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Cells and Cell Division
You were once a single cell – and now look at you! How did you get to be so big?
From one to many……
What happens when skin wears away or damaged tissues need repairing? How do seedlings grow
into gigantic trees?
Throughout the life of multicellular organisms, cell division takes place for the growth,
development, repair and replacement of tissues. In cell division, the nucleus divides. This is called
mitosis (from the Greek word mitos, meaning a thread, because the chromosomes appear like
threads). The following pictures are from cells in the root tip of an onion. The root tip is a growing
area of plants and the cells shown are at various stages of mitosis. The second set of pictures shows
individual cells at each major stage.
Reviewing reproduction …..
Cell division also plays an important role for unicellular organisms in reproduction. In asexual
reproduction, an organism, for example a bacteria, produces a new living organism identical to
itself through mitosis. In this process, two cells identical to the original cell are formed. The new
cells are called daughter cells.
In sexually reproducing organisms, such as flowering plants and humans, gametes are produced in
the sex organs by a different type of cell division called meiosis. During fertilisation, the male
gamete (sperm) fuses with the female gamete (ovum) to form the first cell of the new individual or
zygote. Because genetic instructions have come from two parents, the zygote will not be identical to
either parent. By repeated mitosis, the zygote grows into an embryo and then continues to divide to
become a foetus (about 10 weeks in humans) and then finally develops into the new individual.
Each kind of cell division involves the chromosomes (thread-like structures made of DNA carrying
all of the genetic instructions). What happens to the chromosomes in cell division determines how
instructions are passed on from one generation to the next.
Mitosis
New body cells resulting from cell division must have the same number of chromosomes as their
parent cell if they are to possess all the necessary coding instructions. The process of mitosis (see
the diagram on the next page) ensures this.
The number of chromosomes in each body cell (somatic cell) is called the diploid number. In
normal body cells chromosomes occur in pairs. The diploid number of these cells is represented as
2n. Each species contains a fixed number of chromosomes. A human skin cell (like all human body
cells), for example, has 23 pairs in its cell. Its diploid number is therefore 2n= 2 X 23 = 46
Meiosis
If gametes were produced in the same way as other cells, what would happen to the chromosome
number of the new cell at fertilisation? Meiosis is the kind of nuclear division that prevents the
doubling of the number of chromosomes at fertilisation. If each gamete had 46 chromosomes, a cell
consisting of 92 chromosomes would be produced when the egg nucleus and sperm nucleus fuse
(join) during fertilisation. In meiosis, four gametes, each carrying half the number of chromosomes
of body cells, called the haploid number (n), are produced. The chromosomes are not in pairs in
gametes. (See the diagram above).
Chromosomes carried in the sperm from the father are called paternal. The chromosomes in the
ovum from the mother are called maternal. As a result of fertilisation, the zygote is diploid. It then
divides rapidly by mitosis to form the embryo, each cell carrying a full set of chromosomes.
Note that in both mitosis and meiosis, the DNA on the chromosomes transfers genetic information
to the new cells that are formed.
Varying the results…..
Offspring of sexually reproducing organisms can vary from each other because chance is involved
as to which male gamete will fertilise an ovum. A large assortment of coding instructions is
possible.
Activities
1. Complete the table below which compares mitosis and meiosis.
Mitosis
Similarities
Meiosis
 involves division of the cell nucleus of living organisms
 duplication of DNA or chromosomes occurs
Differences
 used in growth, repair and
replacement of tissues
 used in producing sex cells (gametes)
 occurs in the body cells (somatic
cells) of living things,
 occurs in the gonads (reproductive
organs) of living things,
e.g. skin, bone marrow, buds, capsules of e.g. ovaries and testes of animals,
moss plants
ovaries and anthers of flowering plants
 one cell division completes the
process
 two cell divisions complete the
process
 two cells, which are identical to
each other are formed
 four cells (gametes), which differ
from each other, are formed
 daughter cells contain the diploid
number of chromosomes (2n)
 daughter cells (gametes) contain the
haploid number of chromosomes (n)
 occurs in asexually reproducing
organisms, e.g. cuttings, runners,
bulbs, binary fission
 occurs in sexually reproducing
organisms that reproduce by fusion of
gametes, e.g. vertebrates, flowering
plants
 no variation in new cells of offspring
(unless environment influences); cells
are genetically identical i.e. clones of
each other
 offspring show variation between
them
 used in tissue culture for skin grafts,
cloning plants, i.e. narrowing
variation and diversity
 produces new varieties of animals
and plants, i.e. increasing variation
and diversity
2.
3. (a)
The diagrams below illustrate the major stages of mitosis but they are not in the correct
order. Number them (1 to 6) to place them in the correct order. B, F, A, D, E, C
Complete the table below which shows the number of chromosomes in the body cells
and the gametes of a range of organisms.
Species
cat
dog
Drosophilia fruit fly
earthworm
garden pea
goldfish
horse
housefly
human
mosquito
mouse
onion
chicken
spinach
potato
Number of
chromosomes in each
body cell
38
78
8
36
14
94
64
12
46
6
40
16
18
12
48
Number of
chromosomes in each
gamete
19
39
4
18
7
47
32
6
23
3
20
8
9
6
24
(b) Explain why the number of chromosomes in all body cells must be even while the
number of chromosomes in gametes can be either even or odd.
All chromosomes in body cells come from both parents originally (sperm and
ovum both have half chromosome number) so chromosomes are paired. Gametes
are formed from meiosis and have half the number of chromosomes so that when
they fuse in fertilization they will have the full number and no more.
(c) Do you think that the more chromosomes an organism has, the more intelligent and
complicated the organism is? Explain your answer.
No as the chromosomes can be long or short and even if they have a large number
in their cells, they may not carry much information.