Saturday, June 28, 2014

BIOLOGY FORM 4 NOTES CHAPTER 5 : CELL DIVISION

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
Events
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)

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
Prophase









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 








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