How do Organisms Reproduce?

1. The Importance of Reproduction and Variation

• Purpose: Reproduction is not essential for the survival of an individual but is vital for the continuation of a species.
• DNA Copying: The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA.
• Variation: DNA copying is not absolutely accurate. These inaccuracies lead to subtle variations in the DNA copies.
  • If variations are too drastic, the new cell dies.
  • Mild variations allow surviving cells to be similar but not identical.
• Evolutionary Significance: Variations are beneficial for species survival. If an ecosystem changes drastically (e.g., temperature rise), a population with variations has a better chance that some individuals will survive and repopulate.

2. Modes of Asexual Reproduction

Asexual reproduction involves a single parent and creates genetically similar offspring.

2.1 Fission

• Binary Fission: The parent cell divides into two equal halves (e.g., Amoeba, Leishmania). In Leishmania, division occurs in a definite orientation due to its whip-like structure.
• Multiple Fission: The parent cell divides into many daughter cells simultaneously (e.g., Plasmodium).

2.2 Fragmentation

• Simple multicellular organisms break into smaller pieces upon maturation.
• Each fragment grows into a new individual (e.g., Spirogyra). This only works for organisms with relatively simple body organization.

2.3 Regeneration

• If an organism is cut or broken, specialized cells proliferate and differentiate to regrow the missing parts or form a complete organism.
• Examples: Hydra and Planaria.
• Note: Regeneration is generally not a primary reproductive method but a survival mechanism.

2.4 Budding

• A bud develops as an outgrowth due to repeated cell division at a specific site.
• The bud matures and detaches from the parent to become an independent individual (e.g., Hydra, Yeast).

2.5 Vegetative Propagation

• New plants are obtained from vegetative parts like roots, stems, and leaves.
• Natural Examples: Buds produced in the notches of Bryophyllum leaves fall on soil and grow.
• Artificial Methods: Layering, grafting, and tissue culture (used for sugarcane, roses, grapes).
• Advantages: Plants can bear flowers/fruits earlier; allows propagation of plants that have lost the capacity to produce seeds (banana, jasmine); ensures genetic similarity.

2.6 Spore Formation

• Structures called sporangia (blobs-on-a-stick) contain cells called spores.
• Spores are covered by thick walls to protect them until they find moist conditions to grow (e.g., Rhizopus/Bread mould).

3. Sexual Reproduction

• Significance: Combines DNA from two individuals, generating greater variation which aids survival and evolution.
• Mechanism: To prevent the DNA amount from doubling in every generation, specialized germ-cells (gametes) are formed via meiosis, containing half the number of chromosomes.
• Gametes: Usually, the male gamete is motile and small, while the female gamete is larger and stores food.

3.1 Sexual Reproduction in Flowering Plants

• Reproductive Parts: Located in the flower.
  • Stamen (Male): Produces pollen grains.
  • Pistil (Female): Consists of Stigma (sticky top), Style (tube), and Ovary (swollen base containing ovules).
• Pollination: Transfer of pollen from stamen to stigma. Can be Self-pollination (same flower) or Cross-pollination (different flower via wind, water, animals).
• Fertilisation: A pollen tube grows through the style to the ovary. The male germ-cell fuses with the female germ-cell inside the ovule to form a zygote.
• Post-Fertilisation:
  • Zygote divides to form an embryo.
  • Ovule develops a tough coat and becomes a seed.
  • Ovary ripens to form a fruit.
  • Petals, sepals, and stamens fall off.
• Germination: The seed contains the future plant (embryo) which develops into a seedling under appropriate conditions.

4. Reproduction in Human Beings

4.1 Puberty

• The period during adolescence when reproductive tissues begin to mature.
• Common Changes: Hair growth in armpits/genital area, oily skin, pimples.
• Girls: Breast enlargement, darkening of nipple skin, onset of menstruation.
• Boys: Facial hair growth, voice cracking, enlargement of the penis.

4.2 Male Reproductive System

• Testes: Located in the scrotum (outside the body to maintain lower temperature for sperm formation). Produce sperms and the hormone testosterone.
• Vas Deferens: Tube that delivers sperm from testes.
• Glands (Prostate & Seminal Vesicles): Add secretions to make sperm transport easier and provide nutrition.
• Urethra: Common passage for both sperm and urine.
• Penis: Organ used to transfer sperms.

4.3 Female Reproductive System

• Ovaries: Produce eggs (one released per month) and hormones.
• Fallopian Tube (Oviduct): Carries the egg from the ovary to the womb; site of fertilisation.
• Uterus (Womb): Elastic bag-like structure where the baby develops. Opens into the vagina via the cervix.

4.4 Fertilisation and Development

• Sperms enter through the vagina and reach the fallopian tube to fuse with the egg.
• The fertilised egg (zygote) divides to form an embryo which implants in the uterus lining.
• Placenta: A disc-shaped tissue embedded in the uterine wall. It provides a large surface area for glucose and oxygen to pass from the mother to the embryo and removes waste from the embryo.
• Gestation: Development takes approximately nine months.

4.5 Menstruation

• If the egg is not fertilised, the thick and spongy uterus lining is not needed.
• The lining breaks and comes out through the vagina as blood and mucus.
• This cycle occurs roughly every month and lasts 2–8 days.

5. Reproductive Health

• Sexually Transmitted Diseases (STDs):
  • Bacterial: Gonorrhoea, Syphilis.
  • Viral: Warts, HIV-AIDS.
  • Prevention: Use of condoms helps prevent transmission to some extent.
• Contraception (Methods to avoid pregnancy):
  • Mechanical Barriers: Condoms, cervical caps (prevent sperm from reaching egg).
  • Chemical Methods: Oral pills (change hormonal balance to prevent egg release; may have side effects).
  • Intra-Uterine Contraceptive Devices (IUCD): Copper-T, loops placed in the uterus (can cause side effects/irritation).
  • Surgical Methods: Vasectomy (blocking vas deferens in males) or Tubectomy (blocking fallopian tubes in females).
• Social Issues:
  • Illegal sex-selective abortion (female foeticide) leads to a declining child sex ratio.
  • Population size affects the standard of living.

Summary based on Chapter 7: How do Organisms Reproduce?