Reproduction:Reproduction is the process by which all living organisms produce new individuals of their own kind. Reproduction is essential for the survival of species on this earth.
DNA Copying in Reproduction
→ The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA.
→ The process of copying the DNA may have some variations each time. As a result, the DNA copies generated will be similar, but may not be identical to the original.
Importance of copying of DNA during reproduction
The copying of DNA during reproduction is important because:
(i) It’s helps in transferring of characters from parents to offspring
(ii) It maintains the characteristics in different generations of the species.
(iii) It also produces variations which are useful for the survival of species for long time..
What is the importance of variation?
(i) Variation helps organisms to adapt to the changes in environment.
(ii) It contributes to evolution
(iii) It is the basis of heredity.
(iv) It helps species to be resistant to diseases.
Types of Reproduction
It is mainly of two types:
1.Asexual reproduction: The production of new organism from a single parent without the involvement of sex cells (or gametes) is called asexual reproduction. 2.Sexual reproduction: The process of production of new organism from two parents by using sex cells (called gametes) is called sexual reproduction.
Modes of Asexual Reproduction
(i) Fission: In this the parent cell divides into daughter cells.
→ Binary fission: When the fission results in two daughter cells, it is binary fission. For example – Amoeba and Paramecium.
Reproduction in amoeba: When the amoeba cell reaches its maximum size, then first the nucleus of amoeba lengthen and divide into two parts following the division of cytoplasm of parent to form two smaller daughter cells .
→ Multiple fission: When fission results in many daughter cells, it is called multiple. For example – Plasmodium.
Reproduction in plasmodium: During unfavourable condition a cyst or protective wall is formed around the cell of plasmodium. Inside the cyst, the nucleus of cell splits several times to form many daughter nuclei. Each daughter nuclei is surrounded by the cytoplasm collected inside a thin membrane. Thus, a number of new daughter cells are formed within the cyst. When the favourable conditions arrive the cyst breaks open and daughter cells are released each forming a new organism.
(ii) Fragmentation: In multi-cellular organisms, the organism breaks-up into smaller pieces upon maturation, each piece develops into new individual. For example – Spirogyra.
(iii) Regeneration: In this, a few organisms may give rise to new individual organisms from their body parts. For example – Hydra and Planaria.
(iv) Budding: In some organisms, a bud is formed which develops into tiny individual. It detaches from parent body upon maturation and develops into a new individual. For example: Hydra
(v) Vegetative Propagation: It is the method of reproduction in which plants reproduce by their vegetative parts such as roots, stems and leaves.
It is of two types:
(a) Natural vegetative propagation: It can be done by the following methods
Cutting: A small vegetative part of the plant removed by cutting with sharp knife is called cutting. It is then planted in the soil to grow a new similar plant. For example – rose and sugarcane are grown by cutting. Layering: In this method, a branch of plant is pulled towards the ground and a part of it is covered with moist soil leaving the tip of branch exposed above the ground. After some time, new roots develop and then this branch is cut off from parent plant and it grows into a new plant. For example- jasmine and strawberry are grown by layering. Grafting: In this method, the stems of two different plant are cut, one with roots and other without roots. Both are joined together and the part with roots is sown in the soil. This results into a new lant that has the characteristics of both the plants. The cut stem of plant having root is called stock and the cut stem of another plant is called scion. For example- apple and pear are grown by grafting.
In tissue culture, new plants are grown by removing tissue or separating cells from the growing tip of a plant. The cells are then placed in an artificial medium where they divide rapidly to form a small group of cells or callus. The callus is transferred to another medium containing hormones for growth and differentiation. The plantlets are then placed in the soil so that they can grow into mature plants
Importance of vegetative propagation:
→ Plants can bear flowers and fruits earlier.
→ Plants which have lost the ability to produce viable seeds can also reproduce by vegetative propagation.
→ All plants are genetically almost similar to the parent plant.
→ Seedless varieties can be obtained.
(vi) Spore formation: In this method, the parent plant produces a number of microscopic reproductive units called spores closed inside a spore case. When the spore case burst, the spores spread into air. When these air borne spores land on food or soil, they begin to germinate under favourable conditions and produce new plants. For example – Most of the fungi such as Rhizopus ( bread mould) and non-flowering plants.
It involves two parents – male and female. A male gamete fuses with a female gamete to form a new cell called zygote. This zygote then grows and develops into a new organism.
Sexual reproduction in flowering plants:
Flowers are the reproductive organs of plants. It mainly consists of four parts – sepals, petals, stamen and pistil.
Stamen is the male reproductive part and produces pollen grains that contain male gametes. Each stamen has two parts – Filament and anther.
Carpel is the female reproductive part and produces ovules that contain female gametes. It has three parts – Stigma, style and ovary.
There are two type of flowers:
Unisexual: A flower that contains either male or female reproductive parts. For example – Papaya and watermelon.
Bisexual: A flower that contains both male and female reproductive parts. For example – Hibiscus and mustard.
Pollination: Transfer of pollen grains from the anther to the stigma of the flower is known as pollination.
It is of two types:
(i) Self-pollination: The transfer of pollen grains from the anther to the stigma of the same flower or another flower of the same plant. (ii) Cross-pollination: The transfer of pollen grains from the anther to the stigma of another flower or another flower of a different plant of the same species. It is carried out with the help of external agents like insects, birds, wind and water.
Fertilization: Fertilization is the process of fusion of male and female gamete to form a zygote during sexual reproduction.
→ Pollen grains produced in the anther are transferred to the stigma of same flower or stigma of another flower.
→ A male gamete present in the pollen grains moves down the pollen tube.
→ The pollen tube enters the ovule in the ovary where the male gamete combines with the female gamete present in ovule to form a fertilized egg called zygote.
→ Zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and changes into seed gradually.
→ Ovary grows rapidly and changes into a fruit. The other parts of flower fall off.
Reproduction in Human Being: Human beings show sexual reproduction. Male parent produces male gametes called sperms. Female parent produces female gametes called ova. Sperms have tail and are therefore, motile. They are produced in large numbers in the testes. Ovum is bigger, non-motile and only one ovary produces one ovum in one month. There is no food stored in the sperms whereas ova contain stored food. Both the gametes are microscopic unicellular and have half the number of chromosomes as compared to the body cells.
Human beings become reproductively active from the onset of puberty. Puberty is the period during adolescence when the rate of general body growth begins to slow down and reproductive tissues begin to mature. Onset of puberty in human males is between 11 to 13 yrs of age, while in human females is between 10 to 12 yrs. of age. Puberty is associated with many physical, mental, emotional and psychological changes in boys and girls which occur slowly over a period of time. These are called secondary sexual characters. For instance thick dark hair start growing in new parts of the body such as arm pits and genital area between the thighs. Thinner hair appear on legs, arms and face. Skin becomes oily and pimples may appear on the face. Individuals become more conscious of their bodies become more independent, more aggressive etc.
In case of boys beard and mustache start appearing, voice begins to crack, reproductive organs develop and start producing releasing sperms. In case of girls, breast size begins to increase, skin of the nipples darkens, menstruation starts. The act of mating between the male and female partner is termed as copulation.
Male Reproductive System: Male reproductive system consists of the following components
1 pair of testes
A system of ducts
Vas deferens or the sperm duct
A system of glands
A copulatory organ called a penis.
One pair of testes are present in a bag-like structure called scrotum which lies outside the abdominal cavity, hence they are extra abdominal in position. This is so because the testes have to be maintained at 1-3 degree lesser temperature than the body in order to produce functional sperms.
Functions of testes
To produce male gametes i.e. the sperms.
To produce a male reproductive hormone called testosterone which is responsible for producing sperms as well as secondary sexual characteristics in males.
Attached to each testis is a highly coiled tube called epididymis. The sperms are stored here and they mature in the epididymis. Each epididymis leads into the sperm duct or the vas-deferens. Each vas-deferens rises up and enters into the abdominal cavity. It unites with the duct coming from the urinary bladder to form a common duct called urethra which passes through the penis and opens to the outside. Along the way the ducts of the three glands also open and pour their secretions into the vas deferens.
Function of the vas-deferens: It is meant for the passage of the sperms in the male body.
Functions of the glands: They produce different secretions which provide nutrition as well as medium for locomotion to the sperms. The secretions of the three glands along with the sperms is known as semen.
Function of the urethra: It is the common passage for both semen and urine from the body to. the outside.
Penis: It is the organ which is used to introduce semen into the female body. It is richly supplied with blood vessels.
Female Reproductive System: It consists of the following components
1 pair of ovaries
1 pair of fallopian tubes or oviducts
A vagina/birth canal.
Each ovary is almond shaped and present inside the abdominal cavity. At the time of birth each girl child already contains thousands of immature ova. These ova start maturing only from the time of puberty. Only one ovum is produced by one ovary in one month and each ovary releases an ovum in alternate months. The release of an ovum from the ovary into the abdominal cavity is known as ovulation.
Functions of ovary
To produce and release ova
To produce female reproductive hormones: estrogen and progesterone.
There are two fallopian tubes. The end lying close to the ovary has finger like structures called fimbriae. The two fallopian tubes unite to form an elastic bag like structure called uterus.
Function of the fallopian tubes: It is the site of fertilization between the male and the female gametes and formation of the zygote early embryo. The inner lining of the uterus is richly supplied with blood vessels and is known as endometrium. The narrow end of the uterus is called cervix.
Function of the uterus: The embryo formed in the fallopian tube comes down and gets attached to the endometrium (implantation) and develops for the next nine months till the baby is delivered.
Vagina: The uterus opens into the vagina through the cervix. The vagina is a muscular tube through which the baby is delivered at the end of nine months. It also serves as the canal for receiving the semen at the time of copulation.
The semen is discharged into the vaginal tract during copulation. The sperms travel upwards and reach the fallopian tube where one sperm fuses with the ovum to form the zygote. The zygote divides and redivides as it descends into the uterus and the embryo gets implanted in the endometrium. The endometrium thickens so as to receive the embryo.
The embryo gets nutrition from the mother’s blood with the help of a special tissue called placenta, which is a disk-like structure embeded in the uterine wall. It contains finger-like villi on the embryo side, while on the mother’s side blood spaces surround the villi. Villi provides a large surface area for glucose and oxygen to pass from the mother to the developing embryo and the wastes to pass from the embryo to the mother through the placenta. When the embryo starts resembling a human is formed, it is termed as a foetus. The foetus continues to develop inside the uterus for almost nine months after which the baby is delivered as a result of rhythmic contractions of the uterine muscles.
Menstruation: It is the loss of blood, mucous along with the unfertilized ovum and the ruptured cells and tissues of the endometrium through the vagina of the female. It is a 28-day cycle which occurs in every reproductively active female (from puberty). The flow of blood continues for 2 to 8 days. If the ovum does not get fertilized, then the endometrium starts sloughing off and there is loss of blood and mucous etc. through the vagina. In case the ovum gets fertilized, then the endometrium becomes thick and spongy for nourishing the embryo and hence menstruation does not occur. A lady with a developing embryo in her womb is termed as pregnant. The beginning of menstruation at puberty is known as menarche. The stopage of menstruation when the woman is 45-55 yrs of age is called menopause.
Reproductive Health: Sexually transmitted diseases and birth control. A number of diseases occur as a result of sexual intercourse if one of the partners is infected. These are known as sexually transmitted diseases (STD’s). They can be caused by bacteria for example; syphilis, gonorrhoea; or caused by a virus for example; HIV-AIDS, warts etc. The transmission of these diseases can be avoided by using birth control measures such as wearing a condom during the sexual act.
Birth control measures: They can be mechanical, chemical and surgical.
Mechanical methods: These are used to prevent the passage of semen to the follopian tube : (i) Use of condoms: Condoms are thin rubber tubes worn over the penis before sexual intercourse. The semen gets collected in this and is not discharged into the vagina. (ii) Diaphragm: It is a thin rubber fixed over a flexible metal ring which is fitted over the cervix in a woman’s body by a doctor. (iii) Intra Uterine Contraceptive Device (IUCD) or loop: It is inserted in the uterus and its insertion causes certain secretion which prevents the implantation of the embryo in the uterine wall. Both methods (ii) and (iii) cause side effects.
Use of spermicides: These are strong sperm-killing chemicals available in the form of creams, jellies etc. which are injected into the vagina just before copulation.
Oral contraceptive pills: These are hormonal pills which prevent ovulation but do not stop menstruation.
Vasectomy: It involves cutting and ligating the vas deferens in males.
Tubectomy: It involves cutting and ligating Reproductive organs the fallopian tubes in females.
Medical termination of pregnancy (MTP) or abortions is carried out to eliminate the developing embryo. This practice can, however, be misused to carry out female foeticide which involves the killing of the female foetus. It should be avoided at all cost as it disturbs the male-female ratio in a population.