Meiosis and Genetic Variation PDF

## What is Meiosis? How Does It Generate Variation in Organisms? - As opposed to mitosis, meiosis occurs only in gametes, for sexual reproduction - Unique events in meiosis cause genetic variation: - Synapsis - Crossing-over - Random Chromosome Assortment ### Stages of Meiosis | Stage...

## What is Meiosis? How Does It Generate Variation in Organisms? - As opposed to mitosis, meiosis occurs only in gametes, for sexual reproduction - Unique events in meiosis cause genetic variation: - Synapsis - Crossing-over - Random Chromosome Assortment ### Stages of Meiosis | Stage | Description | |-----------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Prophase I | The nuclear envelope disintegrates. The chromosomes condense. The centrosomes move to opposite poles. | | Metaphase I | Spindle fibers attached to the centromeres will align the chromosomes at the center. | | Anaphase I | The spindle fibers will separate each chromosome’s sister chromatids and move them to the opposite poles. | | Telophase I & Cytokinesis | The chromosomes decondense. The nuclear envelope forms again, and the cell divides into two. After telophase I, there are four haploid daughter cells. | | Prophase II | The nuclear envelope disintegrates. The chromosomes condense again. The centrosomes move to opposite poles. | | Metaphase II | Spindle fibers attached to the centromeres will align the chromosomes at the center. | | Anaphase II | The spindle fibers separate each chromosome's sister chromatids and move them to the opposite poles. | | Telophase II & Cytokinesis | The chromosomes decondense. The nuclear envelope forms again, and the cell divides into two. After telophase II, there are four haploid daughter cells. | ### Fertilization - Although fertilization is not part of meiosis, it needs to be mentioned because meiosis produces the sex cells needed for fertilization. - Fertilization is a random event. The first fit sperm to enter the egg cell is unique; hence, each parent provides millions of possible combinations of chromosomes and genes. This increases genetic variation in a population. ### Genetic Variation - After discussing the stages of meiosis, we can answer the question "are we unique?" - The answer is yes! We are unique because of meiosis and other biological events. - Species that reproduce through sexual reproduction are the ones with individuals that are different from one another. - Therefore, each human is unique because of genetic variation - **Crossing-over** in prophase I of meiosis enables homologous chromosomes to exchange arms with one another. - Remember that in each cell, one set of chromosomes comes from the father and another set comes from the mother. - Thus, the daughter's traits are a combination of theirs. - That explains how a father with curly hair and black eyes and a mother with straight hair and brown eyes can have a child with curly hair and brown eyes. - **Random Chromatid Assortment** - In metaphase I the homologous chromosomes of each cell align at the center. - The orientation of each homologous pair is random. Therefore, once they separate, each daughter cell’s resulting chromosome combination after meiosis I will be purely arbitrary. - Likewise, during meiosis II, the assortment of sister chromatids is also random. - This means that each gamete has a unique combination of chromosomes. - Therefore, no one is alike (except in the case of twins). ### Stages of Meiosis I - The first division aims to separate the homologous chromosomes, reduce the chromosome number of the cells, and initiate genetic recombination. - Prophase I has five substages. - Leptotene: -The chromosomes start to condense - Zygotene: - Homologous chromosomes pair to produce a tetrad or bivalent. - Pachytene: - Genetic recombination happens through the crossing over of genetic material between non-sister chromatids. - Diplotene: - Crossing-over and synapsis end with the disappearance of the synaptonemal complex. - Diakinesis: - The chromosomes become fully condensed. The nuclear membrane disintegrates. ### Stages of Meiosis II - The second division aims to separate the sister chromatids. - Unlike in meiosis I, the chromosome number will be the same before and after meiosis II. - Therefore, the latter is much like mitosis. ### Cell Division: Mitosis vs. Meiosis - Mitosis is for growth, repair, and development, while meiosis for the production of sex cells or gametes - Mitosis and meiosis are different from mitosis in terms of function, steps, and ploidy of resulting cells. ### Integrate **(In the Philippines)** - Marrying one's siblings or relatives is a social taboo. - However, in some island communities and ethnic groups, marrying relatives such as cousins is a common practice. **(In cytology)** - It is highly discouraged to mate or produce offspring with family members or relatives. - This is because inbreeding can lead to less genetic variation. - The disadvantage of a community with low genetic diversity is not mentioned.