Grade-11 Paramecium (ZOOLOGY) || Binary Fission and Conjugation

 Paramecium

What is paramecium?

 Kingdom: Protista 

Phylum: Protozoa

Class: Ciliate

Genus: Paramecium •

Species: caudatum

 

The paramecium is an oval, slipper-shaped micro-organism rounded at the top/front and pointed at the back/bottom. The pellicle, a stiff but elastic membrane that gives the paramecium a definite shape but allows some small changes. Covering the pellicle are many tiny hairs, called cilia.

What are the different parts of paramecium?

Pellicle - a membrane covering that protects the paramecium like skin

Cilia - hair-like appendages that help the paramecium move food into the oral groove

Oral Groove - collects and directs food into the cell mouth

Cell Mouth - opening for food

Anal Pore - disposes of waste

Contractile Vacuole - contracts and forces extra water out of the cell

Radiating Canals - paths to the contractile vacuole

Cytoplasm - intercellular fluid needed to contain vital cell parts

Trichocyst - used for defense

Gullet - forms food vacuoles

Food Vacuole - storage pocket for food

Macronucleus - larger nucleus which performs normal cell functions

Micronucleus - smaller nucleus which is responsible for cell division.

Paramecium performs both sexual and asexual reproduction

List the Various processes of reproduction that occur in paramecium.

1. Binary fission - Asexual reproduction

2. Conjugation - Sexual reproduction by cross-fertilization

3. Autogamy - Sexual reproduction by self-fertilization

4. Endomixis - *Nuclear organization and multiplication *Degeneration of old macronucleus and formation of new by fusion of micronuclei

 

Describe binary fission in paramecium.

1. BINARY FISSION (Asexual Reproduction)

A common method of reproduction

. Occurs transverse at the right angle to the longitudinal axis of the body

. Stops feeding before division and oral groove disappears

 

Macronucleus divides by amitosis and micronucleus divides by mitosis then they move towards the opposite end

At the same time, the constriction develops at the middle part of the body which divides cytoplasm into two equal parts

A fully grown Paramecium is thus divided into two daughter individuals

 

. Daughter from the anterior end is called protor and from the posterior end is called opisthe

. They are of equal size and have a complete set of cell organelles

. Oral groove and cytopharynx are newly formed in both the daughters

. One contractile vacuole goes to protor and another to opisthe.

. The other two contractile vacuoles are newly formed.

. The whole process is completed within 2 hours and may occur one to four times a day.

 

CONJUGATION (A form of sexual reproduction)

Continuous multiplication by binary fission is interrupted by conjugation as it is necessary for the survival and rejuvenation of the race.

Unfavorable conditions like some degree of starvation, shortage of food, a particular bacterial food, a certain range of light and temperature, and certain chemicals induce conjugation.

Definition: Temporary union of two individuals of the same species for mutual exchange of genetic materials is called conjugation.

Steps:

                                 

. The two Paramecium of opposite mating types come in contact by their ventral surface during swimming

. They stick together through their oral groove region

. Cilia, Trichocysts, and feeding apparatus degenerate

. Pellicle and ectoplasm degenerate at the point of contact and a cytoplasmic bridge/conjugation bridge is formed

. United paramecia are now conjugants

Changes in Macronucleus

The macronucleus breaks up into fragments and these are later absorbed into the cytoplasm.

Changes in micronucleus:

. The micronucleus in each conjugant grows in size and then divides by meiosis to produce four micronuclei

. Out of these four micronuclei, three degenerate and disappear

. The remaining micronucleus divides by mitosis to form two unequal gamete nuclei

. The smaller gamete nuclei are active in migratory in nature and are called migratory gamete nucleus

. The larger gamete is passive and stationary in nature and is called the stationary gamete nucleus

. The migratory gamete nucleus of one conjugant passes through the protoplasmic bridge into

another conjugant and fuses with its stationary gamete nucleus.

. This fusion results in the formation of a single diploid zygote nucleus (synkaryon) and this

process are called *amphimixis.

(*The process of complete fusion of two nuclei from two different individuals to form a zygote

the nucleus is called amphimixis.)

After the formation of the zygote nucleus, the attached conjugants separate and now they are

called ex-conjugants.

. The zygote nucleus in each ex-conjugants divides thrice by mitosis forming eight nuclei

. Out of eight nuclei, four grow in size to become macronuclei and the rest four small nuclei are called micronuclei

. Then three micronuclei degenerate and disappear.

. The remaining one micronucleus divides mitotically into two

. The ex-conjugants also divide into two daughter paramecia

. Now each daughter paramecia have 2 macronuclei and I micronucleus

. Micronucleus divides mitotically with the division of the cytoplasm

. This resulted in four daughter paramecia each with one macronucleus and one micronucleus.

. In this way, 8 paramecia are produced as a result of conjugation four from each conjugant