5.1
MITOSIS
5.1.1
What is Mitosis?
Mitosis is a
division of the nuclear to produce two new daughter cells containing
chromosomes identical to the parent cell.
5.1.2
Significant of Mitosis.
1.
Growth:
·
Mitosis increases the number of cells
in all living organisms, thus allowing growth and development in living
organisms.
2.
Repair and replacement:
·
Mitosis allows dead or damaged cells
to be repaired, replaced and generated.
3.
Asexual reproduction:
·
Mitotic cell division forms the basic
of asexual reproduction, in which the daughter cells produced are genetically
identical to the parent cell.
5.1.3
Chromosomes and chromosomal number
1.
Two types of cells in a sexually
reproducing organism:
(a)
Somatic cell
-
all body cells except the reproductive
cells, which formed through mitosis
(b)
Reproductive cell
-
reproductive cells or gametes, which
formed through meiosis
2.
Chromosomes consist of DNA molecules,
which carried genes that determine the individual characteristics of an
organism.
3.
The number of chromosomes present in
the nuclear of each cell is constant for the species concerned. The
characteristics number is referred to as the chromosomal number.
-
For example: Onion
Cell has 16 chromosomes
Fruit
fry has 8 chromosomes
Human
being has 46 chromosomes
4.
Somatic cells have 2 sets of
chromosomes, one set inherited from each parent. Since the chromosomes in the
nuclear exist in pairs, the chromosomal number is said to be diploid (2n).
-
for example: the nucleus of a typical
human somatic cell has 46 chromosomes arranged in 23 pairs or 2n = 46.
-
the two chromosomes in each pair have
the same structural features and referred to as the homologous chromosomes.
2.
The gametes contain only half the
number of chromosomes, that is one set of unpaired chromosomes, the chromosomal
number is said to be haploid (n).
-
for example: the nucleus of a typical
human reproductive cell / gamete has 23 chromosomes or n = 23.
5.1.1
The Cell Cycle
1.
The cell cycle is the period that
extends from the time a new cell is produced until the time the cell completes
a cell division.
2.
The cell cycle can be divided into two
major phases:
(i)
Interphase (G1, S and G2)
(ii)
mitotic cell division or the M phase
1.
The Interphase
-
accounts for 90% of the cell cycle
-
is the stage for cells to grow larger
and prepare for cell division
-
occurs gradually and continuously for
8 to 24 hours
-
the interphase is divided into three 3
shorter stages of subphases:
(i)
G1 (gap or growth phase 1)
(ii)
S (DNA synthesis)
(iii)
G2 (gap or growth phase 2)
-
During interphase:
·
The nucleus is big and well-defined
·
The chromosomes are not condensed and
are visible as thread-like structures called chromatin
·
A pair of centrosomes (found only in
animal cells) is formed. Each centrosome consists of a pair of centrioles.
·
Each pair of centriole will later
migrate towards the opposite poles of the cell and help in the formation of the
spindle fibres.
|
Interphase
|
Events
|
|
G1
(gap or growth phase 1)
|
·
Protein and new cytoplasmic
organelles such as mitochondria and chloroplasts are synthesized.
·
The metabolic rate of the cell is
high.
·
The chromosomes are not condensed
and are visible as thread-like structures called chromatin
|
|
S (DNA synthesis)
|
·
Synthesis of DNA
·
The DNA in the nucleus undergoes
replication
·
Each duplicated chromosomes are now
consists of two identical sister chromatids.
|
|
G2 (gap or growth phase 2)
|
·
The cell continues to grow and
remain metabolically active
·
The cell accumulates energy and completes
its final preparations for the next stage of cell division.
|
1.
The M phase
|
Phases
|
|
|
PROPHASE
|
·
Chromosomes
condense and become visible under light microscope.
·
Nucleolus
and nuclear membrane disappears.
·
Centrioles
move to the opposite poles.
·
Spindle
fibres form.
|
|
METAPHASE
|
·
Chromosomes
line up at equator.
·
Each
centromere attaches itself to the spindle fibre.
.
|
|
ANAPHASE
|
·
Centromeres
divide.
·
Sister
chromatids separate and move to the opposite poles.
|
|
TELOPHASE
|
·
Chromosomes
reach the opposite poles.
·
Spindle
fibres disappear.
·
Nuclear
membrane re-forms and nucleolus reappears in each nucleus.
|
|
CYTOKINESIS
|
·
Cell
membrane at the midpoint of the parent cell constricts.
·
Cleavage
furrow is formed.
·
Two separated
identical daughter cells are formed.
|
CYTOKINESIS IN PLANT CELL:
·
Vesicles
(produced by Golgi apparatus) containing carbohydrates gather at the equator of
the parent cell.
·
The
vesicles fuse together to form cell plate.
·
Cellulose
builds on each side of the cell plate and forms two cell walls of two daughter
cells.
5.1.1
Compare and
contrast Mitosis and Cytokinesis in animal cell and plant cell:
|
SIMILARITIES
|
||
|
|
||
|
DIFFERENCES
|
||
|
Aspects
|
Animal cells
|
Plant cells
|
|
Presence of Centriole
|
|
|
|
Forming of cell plate
|
|
|
|
Forming of cleavage furrow
|
|
|
5.1.1
The important of controlled Mitosis
-
The cells must divide in a controlled
and orderly manner because:
(i)
The genetic information carried by the
chromosomes is necessary for the proper functioning of an organism
(ii)
Mitosis ensures that the genetic
content and the number of chromosomes in the parent cells are maintained in the
daughter cells from one generation to the next.
(iii)
The rate and timing of cell division
in animals and plants are important for normal growth, development and
maintenance.
5.1.2
The effect of uncontrolled Mitosis
-
When a cell divides through mitosis
repeatedly, without control and regulation, it can produce cancerous cell.
-
A cancerous cell divides
uncontrollably to form a tumour, that can invade and destroy neighbouring
cells.
5.1.3
The application of knowledge on
mitosis in cloning and the tissue culture technique.
1. Animal Cloning:
(i) What is cloning?
Cloning is a
process of producing clones or genetically identical organisms through asexual
production.
(ii) What is animal cloning?
Animal cloning involves transfer of the
nucleus from a somatic cell to an ovum with
nucleus removed.
(iii) The outline of animal cloning by using the
example of the cloned sheep Dolly. (How is animal cloning carried out?)
2. The Tissue
Culture Technique in Plants
(i)
What is tissue culture technique?
·
Tissue culture technique is a method
to culture the plants asexually from small pieces of tissues taken from the
parent plant.
·
Tissue culture is the growth of
tissues of living organisms in a suitable and sterile culture medium,
containing nutrients and growth hormones.
(ii)
Write down the outline of the tissue
culture technique. (How is the tissue culture technique carried out?)
(i)
The advantages and disadvantages of
cloning, including examples if possible
|
Adventages
|
Disadvantages
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
5.1
MEIOSIS
5.1.1
What is Meiosis?
Meiosis is the
process of nuclear division that reduces the number of chromosomes in new cells
to half the number of chromosomes in the parent cell.
Meiosis occurs in the
reproductive organs: testes in males and ovaries in females in human; in
plants, meiosis occurs in the anthers and ovaries of the flowers.
5.1.2
Significant of Meiosis.
1.
Sexual reproduction:
·
Meiosis produces haploid reproductive
cells or gametes.
·
Each gamete receives one chromosome
from every pair of homologous chromosomes.
·
Sexual reproduction involves the
fusion of two haploid (n) gametes during fertilization.
·
This results in the formation of a
diploid zygote (2n).
·
Thus, meiosis ensures that the diploid
number of chromosomes is maintained from generation to the next.
2. Genetic variation:
·
Meiosis produces genetic variation in
the offspring so that they can survive in a constant changing environment,
through:
(i)
Crossing over
-
during prophase I, equivalent portion
of homologous chromosomes may be exchanged, leading to the formation of new
combinations of genes on the chromosomes of the gametes.
(ii)
Independent assortment of chromosomes
-
during metaphase I, the pair of
homologous chromosomes arrange themselves randomly on the equator of the
spindle. The independent assortment of chromosomes produces new genetic
combinations.
(iii)
Random fusion of gametes
-
any male gamete is potentially capable
fusing with any female gamete.
5.2
The stages of Meiosis
1.
Meiosis consists of two separate
nuclear divisions:
(a) Meiosis I
(propahse I, metaphase I, anaphase I and telophase I)
(b) Meiosis II (propahse II, metaphase II, anaphase II and telophase II)
(b) Meiosis II (propahse II, metaphase II, anaphase II and telophase II)
2.
Meiosis I begins with a single diploid
parent cell. At the end of meiosis II, four haploid daughter cells are
produced, each genetically different from the others and from the parent cell.
|
Phases
|
Events
|
|
INTERPHASE
|
·
Chromosomes are not clearly visible.
·
Replication of DNA and Duplication
of chromosomes occur.
|
|
PROPHASE I
|
·
Chromosomes condense and become
shorter, thicker and visible under light microscope.
·
Chromosomes duplicate into two
chromatids.
·
Homologous chromosomes come together
to form pairs of bivalents through synapsis.
·
Non-sister chromatids exchange
segment of DNA (Clossing-over occurs).
·
The point where closing-over occurs
is chiasmata.
·
Closing over result in new combination
of genes on chromosomes.
·
At the end of prophase 1, the
nucleolus and nuclear membrane disappear.
·
The centrioles move to the opposite
poles of the cells.
·
Spindle fibres form.
|
|
METAPHASE I
|
·
The homologous chromosomes line up
at the equator of the cell.
|
|
ANAPHASE I
|
·
Homologous chromosomes separate.
·
Chromosome with two sister
chromatids move to the opposite poles.
|
|
TELOPHASE 1
|
·
Chromosomes reach the opposite poles
of the cell.
·
Spindle fibres disappear.
·
Nucleus membrane reforms.
·
Nucleolus reappears in each nucleus.
|
|
CYTOKINESIS
|
·
Cytokinesis occurs simultaneously
with telophase 1, resulting in two haploid daughter cells.
|
|
PROPHASE II
|
·
Chromosomes thicken and shorten.
·
Each chromosome appear as two
chromatids, connected at the centromere.
·
The nucleoli and the nuclear
membrane of the daughter cell disappear.
·
The centrioles replicate and move to
the opposite poles.
·
Spindle fibres reform.
|
|
METAPHSE II
|
·
Chromosomes with two sister
chromatids line up at the equator of the cell.
|
|
ANAPHASE II
|
·
Centromeres divide.
·
Sister chromatids separate into
individual chromosmes.
·
Each individual chromosome moves to
the opposite poles of the cells.
|
|
TELOPHASE II and CYTOKINESIS
|
·
Chromosomes reach the poles.
·
The nucleoli and nuclear membrane
reform.
·
The spindle fibres disappear.
·
Cytokinesis follows.
·
Four haploid cells are formed.
·
Each haploid cell contains half a
number of chromosomes and is genetically different from the parent diploid
cell.
|
5.1
Comparing and contrasting
(a) Meiosis I and
Meiosis II
|
SIMILARITIES
|
||
|
|
||
|
DIFFERENCES
|
||
|
Phase
|
Meiosis
I
|
Meiosis
II
|
|
|
|
|
|
Metaphase
|
|
|
|
Anaphase
|
|
|
|
Telophase
|
|
|
(b) Mitosis and Meiosis
|
SIMILARITIES
|
||
|
|
||
|
DIFFERENCES
|
||
|
Feature
|
Mitosis
|
Meiosis
|
|
Type of cell that undergo the process
|
|
|
|
Behavior of homologous chromosomes during
prophase
|
|
|
|
Chiasmata
|
|
|
|
Crossing over
|
|
|
|
Behavior of homologous chromosomes during
metaphase
|
|
|
|
Number of nuclear divisions per DNA
replication
|
|
|
|
Number of daughter cells produced at the
end of the process
|
|
|
|
Chromosomal number of the daughter cells
|
|
|
|
Genetic content
|
|
|
|
Genetic variation
|
|
|
|
Purpose
|
|
|
|
Significant
|
|
|
No comments:
Post a Comment