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Reproduction Process Essay, Research Paper

reproduction

process by which organisms replicate themselves.

In a general sense reproduction is one of the most important concepts in biology: it means

making a copy, a likeness, and thereby providing for the continued existence of species.

Although reproduction is often considered solely in terms of the production of offspring in

animals and plants, the more general meaning has far greater significance to living organisms.

To appreciate this fact, the origin of life and the evolution of organisms must be considered.

One of the first characteristics of life that emerged in primeval times must have been the ability

of some primitive chemical system to make copies of itself.

At its lowest level, therefore, reproduction is chemical replication. As evolution progressed, cells

of successively higher levels of complexity must have arisen, and it was absolutely essential

that they had the ability to make likenesses of themselves. In unicellular organisms, the ability

of one cell to reproduce itself means the reproduction of a new individual; in multicellular

organisms, however, it means growth and regeneration. Multicellular organisms also reproduce

in the strict sense of the term–that is, they make copies of themselves in the form of

offspring–but they do so in a variety of ways, many involving complex organs and elaborate

hormonal mechanisms.

Reproduction of organisms

In single-celled organisms (e.g., bacteria, protozoans, many algae, and some fungi),

organismic and cell reproduction are synonymous, for the cell is the whole organism. Details of

the process differ greatly from one form to the next and, if the higher ciliate protozoans are

included, can be extraordinarily complex. It is possible for reproduction to be asexual, by

simple division, or sexual. In sexual unicellular organisms the gametes can be produced by

division (often multiple fission, as in numerous algae) or, as in yeasts, by the organism turning

itself into a gamete and fusing its nucleus with that of a neighbour of the opposite sex, a

process that is called conjugation. In ciliate protozoans (e.g., Paramecium), the conjugation

process involves the exchange of haploid nuclei; each partner acquires a new nuclear

apparatus, half of which is genetically derived from its mate. The parent cells separate and

subsequently reproduce by binary fission. Sexuality is present even in primitive bacteria, in

which parts of the chromosome of one cell can be transferred to another during mating.

Multicellular organisms also reproduce asexually and sexually; asexual, or vegetative,

reproduction can take a great variety of forms. Many multicellular lower plants give off asexual

spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or

multinucleate. In some cases the reproductive body is multicellular, as in the soredia of lichens

and the gemmae of liverworts. Frequently, whole fragments of the vegetative part of the

organism can bud off and begin a new individual, a phenomenon that is found in most plant

groups. In many cases a spreading rhizoid (rootlike filament) or, in higher plants, a rhizome

(underground stem) gives off new sprouts. Sometimes other parts of the plant have the

capacity to form new individuals; for instance, buds of potentially new plants may form in the

leaves; even some shoots that bend over and touch the ground can give rise to new plants at

the point of contact.

Among animals, many invertebrates are equally well endowed with means of asexual

reproduction. Numerous species of sponges produce gemmules, masses of cells enclosed in

resistant cases, that can become new sponges. There are many examples of budding among

coelenterates, the best known of which occurs in freshwater Hydra. In some species of

flatworms, the individual worm can duplicate by pinching in two, each half then regenerating the

missing half; this is a large task for the posterior portion, which lacks most of the major

organs–brain, eyes, and pharynx. The highest animals that exhibit vegetative reproduction are

the colonial tunicates (e.g., sea squirts), which, much like plants, send out runners in the form

of stolons, small parts of which form buds that develop into new individuals. Vertebrates have

lost the ability to reproduce vegetatively; their only form of organismic reproduction is sexual.

In the sexual reproduction of all organisms except bacteria, there is one common feature:

haploid, uninucleate gametes are produced that join in fertilization to form a diploid, uninucleate

zygote. At some later stage in the life history of the organism, the chromosome number is

again reduced by meiosis to form the next generation of gametes. The gametes may be

in size (isogamy), or one may be slightly larger than the other (anisogamy); the majority of

forms have a large egg and a minute sperm (oogamy). The sperm are usually motile and the

egg passive, except in higher plants, in which the sperm nuclei are carried in pollen grains that

attach to the stigma (a female structure) of the flower and send out germ tubes that grow down

to the egg nucleus in the ovary. Some organisms, such as most flowering plants, earthworms,

and tunicates, are bisexual (hermaphroditic, or monoecious)–i.e., both the male and female

gametes are produced by the same individual. All other organisms, including some plants (e.g.,

holly and the ginkgo

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