"AKC SCIENCE CLASSES"
CLASS 10 TH (CBSE AND MP BOARD)
CHAPTER 08
HOW DO ORGANISMS REPRODUCE?
BASIC CONCEPTS
1) Why do organisms reproduce? The process of reproduction is necessary for prevention and increase in population of a species. If the organisms would not reproduce the species to which they belong would be extinct once they are dead. In order to maintain the species, an organism reproduces. Reproduction process is not necessary for an individual to live but necessary for perpetuation of its species.
2) Reproducing organisms create new individuals that look very much like themselves.
3) DNA found in chromosomes in the nucleus of the cells has the information to create proteins which leads to body design of an organism. If the organisms are to make exact copies of themselves, the DNA should replicate to make an exact copy of itself. DNA replicates in the cell with the help of various enzymes and this is accompanied by division of the basic unit of every organism i.e., the cell. During replication (reproduction) each cell divides into two daughter cells having similar components of chromosomes as in the parent cell. This cell division is known as mitosis.
4) DNA copying :- The extent of similarity of the daughter cell with the parent cell depends upon the accuracy of DNA copying. Any inaccuracy of DNA replication results in the daughter cell being different from the parent cell. As its biochemical reaction is absolutely reliable, there are variations each time DNA is copied. Thus with each generation, daughter cells are subtly different from parent cells. This leads to variation in the species which the basis of evolution.
5) Variation is useful for the survival of species over time. For example bacteria living in temperate water would perish if the water temperature water would perish if the water temperature is increased. However, a few variant of bacteria which have become suited to higher water temperature may survive and thrive.
6) Asexual reproduction is the reproduction which does not involve sex cells. It is of many kinds :-
- Fission is the cell division occurring in unicellular organisms like Amoeba, Euglena, Bacteria, Leishmania, etc. It involves splitting of the cell into two individual cells.
- Fragmentation :- This types of reproduction occurs in multicellular organisms with simple body organization as in algae like Spirogyra. Here the filaments break into parts and each part grows into a new individual.
- Regeneration :- Multicellular, fully differentiated organisms like Hydra and Planaria have the capacity to regenerate full body from a part of their body. This is known as regeneration.
- Budding :- In organisms like Hydra a bud develops as an outgrowth due to repeated cell division at specific site which after developing into tiny individuals detach from the body of the parent to become new individuals.
- Vegetative propagation :- In plants various parts such as roots, stem and leaves can regenerate into full individual plant under suitable conditions. Plants like sugarcane, roses, grapes, etc., are grown by vegetative propagation. The plants produced by this methods are genetically similar to the parent plant. In Bryophyllum, the leaves can generate full plants.
- Spores :- Many fungi propagate by the means of spores. In Rhizopus, spores develop in blob like structures called sporangia and are spread by the rupture of sporangia. The spores are covered by a thick wall. When the spores come into contact with moist surface the wall breaks and the cell develops into a full individual.
7) Sexual reproduction is the type of reproduction in which both male and female sexes are needed to propagate new generation. Sexual reproduction allows for more variation than asexual reproduction. In sexual reproduction male and female gametes fuse with each other to form a zygote. The gametes are specialized cells which have only half the number of chromosomes complement. The motile gamete is called the male gamete is called the male gamete whereas the immotile gamete with food-stores is called the female gamete. The gametes are formed from germ cells by a type of a cell division named meiosis.
8) Sexual reproduction in angiosperms are by the means of flowers. Flowers are reproductive parts which have sepals, petals, stamens and carpels. Stamens are male reproductive parts whereas carpels are female reproductive parts.
9) Sexual reproduction in human beings takes place by the fusion of sperm (male gamete) with the egg (female gametes) in the fallopian tube of the female. This is brought about by an extensive structural and functional arrangement of body parts in humans.
10) The male reproductive system of human beings consists of testis, seminal vesicles glands, urethra and penis. The female reproductive system of human beings consists of ovaries, fallopian tubes (oviducts), uterus, vagina and vulva.
11) The ovary releases one ovum or egg cell after every 28 days after the age of 10-11 years. Before the release of ovum, the cellular lining of the uterus with blood vessels is shed off. During this process bleeding occurs which passes through vagina and vulva. This process continues for 3-4 days and is called menstruation.
12) The fertilization of the ovum takes place in the oviduct. The fertilized ovum (zygote) develops into embryo. The embryo gets attached with the wall of the uterus. A structure called placenta is formed by the tissues of the uterus wall and the tissues of the embryo.
13) The term menarche is for the commencement of menstruation and beginning of reproductive life of a woman at puberty.
14) Menopause :- The stoppage of menstrual flow and ovarian events in a women is called menopause.
15) Parthenogenesis is the process of development of an organism from an unfertilized egg, e.g., drones (male honey bees).
16) Human population can be controlled by adopting various methods which prevent pregnancy. These may be :-
- Barrier method
- Chemical method
- IUCDS and
- Surgical methods.
