Introduction to Reproduction
|Biology - Reproduction|
One of the characteristics of living things is their ability to reproduce themselves. The drama of reproduction occurs wherever life is present. Either a species reproduces itself or becomes extinct. Organisms reproduce themselves in a surprising variety of ways, some relatively simple, others extremely complex.
In order to understand reproduction, we must first know something of how organisms store the information which controls what cells do and when they do it. This information is stored in an intricate molecule called DNA, which is short for deoxyribonucleic acid. Most of the cell's DNA is found in the nucleus. When viewed under a microscope, dividing cells reveal worm-like bodies called chromosomes. Chromosomes contain most of the DNA in cells undergoing division. All cellular processes, including not only reproduction, but activities such as respiration and assimilation, are under the control of the DNA.
Single-celled organisms can reproduce themselves by the most basic of all reproductive processes, cell fission. An example of fission can be seen in this single-celled organism called an amoeba. Here, a gradual pinching-off occurs in the middle of the cell until the two parts are joined by only the finest thread. Once separated, the two cells go their own way as if nothing has happened. Cell fission involves no separate male and female parents to supply genetic materials, only one "parent" cell is involved, and thus, cell fission is said to be an "asexual" form of reproduction. Reproduction which requires separate male and female parents is called "sexual" reproduction.
Let's look more closely at asexual reproduction as it occurs in a type of single-celled algae called a diatom. Here, as the nuclear membrane breaks down, the chromosomes appear. Each chromosome is duplicated, and the duplicated chromosomes separate. Immediately the cell divides by fission. The result is two independent organisms.
A second method of asexual reproduction common to many plants and some lower animals is called fragmentation. A good example of this process can be found in potatoes, which are usually propagated not by seed, but by planting a portion of the adult plant, a piece of a tuber. New plants then rapidly develop from these "fragments" of the adult.
Besides potatoes, most fruit-bearing plants are propagated from pieces of the mature plant. Even some multicellular animals can reproduce asexually. The pond animal called hydra is a good example. When an abundance of food is available to a hydra, cells migrate and accumulate along its column-shaped body, producing "buds" from which new hydras develop.
The Process of Mitosis
In more highly structured organisms, which reproduce sexually, such as this fertilised toad's egg, the process by which cells divide is repeated over and over during development. The cell divisions lead to the formation of specialised cells, which form components of the body such as heart tissue and brain tissue. This complex process is called cellular differentiation.
In mature multicellular organisms, the process of cell division continues even after growth and differentiation have ceased, mainly to replace worn out or damaged cells. Sometimes the genetic material of cells can become damaged. This can happen to lung cells exposed to cigarette smoke. These damaged cells then begin to reproduce at an uncontrolled rate. Such cells are said to be cancerous.
If the normal process of cell division results in daughter cells that have the same number of chromosomes as the parent cell, the process is called mitosis. If the cell division happens without the duplication of the original genetic material, as is the case with sperm and egg cells, the process is called "meiosis," or "reduction division." Let's follow the process of mitosis, step by step. Although the process is virtually identical in plants and animals, we will use animal cells growing in tissue culture for our first live-action example.
The resting stage between divisions is called interphase. The nucleus is clearly visible, but the chromosomes cannot be seen. Instead, the DNA-containing material called chromatin is dispersed throughout the nucleus. During the next stage, called prophase, the nuclear membrane breaks down and the chromosomes appear as double strands known as chromatids. They are joined at a point called the kinetochore. These double strands are the result of the DNA having duplicated itself.
Also during prophase, fibres made up of tiny tubes form a spindle which is oriented toward two poles called mitotic centres. The next stage, called metaphase, finds the paired chromosomes lined up along the equator of the spindle. Then, during anaphase, the fibres of the spindle, which are attached to the kinetochore of each chromosome, shorten and pull the two sets of chromosomes apart. During the final stage, called the telophase, nuclear membranes reform and the chromosomes disappear, returning to the form of genetic material called chromatin. Then cell division occurs, and two identical daughter cells, both of which are in interphase, are produced. Let us review the process from beginning to end, this time in plant type cells. First is interphase, then prophase, metaphase, then anaphase, telophase, and finally cell division.
In most highly developed organisms, chromosomes exist in pairs. This is called the diploid condition. In humans, 99.99% of the cells have the diploid number of 46 chromosomes. Only the sex cells, sperm and eggs, contain the haploid number, 23 chromosomes. When sex cells are produced by the process of meiosis, two mitotic divisions occur; but in the second prophase, the genetic material is not duplicated.
The result is that each sperm or egg cell has only 23 chromosomes. In humans, males have a matching pair of chromosomes called the XY chromosomes. Human females have XX chromosomes as a matching pair. If during fertilisation the male sperm contains the X chromosome, a girl will result. But if the sperm fertilising the egg contains a Y chromosome, the child will be a boy.
Vertebrates, like human beings, only reproduce sexually. None of the vertebrates can reproduce by asexual means. We already examined asexual reproduction in algae, but, as surprising as it may seem, many algae which reproduce asexually can reproduce sexually as well. In the case of the alga called - spirogyra, which is a colony of individuals linked into strands, a male haploid strand and a female haploid strand join together by conjugation tubes; and the entire contents of the male strand pour into the female cells, leaving empty cell walls behind in the male strand. This process, which usually occurs near the beginning of spring, results in a durable three-walled diploid spore. The algae remain protected in this form until eventually the spore germinates and it undergoes meiosis, forming the haploid strands again.
Another type of alga, called a "desmid," mates in much the same way as spirogyra, except that both parental female and male cell walls are left behind. The alga called - oedogonium - has an even more advanced reproductive process: the male strand of this algae creates specialised cells called - antheridia, which produce free-swimming sperm. These sperm swim to the egg produced as a specialized cell in an area of the female strand called the - oogonium. The fertilised egg, called an - oospore, is capable of surviving through long dormant periods, which might occur because of cold or dry conditions. Following such a dormant period, the oospore germinates, undergoes meiosis, and produces spores capable of developing into new, mature strands.
The process of reproduction we have just seen in the alga oedogonium is not unlike the process by which seeds are produced in the more highly advanced flowering plants. A typical flowering plant has male organs called anthers, which produce pollen grains that contain sperm. These are transferred by wind or insects to the female organ called the pistil, which has a sticky stigma to which the pollen attach. A long pollen tube then grows down through the portion of the pistil called the style, which is seen here in a flower whose petals and anthers have been removed.
Haploid sperm travel down the pollen tube where they join with the haploid egg, produced in the swollen ovary at the base of the style. As a result of this fertilisation, a seed is formed, which, like a spore, has a thick wall and can withstand long periods of dormancy. Once the seed is germinated, a new plant develops, eventually producing seeds itself which will assure that this species of plant will continue on into the future. In multicellular animals like amphibians, the eggs are haploid.
At the moment of fertilisation, as the first sperm joins with the egg, a membrane is thrown up so no other sperm can penetrate the egg. The sperm are too small to be visible in this picture, but the events that accompany fertilisation are obvious. Here, we see waves of change in the pigmentation under the fertilisation membrane. The single fertilised egg is now diploid and begins to undergo mitosis and cell division in a precise sequence. Two cells form four, four form eight, eight yield sixteen, and so on until eventually tissue layers and organs form. In time a recognisable tadpole appears, and this tadpole itself eventually will metamorphose into an adult capable of becoming a parent.
A Review of Reproduction
All organisms rely on methods of reproduction, which can be traced back to the simplest processes found in single-celled plants and animals. These various methods of reproduction seem, on the surface, to be extremely diverse, but at the level of the single cell, they are incredibly similar.
What is clear is that living organisms have evolved ingenious ways of ensuring the continuation of their species, mature organisms reproducing, so that, as they become worn out and pass away, new members of their species can grow, develop, reproduce, and, in their turn, continue the adventure of life on Earth.