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CBSE Class 12 BiologyRevision Notes
CHAPTER-01
REPRODUCTION IN ORGANISMS
Reproduction is a biological process of formation of new offsprings from the pre-existing
organism.Reproduction becomes a vital process without which species cannot survive for
long It ensures continuity of species generation after generations as older individuals
undergo senescence and ultimately they die.
Life span - • The period from birth to the natural death of an organism represents its life
span. Life span of organisms varies from few days (Butterfly-1to 2 weeks) to thousands of
years (Banyan tree).
Types of Reproduction:
Based on whether there is one or two organisms taking part in the process of reproduction
ASEXUAL REPRODUCTON
SEXUAL REPRODUCTION
When the offspring is produced by single parents with or without the involvement of gamete
formation, the reproduction is called asexual reproduction.
When two parents (opposite sex) participates in reproduction process and also involves the
fusion of male and female gametes, it is called sexual reproduction.
Asexual Reproduction
1. Usually followed by organisms with relatively simpler organizations.
2. Off springs produced by single parent.
3. With/without involvement of gamete formation.
4. Off springs produced are genetically and morphologically similar to each other and to the
parent, i.e. they are clones.
• In Protista and Monera, the parent cells divides into two to give rise to new individuals.
Thus, in these organisms cell division is the mode of reproduction itself.
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,• Binary fission- in this method of asexual reproduction, a cell divides into two halves and
rapidly grows into an adult. Ex- amoeba, paramecium.
• Budding- small buds are produced that remain attached initially with parents and get
separated on maturation. Ex. Yeast.
• Fungi and simple plants like algae reproduce through special reproductive structures like
zoospores (motile structure), conidia (penicillium), buds (hydra) and gemmules (sponges).
• In plants, vegetative reproduction occurs by vegetative propagules like runner, rhizome,
sucker, tuber, offset and bulb.
WATER HYACINTH (Terror of Bengal)
One of the most invasive weeds
Grows wherever there is standing water
Drains oxygen from water- leads to death of fishes.
Introduced in India because of its pretty flowers & shape of leaves
Vegetative propagation occurs at a phenomenal rate
Asexual reproduction is the most common method of reproduction in organisms having
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,simpler body like in algae and fungi but during unfavorable condition they shift to sexual
reproduction.
SEXUAL REPRODUCTION:
Involves formation of male and female gamete by two individuals of the opposite sex.
Offspring produced by fusion of male and female gametes not identical to each other
or to the parents.
All sexually reproducing organisms share a similar pattern of reproduction.
• In sexual reproduction, fusion of male and female gametes results in offspring that are not
identical to parents.
DIFFERENT PHASES IN SEXUAL REPRODUCTION:
a. Juvenile phase - The period between birth and sexual maturity is called juvenile phase. In
plants it is known as vegetative phase.The end of juvenile/vegetative phase marks the
beginning of the reproductive phase.
b. Reproductive phase-
• Some plants show flowering in particular season and some other flowers in all seasons.
Some other plants like bamboo species flowers once in life time (after 50-100 years),
Strobilanthus kunthiana (neelakuranji),flowers once in 12 years.
• The female placental animals exhibit cyclic change in activities ovaries and accessary
glands as well as hormone during the reproductive phase.
Menstrual cycle
• It occurs in monkeys, apes and human beings.
• Cycle consists of 3 phases-menstrual, proliferative and secretory phase.
• Blood flows in the last few days of the cycle. The broken endometrium is passed out during
menstruation.
• Female does not permit copulation during menstrual phase of the cycle.
Oestrous cycle
• It occurs in non primates like cow, sheep, rat, deer, dog, tiger etc.
• It consists of a short period of oestrous or heat. it is 12-24 hours in cow followed by
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, anoestrous or passive period.
• Blood does not flow in this cycle. The broken endometrium is reabsorbed.
• Female permits copulation only during oestrous period.
• Both in plants and animals, hormones are responsible for the transition between different
phases of life cycle. Interaction between hormones and environmental factors regulate the
reproductive processes.
c. Senescent phase -
It is the end of reproductive phase.
Old age ultimately leads to death
Events in Sexual Reproduction : Pre-fertilisation, Fertilisation, Post-fertilisation
Pre-fertilisation- all the events prior to fusion of gametes are included in it. It includes
gametogenesis and gamete transfer.
a. Gametogenesis is the process of formation of male and female gametes. Gametes are
haploid cells which may be similar or dissimilar in structure. In algae, both gametes are
similar in structure called homogametes (isogametes). In higher organism that reproduces
sexually, two morphologically distinct gametes are formed called heterogametes, male
gametes are called antherozoid or sperm and female gametes are called ovum or egg.
Isogametes. heterogametes
In fungi and plants, homothallic and monoecious terms are used to denote the bisexual
condition and heterothallic and dioecious are used for unisexual condition. In flowering
plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is
pistillate or bearing pistils.
• In animals, species which possess both male and female reproductive organs in same
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