Reproduction

Mitosis and Cytokinesis

Binary Fission

Budding

Regeneration and Fragmentation

Parthenogenesis

Sexual reproduction is characterized by meiosis and fusion of two genetically unique gametes to form a unique offspring. Self pollination of a flower is a form of sexual reproduction, because the flower has both male and female reproductive structures that produce unique gametes.

Organisms that reproduce sexually must reduce their number of chromosomes by half via meiosis at some point during their life cycle. Sexual reproducers have one of three distinct life cycles:

Zygotic Life Cycle:

The zygote is the only diploid form of the organism. It quickly undergoes meiosis to produce the dominant haploid form. All sexually reproducing fungi and some algae have zygotic life cycles.

Sporic Life Cycle:

There is a true alternation of generations, such that both the diploid and haploid forms undergo mitosis to produce multicellular forms. Some algae and all plants have sporic life cycles. In primitive nonvascular plants, such as mosses, the haploid form is dominant. In seed plants, the diploid form is dominant.

Gametic Life Cycle:

The gamete is the only haploid form of the organism, and it is always unicellular. Most animals, including humans, have gametic life cycles.

• Mutation is the ultimate source of genetic variation in all living things. The diversity of life is the result of billions of years of change in the genetic code of organisms via mutation acted on by natural selection.
• Transformation is a process by which bacteria can take up fragments of DNA from a surrounding medium during cell division. Since many species of bacteria usually coexist, exchange of genetic information via transformation is thought to be significant.
• Transduction occurs when a viral bacteriophage incorporates its DNA into a bacterial cell.
• Conjugation occurs when bacteria transfer plasmids, small rings of DNA distinct from the bacterial chromosome. Bacteria resistance to antibiotics may be transferred via conjugation.

When gametes are produced via meiosis in sexually reproducing organisms, genetic variation is introduced in several ways.

Syngamy produces a unique diploid zygote.

Crossing-over of homologous chromosomes during prophase 1 leads to genetic recombination and formation of unique chromosomes.

Asexual reproduction is advantageous in several ways. It allows very fast, energetically inexpensive production of many genetically identical offspring in organisms, such as bacteria and simple eukaryotes. If the organisms in question are well-adapted to a relatively stable environment, production of many clonal offspring is advantageous. Asexual reproduction is also favored by some plants and animals under stressful conditions, when mates or pollinators may be scarce.

Sexual reproduction introduces genetic diversity, which is generally advantageous. Genetically diverse populations are better able to tolerate environmental changes and are less susceptible to catastrophic losses when changes do occur. Genetic variation is the core of evolution by natural selection, and has produced the marvelously adapted organisms on Earth today.

Summary

• Mitosis followed by cytokinesis is the process by which eukaryotic cells reproduce themselves to create genetically identical copies.
• Asexual reproduction involves production of a genetically identical offspring from a single parent.
• Sexual reproduction is characterized by meiosis and fusion of two genetically unique gametes to form a unique offspring.
• Sexual reproduction introduces genetic variation via recombination, independent assortment, and syngamy. Genetic variation is usually evolutionarily advantageous.