CHAPTER 6 -MODES OF REPRODUCTION IN ORGANISMS

CHAPTER 6 - REPRODUCTION IN ORGANISMS

Reproduction

•  Reproduction is the biological process in which an individual gives rise to an offspring similar to itself.

Types of Reproduction:
Based on whether there is one or two organisms taking part in the process of reproduction

• ASEXUAL REPRODUCTON
• 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.

Modes of asexual reproduction:

1.Binary fission
Parent cell divides into 2 daughter cells.

E.g.. Amoeba, Paramecium

2.BUDDING
Parent cell divides to form large cell and small cell, called bud, attached to the large cell; the bud gets separated and grows into an adult. E.g., Yeast

• In fungi and algae specialized asexual reproductive units are formed

        Zoospores in many algae and certain aquatic fungi, Conidia (Penicillium), Buds (Hydra), Gemmules (Sponge).

• In higher plants, vegetative propagation is followed using units such as rhizome, sucker, offset, bulb, tuber etc. – known as vegetative propagules.

 
Examples of Vegetative Propagation:

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

II 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.

DIFFERENT PHASES IN SEXUAL REPRODUCTION:

1. JUVENILE PHASE
   It is a period of growth and maturity of an organism before it can reproduce sexually.

 2. REPRODUCTIVE PHASE
   Period in which organism is capable of reproducing sexually

• Easily seen in higher plants when they come to flower.
• Some plants flower seasonally & some through out the year
• Few plants exhibit unusual flowering phenomenon

 For e.g.. 1. Bamboo species flower only once in their lifetime. 2. Srobilanthus kunthiana (neelakurinji) found in hilly areas of Kerala, Karnataka & Tamil Nadu flower once in 12 years the latest being in September 2006.

• In animals sexual reproduction is usually seasonal
• In placental mammals there is occurrence of cyclical changes in the activities of ovaries & accessory ducts as well as hormones.

• In non primate mammals like dogs, rats etc these cyclical changes during reproduction is called oestrus cycle – no bleeding occurs.
• In primate mammals like apes & humans, these cycles are called menstrual cycles.

3. SENESCENT PHASE

• It is the end of reproductive phase.
• Old age ultimately leads to death

EVENTS IN SEXUAL REPRODUCTION:

1.       PRE-FERTILIZATION EVENTS:
a)      GAMETOGENESIS

• The gametes are usually haploid
• Gametes called homogametes / isogametes when both have same appearance. 
• When gametes are different, they are called heterogametes; male gamete known as antherozoid /sperm, female is called ovum / egg.
• Bisexual organisms called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.

b)GAMETE TRANSFER

• In most of the organisms, male gametes are motile & female gametes are stationary.
• In algae, bryophytes & pteridophytes water is the medium for gamete transfer.
• Pollination is the method of gamete transfer in higher plants as pollen grains are carriers of male gametes.

2.       SYNGAMY / FERTILIZATION:

• Syngamy results in formation of a diploid zygote.
• In certain animals, female gamete develops into the adult organism without fertilization; such a process is called parthenogenesis - seen in rotifers, lizards turkeys etc.
• Fertilisation may be external or internal

EXTERNAL FERTILIZATION Syngamy occurs outside the body of the organism Large numbers of gametes are released in the surrounding medium. Ex. Bony fishes and Amphibians.

INTERNAL FERTILIZATION Syngamy occurs inside the body of the organism Numbers of ova produced are less, but large numbers of male gametes are released and they travel towards the ovum. Ex. Birds and Mammals.

3.       POST FERTILIZATION:

• Events in sexual reproduction after the formation of zygote
• Zygote – vital link that ensures continuity of species between organisms of one generation and the next.
• Development of zygote depends on 

             - the type of life cycle of the organisms.
             - the environment it is exposed to.

• Zygote develops into embryo.

Embryogenesis involves:
             - cell division to increase the number of cells
             - cell differentiation for the formation of different kinds of tissues

• Based on whether the development of zygote occurs inside or outside the body of the female parent, animals are categorized into oviparous and viviparous.

OVIPAROUS These animals lay eggs. The fertilized eggs have a calcareous shell to protect them from harsh environment.

VIVIPAROUS These animals give birth to young ones Embryo protected inside the mother’s body.

• In flowering plants the zygote formed inside the ovule; zygote develops into embryo after which the ovule becomes the seed and ovary becomes the fruit.