(IGP) GS Paper 1 - General Science - "The Reproductive System"

Subject - General Science
Chapter : The Reproductive System

Asexual Reproduction

• Asexual reproduction allows an organism to rapidly produce many offspring without the time and resources committed to courtship, finding a mate, and mating.

• Fission, budding, fragmentation, and the formation of rhizomes and stolons are some of the mechanisms that allow organisms to reproduce asexually.

• The hydra produces buds;

• starfish can regenerate an entire body from a fragment of the original body.

• The lack of genetic variability in asexually reproducing populations can be detrimental when environmental conditions change quickly.

Sexual Reproduction

• In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote.

• Sperm are male gametes, ova (ovum singular) are female gametes.

• Meiosis produces cells that are genetically distinct from each other.

• fertilization is the fusion of two such distinctive cells.

Human Reproduction and Development

• Human reproduction employs internal fertilization, and depends on the integrated action of hormones, the nervous system, and the reproductive system.

• Gonads are sex organs that produce gametes. Male gonads are the testes, which produce sperm and male sex hormones. Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.

The Male Reproductive System

• Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development.

• Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. About 250 meters (850 feet) of tubules are packed into each testis.

• Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm.

• Sperm production begins at puberty and continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.

Male Sex Hormones

• The anterior pituitary produces follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

• Action of LH is controlled by the gonadotropin-releasing hormone (GnRH).

• LH stimulates cells in the seminiferous tubules to secrete testosterone, which has a role in sperm production and developing male secondary sex characteristics

• FSH acts on cells to help in sperm maturation. Negative feedback by testosterone controls the actions of GNRH.

Sexual Structures

• Sperm pass through the vas deferens and connect to a short ejaculatory duct that connects to the urethra.

• The urethra passes through the penis and opens to the outside

• Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. The prostate gland secretes a milky alkaline fluid.

• The bulbourethral gland secretes a mucus-like fluid that provides lubrication for intercourse

• Sperm and secretions make up semen.

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The Female Reproductive System

• The female gonads, are ovaries, which are located within the lower abdominal cavity

• The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells.

• At birth each female carries a lifetime supply of developing oocytes, each of which is in ProphaseI.

• A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.

Hormones and Female Cycles

• The ovarian cycle is hormonally regulated in two phases. The follicle secretes estrogen before ovulation; the corpus luteum secretes both estrogen and progesterone after ovulation.
• Hormones from the hypothalamus and anterior pituitary control the ovarian cycle. The ovarian cycle covers events in the ovary; the menstrual cycle occurs in the uterus.
• Menstrual cycles vary from between 15 and 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menstruation.
• During menstruation the uterine lining is broken down and shed as menstrual flow.
• FSH and LH are secreted on day 0, beginning both the menstrual cycle and the ovarian cycle.

Sexually Transmitted Diseases

• STDs can affect the sex partners, fetus, and newborn infants. STDs are grouped into three categories.
• Category One : STDs that produce inflammation of the urethra, epididymis, cervix, or oviducts. Gonorrhea and chlamydia are the most common STDs in this category. Both diseases can be treated and cured with antibiotics, once diagnosed.
• Category Two : STDs that produce sores on the external genitals. Genital herpes is the most common disease in this class. Symptoms of herpes can be treated by antiviral drugs, but the infection cannot be cured. Syphilis is a bacterially caused infection, and can, if left untreated, cause serious symptoms and death. However, the disease is curable with antibiotics.
• Category Three : This class of STDs includes viral diseases that affect organ systems other than those of the reproductive system. AIDS and hepatitis B are in this category. Both can be spread by sexual contact or blood. Infectious individuals may appear symptom-free for years after infection.

Fertilization And Cleavage

Fertilization has Three Functions

1. transmission of genes from both parents to offspring
2. restoration of the diploid number of chromosomes reduced during meiosis
3. initiation of development in offspring

Steps in Fertilization

• Contact between sperm and egg
• Entry of sperm into the egg
• Fusion of egg and sperm nuclei
• Activation of development

Cleavage

Cleavage is the first step in development of all multicelled organisms. Cleavage converts a single-celled zygote into a multicelled embryo by mitosis. Usually, the zygotic cytoplasm is divided among the newly formed cells.

Various Stages of Fertilization

• Fertilization, the fusion of the sperm and egg, usually occurs in the upper third of the oviduct. Thirty minutes after ejaculation, sperm are present in the oviduct, having traveled from the vagina through the uterus and into the oviduct. Sperm traverse this distance by the beating of their flagellum. Of the several hundred million sperm released in the ejaculation, only a few thousand reach the egg.
• Only one sperm will fertilize the egg. One sperm fuses with receptors on the surface of the secondary oocyte, triggering a series of chemical changes in the outer oocyte membrane that prevent any other sperm from entering the oocyte. The entry of the sperm initiates Meiosis II in the oocyte. Fusion of the egg and sperm nuclei forms the diploid zygote.

Facts from NCERT

• Fertilization which takes place inside the female body is called internal fertilization e.g. - humans, cow’s, dogs etc.
• Fertilization in which the fusion of a male and female gamete takes place outside the body of the female is called external fertilization e.g.:- it is common in aquatic animals such as fish, starfish, frogs etc.
• The animals which give birth to young ones are called vivi parous animals.
• Those animals which lay eggs are called oviparous animals.
• Life cycle of the silk worm: - egg-larva or caterpillar–pupa-adult.
• Life cycle of a frog—egg—tadpole (9larva)—adult.
• The transformation of the larva into an adult through drastic changes is called metamorphosis.
• Cloning of an animal was successfully performed for the first time by Ian wilmat and his team in Scotland. They cloned a sheep named dolly in 1997.

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