Science Grade 8

Science Grade 8

So far, we have already finished our discussions on Chapter 6 and 7 of your textbook. The following are notes for Chapter 8. Thank you and enjoy learning.

Meiosis Cell Division – it is a special type of cell division that will take place only on our sex cells/gametes. It is characterized by two successive cell division: meiosis I and meiosis II

Meiosis I

1.       Prophase I

·         Thread-like structure chromatin condensed to form chromosomes

·         Nuclear membrane starts to disappear

·         Centrioles appear and moved to the opposite poles

·         Spindle fibers are released

·         Homologous chromosomes (chromosomes with the same length and the same position of the centromere) paired up in the process known as synapsis

·         Exchange of genetic information takes place also known as crossing-over.

2.       Metaphase I

·         Spindle fibers from the centrioles will attach to the centomere of the chromosomes

·         Homologous chromosomes will align themselves in the metaphase plate (middle part) of the cell

3.       Anaphase I

·         The homologous chromosomes will move to the opposite poles by the aid of the spindle fibers.

4.       Telophase  I

·         The centrioles and spindle fibers will disappear ( sometimes they don’t, instead they proceed to undergo Prophase II right away)

·         the nuclear membrane will reappear and divide in the process known as karyokinesis

·         followed by the division of the cytoplasm or cytokinesis.

Note:  At the end of the telophase I, we now have 2 cells with half of the original number of chromosomes. Humans for example has 46 chromosomes, so after meiosis I the number of chromosomes is now reduced to 23. That’s why meiosis I is also known as reductional division.

The cells is now ready to undergo Meiosis II

1.       Prophase II

·         Thread-like structure chromatin condensed to form chromosomes

·         Nuclear membrane starts to disappear

·         Centrioles appear and moved to the opposite poles

·         Spindle fibers are released

2.       Metaphase II

·         Spindle fibers from the centrioles will attach to the centomere of the chromosomes

·          chromosomes will align themselves in the metaphase plate (middle part) of the cell

3.       Anaphase II

·         Centromeres that hold sister chromatids together will break and the sister chromatids will move to opposite poles

4.       Telophase II

·         The centrioles and spindle fibers will disappear

·         the nuclear membrane will reappear and divide in the process known as karyokinesis

·         followed by the division of the cytoplasm or cytokinesis.

Note: the number of chromosomes will not be reduced further. It will still be 23 for humans. Thus, meiosis II is known as equational division.

Why is Meiosis cell division so special? Because they make sure that when fertilization takes place the cell will retain the normal chromosomes number of a specie. For humans, 23 from egg + 23 from sperm = 46. The normal chromosome number for humans. Otherwise, if reduction in the number of chromosomes will not happen in meiosis, the next generation will have double the number of chromosomes.

Comparison between Mitosis and Meiosis Cell Division

Basis of Comparison	Mitosis	Meiosis
Type of cell	Body cells/somatic cells	Sex cells/gametes
Number of daughter cells produced	2	4
Number of chromosomes in the daughter cells	2n ( diploid)	n ( haploid)
Number of times the cell divides	once	twice

Gametogenesis – the process of gamete formation

2 types: Spermatogenesis and Oogenesis

1.       Spermatogenesis – the process of sperm formation

2.       Oogenesis – the process of egg formation