Topical Assessment

SECTION A: Objective Questions

1          What is the advantage of reproducing plants through cloning as compared with reproduction through seedlings?

A  New plant varieties can be produced

B  More similar plants can be produced

C  New plants have a better quality than the parent plants

       D  New plants have a high resistance towards diseases

2          Which of the following plant structures contain haploid nucleus?

A  Epidermis cell

B  Pollen grains

C  Root end cell

D  Xylem

3          Which of the following does not carry out mitosis?

A  Gamete

B  Skin

C  Meristem

D  Cambium

4          Which of the following statements about the chromosome most accurately explains the production of similar daughter cells in mitosis?

A  The chromatid pairs stick at the centromere

B  The chromosomes shorten and thicken at the prophase stage

C  A chromatid from each chromosome moves to the opposite poles

D  The chromosomes are arranged at the equatorial plane on a cell that is dividing

SECTION B:

1.      List eight stages of the cell cycle in the correct sequence.
2.      List five stages of cell division in the correct sequence.


ANSWERS:

SECTION A:

1.B
2.B
3.A
4.C

SECTION B:

1. Growth1 - [DNA] Synthesis - Growth2 - Prophase - Metaphase - Anaphase - Telophase - Cytokinesis
2. Prophase - Metaphase - Anaphase - Telophase - Cytokinesis

Video - mitosis

Application of Mitosis in Cloning

The knowledge of mitosis is applied in cloning and the tissue culture technique.

Cloning is a process of artificially creating a new individual that is genetically identical to an existing individual.

Tissue culture technique involves the growth of cells or tissues outside the organisms in a suitable culture medium, which contains nutrient and growth hormones.

Cloning

Fusion cell cloning involves replacing the nucleus of an unfertilised egg with the nucleus from a different cell. The replacement nucleus can come from an embryo, but if it comes from an adult cell, it is called adult cell cloning.

Dolly the sheep was the first mammal to be cloned using adult cell cloning. She was born in the UK in 1996 and died in 2003. Here's how she was produced:

1.      An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed.

2.      The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell.

3.      The fused cell then began to develop normally, using genetic information from the donated DNA.

4.      Before the dividing cells became specialised, the embryo was implanted into the uterus of a foster mother sheep. The result was Dolly, genetically identical to the donor sheep.

Tissue culture technique

Another way of cloning plants is by tissue culture, which works not with cuttings but with tiny pieces from the parent plant. Sterile agar jelly with plant hormones and lots of nutrients is needed. This makes tissue culture more expensive and difficult to do than taking cuttings.

Tissue culture involves the following steps:

1.      Small amounts of parent tissue or a number of cells are taken

2.      The plant material is transferred to plates containing sterile nutrient agar jelly

3.      Plant hormones are added to stimulate the cells to divide

4.      Cells grow rapidly into small masses of tissue

5.      More growth hormones are added to stimulate the growth of roots and stems

6.      The tiny plantlets are transferred into potting trays where they develop into plants

The importance of controlled mitosis

n  Controlled mitosis: the ability of the cell to divide at its own rate and time.

n  Crucial to normal growth, development and maintenance for the perpetuity of living things.

n  Eg:

1.      In plants à controlled mitosis occurs at shoots results in growth of plants.

2.      In humanà Malphigian layer of skin divides actively to replace dead and damaged cells.

3.      A lizard with no tail will gain back its tail within some time due to mitosis. Mitosis will produce new cells which will help the lizard to get back its tail.

4.      If our skin is scraped, mitosis will produce new cells to replace back the worn out cells.

5.      If we donate a part of our liver, we will gain back that part of liver too because of mitosis.

Overall, mitosis is required for the normal growth, development and maintenance.

 

Uncontrolled Mitosis-Cancer

Cancer is any malignant growth or tumor caused by abnormal and uncontrolled cell division; it may spread to other parts of the body through the lymphatic system or the blood stream. Cancer is the result of cell not listening to the signals telling it to stop dividing. Cancer cells have a defect in 2 genes ( a gene that allows cancer and a gene that destroys the cancer cell).When mitosis gets out of control, cells divisions are being made continuously, without an end point. As a result, this problem could lead to various types of cancer. If mitosis became uncontrolled tumor grows .

Causes of Cancer

n  Genetic- some forms of cancer like prostate, colon, breast, skin, ovary are suspected to be inherited from the parents

n  Carcinogens- these are chemicals which affect genetic activity and cause cancer, e.g. of carcinogen a diesel exhaust, cigarette smoke, hair dyes, soot, arsenic, benzene and formaldehyde.

n  Radiation- excess exposure to x-ray, gamma-rays and ultra violet rays lead to increase cancer risk.

n  Viruses- some viruses (such as the HPV and HIV-1) cause cancer.

Cytokinesis

 à  process of cytoplasmic division to form two daughter cells.

n  Different in animals and plants.

n  Animal cells, the cytoplasm contracts to pull the plasma membrane inwards, forming groove called a cleavage furrow

n  In plant cells, cytokinesis in cells is markedly different.

n  A cleavage furrow does not form. Instead, membrane-enclosed vesicle gather at a plant cell’s equator between the two nuclei.

n  Vesicle fuse to form a cell plate

Phase of Mitosis in Animal Cells

Prophase

Early prophase

n  Chromosome condense and appear shorter and thicker and become visible in a light microscope

n  Each chromosome now consists of a pair of sister chromatids joined together at centromere.

n  Nucleolus disappears & nuclear membrane disintegrates.

n  Paired centrioles move to opposite ends of the cell

Late prophase

n  Nuclear membrane disappears

n  Spindle form

Metaphase

n  Spindle fibres are fully form

n  Sister cromatids line up at the spindle equator/ metaphase plate.

n  Two sister chromatids are still attached to one another at the centromere.

n  At the end of metaphase, the centromers divide.

Anaphase

n  Anaphase begin with the separation of the centromers

n  The sister cromatids are drawn to opposite poles of the cell by shortening of spindle fibres.

n  Once separated, chromatids are referred to as daughter chromosome.

n  End anaphase: the poles of the cell have complete and equivalent sets of chromosome.

Telophase

n  Telophase begins when the two sets of daughter chromosomes have reached the two poles of the cell.

n  The spindle fibres disintegrate, the nuclear membrane forms around each set of daughter chromosomes and the nucleoli reappear.

n  The nucleolus also re-forms in each nucleus.

n  The chromosomes uncoil and become less visible under the light microscope.

n  Mitosis process is now completed.

Cell Cycle and Interphase

Cells that are growing and dividing go through a repeating series of events called the cell division cycle (or cell cycle).Cell cycle is a description of the time course and events in the life of typical cells, divided into four stages, Growth 1, S Phase (DNA synthesis), Growth 2, and cell division (mitosis or meiosis followed by cytokinesis).

The cell cycle is divided into two major phases:

1.Interphase

2.Mitosis

Interphase is the period between division, divided into 3 sub phases (G1, S and G2):

1. G1- cells grow rapidly and new organelle are synthesis
2. S - synthesis of DNA and chromosomes are replicated
3. G2 - cells prepare for mitosis, synthesis protein and mitotic spindle begin to form

CHAPTER 5 CELL DIVISION -Introduction

Cell division is the process by which a cell replicates to form two essentially identical daughter cells; the process includes the four stage nuclear division, mitosis, and cytoplasmic division = cytokinesis; upon completion of the process, each daughter cell contains the same genetic material, exact copies of the DNA, as the original cell and roughly half of its cytoplasm.

Mitosis is the process of nuclear division of either a diploid (2N) or haploid (N) eukaryotic cell whereby two daughter nuclei are produced that are genetically identical to the parent nucleus. Cell division usually follows nuclear division.

Mitosis usually results in the production of two progeny cells that are genetically identical to the parent cell.

Mitosis is a continuous process and divided into 4 main phases based on the appearance and behavior of the chromosomes.

1.      Prophase, P

2.      Metaphase, M

3.      Anaphase, A

4.      Telophase, T

The significance of mitosis

1.      Growth. The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.

2.      Cell Replacement. Cells are constantly sloughed off, dying and being replaced by new ones in the skin and digestive tract. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced so as to retain normal function of cells.

3.      Regeneration. Some animals can regenerate parts of the body, and production of new cells are achieved by mitosis.

4.      Vegetative Reproduction. Some plants produce offspring which are genetically similar to themselves. These offspring are called clones.

Red blood cells have short live spans of about 4 months and need to be replaced constantly by mitosis.