9th Class Biology Chapter 5 Notes English Medium
Important Notes of 9th Class Biology Chapter 5 Notes English Medium written by Honorable Sir Adnan Haider Suib. These notes are very helpful in the preparation of Cell Cycle 9th Class Notes for students of the 9th Class Biology Chapter 5 Notes English Medium these are according to the paper patterns of all Punjab boards.
Summary and Contents:
Topics which are discussed in the notes are given below:
- Here are the detailed Cell Cycle 9th Class to help you prepare for your exams.
- Very Important Multiple Choice Questions (MCQs) of Chapter No.5 Cell Cycle English Medium.
- ii. Metaphase: When spindle fibres have grown to sufficient length, some spindle fibres, known as kinetochore
fibres, attach with the kinetochores of chromosomes. Two kinetochore fibres from opposite poles
attach with each chromosome. Chromosomes arrange themselves along the equator of cell forming
a metaphase plate. A number of other fibres (non-kinetochore) from the opposite centrosomes
attach with each other.
- iii. Anaphase: When a kinetochore spindle fibre connects with the kinetochore of chromosome, it starts to pull
toward the originating centrosomes. The pulling force divides the chromosome’s sister chromatids
and they separate. These sister chromatids are now sister chromosomes, and they are pulled apart
toward the respective centrosomes. The other spindle fibres (non-kinetochore) also elongate. At
the end of anaphase, cell has succeeded in separating identical copies of chromosomes into two
groups at the opposite poles.
- iv. Telophase: Telophase is a reversal of prophase. A new nuclear envelope forms around each set of separated
chromosomes. Both sets of chromosomes, now surrounded by new nuclear envelopes, unfold back
into chromatin. Nuclear division is completed, but cell division has yet one more step to complete.
- Significance Of Mitosis: Importance of mitosis is the maintenance of chromosomal set i.e. each daughter cell receives
chromosomes that are a like in composition and equal in number to the chromosomes of parent
cell. Following are the occasions in the lives of
organisms where mitosis happens.
- Development and growth: The number of cells within an organism
increase by mitosis. This is the basis of the
development of a multicellular body from
a single cell i.e. zygote and also the basis of
the growth of multicellular body.
- Cell replacement: In some parts of body, e.g. skin and digestive tract, cells are constantly sloughed off and replaced
by new ones. New cells are formed by mitosis and so are exact copies of the cells being replaced.
Similarly, red blood cells have short life span (about 4 months) and new red blood cells are formed
by mitosis.
- Regeneration: Some organisms can regenerate parts of their bodies. The
production of new cells is achieved by mitosis. For example;
sea star regenerates its lost arm through mitosis.
- Asexual reproduction: Some organisms produce genetically similar offspring through asexual reproduction. Mitosis is a
mean of asexual reproduction. For example; hydra reproduces asexually by budding. The cells at
the surface of hydra undergo mitosis and form a mass called bud. Mitosis continues in the cells of
bud and it grows into a new individual. The same division happens during asexual reproduction
(vegetative propagation) in plants.
- Errors In Mitosis: Errors in the control of mitosis may cause cancer. All cells have genes that control the timing
and number of mitosis. Sometimes mutations occur in such genes and cells continue to divide.
It results in growths of abnormal cells called tumors. As long as these tumors remain in their
original location, they are called benign tumours. But if they invade other tissues, they are called
malignant (cancerous) tumors and their cells are called cancer cells. Such tumors can send cancer
cells to other parts in body where new tumors may form. This phenomenon is called metastasis
(spreading of disease).
- Phases Of Meiosis: Meiosis was discovered and described for the first time in 1876, by a German biologist Oscar
Hertwig. The preparatory steps of meiosis are identical to the interphase of mitosis. Interphase is
divided into the same three phases i.e. G1, S phase, and G2. Interphase is followed by meiosis I and
meiosis II.