Download Cell Division – An Introduction

CELL DIVISION - AN INTRODUCTION
Dear Reader
In the previous chapter you have read about the diversity in the living world. One of
the fundamental feature of all living organisms is reproduction. Reproduction as you
are aware is the process by which parents can produce young ones of the similar kind.
You are also familiar with asexual and sexual reproduction. Try to recall the differences
between the two.
Asexual Reproduction- only single parent is involved and
genetically identical copies are produced.
Sexual Reproduction- requires fertilization of male and
female gametes; results in variation.
Let us try to recall your early childhood (or of some other child you know). What is the
approximate height of a child who is:
just born
one-month old
one-year old
five-year old
ten-year old?
Compare it with your present height. I am sure you agree that there is an increase in
height at each stage as compared to the previous one.
You might have noticed increase in height of a plant as it grows from a seedling into an
adult plant.
Growth is another fundamental feature of all living organisms.
Did you ever try to find out the mechanism of growth in living organisms? Is growth
the result of an increase in cell size or an increase in cell number?
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One of the changes that is associated with the growth of an organism is increase in the
number of cells. How do cells increase in number?
Cells divide to increase their numbers. Try to recall the process of binary fission in
Amoeba and budding in yeast. You are also aware about the fact that higher plants and
animals cannot divide by binary fission and budding.
Cell division is the key phenomenon for both growth and reproduction. Cell division in
higher organisms is mainly of two types: mitosis and meiosis. Mitosis is an equational
division and meiosis is a reductional division. In this chapter we will study some basic
concepts related with these two types of cell division.
Chromosomes
In eukaryotes, the nucleus of all cells contains thread - like structures called
chromosomes. Each chromosome is further made up of helically coiled DNA. DNA is
the hereditary material in all organisms except some viruses. During cell division, DNA
is transmitted from parent cells to the daughter cells, thus DNA is the unit of
inheritance. You will learn more about the structure of chromosome in Chapter 8- Cell:
The Unit of Life in class XI.
Figure 1: Relationship between Nuclear, Chromosomes, DNA and Genes
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Haploid and Diploid Cells
Try to recall the number of chromosomes in human sperm and ovum.
The number of sets of chromosomes in a cell is called its ploidy. A cell with one set of
chromosomes is called haploid (n), whereas a cell with two sets of chromosomes is
called diploid (2n). Gametes such as sperm and ovum have only one set of
chromosomes, so they are haploid (n). Somatic cells have two sets of chromosomes, so
they are diploid (2n).
sperm (male
gamete)
Haploid(n)
ovum (female
gamete)
Ploidy
Diploid(2n)
somatic cells
Figure 2: Ploidy of cells
Video
link:
http://www.britannica.com/science/diploid-phase/images-videos/The-terms-
haploid-and-diploid-refer-to-the-number-of/68422
Let us know more about somatic cells and gametes present in higher animals.
Somatic Cells and Reproductive Cells
Somatic cells or vegetal cells make up all tissues and organs of the body such as skin,
stomach, intestine, lungs, bones and blood. In a multicellular organism, any cell other
than gametes, germ cells or stem cells is a somatic cell. Somatic cells are diploid and
divide by mitosis or equational division to form more somatic cells which are also
diploid. What is the number of chromosomes in human somatic cells?
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Stem cells (2n) are undifferentiated cells normally present in the bone marrow of an
adult. These cells divide by mitosis to form more cells of the same type from which
some cells can differentiate into specialised cells.
In animals, the egg cell and sperms are the gametes. They are also called reproductive
cells, as they participate in the process of sexual reproduction. Each mature
reproductive cell is haploid. Haploid gametes are formed from diploid germ cells by a
type of cell division called meiosis or reductional division.
Gametes: Haploid
cells (n)
Human egg
cell(n=23)
Human sperm cell(n=23)
Somatic cells: Diploid (2n)
Chromosome number in somatic
cells of human being is
2n = 2 x 23 = 46
Here we have taken the example of animals particularly humans to understand the
basics of cell division, the process is similar in plants also. In higher plants, meristematic
cells divide by mitosis to form simple and complex permanent tissues. Microspore
mother cells in the anther and megaspore mother cell present in the ovule of a flower
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divide by meiosis to form haploid microspore and megaspore, respectively. Further
development inside the microspores and megaspore leads to formation of male &
female gametes respectively.
Homologous Chromosomes
Figure 3: Fertilisation of a haploid male and a female gamete helps to restore diploidy in the zygote
When a haploid sperm (n) fertilises with a haploid ovum(n), it results in the formation
of a diploid zygote(2n). The zygote thus receives one set of paternal(n) and one set of
maternal chromosomes(n). These chromosomes are similar but not truly identical. Thus,
for each paternal chromosome there is a homolog or similar maternal chromosome.
Each of such a pair of similar chromosomes is called homologous chromosome. Each
chromosome of a homologous pair carries same genes in the same order although their
expression may vary. You will study in Chapter 10- Cell Cycle and Cell Division in class
XI that in germ cells, homologous chromosomes pair up at the time of meiotic cell
division.
Zygote undergoes a special type of mitotic cell division resulting in the formation of
diploid daughter cells. Try to recall the term used for this type of cell division from
chapter 1 - How life begins?
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Mitosis and Meiosis
Each human somatic cell contains 23 pairs of chromosomes (2n) whereas each gamete in
humans contains only 23 chromosomes (n).
Try to analyse the fact that although “all the cells in
an organism are formed from the same zygote
but still the somatic cells and gametes vary in
the number of the sets of chromosomes
possessed by them”.
Actually, it happens because of the two different types of cell division taking place in
them.
Somatic cells(2n) divide by the process of mitosis. Mitosis generally takes place when
plants and animals need to make more cells for growth and repair. Mitosis is an
equational division, resulting in the formation of two daughter cells(2n) which are
genetically identical to the parent cell. The chromosome number in each daughter cell is
equal to the number of chromosomes in the parent cell (2n).The daughter cells grow
and behave as parent cell and enter another round of equational cell division and the
process continues as the organism continues to grow.
Thus, a human somatic cell (2n = 2 x 23) will divide to produce two daughter cells each
having 23 pairs of chromosomes.
Video link for mitosis - https://www.youtube.com/watch?v=DwAFZb8juMQ
Diploid (2n) germ cells (also known as meiocytes)present in the testes of a male and
ovary of a female animal divide by the process of meiosis to form haploid male and
female gamete respectively (n).For example, in humans, germ cells in the testes of a
male and germ cells in ovary of a female, each having 23 pairs of chromosomes will
divide by meiosis to form sperm and ovum respectively, each having only 23
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chromosomes respectively (n). Thus, meiosis is a reductional division. It has two main
phases - Meiosis I and II. Meiosis also introduces variations in the next generation.
Reductional division during gametogenesis is essential to restore diploid chromosome
number in the next generation.
What will happen if sperm and ovum are also diploid?
Video link – meiosis- https://www.youtube.com/watch?v=nMEyeKQClqI,
https://www.youtube.com/watch?v=D1_-mQS_FZ0
Video link - mitosis vs. meiosis - https://www.youtube.com/watch?v=Ba9LXKH2ztU
Do you know that chromosome number
is constant for a species?
The number of chromosomes does not correlate with the apparent
complexity of an organism.
Chromosome number (2n) in some common organismsHuman (Homo sapiens) = 46, Gorilla gorilla= 48,
Pea (Pisum sativum) = 14, Onion (Allium cepa) = 16
Types of
cells
Somatic
cell (2n)
Somatic
cell(2n)
Germ
cell(2n)
Mitosis
Somatic
cell (2n)
Gamete
(n)
Gamete
(n)
Meiosis
Gamete
(n)
Gamete
(n)
You will study the details of mitosis and meiosis in Chapter 10 - Cell cycle and Cell
Division in Class XI.
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DO IT YOURSELF
1.
2.
3.
Identify the ploidy of the following cells:
a.
Zygote: _________
b.
Intestinal cells: ________
c.
Pollen mother cells: _______
d.
Ovum: ____________
Which type of cell division will take place in the following cells:
a.
Pollen mother cells: ____________
b.
Skin cells: ____________________
c.
Megaspore mother cells: ____________
d.
Meristematic cells: ____________
Write the chromosome number in the following cells:
S.
No.
Name of organism
Chromosome
number in meiocyte
Chromosome
number in gamete
A.
Onion (Allium cepa)
16
………..
B.
Human being (Homo
sapiens)
46
………..
C.
Fruitfly (Drosophila
melanogaster)
………..
4
D.
Rice (Oryza sativa)
………..
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REFERENCES
http://evolution.about.com/od/Overview/a/Asexual-Vs-SexualReproduction.htm
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