CHAPTER# 05
CELL CYCLE
Q. What do you know about Genes?
GENES
A unit of heredity
which is transferred from a parent to offspring and is held to determine some
characteristic of the offspring.
Q. Define chromosomes. Also describe
the types of chromosomes.
CHROMOSOMES
The term Chromosomes is given by German embryologist Walter
Fleming in 1882.
Chromosome can be define as a thread like structure of nucleic acids and proteins found
in the nucleus of most living cells, carrying genetic information in the form
of genes.
TYPES
OF CHROMOSOMES
Chromosomes are made up of DNA and basic protein, Histones,
appear during the cell division in the shape of rod. It has two parts arms and
centromere. Depending upon position of centromere. These types are:
·
Metacentric
·
Sub metacentric
·
Acrocentric OR Sub -telocentric
·
Telocentric
|
Metacentric |
Chromosomes
with equal arms |
|
Sub metacentric |
Chromosomes
with un equal arms |
|
Acrocentric OR Sub –telocentric |
Rod like
chromosomes with one arm very small and other very long. The centromere is sub
terminal. |
|
Telocentric |
Location
of centromere at the end of chromosomes |
Q. Define chromatids.
CHROMATIDS
In the beginning of cell-division each chromosome is consist
of two genetically identical copies of thread attach with each other called
chromatids or sister chromatids.
Q. What is Cell Cycle?
CELL
CYCLE
The sequence of changes which occurs between one cell division
and the next is called Cell Cycle.
Q. Define
cell division.
CELL DIVISION
The cell cycle undergoes a sequence of changes, which involve
period of growth, replication of DNA followed by cell division.
Q. Explain
cell cycle.
CELL CYCLE
The sequence of changes which occurs between one cell division
and the next is called Cell Cycle.
It has two phases, Interphase, which is the period of
non-division and M-phase, which is a period of cell division.
INTERPHASE
The period of cell cycle between two consecutive divisions is
called Interphase. It is a period of growth and synthesis of DNA. During this
period the cell prepares itself for the M- phase.
The Interphase is divided further into three sub-phases:
·
G1- phase
·
S-phase and
·
G2 -phase
G-(GAP ONE) PHASE: 1
In this phase, cell grows in size, specific enzymes are
synthesized and DNA base units are accumulated for the DNA synthesis. At a
point in G, the cell may enter into a phase called G (G-knot) where cell cycle
stop. It remains for days, weeks or in some cases even for the life time of the
organism.
S-(SYNTHESIS) PHASE
During this phase, replication of DNA occurs. As a result of
it chromatin material is duplicated.
G – (GAP TWO) PHASE: 2 (PRE-MITOTIC PHASE):
In this phase cell grows in size, cell organelles are
replicate in numbers as well as enzyme require for cell-division also synthesized.
Q. What do
you know about Mitosis? Discuss in detail.
MITOSIS
Mitosis is a type of cell division in which parent cell
divides into two daughter cells in a way that the number of chromosomes in the
daughter cells remains the same as in the parent cell.
Mitosis can be divided into two phases:
1.
Karyokinesis - nuclear
division
It is further divided into four phases which are:
·
Prophase
·
Metaphase
·
Anaphase
·
Telophase
2.
Cytokinesis - cytoplasmic
division.
PROPHASE
During early prophase chromatin material condenses and become
thread like structures called chromosomes. Each chromosome at this stage is
already double, consists of two chromatids. The chromatids are attached to each
other at centromere. The nuclear membrane gradually disappears and at the same
time centrosome divides to form two centrioles, each moves towards the opposite
pole of the animal cell and forms the spindle fibers.
METAPHASE
During this phase each chromosome arranges itself on the
equator of the spindle. Each chromosome is attached to separate spindle fiber
by its centromere.
ANAPHASE
In this phase the spindle fiber contract, centromere of a
chromosome divides and the chromatids of each chromosome separates from each
other and begin to move towards the opposite poles.
TELOPHASE
In this stage the chromatids (now called chromosomes) reach at
the respective poles and their movement ceases. Each pole receives the same
number of chromosomes as were present in the parent cell.
The nuclear membrane is reformed around each set of
chromosomes. In this way two daughter nuclei formed in each cell.
CYTOKINESIS
AS the nucleus divides, soon the cytoplasm of the cell also
divides and two daughter cells are formed.
Q. Write
down the significance of Mitosis.
SIGNIFICANCE OF MITOSIS
·
It is responsible for
development and growth of organisms by increasing exact copies of cells.
·
Almost all kinds of
asexual reproduction and vegetative propagation take place by mitosis.
·
The production of new
somatic cells, such as blood cells depends on mitosis.
·
The healing of wounds,
repair of wear and tear within organism is also dependent upon the mitotic
division.
Q. What is
Meiosis? Explain in detail.
MEIOSIS
Meiosis is a type of cell division in which single cell
divides into four daughter cells and number of chromosomes becomes half in each
daughter cell.
In animal meiosis takes place in germ cell to produce gametes
i.e. Sperms and Eggs whereas in plants it takes place in spore mother cells
(S.M.C) to produce spores.
Events of Meiosis: Meiosis is a series of two divisions,
MEIOSIS I and MEIOSIS II which result in the formation of four haploid cells.
MEIOSIS I (FIRST MEIOTIC DIVISION)
First meiotic division is the reduction division during which
the chromosomes number is reduced to half. Meiosis I consists of following four
phases:
·
Prophase I
·
Metaphase I
·
Anaphase I and
·
Telophase I
PROPHASE I
Prophase is the longest phase of meiosis. It can be subdivided
into following sub stages:
·
Leptotene
·
Zygotene
·
Pachytene
·
Diplotene
·
Diakinesis
i.
LEPTOTENE
In this stage the chromatin network break into specific number
of long thin thread called leptotene. Each thread has two morphologically
similar leptene in each cell called homologous structure.
ii.
ZYGOTENE
During this sub stage the Homologous chromosomes are attracted
towards each other and their pairing takes place. The pairing of homologous
chromosomes is known as synapsis, while the paired homologous chromosomes are
known as bivalent.
iii.
PACHYTENE
In this stage the chromosomes uncoil and separate. The
separation is however incomplete and paired chromosomes are in contact with
each other at one or more points, called Chiasmata. Each homologous chromosome
split longitudinally except in the centromere region. Now each bivalent is
composed of four chromatids and therefore is known as bivalent tetrad.
iv.
DIPLOTENE
The homologous chromosomes exchange their parts of chromatid
at Chiasmata. This exchange of segments of chromatids at chiasmata between the
homologous chromosomes is called Crossing Over.
v.
DIAKINESIS
During this sub stage; nucleoli and nuclear membrane are
disappeared, whereas Mitotic Apparatus (spindle) is completed. Chiasmata moves
from the centromere towards the ends of the chromosomes like a zipper.
This type of movement of chiasmata is known as
Terminalization.
METAPHASE I
In this stage following changes takes place:
·
The bivalent line up at
the equatorial plane.
·
The centromere of each
chromosome attaches with same fibers of spindle.
ANAPHASE I
At this stage one chromosome from each member of homologous
pair (bivalent) begins to separate and move towards its respective pole by the
contraction of spindle fibers.
TELOPHASE I
The nuclear membrane form around the chromosomes at each pole
and chromosomes become uncoil. The nucleolus reappears and thus two daughter
nuclei formed.
INTERPHASE
There is a short period called Interphase before meiosis II.
It is similar to Interphase between Mitotic divisions except that DNA
replication does not occur.
SECOND MEIOTIC DIVISION (MEIOSIS II)
The second meiotic division is actually the mitotic division
which divides each haploid cell formed during meiosis I into two daughter
haploid cells.
It includes:
·
Prophase II
·
Metaphase II
·
Anaphase II
·
Telophase II
PROPHASE II
The spindle fibers are formed. The nuclear membrane and the
nucleolus disappear.
METAPHASE II
The chromosomes are attached to half spindle fibers by their
centromere and get arranged at equatorial plane. Each chromosome attach with
separate fiber of spindle.
ANAPHASE II
The spindle fibers attached to the centromeres shorten and the
chromatids of chromosomes are pulled away from one another.
TELOPHASE II
The spindle fibers disappear completely and chromosomes begin
to uncoil. They become longer and indistinct and form group at each pole.
Around each group nuclear envelope is formed.
After the karyo-kinesis in each haploid meiotic cell the
cytokinesis i.e. division of cytoplasm occurs and thus four haploid cells are
formed.
Q. Write
down the significance of Meiosis.
SIGNIFICANCE OF MEIOSIS
·
It maintains the constant
number of chromosomes
·
It is responsible for
genetic variation among species
Q. What is
Apoptosis and Necrosis?
APOPTOSIS OR SELF - DESTRUCTION (AUTOPHAGY)
Programed change which lead to sequence of physiological
changes in cell by which cells commit suicide collectively called Apoptosis.
OR
Apoptosis is
an orderly process in which the cell's contents are packaged into small packets
of membrane for “garbage collection” by immune cells.
NECROSIS
This type of cell death which is caused by external factors
i.e. infection, toxin and tumor i.e. accidental cell death.
