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Reproduction is defined as the formation of new individuals or offspring by sexual or asexual means.
Types of Reproduction
There are two types of Reproduction in plants- asexual Reproduction and sexual Reproduction.
In asexual Reproduction, single parent is involved. The offspring produced are identical to their parents. There are different modes of asexual Reproduction. They are as follows-
- Binary fission in prokaryotes. For example, Amoeba and Paramecium.
- Budding in Hydra.
- For example, gemma in Liverworts
- Vegetative Reproduction. For example, Rhizome in Polypodium, Runners or stolons in grasses, Plants like onion, tulips reproduce by bulbs.
Note: For detailed study of asexual Reproduction, kindly refer to the content “Reproduction in Organisms”.
Sexual Reproduction involves male gamete and female gamete. Male and female gametes fuse to form a zygote. Zygote will then undergo various divisions to form a new organism. The offspring’s produce certain traits from both the parents.
Events in Sexual Reproduction:
- Pre-fertilization events.
- Post fertilization events
Pre-fertilization events include gametogenesis and gamete transfer. Gametogenesis involves the formation of the male and female gametes. Usually male gamete is smaller and motile whereas female gamete is larger and non-motile. Gametes can be identical, known as homogametes such as in algae and they can differ morphologically, such gametes are known as heterogametes such as antherozoid or male gamete and egg or female gamete.
Flower is the reproductive organ in the Flowering Plants. The androecium is the male reproductive part of the plant and gynoecium is the female reproductive part of the plant. The unit of androecium is the stamens. Stamens contain anther and the filament. Anther contain the pollen grains known as male gamete. Gynoecium or carpels is divided into ovary, style, and stigma. Ovary is the site where fertilization occurs. Stigma is the place where pollen lands and reaches to ovary for fertilization.
Fertilization is the process of mating of male gamete and female gamete to form a zygote. This is also known as syngamy. Fertilization is of two types- external fertilization and internal fertilization. When fertilization occurs outside the body of an organism, such a fertilization is known as external fertilization. For example, in aquatic animals. When fertilization occurs inside the body of the female organism, such fertilization is known as internal fertilization. Such as in case of humans.
Post fertilization events includes formation of zygote. Zygote formed will undergo cleavage to form embryo. The process of formation of embryo is known as embryogenesis. The characteristic feature of Flowering Plants is double fertilization.
When pollen lands on the stigma of the flower, it forms the pollen tube that adheres down towards the ovary via style. The pollen tube enters the ovary through the micropyle end. The pollen tube releases the two male gametes in the female gametophyte. One sperm fertilizes the egg cell and the other sperm fuses with the two polar nuclei of the large central cell of the megagametophyte. The haploid male gamete and haploid egg combine to form a diploid zygote, while the other male and the two haploid polar nuclei of the large central cell form a triploid nucleus or endosperm. This is known as double fertilization.
Fig.3. Double Fertilization
Note: For detailed study, kindly refer to the content “Sexual Reproduction in Flowering Plants”.
Humans are sexually reproducing animals. Male reproductive system consists of:
- A pair of testis located in the abdominal cavity. Testis contain sperm, the male gamete
- Glands such as prostate glands, seminal vesicle, and bulbourethral glands. They form a major component of semen
- Accessary ducts such as rete testis, vasa efferentia, epididymis and vas deferens
- Male genitalia
Fig.4. Male Reproductive System
Female Reproductive System consists of:
- A pair of ovaries where ova or egg are formed
- Uterus is a place where zygote is implanted for the growth of the embryo
- External genitalia
The characteristic feature of female mammal is the presence of mammary glands.
The formation of gametes, such as, sperm in males and ova in females is known as gametogenesis. The formation of sperm is known as spermatogenesis whereas formation of ova is known as oogenesis.
Menstrual cycle is the reproductive cycle of female mammals. It is divided into following phases:
- Menstrual phase (from day 1 to 5) - The body discharge the blood vessels from the vagina in the form of menstrual fluid.
- Follicular phase (from day 1 to 13) – In this phase, an ovum matures or prepares itself for ovulation. The lining of the uterus known as endometrium gets thickened due to lining of blood vessels. Luteinizing hormone and follicular stimulation hormone increases in this phase.
- Ovulation phase (day 14)- It is the phase where ovum is released from the ovary.
- Luteal phase (day 15 to 28) – The released ovum remains in the fallopian tube for about 24 hours. If the sperm does not enter the body, the ovum degenerates. The ovum or matured Graafian follicle converts into corpus luteum. If fertilization does not occur corpus luteum degenerates. If fertilization occurs, corpus luteum will secrete a pregnancy hormone known as progesterone, required to maintain pregnancy.
The sperm reaches to the female fallopian tube where fertilization occurs. The ovum released from the ovary also reaches the fallopian tube. The process of fusion of male gamete, sperm with female gamete ovum is known as fertilization.
Fertilization forms a diploid zygote. Zygote undergo mitotic divisions to form mass of cells known as blastomeres. These cells form a compact mass of cells known as morula (8-17 cells). Morula undergo mitotic divisions to form blastocyst. The blastomeres of the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass.
The trophoblast layer will gets attached to the endometrium and the inner cell mass will differentiate as the embryo. The attachment of the blastocyst in the uterus is known as implantation. This marks the beginning of the pregnancy. Blastocyst then later transform into gastrula. It is the stage where 3 germ layers are formed- outermost ectoderm, middle mesoderm, and innermost endoderm. These germ layers form different tissues and organs during embryonic development.
The process of delivery of the baby or better known as childbirth is known as parturition.
Note: For detailed study kindly refer to the content “Human Reproduction”.
A good Reproductive Health is defined as the state of well-being in all matters related to reproductive system. The individuals with good Reproductive Health can have safe sex life as well as can reproduce effectively.
A sudden increase in the size of the population is known as population explosion.
Methods for controlling population explosion/Birth control
- Periodic abstinence
- Withdrawal or coitus interruptus
- Lactational amenorrhea
- Diaphragms, cervical caps and vaults
Note: For detailed study refer to the content “Population explosion and birth control”.
If a couple is unable to produce children even after unprotected coitus, they are said to be infertile. There are certain methods available for couples to have children despite being infertile. They are as follows:
- In vitro fertilization
- Zygote intra-fallopian transfer (ZIFT)
- Intracytoplasmic sperm injection (ICSI)
- Gamete intra-fallopian tube (GIFT)
- Artificial insemination
For detailed study refer to the content “Infertility and sexually transmitted diseases”.
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