Maharashtra Board Class 11 Biology Chapter 7 Cell Division PDF Download

Cell Division

Life of all multicellular organisms starts from single cell i.e. zygote. Growth of every living organism depends on cell division. As stated in the cell theory, every cell arises from the pre-existing cell.

Cell Cycle

Sequential events occurring in the life of a cell is called cell cycle. There are two phases of cell cycle as interphase and M-phase. During interphase, cell undergoes growth or rest as per the need. During M-phase, the cell undergoes division. Interphase alternates with the period of division.

Interphase

Interphase is the stage between two successive cell divisions. It is the longest phase of cell cycle during which the cell is highly active and prepares itself for cell division. The interphase is divisible into three sub-phases as G₁-phase, S-phase and G₂-phase.

G1-Phase

This is also known as first gap period or first growth period. It starts immediately after cell division. Cell performs RNA synthesis (mRNA, rRNA and t-RNA), protein synthesis and synthesis of membranes during this phase.

S-Phase

It is synthesis phase in which DNA is synthesized or replicated, so that amount of DNA per cell doubles. Histone proteins are also synthesized during this phase.

G2 Phase

G2 is the second growth phase, during which nucleus increases in volume. Metabolic activities essential for cell division occur during this phase. Various proteins necessary for cell division are synthesized during this phase. Besides, RNA synthesis also occur during this phase. In animal cells, a daughter pair of centrioles appear near the pre-existing pair.

Teacher's Note

When your fingernail grows, cells divide in G1 and G2 phases before they prepare for division. This is how your body makes new cells every day.

Exam Trick

Remember G1-S-G2 as "Grow-Synthesize-Grow". First you grow, then make DNA copy, then grow again before dividing.

Points to Remember

G1 phase is when cell grows and makes RNA and proteins.
S phase is when DNA is copied and becomes double.
G2 phase is when cell prepares proteins needed for division.
Histone proteins are made only in S phase.
All three phases together make interphase.

M-Phase Or Period Of Division

'M' stands for mitosis or meiosis. M-phase involves karyokinesis and cytokinesis. Karyokinesis is the division of nucleus into two daughter nuclei whereas cytokinesis is division of cytoplasm resulting in two daughter cells.

Types Of Cell Division

Three kinds of cell division are found in animal cells. They are amitosis or direct division, mitosis or indirect division and meiosis or reductional division. Mitosis can be performed by haploid as well as diploid cells but meiosis can be performed by diploid cells only. In honey bee, drones develop from haploid unfertilized eggs whereas in Marchantia, haploid spores form gametophyte by mitosis.

Teacher's Note

During wound healing on your skin, cells divide by mitosis to replace damaged cells. This is why your cuts heal and new skin grows.

Exam Trick

Remember: Mitosis = same cells (like photocopying). Meiosis = half cells (like cutting in half). Meiosis is only for making babies.

Points to Remember

Amitosis is the simplest type of cell division.
Mitosis makes two identical daughter cells from one parent cell.
Meiosis makes four different cells with half chromosomes.
Meiosis only happens in reproductive cells.
Mitosis happens in all body cells for growth and repair.

Amitosis

It is the simplest mode of cell division. During amitosis, nucleus elongates and a constriction appears somewhere along its length. This constriction deepens and divides the nucleus into two daughter nuclei. This is followed by the division of the cytoplasm which results in the formation of two daughter cells. This division occurs in unicellular organisms, abnormal cells, old cells and in foetal membrane cells.

Mitosis

This is a type of cell division in which a cell divides to form two similar daughter cells which are identical to the parent cell. It is completed in two steps as karyokinesis and cytokinesis.

Karyokinesis is nuclear division which is sub-divided into prophase, metaphase, anaphase, and telophase. Although for the sake of convenience above mentioned steps are used, it must be remembered that mitosis is a continuous process that starts with the disappearance of nuclear membrane in prophase and ends with separation of two fully formed cells after cytokinesis.

Prophase

This phase involves condensation of chromatin material, migration of centrosomes, appearance of mitotic apparatus and disappearance of nuclear membrane.

Due to condensation, each chromosome becomes visible under light microscope which can be seen with its sister-chromatids connected by centromere. The nucleolus starts to disappear. Nuclear membrane disintegrates and disappeares gradually. Centrosome which had undergone duplication during interphase begins to move towards opposite poles of the cell. Mitotic apparatus is almost completely formed.

Metaphase

In this phase, chromosomes are completely condensed so that they appear very short. Sister-chromatids and centromere become very prominent. All the chromosomes lie at equatorial plane of the cell. This is called metaphase plate. Mitotic spindle is fully formed. Centromere of each chromosome divides into two, each being associated with a chromatid.

Anaphase

The chromatids of each chromosome separate and form two chromosomes called daughter chromosomes. The formed chromosomes are pulled away in opposite direction by spindle apparatus. Chromosomes being pulled away appear like a bunch of banana during midway of anaphase. Each set of chromosomes reach at opposite poles of the cells marks the end of anaphase.

Telophase

The telophase is the final stage of karyokinesis. The chromosomes with their centromeres at the poles begin to uncoil, lengthen and loose their individuality. The nucleolus begins to reappear. The nuclear membrane begins to appear around the chromosomes. Spindle fibres break down and get absorbed in the cytoplasm. Thus two daughter nuclei are formed in a cell.

Teacher's Note

When you see onion root cells under a microscope, you can see different stages of mitosis happening at the same time in different cells. Some cells are in prophase, some in metaphase, and some in anaphase.

Exam Trick

Remember PMAT: Prophase-Metaphase-Anaphase-Telophase. Think "Please Make Another Two" to remember the order of mitosis stages.

Points to Remember

Prophase is when chromosomes become visible and nuclear membrane disappears.
Metaphase is when chromosomes line up at the middle of the cell.
Anaphase is when chromatids separate and move to opposite poles.
Telophase is when two new nuclei form at opposite ends.
All four phases happen one after another in order.

This is a preview of the first 3 pages. To get the complete book, click below.