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Asexual reproduction is quite simple compared to sexual reproduction in that it requires only one organism; no partner is necessary. Therefore, in most asexual species, every mature individual can reproduce, enabling the population to increase far more rapidly than the otherwise comparable sexual species that require two individuals to reproduce.
Many organisms take full advantage of asexual reproduction. For instance, each time an amoeba divides, it produces two genetically identical replicas of itself. And many species of single-celled as well as multicellular organisms produce asexually reproductive cells known as spores that float in the air or water and eventually produce genetic replicas of the parent.
Another asexual mode of reproduction involves budding. In budding, part of the parent sprouts smaller offspring that separate and become distinct individuals. Many plants reproduce vegetatively, sprouting new plants from leaves, roots, or some other part of the parent. In parthenogenesis, an egg can develop into an adult without being fertilized by a sperm cell.
Another form of asexual reproduction is fragmentation, in which part of an organism separates from the whole, and a new individual regenerates from that part. Such fragmentation sometimes occurs when an organism is in danger; pieces of the injured organism then regenerate into whole organisms. Starfish, for example, have this reproductive capacity. Worms and planaria can also fragment and then regenerate the missing portion of their body.
Cloning is another type of asexual reproduction that involves the production of copies that are genetically identical, although they may not look identical. This happens with many plant species – such as when one plant grows from a seed and then many other plants sprout up from the roots.
Sexual reproduction is costly to a species in that it requires both a male and a female to produce as many offspring as one asexual organism can produce, but there are also benefits to such an expensive reproductive mode.
For sexual reproduction to occur, specialized cells from both the male and female come together and unite. Yet merely combining any two cells is not adequate. Rather, certain cells first undergo a peculiar type of cell division called meiosis, creating gametes called germ cells.
In animals, undifferentiated male germ cells are located in the testes. These cells undergo two meiotic divisions, called meiosis I and meiosis II, creating four sperm cells. Undifferentiated at first, these sperm cells, known as spermatids, undergo differentiation before becoming mature spermatozoa. This process, spermatogenesis, is the result of the division and maturation of a single diploid primary spermatocyte, producing four spermatozoa. In female animals, all the undifferentiated germ cells are located in the ovaries, where oogenesis occurs. Oogenesis comprises the series of steps that produce an egg from a primary oocyte, which is also called an ovum.
In terms of weight, a human egg, though extremely tiny (much smaller than the head of a pin), is approximately 58,000 times heavier than a single sperm cell. In terms of length, a completely differentiated sperm cell is about 1/3 the diameter of a human egg.
Only one sperm cell can fertilize each egg. When the sperm cell penetrates the egg, it contributes its haploid (sometimes called monoploid or 1N) genetic complement of chromosomal DNA to the haploid (1N) egg, creating a diploid (2N) zygote.
II. POST-READING TASKS.
a) Define:
1 asexual reproduction and discuss the different types that exist,
2 sexual reproduction,
3 spermatogenesis and oogenesis.
b) Compare and contrast asexual and sexual reproduction.
c) Write an essay describing asexual and sexual reproduction.
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