lin,:!! V Urn PNA in::! ■. .1 Ml 11: plu k'l-d 1: u ■■■ j vi

"■J ;'. iTJTpll vn J ,v. !.H H-vr-:" :", r ■-Hf- 'JH'iî I" ^ L'^

■ Lb^i I':i;i;' ■-ni'hiv:. " ■■■ r ::■ h^-r-.-

N j : ■ ■ j 1 Nu; I'.'" r-nvr-lopc . : dieilasiPi'.^.

er: - j :. r.i Rihüi-uri-üi---».^ J.- M urhundric-! -

Chlûrùpf&M- - _ " ricjl^r. ^-.ir ■'.:■ _ . ■ ~~~ in i n^-yrn

Ch^rPïukiiry ulin mlli

[ukiiry ulin milt.

^i-l.!I:\fly tmiH, Pram 3 Lo 10 jjm RjelxlivHy EargK. irurrr IQ !□ lûDjim

■.lin,:!! V Urn PNA in::! ■. .1 Ml 11: plu k'l-d 1: u ■■■ j vi

"■J ;'. iTJTpll vn J ,v. !.H H-vr-:" :", r ■-Hf- 'JH'iî I" ^ L'^

■ Lb^i I':i;i;' ■-ni'hiv:. " ■■■ r ::■ h^-r-.-

0 0

Post a comment