CELL DIVISION & CELL DIVERSITY – Human Anatomy and Physiology B. Pharma 1st Semester

CELL DIVISION & CELL DIVERSITY

 

The process by which the cells reproduce themselves is
called cell division.

Somatic cell:
Cell of the body other than germ cell

Reproductive
cell/Gamete/Germ cell:
Precursor cell destined to become gamete Ex: Sperm
or Oocyte

Mitosis: Process
by which nuclear division in case of somatic cell takes place.

Cytokinesis:
process by which the cytoplasm of somatic cell divides.

Meiosis: process
by which the reproductive cells undergo division to produce gametes. 

Cell cycle: an
orderly sequence of events by which a cell duplicates its contents and divides.

Cell division can be classified into 2 types

1) Somatic cell division

2) Reproductive cell division

Somatic cell
division:
A cell undergoes a nuclear division called mitosis and a cytoplasmic
division called cytokinesis to produce two identical cells, each with the same
number and kind of chromosomes as the original cell. Somatic cell division replaces
dead or injured cells and adds new ones during tissue growth.

Reproductive cell division:
This type of cell division produces gametes. This type of cell division
consists of 2 step division called meiosis in which the number of chromosomes
in the nucleus is reduced by half. The cells thus produced will be utilized in
sexual reproduction.

A cell reproduces itself by duplicating all its contents to
pass to the next generation of cells. The cell cycle consists of two steps

1) Interphase: When the cell is preparing its self for
division.

2) Mitotic phase: when the cell is dividing.

Interphase:
during this phase the cell replicates its DNA, produces additional cell organelles
and cytosolic components and prepares itself for cell division.

Interphase consists of 3 steps/phases

a) G1 phase:  is the interval between mitotic phase and the
S phase. During G1 the cell is metabolically active, it replicates most of its
organelles and cytosolic components. 

b) S phase: is
the interval between G1 and G2 phase during S phase the replication of DNA
occurs 

c) G2 phase: is
the interval between S phase and mitotic phase, during G2 cell growth continues
and enzymes, proteins are synthesized for the cell division.

Once a cell completes its activities during the G1, S, and
G2 phases of interphase, the mitotic phase begins.

Mitotic phase

The mitotic (M) phase of the cell cycle consists of a nuclear
division (mitosis) and a cytoplasmic division (cytokinesis) to form two
identical cells.

Mitosis can be divided into 4 phases

A) Prophase

B) Metaphase

C) Anaphase

D) Telophase

Prophase: During
this phase chromatin fibers condense and shorten into chromosomes, each
prophase chromosome consists of a pair of identical strands called chromatids.
A constricted region called a centromere holds the chromatid pair together. At
the outside of each centromere is a protein complex known as the kinetochore.
The mitotic spindle is responsible for the separation of chromatids to opposite
poles of the cell. Then, the nucleolus disappears and the nuclear envelope
breaks down.

Metaphase: During
metaphase, the microtubules of the mitotic spindle align the centromeres of the
chromatid pairs at the exact center of the mitotic spindle. This midpoint region
is called the metaphase plate.

Anaphase: During
anaphase, the centromeres split, separating the two members of each chromatid
pair, which move toward opposite poles of the cell

Telophase: This
is the final stage of mitosis, telophase, begins after chromosomal movement
stops. The identical sets of chromosomes, now at opposite poles of the cell, uncoil
and revert to the threadlike chromatin form. A nuclear envelope forms around
each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic
spindle breaks up.

Cytokinesis

Cytokinesis process of division of a cell’s cytoplasm and
organelles into two identical cells.

This process usually begins in late anaphase with the formation
of a cleavage furrow, a slight indentation of the plasma membrane, and is
completed after telophase.

Reproductive
cell division

Meiosis, the reproductive cell division that occurs in the
gonads (ovaries and testes), produces gametes in which the number of
chromosomes is reduced by half. As a result, gametes contain a single set of 23
chromosomes and thus are haploid (n) cells. Fertilization restores the diploid
number of chromosomes.

Meiosis occurs in two successive stages: Meiosis I and
Meiosis II.

Meiosis I: consist of 4 phases

A) Prophase I

B) Metaphase I

C) Anaphase I

D) Telophase I

Prophase I: Is an
extended phase in which the chromosomes shorten and thicken, the nuclear
envelope and nucleoli disappear, and the mitotic spindle forms.

Two events that are not seen in mitotic prophase occur
during prophase I of meiosis.

 First, the two sister
chromatids of each pair of homologous chromosomes pair off, an event called
synapsis. The resulting four chromatids form a structure called a tetrad.

Second, parts of the chromatids of two homologous chromosomes
may be exchanged with one another. Such an exchange between parts of nonsister
(genetically different) chromatids is termed crossing-over. Crossing-over process
permits an exchange of genes between chromatids of homologous chromosomes. Due
to crossing-over, the resulting cells are genetically unlike each other and
genetically unlike the starting cell that produced them. Crossing-over accounts
for part of the great genetic variation among humans and other organisms that
form gametes via meiosis.

Metaphase I: the
tetrads formed by the homologous pairs of chromosomes line up along the
metaphase plate of the cell, with homologous chromosomes side by side.

Anaphase I: the
members of each homologous pair of chromosomes separate as they are pulled to
opposite poles of the cell by the microtubules attached to the centromeres. The
paired chromatids, held by a centromere, remain together.

Telophase I and
cytokinesis of meiosis
are similar to telophase and cytokinesis of mitosis. 

The net effect of meiosis I is that each resulting cell
contains the haploid number of chromosomes because it contains only one member
of each pair of the homologous chromosomes present in the starting cell.

MEIOSIS II:
consists of four phases

A) Prophase II

B) Metaphase II

C) Anaphase II

D) Telophase II

These phases are similar to those that occur during mitosis

Meiosis I begins
with a diploid starting cell and ends with two cells, each with the haploid
number of chromosomes.

During meiosis II, each of the two haploid cells formed during
meiosis I divides; the net result is four haploid gametes that are genetically
different from the original diploid starting cell.

CELL
DIVERSITY

The body of an average human adult is composed of nearly 100
trillion cells. All of these cells can be classified into about 200 different
cell types. Cells vary considerably in size and shape.

Cells may be round, oval, flat, cube-shaped, column-shaped,
elongated, star-shaped, cylindrical, or disc-shaped. A cell’s shape is related
to its function in the body. 

For example, a sperm cell has a long whip like tail
(flagellum) that it uses for locomotion.

The disc shape of a red blood cell gives it a large surface
area that enhances its ability to pass oxygen to other cells.

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