Reproduction

1. What This Chapter Covers

Reproduction is a necessary life process for the continuation of life by producing offspring — and the book immediately complicates that by asking whether it happens only for continuation, since growth and the repair of worn-out parts after injury use the same machinery.

Some organisms reproduce differently in different situations. Under favourable conditions Paramoecium simply splits into two by binary fission, rapidly, producing many organisms. Under unfavourable conditions two Paramoecia come into contact and exchange nuclear material — conjugation — producing forms more tolerant of adverse conditions.

The time required to reproduce varies enormously: compare yeasts, bacteria, a rat, a cow, an elephant and a human being. During the rainy season swarms of insects suddenly appear, and most insects have life cycles spanning a few days to a few months.

Activity 1. Mix a teaspoon of curd thoroughly with about 60 teaspoons of luke warm milk in one bowl, and the same amount of curd with the same quantity of cold milk in another. Cover both, note the time, and check every hour. Curdling means the Lactobacillus population has increased, and the time difference between the two bowls tells you how strongly temperature affects the rate.

The index gives this chapter 15 periods in October — the joint largest allotment in the book — and runs it from page 123 to page 152.

2. Asexual Reproduction (Textbook 6.1)

Asexual reproduction involves a single parent and no gametes.

Fission (6.1.1). Single-celled organisms such as Paramoecium and bacteria split into two or more offspring, usually symmetrically. Splitting into two is binary fission; when more cells are formed it is multiple fission. This is often the only mode of reproduction in these organisms.

Budding (6.1.2). A growth on the body forms a bud that grows into a nearly identical copy of the parent. When it has fully grown it separates and survives independently. The example is yeast.

Fragmentation (6.1.3). Some organisms grow from a single piece of the parent, taken from any part of the body. It occurs in the simplest organisms — some flatworms, moulds, lichens and Spirogyra — which may also reproduce sexually. It is a common mode in algae, fungi and many terrestrial plants.

Parthenogenesis (6.1.4). Normally sexual reproduction fuses haploid gametes into a diploid zygote, but in certain cases organisms develop directly from unfertilized gametes. From the Greek partheno, virgin, and genesis, production. It occurs commonly in lower organisms such as algae and fungi, for example Spirogyra.

In animals it takes a striking form. In some species of ants, bees and wasps the fertilized ovum develops into a female and the unfertilized ovum into a male. Sperms develop by mitotic division in the male while ova develop by meiotic division in the female.

Parthenocarpy (6.1.5). In some plants, such as banana, the ovary develops directly into a fruit without fertilization, and the fruit is seedless. It is induced in many crops — pomegranate, papaya, tomato — and it is how seedless watermelon and grapes are produced.

Regeneration (6.1.6). Many organisms can give rise to new individuals from body parts: if an individual is cut or broken into many pieces, those pieces grow into separate individuals. The book's example is Planaria, and it asks you to argue whether regeneration is a type of fragmentation.

3. Vegetative Propagation (Textbook 6.2)

Higher plants can reproduce from their vegetative parts — leaves, stems and roots. This may be natural or artificial.

Natural propagation (6.2.1)

Leaves. In Bryophyllum, small plants grow at the edge of the leaves.

Stems. Aerial weak stems such as runners and stolons give off adventitious roots where they touch the ground; when the connection with the parent is broken, that stem portion with its roots becomes an independent plant.

Roots of Murraya, guava and Millingtonia produce radical buds that grow as new plants.

Artificial propagation (6.2.2)

Stem cuttings. A piece of the parent plant bearing a bud is cut off and its lower part buried in moist soil. After a few days the cutting develops roots and grows as an individual plant. Examples: rose, hibiscus.

Layering. A branch with at least one node is bent to the ground and part of it covered with moist soil, leaving the tip exposed. After a few days new roots develop from the buried part, which is then cut off from the parent. Examples: nerium, jasmine.

Grafting. Two plants are joined so that their stems grow as a single plant. The one attached to the soil is the stock; the cut stem of the other, without roots, is the scion. Both are tied with twine and covered with a polythene cover.

Grafting is used to obtain a plant with desirable characters, and it makes a very young scion flower and fruit earliest. It is used on mango, citrus, apple and rose. Cutting, grafting and layering are the traditional methods; pineapple, orange, grape and rose are propagated this way.

For commercial purposes the traditional methods are being replaced by tissue culture, in which a few plant cells or tissues are placed in a growth medium containing plant hormones and grow into new plants. Thousands of plants can be grown in a very short interval of time.

4. Spore Formation (Textbook 6.3)

The whitish thread-like structures and blackish powder on rotten fruit and bread are the reproductive spores of a fungus. The example is Rhizopus.

Rhizopus produces hundreds of microscopic spores. When the spore sac, the sporangium, bursts, the spores spread into the air. Landing on food or soil in damp and warm conditions, they germinate and produce new individuals. Rhizopus, Mucor, a few bacteria and non-flowering plants such as ferns and mosses reproduce mostly this way.

Lab Activity — growing and slide-mounting a mould

Leave a soft bread or roti in the open air for about an hour so it is exposed to contaminants. Put it in a plastic bag, sprinkle water on it for dampness, seal the bag leaving some air inside, and keep it in a dark, warm place.

Mould starts growing in 2 to 3 days but takes a week or more to form spores. Check every two days and add water if it is drying. The book warns that this should not be done by anyone with a mould allergy or severe asthma.

To make the slide: place a drop of water in the centre, scrape a very little mould onto it with a toothpick, and lower a cover slip at an angle so one edge touches the water first and no air bubbles are trapped. Blot excess water at the edges with tissue paper and view first under low power, 10x.

You will see fine thread-like projections, the sporangiophores, and thin knob-like structures, the sporangia, each containing hundreds of minute spores.

Sporophyll (6.3.1). A fern leaf is called a sporophyll. On its lower surface are clusters of dot-like structures, the sori, containing sporangia. Rupture a sorus gently with a needle and observe the spores under a compound microscope.

Paramoecium does both, and which one it uses depends on the conditions — fission when things are favourable, conjugation when they are not. That single example is the book's whole argument for why variation is worth its cost.

5. Sexual Reproduction (Textbook 6.4)

Sexual reproduction is reproduction in which gametes fuse, by fertilization. It may be external, outside the female's body, or internal, inside it.

In land animals such as mammals the eggs are fertilised inside the body. External fertilization occurs in aquatic animals such as most fishes and amphibians: the female lays a vast number of eggs in water and the male releases millions of sperms over them. Because the chance of fertilization outside is left to nature, producing vast numbers of both is unavoidable.

Importance of sexual reproduction. Asexual reproduction produces organisms that are normally copies of the single parent; sexual reproduction requires two parents and produces a combination of characters of both. Asexual reproduction looks more efficient, since only one parent is needed and no time or energy is spent finding a mate — but sexual reproduction helps organisms develop characters that let them adapt better to their surroundings.

6. The Male Reproductive System (Textbook 6.6)

Nine structures make up the system: a pair of testes, vasa efferentia, epididymis, vasa deferentia, seminal vesicles and ejaculatory ducts, plus the prostate gland, a pair of Cowper's glands, and the urethra.

Testes. Located outside the abdominal cavity in a pouch called the scrotum. Each testis contains highly coiled seminiferous tubules, where sperms are produced by meiosis in hundreds of millions. The male sex hormone testosterone is also produced here. The scrotum keeps the testes 2 to 2.5 °C lower than body temperature, which is necessary for sperm formation.

Vasa efferentia. The seminiferous tubules open into these, which carry sperms to the epididymis.

Epididymis. A highly coiled tube along the posterior side of each testis, where sperms are stored.

Vasa deferentia. From each epididymis a vas deferens ascends into the abdominal cavity, looping around the ureter.

Seminal vesicles. They open into the vas deferens and produce seminal fluid, the source of energy for sperms once outside the body.

Prostate gland. Secretes prostate fluid, a component of semen, which supplies nutrients to the sperm.

Cowper's glands. Their secretions neutralise the acidity in the walls of the urethra and allow free flow of the sperm cells.

Ejaculatory duct. A duct from the seminal vesicles joins the vas deferens and continues as the ejaculatory duct; the two ejaculatory ducts join at the centre of the urethra.

Urethra. In males it transports both urine and sperms, so it is also called the urino-genital duct.

Passage of spermatozoa (6.6.1): seminiferous tubules, vasa efferentia, epididymis, vasa deferentia, ejaculatory duct, urethra.

The sperm (6.6.2)

The head bears the acrosome, which helps the sperm penetrate the ovum, and in the middle of the head is the male nucleus, which fuses with the female nucleus at fertilisation.

Head and middle piece are joined by the neck. Mitochondria in the middle piece produce the energy for movement, and the tail propels the sperm.

Fluids from the seminal vesicles, prostate and Cowper's glands together make seminal plasma; seminal plasma with sperm is semen, and sending semen out of the body is ejaculation.

For normal fertility, 60 per cent of total sperm should have normal shape and size and 40 per cent must have vigorous motility. Men produce sperm from about the age of 13 or 14 and continue until old age, though production decreases with age.

7. The Female Reproductive System (Textbook 6.7)

The main parts are a pair of ovaries, a pair of fallopian tubes, the uterus and the vagina, with the functional mammary glands integrated with the system.

Ovaries. A pair in the abdominal cavity. Ova develop in tiny cellular structures called graafian follicles, which at first look like cellular bubbles. As a follicle grows it develops a fluid-filled cavity, and each follicle contains a single ovum formed after meiotic division. When the ovum matures the follicle ruptures at the surface and the ovum is flushed out — ovulation.

Fallopian tubes. The ovum enters the widened funnel of an oviduct, which runs from near the ovary to the muscular, thick-walled uterus. Fertilization occurs as the ovum passes through the fallopian tube. Usually a single sperm fertilizes a single ovum, forming a zygote, which undergoes mitotic division as it travels down and becomes a solid ball of cells by the time it reaches the uterus.

Uterus. An inverted pear-shaped structure whose inner layer is the endometrium. Its thickness increases gradually soon after menstruation, readying it to receive the embryo. If there is no fertilization the endometrium disintegrates and flows out as menstrual fluid; if there is, the thickness continues and the endometrium nourishes the developing embryo and disposes of its wastes.

Implantation and the embryonic membranes

The fertilized ovum divides as it moves down the oviduct and finally attaches to the soft tissues of the uterus, sinking into the inner wall. This is implantation. Certain embryonic cells then form four membranous structures.

Chorion. Tiny finger-like projections grow from this outer membrane into the soft uterine tissue, and small pools of rapidly moving blood form around them. The chorion tissue together with the adjacent uterine tissue makes the placenta.

The placenta is formed from cells of both the embryo and the mother, at around 12 weeks of pregnancy. Under normal conditions there is no direct flow of blood between mother and young: the two blood systems are separated by thin membranes of cells that allow exchange mainly by diffusion of oxygen, carbon dioxide, nutrients and wastes.

Amnion. Grows around the embryo itself; its cavity fills with amniotic fluid, which keeps the embryo moist and protects it from minor mechanical injury.

Allantois. Arises from the gut of the embryo. The edges of the amnion join at the cord of the allantois to form a long tube, the umbilical cord, carrying the blood vessels that connect embryo to placenta and supply food materials from mother to child.

Yolk sac. Encloses a fluid-filled cavity and has no specific function in placental mammals.

From the third month of pregnancy the embryo is called a foetus. Pregnancy lasts on average 9 months, or 280 days — the gestation period.

8. Child Birth (Textbook 6.8)

As pregnancy progresses the uterus increases in diameter, and at about the ninth month the head of the foetus turns down towards the opening of the uterus. During birth the head usually comes out first; when the feet come first the delivery is more difficult. The book is candid that we still do not know much about the mechanism of childbirth or how it is triggered.

Childbirth begins when the muscle layers of the uterus start to contract and relax rhythmically, felt as labour pains. At first this is just strong enough to move the baby slowly towards the vagina. Generally at this stage the amnion breaks and its fluid is released, which is a good sign that labour is well on its way. The contractions then become stronger and more frequent and the baby is pushed out.

The umbilical cord is tied off and cut; the piece left attached to the baby shrivels and falls off within a few days, and the navel marks where it entered. Contractions continue until they push out the placental tissues, commonly called the afterbirth.

Towards the end of pregnancy a watery, yellowish, lymph-like fluid called colostrum accumulates in the mammary glands. For the first few days after birth the glands secrete only colostrum, and it is very important to feed it to the newborn because it helps develop the child's immune system. Milk follows, and the menstrual cycle resumes once milk production stops.

9. Sexual Reproduction in Flowering Plants (Textbook 6.9, 6.10)

There are nearly 3,50,000 species of flowering plants, and with few exceptions they produce seeds enclosed in fruits. They range from trees weighing many tons to water plants the size of a rice grain.

The flower

The reproductive parts of a flowering plant are in the flower. Of the four parts — sepals, petals, stamens and carpels — the ones carrying sex cells are the stamens and carpels.

Flowers with either stamens or carpels are unisexual, for example bottle gourd and papaya. Those with both are bisexual, for example Datura.

The stamens together are the androecium, producing male sex cells in the pollen grain; the gynoecium produces female sex cells in ovules inside the ovary. A carpel has three parts: the stigma, which receives the pollen; the style, the passage for compatible male sex cells; and the ovary, where fusion occurs to form the zygote.

Pollination

Self-pollination is the transfer of pollen from the anther to the stigma of the same flower, as in the pea plant. The book gives a structural clue for spotting it: if the anthers are below the stigma, self-pollination may not occur.

Cross pollination is the transfer of pollen from one flower to the stigma of another flower of the same plant, or of another plant of the same species, with birds and insects as agents.

Darwin showed in 1876 that plants when isolated had the greatest tendency to self-fertilize, while when surrounded by varieties of the same flower they readily cross-fertilize.

Activity 2. Put a drop of water on a slide, tap the anther of a hibiscus, tridax or marigold flower over it, and observe the small dot-like pollen grains first with a hand lens and then under a compound microscope. A permanent slide shows the germinating grain with two nuclei in the pollen tube. The pollen grain germinates only on the stigma.

10. The Ovule and Double Fertilisation (Textbook 6.11)

An ovule is an egg-shaped structure attached by a stalk to the inner side of the ovary, and an ovary may hold one, two, several or hundreds of them. At the centre of each is a microscopic embryo sac filled with food and water, composed of gametophytic cells.

Most angiosperms have an embryo sac of 7 cells and 8 nuclei. The megaspore undergoes three mitotic divisions to reach the eight-nucleate stage. Three cells move to the top end and are the antipodals. Three group at the micropylar end, where the pollen tube enters: two are the synergids and the central one is the egg cell. One large cell holds two polar nuclei and is called the secondary nucleus.

Cells on the surface of the stigma secrete a sticky nutrient fluid containing sugars, which helps the pollen grain germinate into a pollen tube bearing two nuclei. When the tip of the tube enters the embryo sac, its end ruptures and releases both nuclei.

One nucleus fuses with the egg to form a zygote — that is fertilization. The second male nucleus moves to the centre and unites with the secondary nucleus, which stimulates the formation of a new tissue, the endosperm. The two unions together are double fertilization, a characteristic feature of angiosperms.

From ovule to seed

After double fertilization the ovule rapidly increases in size as endosperm tissue forms by mitosis, and the embryo develops with one or more cotyledons.

The cotyledons develop by using the food stored in the endosperm. In some plants such as beans they digest, absorb and store the food as the ovule matures, so the cotyledons become greatly enlarged and the endosperm disappears almost completely. In others such as corn or castor the endosperm tissue keeps growing as the ovule matures into a seed.

The zygote divides several times to form an embryo within the ovule; the ovule develops a tough coat and becomes a seed; the ovary grows rapidly and ripens into the fruit; and the other floral parts shrivel and fall off.

Activity 3. Soak groundnut or Bengal gram seeds overnight, drain the excess water, cover them with a wet cloth and leave them a day, sprinkling water so they do not dry. Open them next day and identify the plumule, radicle, cotyledon and seed coat.

The book's life cycle diagram closes the loop: pollination, zygote, embryo, seed, fruit, germination, seedling, mature plant.

11. The History of Cell Division (Textbook 6.12)

Virchow (1821-1902), a proponent of cell theory, is credited with Omnis cellula e cellula — cells arise from pre-existing cells.

In 1852 the German scientist Robert Remak published observations on cell division based on embryos, one of the first attempts to understand its mechanism, and stated that binary fission of cells was how animal cells reproduce.

In 1879 Walther Flemming (1843-1905) examined many animal and plant cells, selected those that were dividing, and reported string-like structures in the nucleus which split longitudinally. He named the process mitosis, from mitos, fine threads.

A decade later those structures were named chromosomes, coloured bodies, because attempts to see them relied on dyes and these structures took the stain most often.

Wilhelm Roux (1850-1924) proposed that chromosomes carry a set of heritable elements, and that the longitudinal splitting Flemming saw ensures their equal division. Combined with the rediscovery of Gregor Mendel's 1866 paper on heritable elements in peas, this put chromosomes at the centre of heredity.

Weismann's problem

If cell division were always mitosis, human egg and sperm cells would each contain 46 chromosomes, and fertilization would produce 92, then 184, then 368, generation after generation. That does not happen.

August Weismann (1834-1914) hypothesised that individuals of the same species have the same number of chromosomes in successive generations, and that in successive cell divisions the number remains constant.

The scheme of meiotic division was confirmed in 1904 by Theodor Boveri (1862-1915). The chemical nature of the genetic material was settled over the next fifty years, culminating in the structure of DNA in 1953 by James Watson and Francis Crick.

12. The Cell Cycle (Textbook 6.13, 6.14)

Cell division transforms a fertilized egg into a baby in 9 months and into an adult over the next 20 years, and in a multicellular organism it is highly regulated — it occurs only when there is need for it.

Cells in organs such as the heart and brain divide slowly; bone marrow cells divide actively to produce red blood cells, which live only about 120 days. Cut a finger and a clot forms, bringing chemicals that stimulate skin cells to divide and heal the wound — and division ceases as the wound heals. Cancer cells do not respond to such growth regulating factors and divide continuously at the expense of normal cells, ultimately killing the host.

Division itself is a small part of the cycle and completes in 40 to 60 minutes. The period between two divisions is interphase, when DNA makes its copy.

Potu Narasimha Rao and Johnson worked out the functional relationship between these phases using the cell fusion technique, combining two cells under experimental conditions. They revealed for the first time the structure of interphase chromosomes, which are not ordinarily visible under the microscope, and showed that cells progress through the cycle in a sequential, unidirectional and controlled way by a series of chemical signals that diffuse freely between nucleus and cytoplasm. These experiments are considered a milestone.

The book's annexure records that Dr Potu Narasimha Rao came from a poor family in Muppalla village, Guntur district, graduated in Agriculture, did his MS at IARI New Delhi, went to the USA, worked on the cytogenetics of tobacco, received his PhD in 1963 and then turned from plant cytogenetics to cancer cells. He found that human cells, normal or cancerous, in culture usually divide every 20 to 24 hours while mitosis itself takes only 40 to 60 minutes.

Mitosis

Activity 4 asks you to observe permanent slides of the stages under the microscope and compare your drawings with the book's figures.

Division of the nucleus is karyokinesis and division of the cytoplasm is cytokinesis; together they produce two daughter cells.

Meiosis (6.14.1)

Unlike mitosis, which is continuous in most cells, meiosis occurs only during the formation of gametes.

It has two phases. In the first, the parent cell with two sets of chromosomes, diploid, divides and daughter cells with one set, haploid, are formed. The second phase is similar to normal mitosis. So the four daughter cells have half the chromosome number of the parent, which is why meiosis is also called reduction division.

13. Reproductive Health (Textbook 6.15)

Sexual maturation is gradual and takes place while general body growth is still going on, so some degree of sexual maturation does not mean the body or the mind is ready for sexual acts, or for having and bringing up children.

The book is direct about the pressures involved: from friends, to participate in activities whether or not we want to; from families, to marry and start having children; and from government and voluntary organisations, to protect child rights and prevent child marriages. In these situations, right choices are important.

Because the sexual act is an intimate connection of bodies, many diseases can be sexually transmitted: bacterial infections such as gonorrhea and syphilis, and viral infections such as AIDS, Acquired Immuno Deficiency Syndrome. These spread by unsafe sexual contact, infected devices, infected blood transfusion, and from an infected mother to child.

Telangana has a high incidence of HIV-positive cases. The factors experts name are illiteracy, poor health, unemployment, migration, non-traditional sex practice, unethical contacts and trafficking. The government has established ART, Anti Retroviral Therapy, centres to supply medicine, and programmes such as ASHA, the Accredited Social Health Activist scheme, and the Red Ribbon Express create awareness of the risks and symptoms.

The book asks you to invite a local health worker to school to discuss HIV, and to argue why social discrimination against AIDS patients is itself a social evil.

14. Birth Control Methods (Textbook 6.16)

Preventing pregnancy by arresting fertilisation is contraception, and any device or chemical that prevents pregnancy is a contraceptive.

Physical devices such as condoms and the diaphragm or cap prevent sperms reaching the ovum. These also prevent transmission of some sexually transmitted diseases such as gonorrhea, syphilis and AIDS — and the book is emphatic that no other method of contraception provides protection against sexually transmitted diseases.

Chemical methods are pills taken orally or devices inserted into the vagina. Pills contain hormones that stop the ovaries releasing ova into the oviducts. Pills for males are also available now; these kill the sperms and are called spermicides.

Intra-uterine devices such as the copper-T and the loop are very effective at preventing pregnancy, but the book notes explicitly that a woman using a copper-T is not protected from acquiring sexually transmitted diseases.

Surgical methods. In males a small portion of the vas deferens is removed by operation and both ends tied, preventing the release of sperms — vasectomy. In females a small portion of the fallopian tube is removed and the cut ends tied, preventing the ovum from entering the oviduct — tubectomy.

15. Fighting Social Ills (Textbook 6.17)

Teenage motherhood. Childbirth needs maturity of mind and body. Illiteracy, poverty and superstition are the main reasons for early child marriage. A girl can be said to be prepared only after 18 years of age, and even then the risk is real: according to the department of family welfare, 21 per cent of teenage mothers die during delivery, with malnutrition among the reasons. Girls below 18 should not be married off.

Stop female foeticide. Reckless female foeticide is making the male-female child sex ratio decline at an alarming rate in some sections of society. The government has enacted laws banning determination of the sex of a foetus, and in spite of those laws it remains a social responsibility to prevent it.

Key words from the chapter

Progeny, cyst, fragmentation, regeneration, vegetative propagation, artificial propagation, parthenocarpy, parthenogenesis, stem cuttings, layering, grafting, stock, scion, desirable characters, tissue culture, amniotic fluid, placenta, umbilical cord, mitosis, meiosis, chromatids, chromosomes, foeticide, HIV-AIDS, vasectomy, tubectomy.

16. Summary

Reproduction is necessary for the perpetuation and continuation of life, and it is of two types, sexual and asexual. In sexual reproduction only half of each parent's chromosomes pass to the next generation.

Fission, budding, fragmentation, regeneration and spore formation are the ways of asexual reproduction. Plants can also be grown from vegetative parts such as stems, roots and leaves, which is vegetative propagation; it may be natural or man-made and has economic importance.

Grafting lets us acquire desirable characters of two plants in one, and tissue culture is the modern technique, growing more plants in less time and space.

Sexual reproduction in higher animals uses specialised, distinctively male and female reproductive systems.

Cells divide for growth, to repair and replace worn-out cells, and to form gametes. Division is of two types: mitosis, somatic cell division, and meiosis, reproductive cell division. Body cells are either somatic cells or germ cells that take part in gamete formation.

G1, S, G2 and M are the stages of the cell cycle, and the synthesis phase is the longest, being where the genetic material is duplicated. Mitosis runs through prophase, metaphase, anaphase and telophase and ends with two daughter cells carrying the same chromosome number as the parent; division of the cytoplasm is cytokinesis. In meiosis the parent cell divides twice and four daughter cells are formed.

Reproductive health matters because a sound mind needs a sound body, and one should know the facts about how sexually transmitted diseases are transmitted. There is no cure for AIDS, so prevention is the only way to avoid it.

Various methods of contraception are available to control child birth. Determination of sex before birth is illegal, and building a healthy society is a shared responsibility.