Modes of Reproduction in Organisms | Term Paper | Zoology | Biology

Here is a term paper on the asexual and sexual modes of reproduction in organisms.

Modes of Reproduction in Organisms

Term Paper # 1. Asexual Mode of Reproduction in Organisms:

In an asexual reproduction, an individual can reproduce in the absence of another individual of similar type with an opposite sex. The division of a bacterial cell into two daughter cells is the best example of asexual reproduction.

Asexual reproduction is found in single-celled organisms along with multicellular ones. Most plants have the ability to reproduce asexually. The ant species Mycocepurus smithii reproduces entirely by asexual method. Asexual reproduction is a mode of reproduction by which offspring arise from a single organism and inherit the genes of that parent only.

The offspring will be exact genetic copies of the parent as only one parent is involved, except in the specific cases of automixis. Asexual reproduction is the primary form of reproduction for single-celled organisms such as the Archaebacteria, Eubacteria, and Protists. Many plants and fungi reproduce asexually as well.

I. Asexual Reproduction in Single Celled Organism:

Fission:

Fission is also referred as cell division. Cell division can be either Binary or a multiple fission.

1. Binary fission:

In cell division, a mother cell divides to produce two daughter cells. This is called as Binary fission. The cell divides in transverse or longitudinal symmetry. In unicellular organisms like Amoeba, the division of the cells during reproduction can be across any plane.

This is called transverse binary fission. In Euglena, binary fission occurs in a definite plane in relation to the whip-like structures located at one end of the cell. This is called longitudinal binary fission.

2. Multiple fission:

The mother cell divides into many daughter cells at the same given time. The malarial parasite, Plasmodium, a unicellular organism, reproduces by multiple fission. The nucleus divides first in many copies followed by cell organelles and finally cytoplasm covers all the nucleus dividing the cell into new daughter cells.

II. Asexual Reproduction in Multicellular Organism:

1. Fragmentation:

It is a process in which the organism is split into fragments and each fragment then develops into a fully mature organism. These organisms develop special zones or weak zones that can be easily broken naturally, by environment or by man. Fragmentation occurs in spirogyra, moulds, cyanobacteria, sea stars, coral, sponges etc.

2. Regeneration:

Regeneration is the ability of an organism to give rise to new individuals from their body part which gets separated due to some injury. In Hydra and Planaria, the organism when cut into parts, each part can grow into an individual.

Regeneration is carried out by specialized cells that proliferate and produce large numbers of cells. From this mass of cells, cells differentiate to form tissues of various cell types. These changes take place in an organized sequence referred to as development.

Regeneration is fundamentally regulated by asexual cellular processes. Regeneration is different from reproduction. For example, Hydra performs regeneration but reproduce by the method of budding. It is gaining a lost part of the body for survival.

3. Budding:

Budding is a process in which a new organism develops from an outgrowth, a small projection which is called as a bud, due to cell division at one particular site. A small protruding process or outgrowth i.e. bud appears at one end of parent cell and enlarges in whole organism.

Nucleus enlarges dividing in two parts of which one goes in the bud. The new organism remains attached to parent as it grows, separating from the parent organism only when it is mature, leaving behind scar tissue at its joint.

In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and when fully mature, detach from the parent body and become new independent individuals.

Yeast can put out small buds that separate and grow further.

4. Vegetative Propagation:

Vegetative Propagation is a process in which reproduction of new plant does not occur from seed or spores but from roots, stem, and leaves. This property of vegetative propagation is used in methods such as layering or grafting to grow many plants like sugarcane, roses, or grapes.

Plants raised by this method give flowers and fruits earlier than those produced from seeds. Plants that have lost the capacity to produce seeds, also are propagated or reproduced by this method such as banana, orange, rose and jasmine.

Another advantage is that all the plants produced are genetically similar to the parent plant. Similarly, buds produced in the notches along the leaf margin of Bryophyllum fall on the soil and develop into new plants. Rhizome is a horizontal plant stem with shoots above and roots below serving as a reproductive structure.

5. Spore Formation:

Have you ever noticed a bread getting whitish green patches (fungus on the bread)? The fungus is Rhizopus. It reproduces through spores. The reproduction from a spore is called sporogenesis and method is spore formation.

Rhizopus has thread like structure called hyphae, which spread on the bread surface. From hyphae, stem like structure are formed which bear a ball or bob like structure called as sporangium.

Sporangium is a thick walled sac that contain tiny spores which are the reproductive parts. These spores are released in the air when sporangia come in contact with moisture in the air. Spore that is attached to substratum then germinate into an individual Rhizopus.

Illustration 1:

How does bacteria reproduce?

Sol:

Bacterial asexual reproduction most commonly occurs by a kind of cell division called binary fission. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics.

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Term Paper # 2. Sexual Mode of Reproduction in Organisms:

Sexual Reproduction is a process which mainly occurs in eukaryotes, where two individuals of opposite sex produce an offspring that have genetic characteristics from both the parents. There are variations among generations as the two parents are involved, and gametes are produced from each parent. The male gamete is produced from the testis and called as sperm while ova produces female gamete egg. Both gametes are haploid.

Example:

A male dog alone cannot give birth to puppies. Thus, it takes a male and a female dog to produce its offspring.

Gametes are also called as the sex cells that are produced by a cell division called as Meiosis. Gametes need motility to be able to meet and unite and food to nourish the developing embryo.

Illustration 2:

In a sexual reproduction, the offsprings produced contain double chromosomes than the parents. The statement is true or false?

Sol:

The gametes or the sex cells are haploid. The cells have half the number of chromosomes present in normal cell. And thus, the offsprings receive half the chromosome (one set of chromosome) from a male parent and the other set of chromosomes from a female parent. That is why the offspring (zygote) contains two set of chromosomes like its parent cells. Mostly, the male sex cell is called as spermatozoa, which is motile.

The female gamete is called as ovum and is non-motile, contains stored food and relatively larger than male gamete. After fertilization of the female gamete with male gamete, a zygote is formed which contains two sets of chromosomes, one from each parent.

Note:

Haploid is the term used when a cell has only one set of chromosomes.

I. Sexual Reproduction in Plants:

The flower is a reproductive organ of a flowering plant. To understand this, we need to first study the structure of a flower.

Parts of a typical flower:

A flower is a modified shoot with a limited growth. Flowers vary in size, shape, structure and colour.

The main parts of a flower are:

1. Calyx:

Outermost whorl of flower protects petals, it is also known as sepals.

2. Corolla:

Petals attract insect or bird pollinators by their colour, scent, and nectar, which may be secreted in some part of the flower.

3. Androecium:

Androecium is the male part of a flower. It is a group of stamens. Each Stamen consists of a stalk called the filament and a small sac like structure called the anther at the tip. The pollen grains are contained in the anther, within the pollen sacs.

4. Gynoecium:

Gynoecium is the female part of the flower and consists of the carpels or ovary.

Gynoceium has three parts:

(1) Stigma

(2) Style and

(3) Ovary.

The ovary contains the ovules and each ovule carries within it, an embryo sac, within which lies the egg cell or the female Ovary gamete.

The sexual reproduction in the flowering plants involves:

1. Pollination

2. Fertilization.

A. Pollination:

Transfer of pollen grains from the anther to the stigma is called pollination. Pollen grains are transferred mainly by the wind, water and insects. They are called as pollinating agents.

Types of Pollination:

Pollination is of two types.

They are:

1. Self pollination:

The transfer of pollen grains from the anther of a flower to the stigma of the same flower called as autogamy or another flower of the same plant is known as self-pollination.

Advantages of self-pollination are:

a. Self-pollination is certain in bisexual flowers.

b. Flowers need not depend on agents of pollination.

c. There is no wastage of pollen grains.

Disadvantages of self-pollination are:

a. The seeds are less in number.

b. The Endosperm is minute. Therefore, the seeds produce weak plants.

c. New varieties of plants cannot be produced.

2. Cross-pollination:

The transfer of pollen grains of a flower to the stigma of another flower of a different plant of the same species is called cross-pollination.

Advantages of cross-pollination:

a. The seeds produced as a result of cross-pollination. Seeds develop, germinate properly and grow into better plants, i.e., cross-pollination leads to the production of new varieties of plants.

B. Fertilization:

After the pollen attaches with a suitable stigma, it reaches the female germ-cells which are in the ovary. The pollen tube enters into the embryo sac through the micropyle. At this time, the pollen tube bursts open, two gametes are released from the pollen tube that enter the embryo sac.

One of the gametes fuse with the egg, and the other fuses with the secondary nucleus in the ovule. The fusion of a male gamete with egg is known as fertilization. The fertilized egg is known as zygote, which develops into an embryo.

C. Germination:

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.

Zygote divides to form endosperm giving free nuclei. Cell wall and cellular endosperm are formed. The food material is intact with endopserm to nourish embryo. Embryo develops in heart shaped structure. This forms fruits with fresh seeds and covering.

II. Reproduction in Human Beings:

Puberty is the stage of a boy and a girl for their specific development of body, mind and hormones. Noticeable changes like oily face, growth of hair on underarms, hands and legs, development of breast (in girls), cracking of voice and size and shape change in penis (in boys) are seen.

This is called as sexual maturation. During adolescence or puberty, the rate of growth of the body in weight slows down and reproductive tissues begin to mature. The female and male reproductive systems are involved with producing, nourishing and transporting either the egg or sperm and they are different in shape, structure and function. The male has reproductive organs, or genitals, that are both inside and outside the pelvis, while the female has reproductive organs entirely within the pelvis.

Let us study the male and female reproductive system in details:

A. Male reproductive system:

The male reproductive system consists of a pair of testes and a series of ducts and glands. Sperms are produced in the testes and are transported through the reproductive ducts. These ducts include the epididymis, vasa deferens, ejaculatory duct and urethra.

The reproductive glands produce secretions that become part of the semen. The semen is the fluid that is ejaculated from urethra. These glands include the seminal vesicles, prostate gland.

1. Testes:

The testes, a pair of oval glands (singular, testis) are located in the scrotum (a sac of loose cutaneous tissue between the upper thighs). It has around 200 lobules and testosterone is produced in the testes, which stimulates the production of the sperms as well as give secondary sex characteristics at the beginning of the puberty. Its temperature is maintained by muscle fibres hence testes are placed outside body.

2. Vas deferens:

Vas deferens is a spermduct ascending from the posterior border of testes. Vas deferens then loops with the tube coming from the urinary bladder and thus is a common passage for sperm and urine.

3. Seminal vesicles:

The pair of seminal vesicles are posterior to the urinary bladder. They secrete fructose to provide an energy source for the sperm and alkalinity to enhance sperm mobility in a female system making 60% of the semen.

4. Prostate glands:

The prostate gland is a muscular gland doughnut in shape that surrounds the urethra as it emerges from the bladder. The smooth muscle of the prostate gland contracts during ejaculation to contribute to the expulsion of the semen from the urethra. 20% of the semen is secreted from this.

5. Penis:

The penis is an external genital organ made of erectile tissues. The distal end of the penis is called the glans penis which is covered by a fold of skin called the prepuce or foreskin. It consists of a framework of smooth muscle and connective tissue that contains blood sinuses which makes it rigid known as penis erection.

6. Urethra:

The urethra, which is the last part of the urinary tract, lies on the tip of the glans penis. It is both a passage for urine and for the ejaculation of semen.

7. Sperm:

The sperms are tiny bodies that consist of mainly genetic material in head, middle piece has mitochondria to make ATP from fructose for movement and a long tail that helps them to move towards the female germ-cell. Head has a specialised lysosome, acrosome to digest a limiting membrane of ovum and facilitates the sperm entry in egg.

B. Female reproductive system:

1. Ovaries:

Ovaries are a small pair of organs located near the lateral walls of pelvic cavity of the body that produces hormones and female germ cells called as ova or eggs. When a girl is born, many eggs are produced which are attached to the wall of the ovaries.

On reaching puberty, these eggs mature one by one and are detached from the wall. These eggs then leave the ovary, travelling through oviducts, fallopian tubes, uterus, cervix vagina with some amount of blood, mucous and uterine lining. This is called as menstruation. This occurs every 28 days or 31 days from puberty.

2. Fallopian tube:

The Fallopian tubes or oviducts are two tubes connecting the ovaries of female mammals to the uterus. The ovum on maturity is released from the ovaries and in the fallopian tubes, where the cilia facilitate the movement of the ovum into the uterus.

3. Uterus:

The uterus or womb is a pear-shaped hollow muscular organ. Its major function is to welcome a fertilized ovum which is implanted into the endometrium which has secretory ciliated cells, and derives nourishment from blood vessels, which is present majorly for this. The fertilized egg or zygote develops into foetus and stays in uterin cavity for 9 months till child birth. If the egg is not fertilized, it is removed from the body during menstruation.

4. Cervix:

The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall.

5. Vagina:

The vagina is a fibro-muscular tubular tract made of muscular tissue leading from the uterus to the exterior of the female body. The vagina is a canal that joins the cervix (the lower part of the uterus) to the outside of the body. It also is known as the birth canal. During sexual intercourse its smooth muscle stretch helping penis insertion in vagina depositing the semen in it.

6. Vulva:

It is the external covering of vagina. It has mons pubis: pubic hair and urethral opening, labia majora and minora: covering vagina between them, clitoris: erectile tissue and vestibule: a cleft.

7. Ovum:

The egg cell is typically not motile, and it is much larger (visible to the naked eye) than the motile sperm cells. When the egg and the sperm fuse, a diploid cell (the zygote) is formed, which gradually develops into a new organism. Zygote starts dividing and gets implanted in the lining of the uterus.

Note:

Diploid – an organism or cell having double (2n) the basic haploid (n) number of chromosomes.

8. Placenta:

The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta. It provides a large surface area for glucose and oxygen to pass from the mother to the embryo and removal of the waste generated by the embryo. After the completion of gestation period (9 months), rhythmic contraction in the uterine wall helps in the child birth.

Ovulation:

The Ovary contains connective tissues with ovarian follicles. These are immature eggs that develop in ovary.

When the egg grows, the ovary is filled with a fluid to increase size. Large sac of fluid and maturing egg in called graafian follicle which produces and secretes estrogen hormone.

Graafian follicle ripes from puberty stage and projects from the surface of ovary, bursts and releases ovum in oviduct funnel. Cilia in funnel helps to grab the ovum.

Corpus luteum:

The remains after the follicle ruptures converts in yellow mass of endocrine tissue called corpus luteum. It secrets oestrogen, progesterone, relaxin.

Fertilisation:

It is union of male and female gamete which occurs when the ovum meets the sperm in the fallopian tube, the site of fertilisation in human females. Approximately, 300 million/day sperms are produced and matured, having life of 48 hrs when ejaculated.

Fertilisation in human is a chance process as thousands of sperms move to ovum however only one sperm can fertilise it. After its attachment to ovum, it holds strongly. Lysosomal enzymes of the sperm cells digest the limiting membrane of ovum and the entire sperms penetrates the egg. For fertilisation, the ovum has only 24 hours after its release from ovary.

Implantation:

The fertilised egg has microscopic amount of food for it which can nourish it for 5-7 days. Zygote that divides to form hollow cells, called blastocyst, now implants itself to the roof of uterus and nourishes itself from mother. This attachment of blastocyst to the endometrium occurs 7-8 days after fertilisation with sperm in oviduct is called the implantation.

Development and Role of Placenta:

Placenta is association between the maternal and foetal tissue for a physiological (blood, food, etc.) exchange. As blastocyst has vanishing food reverse, a special layer trophoblast or feeding layers is formed on outer edge of blastocyst.

These tissues burrow into the digest tissue of uterus forming a productive tissue called the placenta which feeds the embryo till birth. Villi from placenta pretrude into the uterine wall, capillaries in the placenta are connected to the artery and vein which run into the umbilical cord from the embryo’s abdomen.

The Umbilical cord is a tough structure that serves as a blood vascular connection between the foetus and the uterine wall. Amnion (water sac) surrounds the embryo except at umbilical cord.

This fluid protects embryo against any mechanical jerks or blows that may strike the mother’s womb. Maternal blood carries nutrients, oxygen, hormones and water to placenta, where they are absorbed and passed on to the embryo through blood vessels of the umbilical cord.

Waste products from embryo are brought to the placenta that transfers it to the mothers body. In no case mothers blood circulate through the embryo. This protects the tissues of the embryo from damage by high pressure blood.

Labour & Child Birth:

Female body has many function and thus it prepares itself for childbirth. The blood volume and the heart size increases 25% for requirements of the baby. Mouth of the womb, becomes soft and pliable.

This is because the cervix has a very small opening that dilates by 5-6 inches to allow it to deliver the baby. Muscle fibers of birth canal elongate and become elastic to withstand stress of the delivery. New blood vessels develop.

The birth tissues produce lactic acids for its protection against the bacteria. The ligaments, holding the pelvic bones, soften for delivery by the hormones. Breasts size also enlarges, the milk ducts are develop and they are nourished by the blood.

Stages in Labour:

1) First stage (8-20 hrs):

Uterus contract to push the head of the baby against cervix. Pressure dilates the opening of the cervix. The amniotic sac breaks when it is dilated that acts as stimulant of labour.

2) Second stage (45 min-2 hrs):

The Baby is delivered. Few minutes after birth, the blood surge from placenta into the baby’s body and umbilical cord is cut. Expulsion of the foetus from the uterus is called the parturition.

3) Third stage (30 mins):

The Uterus restarts a kneading motion to expel the placenta.

Menstrural Cycle:

For a fertilised egg to be received, implanted and nourished, the uterus undergoes many changes under the influence of hormones. Endometrium thickens with increased blood vessels and mucus. If fertilisation does not occur, endometrium is shed in form of menstrual flow. Duration of menstrual cycle is 24-35 days.

It has 4 phases:

(1) Menstrual phase: Menstruation/Menes:

Discharge of 25-26 ml of blood and mucues due to reduction in hormones. It lasts for 3-6 days. 1^st day of menstruation is 1^st day of the ovarian cycle. Primary follicular development begins. Low levels of estrogen is produced.

(2) Provulatery (follicular) phase:

Between menstruation and ovulation, 6-13 days in the 28 days cycle, the secondary follicle matures into Graafian follicle. Oestrogen  increases.

(3) Ovulation:

Repulses of the graafian follicle, mature ovum releases in the oviduct. Prior to ovulation, oestrogen level is maximum. The graafian follicle collapse into corpus luteum, secreting oestrogen and progestrone.

(4) Post ovulatory (luteal) phase:

Between the ovulation and onset of menses. Corpus luteum produces pregsterone which is prominent.

Endometrium is prepared to produce fertilised egg. When the fertilisation and implantion does not take place, corpus luteum disintegrates. The Hormones decrease and menses start.