The nucleus of a cell contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribonucleic Acid) molecules.
The DNA in the cell nucleus is the information source for making proteins.
If the information is changed, different proteins will be made. Different proteins will eventually lead to altered body designs.
1. DO ORGANISMS CREATE EXACT COPIES OF THEMSELVES?
Organisms look similar because their body designs are similar. If body designs are to be similar, the blueprints for these designs should be similar. Thus, reproduction at its most basic level will involve making copies of the blueprints of body design.
A basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA.
1.1 The Importance of Variation
- If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive.
- Example:- Thus, if there were a population of bacteria living in temperate waters, and if the water temperature were to be increased by global warming, most of these bacteria would die, but the few variants resistant to heat would survive and grow further. Variation is thus useful for the survival of species over time.
2. MODES OF REPRODUCTION USED BY SINGLE ORGANISMS
The modes by which various organisms reproduce depend on the body design of the organisms.
Types of Asexual mode of reproduction
This methode of reproduction is used by unicellular organisms.
In this mode of reproduction cell division, or fission, leads to the creation of new individuals.
Fission mode of reproduction is divided into two parts
1. Binary fission: In binary fission, a single organism produces two parts. Example: Amoeba , paramecium
Binary fission in Amoeba
2. Multiple fission: in which a single organism produces multiple parts. Example: Leishmania , Plasmodium
Multiple fission in Plasmodium
It is a form of asexual reproduction wherein a parent organism breaks into fragments, each capable of growing independently into a new organism.
Many fully differentiated organisms have the ability to give rise to new individual organisms from their body parts. That is, if the individual is somehow cut or broken up into many pieces, many of these pieces grow into separate individuals.
For example, simple animals like Hydra and Planaria can be cut into any number of pieces and each piece grows into a complete organism. This is known as regeneration
Regeneration in Planaria
Budding is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site.
These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.
Example : Budding in Hydra
Budding in Hydra
5. Vegetative Propagation
In this mode of reproduction the parts of many plants like the root, stem and leaves develop into new plants under appropriate conditions.
This property of vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes for agricultural purposes.
Plants raised by vegetative propagation can bear flowers and fruits earlier than those produced from seeds. Such methods also make possible the propagation of plants such as banana, orange, rose and jasmine that have lost the capacity to produce seeds.
Another advantage of vegetative propagation is that all plants produced are genetically similar enough to the parent plant to have all its characteristics.
Example: sugarcane, rose, grapes, bryophyllum
Leaf of Bryophyllum with buds
6. Spore Formation
A mode of reproduction resembling multiple fission, common among Protozoa, in which the organism breaks up into a number of pieces, or spores, each of which eventually develops into an organism like the parent form.
Spore formation in Rhizopus
3. SEXUAL REPRODUCTION
Sexual reproduction means production of new living organisms by combining genetic information from two individuals of different types (sexes).
Why the Sexual Mode of Reproduction?
Variation : Every individual organism cannot be protected by variations, but in a population, variations are useful for ensuring the survival of the species.
Sexual Reproduction in Flowering Plants
- The reproductive parts of flowering plants (angiosperms) are located in the flower.
- Different parts of a flower – sepals, petals, stamens and pistil. Stamens and pistil are the reproductive parts of a flower which contain the germ-cells.
- The flower may be unisexual (papaya, watermelon) when it contains either stamens or pistil or bisexual (Hibiscus, mustard) when it contains both stamens and pistil.
- Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour.
- Pistil is present in the centre of a flower and is the female reproductive part. It is made of three parts.
- The swollen bottom part is the ovary, middle elongated part is the style and the terminal part which may be sticky is the stigma. The ovary contains ovules and each ovule has an egg cell.
- The male germ-cell produced by pollen grain fuses with the female gamete present in the ovule. This fusion of the germ-cells or fertilisation gives us the zygote which is capable of growing into a new plant.
- Thus the pollen needs to be transferred from the stamen to the stigma. If this transfer of pollen occurs in the same flower, it is referred to as self-pollination. On the other hand, if the pollen is transferred from one flower to another, it is known as cross-pollination. This transfer of pollen from one flower to another is achieved by agents like wind, water or animals.
Germination of pollen on stigma
After the pollen lands on a suitable stigma, it has to reach the female germ-cells which are in the ovary. For this, a tube grows out of the pollen grain and travels through the style to reach the ovary.
After fertilisation, the zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is gradually converted into a seed. The ovary grows rapidly and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, style and stigma may shrivel and fall off.
The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. This process is known as germination.
Reproduction in Human Beings
In our early teenage years, a whole new set of changes occurs that cannot be explained simply as body enlargement. Instead, the appearance of the body changes. Proportions change, new features appear, and so do new sensations.
Some of these changes are common to both boys and girls. We begin to notice thick hair growing in new parts of the body such as armpits and the genital area between the thighs, which can also become darker in colour.
Thinner hair can also appear on legs and arms, as well as on the face. The skin frequently becomes oily and we might begin to develop pimples. We begin to be conscious and aware of both our own bodies and those of others in new ways.
On the other hand, there are also changes taking place that are different between boys and girls. In girls, breast size begins to increase, with darkening of the skin of the nipples at the tips of the breasts.
Also, girls begin to menstruate at around this time. Boys begin to have new thick hair growth on the face and their voices begin to crack. Further, the penis occasionally begins to become enlarged and erect, either in daydreams or at night.
All of these changes take place slowly, over a period of months and years. They do not happen all at the same time in one person, nor do they happen at an exact age. In some people, they happen early and quickly, while in others, they can happen slowly.
All of these changes are aspects of the sexual maturation of the body.
(a) Male Reproductive System
The male reproductive system consists of portions which produce the germ-cells and other portions that deliver the germ-cells to the site of fertilisation.
Formation of germ-cells or sperms
- The formation of germ-cells or sperms takes place in the testes. These are located outside the abdominal cavity in scrotum because sperm formation requires a lower temperature than the normal body temperature.
- Testes secrets the hormone, testosterone. In addition to regulating the formation of sperms, testosterone brings about changes in appearance seen in boys at the time of puberty.
Portions that deliver the germ-cells to the site of fertilisation.
- The sperms formed are delivered through the vas deferens which unites with a tube coming from the urinary bladder. The urethra thus forms a common passage for both the sperms and urine.
- Along the path of the vas deferens, glands like the prostate and the seminal vesicles add their secretions so that the sperms are now in a fluid which makes their transport easier and this fluid also provides nutrition.
- The sperms are tiny bodies that consist of mainly genetic material and a long tail that helps them to move towards the female germ-cell.
(b) Female Reproductive System
The female germ-cells or eggs are made in the ovaries. They are also responsible for the production of some hormones.
- When a girl is born, the ovaries already contain thousands of immature eggs. On reaching puberty, some of these start maturing. One egg is produced every month by one of the ovaries.
- The egg is carried from the ovary to the womb through a thin oviduct or fallopian tube.
- The two oviducts unite into an elastic bag-like structure known as the uterus. The uterus opens into the vagina through the cervix.
The sperms enter through the vaginal passage during sexual intercourse. They travel upwards and reach the oviduct where they may encounter the egg. The fertilised egg (zygote) starts dividing and form a ball of cells or embryo. The embryo is implanted in the lining of the uterus where they continue to grow and develop organs to become foetus.
The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. This is a disc which is embedded in the uterine wall. It contains villi on the embryo’s side of the tissue. On the mother’s side are blood spaces, which surround the villi. This provides a large surface area for glucose and oxygen to pass from the mother to the embryo.
The developing embryo will also generate waste substances which can be removed by transferring them into the mother’s blood through the placenta. The development of the child inside the mother’s body takes approximately nine months. The child is born as a result of rhythmic contractions of the muscles in the uterus.
(c) What happens when the Egg is not Fertilised?
- If the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus its lining becomes thick and spongy.
- This would be required for nourishing the embryo if fertilisation had taken place. Now, however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous.
- This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days.
(d) Reproductive Health
- Since the sexual act is a very intimate connection of bodies, it is not surprising that many diseases can be sexually transmitted.
- These include bacterial infections such as gonorrhoea and syphilis, and viral infections such as warts and HIV-AIDS.
- Using a covering, called a condom, for the penis during sex helps to prevent transmission of many of these infections to some extent.
Many ways have been devised to avoid pregnancy.
Creation of a mechanical barrier
- These contraceptive methods fall in a number of categories. One category is the creation of a mechanical barrier so that sperm does not reach the egg.
- Condoms on the penis or similar coverings worn in the vagina can serve this purpose.
Changing the hormonal balance of the body
- Another category of contraceptives acts by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur.
- These drugs commonly need to be taken orally as pills. However, since they change hormonal balances, they can cause side-effects too.
- Other contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy. Again, they can cause side effects due to irritation of the uterus.
- If the vas deferens in the male is blocked, sperm transfer will be prevented. If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus. In both cases fertilisation will not take place. Surgical methods can be used to create such blocks.
- While surgical methods are safe in the long run, surgery itself can cause infections and other problems if not performed properly.
- Surgery can also be used for removal of unwanted pregnancies. These may be misused by people who do not want a particular child, as happens in illegal sex-selective abortion of female foetuses.