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My name is Poon Cheng Moh. I have been teaching biology in SMK(P) Raja Zarina, Port Klang for 26 years. I sincerely hope that this blog on SPM Biology will be useful to both teachers and students.

Saturday, June 22, 2013

How meiosis work?



Stages of meiosis

Prophase I
• The chromosomes begin to condense. They become shorter, thicker and clearly visible.
• The homologous chromosomes come together to form bivalent through a process called synapsis.
• Non-sister chromatids exchange segments of DNA in a process known as crossing over.
• Crossing over results in a new combination of genes on a chromosome.
• The points at which segments of chromatids cross over are called chiasmata.
• At the end of prophase I, the nucleolus and nuclear membrane disappear.
• The two pairs of centrioles migrate to the opposite poles of the cells.
• These features are similar to those of prophase during mitosis.
Metaphase I
• The chromosomes are lined up side by side as tetrads on the metaphase plate.
• The chromosomes are still in homologue pairs.
• The centromere does not divide.
Anaphase I
• The spindle fibres pull the homologous chromosomes away from one another and move them to the opposite poles of the cell.
• Each chromosome still consists of two sister chromatids which move as a single unit.
• Although the cell started with four chromosomes, only two chromosomes (each with two sister chromatids) move towards each pole.
Telophase I
• The chromosomes arrive at the poles.
• Each pole now has a haploid daughter nucleus because it contains only one set of chromosomes.
• The spindle fibres disappear.
• The nuclear membrane reappears to surround each set of chromosomes.
• The nucleolus then reappears in each nucleus.
• Cytokinesis usually occurs simultaneously with Telophase I, resulting in two haploid daughter cells, each receiving one chromosome from the homologous pair.
• Meiosis II follows immediately after cytokinesis, usually with no interphase between them.
• DNA replication does not occur and the chromosomes remain in a condensed state.
Prophase II
• The nuclear membranes of the daughter cells disintegrate again.
• The spindle fibres reform in each daughter cell.
Metaphase II
• The chromosomes, each still made up of sister chromatids, are positioned randomly on the metaphase plate with the sister chromatids of each chromosome pointing towards the opposite poles.
• Each sister chromatid is attached to the spindle fibres at the centromere.
Anaphase II
• The centromere of the sister chromatids finally separate, and the sister chromatids of each chromosome are now individual chromosomes.
• The chromosomes move towards the opposite poles of the cell.
Telophase II
• Finally, the nucleolus and nuclear membranes reform. The spindle fibres break down.
• Cytokinesis follows and four haploid daughter cells are formed, each containing half the number of chromosomes and is genetically different from the parent diploid cell.
• These haploid cells (n) will develop into gametes.

Thursday, June 20, 2013

Cytokinesis in animal and plant cell

How cancer develops - Uncontrolled cell division

This video demonstrates how cancer growth happens in human body. It is due to uncontrolled cell division.

Cloning Of Dolly, the sheep

Tissue culture

Step 1: Weigh out 2.5 g of nutrient agar and add to 250 ml of distilled water.
Step 2 : Then add hormone into the agar solution. Heat and stir until the agar dissolves.
Step 3 :Pour the hot nutrient agar into several test tubes about 5 cm3 in height.
Allow the agar to cool and solidify in each test tube.
Step 4 :Germinate some mustard seed in a warm lighted place until the cotyledon starts to unfold.
Step 5 : Cut the tips of the seedling just above the shoot apex (meristem).These tissues are called explants.
Step 6 : Put one explant in each test tube.
Step 7 : Cover the tubes with a cling film and place the tubes into a rack by the window.
Step 8 : The tissue cells are left to divide by mitosis to produce a mass of loosely arranged and undifferentiated cells called callus. The callus will then grow into plantlets(little plants) which will grow into adult plants.