Importance of variation
The consistency of DNA copying during reproduction is important, which leads to the stability of population species. Changes in an ecological system, is beyond our control like varying temperature, varying water level and natural calamities, can wipe out the population species. So, if there are some variations that are present in some individual of the population species then there is a chance for the survival of these species over time. The process of reproduction results in the production of offspring’s which are exactly similar to the parents.
Do organisms create exact copies of themselves?
Organisms look similar because their body designs are similar. If the body designs are to be similar, the blueprints for these designs should be similar. The reproduction involves making the blueprints of body design.
The chromosomes of the nucleus of a cell contain information for the inheritance of features from parents to next generation in the form of DNA molecules. The DNA molecule present in the chromosome defines the characteristics of organisms. 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 more the difference in DNA more the difference between organisms.
Therefore, DNA is responsible for inheritance. During reproduction, there is the formation of new cells which must carry the same amount and type of hereditary information as present in parent cell. This is accomplished by DNA copying, prior to each cell division. As a result, DNA division takes place; very often the DNA does not divide to form exact copies. The process of copying DNA will have some variation each time. The DNA copies generated will be similar but may not be identical to the original.
This is because the DNA division is a biochemical reaction. When a cell divides the two copies of the cell may be similar but not be identical. When DNA of a cell is changed, it will behave in a slightly different manner, such differences that take place during the division of DNA are called variation and this is the reason why organisms cannot make identical copies of themselves.
Reproduction is a process by which living organisms produce more living organisms of their own kind. There are two main types of reproduction. They are asexual reproduction and sexual reproduction. a) Asexual reproduction: the process of reproduction in which new individuals are produced from a single parent. Eg: microorganism.
- a) Asexual reproduction is found in single-celled organisms such as archaebacteria, eubacteria, and protists. Many plants and fungi reproduce asexually as well.
- b) Sexual reproduction: the process of reproduction in which two individuals are involved to produce a new individual. Eg: human, tiger.
Types of asexual reproduction
- Fission is defined as the splitting of the organisms into two equal halves and each half give rise to a new organism. For example, Amoeba, bacteria. The most common type of fission in binary fission. It is a division of the organism such as bacteria into two or more parts. Binary fission can be irregular (division can take place in any plane, eg: Amoeba), longitudinal (division occurs longitudinally, eg: Leishmania which causes kala-azar, euglena), transverse (division occurs transversely, e.g: Paramecium) and can also be oblique (division occurs obliquely).
Fig.1. Binary fission in Amoeba
Multiple fission: in multiple fission, the nucleus of the parent cell unedges repeated divisions to produce many daughter nuclei. The cytoplasm also divides and cleaves around each nucleus forming several daughter cells in the parent cell. In the favourable conditions when parent cell membrane raptures all the daughter individuals are released. Eg: plasmodium (malarial parasite)
- Fragmentation is another mode of asexual reproduction in which an organism breaks into pieces and each piece give rise to a new organism. The splitting might be or might not be deliberate. fragmentation occurs in multicellular organisms with simple body organisation. For example,
Fragmentation in spirogyra
- Regeneration is the ability to form a new organism from the body parts. Many fully differentiated organisms have the ability to give 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. This is known as regeneration. For example, Hydra,
- Regeneration is carried out by specialised cells. These cells proliferate and make large number of cells. From this mass of cells, different cells undergo changes to become various cell types and tissue. These changes take place in an organised sequence referred to as development. However, regeneration is not same as reproduction, since most organism would not normally depend on being cut up to be able to reproduce.
Regeneration in planaria
Regeneration in Hydra
- Budding is defined as an outgrowth from the body of the organism. This outgrowth then detaches from the body and form a new independent organism. For example, Hydraand Yeast.
Budding in Hydra
- Vegetative reproduction is another method of asexual reproduction. In this form of reproduction, stem, root and leaves are used to form plants when provided with suitable conditions. Layering and cutting are the two common methods used for vegetative propagation. For example, banana, rose, jasmine etc. The plant produced through vegetative propagation is genetically identical to the parent plant.
- The tuber of potato: the potato tuber is a modified stem. Many notches can be seen on the surface of the potato. These are called ‘eyes’ of the potato. Each ‘eye’ of a potato can give rise to anew potato plant under suitable condition.
- Rhizomes of ginger and turmeric: rhizomes are examples of modified underground stems for food storage. These contain nodes, internodes and scaly leaves. When buried under the soil, the rhizomes produce new plants.
- Leaf of Myriophyllum: Leaves of Myriophyllum have notches on the margin. If a leaf is put under the soil, small saplings grow from notches of the leaf margin.
- Vegetative propagation can also be done artificially. Artificial propagation includes growing plants by man-made methods and the commonly used methods are:
- Cutting in which a stem is given an oblique cut and the cut stem is kept under appropriate conditions to give rise to an entirely new plant, eg, Rose and China rose.
- Layering is a process in which the stem, while still attached to the mother plant is buried under the soil for a while, till it shrinks new roots. It is then detached from the mother plant, eg Jasmine.
- Grafting is a method in which the cut stems of two different plants (one with the roots and the other without roots) are joined together in such a way that the two stems join and grow as a single plant. The cut stem of a plant having roots (and fixed in soil) is called stock. The cut stem of another plant (without roots) is called the scion.
- Spore formation, the parent plant produces hundreds of microscopic reproductive units called ‘spores’. Spores are formed in special spore cases called sporangia. When the spore mature, the sporangium bursts open to release them. When these air- borne spores land on food (or soil) under favourable conditions, they germinate and produce new plants. Spores can survive in extreme conditions because of the protective hard coat.
- It involves specific reproductive parts such as hyphae in Rhizopus and blob-on-a stick structure in Rhizopusare involved in reproduction.
Spore formation in Rhizopus
The production of new plants from a small piece of plant tissue or cells removed from the growing tips of a plant in a suitable growth medium is called tissue culture or culture solution. In this process the growth medium or culture solution is very important as, it is used for growing plant tissue because it contains various plant nutrients in the form of ‘jelly’ known as agar and plant hormones which are necessary for the growth of the plant.
The process of tissue culture for producing new plants is as follows:
1. A small piece of plant tissue is taken from the growing point of the plant or from the tip of the plant and placed on a sterile jelly which contains nutrients and plant hormones. The hormones make the cells in the plant tissue divide rapidly producing many cells which forms a shapeless lump of mass called ‘callus’.
2. The callus is then transferred to another jelly containing suitable plant hormones which stimulate the callus to develop roots.
3. The callus with developed roots is then put on a yet another jelly containing different hormones which stimulate the development of shoots.
4. The callus having roots and shoots separates into tiny plantlets. In this way, many tiny plantlets are produced from just a few original plant cells or tissue.
5. The plantlets thus produced are transplanted into pots or soil where they can grow to form mature plants.
Advantages of Tissue Culture
1. Tissue culture is a very fast technique. Thousands of plantlets can be produced in a few weeks’ time from a small amount of plant tissue.
2. The new plants produced by tissue culture are disease-free.
3. Tissue culture can grow plants round the year, irrespective of weather or season.
4. Very little space is needed for developing new plants by tissue culture.
5. It helps to speed up the production of new varieties into the market place.
6. In the case of the seed potato industry, this technique helps in maintaining and establishing virus-free stock.
- Development of a seedling from a seed
Sexual reproduction it involves two sexes, namely male and female in the form of gametes produced by two different parents.
Significance of Sexual Reproduction
Sexual reproduction is the source of variation. Mixing of two organisms gives rise to new recombinants or variants. Sexual reproduction involves the mating of germ cells also known as gametes. These gametes are haploid in nature, that is, they have a half set of chromosomes. These gametes are formed through the process of meiosis. When male gametes and female gamete each with a haploid set of chromosomes combine, they will form a diploid zygote. Zygote undergoes repeated divisions to form a new organism.
In human male gamete is small and motile whereas female gamete is large and non-motile.
Sex cells– gametes, also known as sex cells or germ cells, are that come together during fertilization or conception un organisms that reproduce sexually. Their genetic complement consists of a single set of unpaired chromosomes.
Sexual reproduction in flowering plants
The flower is the reproductive structure found in angiosperms. Angiosperms (flowering plants) are seed-bearing vascular plants. Their reproductive structures are flowers in which the ovules are enclosed in an ovary. The flower consists of sepals, petal, stamens and pistils.
Structure of flower
A flower is said to be unisexual if it contains either stamens or pistils whereas if both stamens and pistils are present, it is known as bisexual. Papaya and watermelon are unisexual whereas Hibiscus and mustard are bisexual in nature.
Sepals resemble like small leaves and they protect the flower in the bud.
Petals are the coloured and scented part of the flower. Petals attract insects and birds. Metals also protect the other delicate parts of the flower like the anther.
Pistil/carpel is the female reproductive structure which consists of swollen basal part ovary, middle elongated style and terminal stigma. Ovary contains ovules and each ovule bear an egg cell.
Stamen is the male reproductive part and it consists of the anther and the stigma. Anther contains pollen grains that are yellowish in colour which fuses with a female gamete, that is, egg cell. Fusion leads to zygote formation which forms a new plant.
Transfer of pollen grains from the anther to the stigma of the flower is known as pollination. When the pollen and the stigma is of the same flower, it is known as self-pollination. When pollen from one flower lands on the stigma of another flower it is known as cross-pollination. This transfer is achieved by agents like wind, water, or animals.
It occurs when male gamete present in pollen grain joins the female gamete or egg present in the ovule.
1)When pollen grains fall on the stigma of the carpel, it burst open and grows a pollen tube downwards through the style towards female gamete in ovary.
2)A male gamete moves down the pollen tube. The pollen tube enters the ovule in ovary. The tip of pollen tube bursts open and male gamete comes out of pollen tube. In ovary, male gamete of pollen combines with nucleus of female gamete or egg present in ovule to form a fertilised egg (Zygote).
Formation of Fruit and Seeds
1)The zygote divides several times to form an embryo within the ovule.
2)The ovule develops a tough coat around it and gradually converted into a seed.
3)The ovary grows rapidly and ripens to form a fruit (with seed inside it) 4). The other part of flower like sepals, petals, stamen, stigma, style dry up and fall off. 5)The place on plant where we had flower, we now have a fruit. A fruit protect the seed.
6)The seed contain a baby plant (or embryo) and food for the baby plant.
The part of the seed which contains stored food for the baby plant is called cotyledon. The baby plant present inside a seed is in an inactive form, but when it gets suitable conditions to germinate it forms new plant out of the seed.
In flowering plants, one sperm fertilizes the egg cell, whereas the other sperm fuses with the two polar nuclei forming endosperm. This is known as double fertilization as two fertilization events are taking place. Two polar nuclei fuse with one sperm and other sperm fuses with the egg cell to form the zygote.
Sexual reproduction in animals
An animal having male sex cells called ‘sperms’ in its body is called male.
An animal having female sex cells called ‘ovum’ in its body is called female.
Gametes the cells involved in sexual reproduction is called gametes. The male gamete in the animal is called sperm and the female gamete in animal is called ovum.
Fusion of gametes give rise to a single cell called zygote.
Fusion of sperm with an ovum to form a zygote during sexual reproduction is called fertilization.
The fertilization which occurs inside the female body is called internal fertilization. In mammals, birds and reptiles internal fertilization takes place.
The fertilization which occurs outside the female body is called external fertilization. In amphibians like frog and toads and fishes’ external fertilization takes place.
Why the amount of DNA does not get doubled during the sexual reproduction?
The gametes are special type of cells called reproductive cells which contain only half the amount of DNA (or half the number of chromosomes) as compared to the normal body cellf of an organism. So, when a male gamete combines with female gamete during sexual reproduction, then the new cell ‘zygote’ will have the normal amount of DNA. For example, human sperm has 23 chromosomes and human egg has also 23 chromosomes.so when sperm and egg fuse together during fertilization, then the zygote formed will have 23+23=46 chromosomes, which is normal number of chromosomes.
The age at which males and females become sexually mature is called puberty. The beginning of puberty is characterized by production of gametes (eggs and sperms), maturity of sex organs and number of visible changes in both males and females.
The onset of puberty starts much earlier in girls, between 8 and 13 years of age, whereas it starts between 10 and 14 years of age in boys and lasts up to 2 to 4 years.
- a) Changes in boys:
Boys increase in height as legs grow faster than other parts of the body.
Development of strong muscles.
Shape of the body changes as a result of broadening of shoulders and widening of chest.
There is a typical change in the voice due to enhanced growth in larynx forming a protruding structure called as Adam’s apple. Boys develop deep voice.
Active working of sweat glands resulting in the formation of sweat.
Sebaceous glands secrete much of oil which makes the skin glowing. Sometimes it also results in acne.
Intellectual, emotional and psychological maturity is attained.
Development of reproductive organs to produce gametes and secrete reproductive hormones.
Growth of facial hair and body hair under the armpits and pubic region.b) Changes in girls:
Girls exhibit a rapid increase in height due to the faster growth of legs.
Development of shaped muscles.
Development of broad shoulders and narrow waist.
Development of high pitched voice.
Sweat glands are active secreting lot of sweat.
Sebaceous glands secrete lot of oil and lead to the development of pimples.
Growth of hair in the pubic region and under armpits.
Ovaries are active in the formation of follicles. They also secrete female sex hormones. Females are mature and active in their reproductive phase.
Development of mammary glands under the influence of oestrogen hormone.
Females exhibit menstrual cycle during their reproductive phase. First menstrual flow is called as ‘menarche’. Termination of menstruation is called as ‘menopause’. Secondary sexual characters: These are the characters that are used to distinguish a male from a female.
Growth of facial hair and body hair in males. Growth of hair in the pubic region in females.
Broadening of shoulders and widening of the chest in males. Widening of waist and narrowing of the hip in females.
Breast development in females.
Mood swings in both the sexes. Both boys and girls attain mental and emotional maturity. The brain is active and the brain has the capability of learning more.
HUMAN REPRODUCTIVE SYSTEM
Human Male Reproductive system
The human male reproductive system consists of the following parts:
- Occur in pairs
- Produce sperms
- Produce male hormone, testosterone
- Extension of the abdominal cavity containing testes
- Protects testes
- Maintains a temperature lower than body temperature
- Complex tube-like structure in the scrotum
- Stores sperms, help in the passage of sperms
- Vas deferens
- Tubelike structure emerging from the lower part of the epididymis
- Opens to the ejaculatory duct
- Muscular, copulatory organ
- Discharge sperms when stimulated
- Tubelike structure
- Common passage for both sperms & urine
- Prostate gland
- Secretes milky fluid that helps in sperms mobility
- Seminal vesicles
- Produce viscous fluid which helps in sperm mobility in female
Sperms: The male gametes
- Tiny bodies with the long tail
- Mainly composed of genetic material
- Sperms are produced in testes. Around 12 billion sperms are produced per month.
- Sperms move from Testes to Epididymis where they mature
- Before intercourse, Penis gets filled with blood and becomes erect.
- Mature sperms move from Epididymis to Vas deferens
- Ducts of seminal vesicle and prostate gland combine with Vas deferens to form the Ejaculatory duct
- The liquid mixture (consisting of sperm) from the Ejaculatory duct joins the urethra and gets ejaculated.
- Of 300 million sperms ejaculated, only 200- 300 survive to reach the egg cell & only one succeed to fertilize it.
Human Female reproductive system
The female reproductive system consists of the following parts:
- A pair of Ovaries
- One on each side of uterus
- Produce female sex cell (ovum/ova)
- Secrete female hormones estrogen & progesterone
- Fallopian tube
- Exist in pairs, originating from uterus extending up to each ovary
- Receives ovum from the ovary
- The pear-shaped muscular hollow structure
- Foetus develops here
- The narrow lower part of the uterus
- Tubelike structure
- Sperm discharge occurs here
- Acts as the birth canal
Ovum: The female gamete
- Ovum is the female sex cell. Also called Egg cell
- Ova are produced by epithelium in the ovary
- Release of the ovum from the ovary is termed as Ovulation
- Ovary gets matured by puberty
- Ova are produced by germinal epithelium of the ovary
- Formation of ova starts in foetus even before birth. At birth, there are 2-3 lakhs immature eggs. After attaining puberty, these immature eggs start getting matured under the influence of hormones from the pituitary gland.
- One matured ovum is released from either ovary at every 28 days. This process of release is Ovulation.
Fertilization in Human beings
Fertilization is internal in human beings.
- This process occurs in the fallopian tube.
- Ovum comes from the ovary to Fallopian tube. The unfertilized egg remains alive for approx. 24 hours.
- Sperms enter through the vagina and reach the Fallopian tube. Sperm must fertilize the egg within the above time.
- Sperm enters the egg.
- The fusion of nucleus of sperm & ovum is termed as Fertilization.
- Fertilization leads to the formation of single-celled Zygote
- The zygote undergoes repeated divisions and gets implanted in the uterus. This is now called as Embryo. Embryo in advanced stages of development is termed as Foetus.
- Uterus also prepares itself to receive the embryo. It thickens its walls. Embryo descends into the uterus in 4-5 days from the day of fertilization.
- The embryo in the uterus receives nutrients from the mother’s body through the Umbilical cord which in turn is connected to the uterine wall of the mother through Placenta. Placenta: is a structure formed by specialized tissues of fetus & uterine wall of mother.
Nutrients enter through this cord, and wastes go out of the foetus body. The umbilical cord also has arteries & veins for exchange of oxygenated and deoxygenated blood.
Sexual cycle in Females: Menstruation
- Usually, one mature ovum is released from the ovary into the oviduct once every 28 days. This is called
- An egg is released from either of the ovaries every 28 days. The unfertilized egg is alive for 24 hours after ovulation. Uterus prepares itself to receive the foetus. As a result, the Endometrium thickens.
- If fertilization doesn’t take place, then no zygote is formed. Therefore, endometrium breaks which result in bleeding through the vagina.
- Process of bleeding through the vagina due to breakage of the endometrium is termed as ‘Menstruation’. This bleeding lasts for 2-6 days. This occurs approx. 14 days after ovulation. Since Menstruation occurs every 28 days; this is also referred to as Menstrual cycle.
- The first occurrence of menstruation at puberty is called menarche.
- Menstrual cycle in a woman continues till 45-55 years of age, post which ovary becomes dormant. No ovulation, no menstruation occurs after this. This is termed as “Menopause”
Some people give birth to a child every year; others end up with 10- 11 children. It is very important to understand the disadvantages of overpopulation and proactively act to control the same. Following points can ensure that the population does not increase beyond the limit:
- Control the number of children in a family to one/two depending upon the family’s income
- No differentiation between male & female child. Some people end up giving birth to 6-7 daughters just to get a son.
- The proper age gap between children must be maintained so that enough care is provided to the first child even before the second one arrives.
- Parents should plan a baby at the right age. If parents are too old in the 40s, they themselves become too old to take care of kids. Similarly, parents as young as 19- 20 years old couples are too young to manage kids.
The prevention of pregnancy in women is called contraception.
There are several means to avoid unwanted pregnancy or maintain the proper gap between children. Some of the ways are as follows:
- Barrier method: Condoms
- Balloon like elastic covering that tightly fits over the penis
- Collects ejaculated sperms during intercourse
- The diaphragm is used by females by putting it in the vagina to cover the cervix
- Chemical method: Oral contraceptive pills
- Does not allow ovulation to occur
- Surgical method:
- Vasectomy: Removal of a small portion of Vas deferens
- Tubectomy: Removal of a small portion of the fallopian tube
Sexually Transmitted Diseases (STDs)
There exist many communicable diseases which spread by sexual contact during intercourse.
Some of these STDs are:
- Caused by bacteria and can be treated with antibiotics
- HIV AIDS
- Caused by the HIV virus which destroys the immune system of the body. There is no cure for AIDS till date. The person suffering from AIDS eventually dies due to infection.
- Genital & anal warts
To prevent STDs, ensure the following:
- Use of condoms during intercourse
- Wash before & after intercourse
- Confirm that a new partner is HIV tested before intercourse