Week 9.3_Meiosis (1)

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Meiosis Week 9 – Friday October 20, 2023
Today’s Learning Objectives Understand the process of meiosis including the individual stages and the end product. Be able to compare and contrast mitosis and meiosis.
Mitosis Review Mitosis occurs in somatic cells. It is the division of a parent cell to form two daughter cells that are genetically identical. We started mitosis with chromosomes that have replicated, giving rise to sister chromatids that are held together at their centromeres. The main goal for mitosis is to pull the daughter chromatids apart and place each in their own nucleus and cell. Equator of cell
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There Are Five Mitotic Phases Cytokinesis is cell division and begins during anaphase. Cytokinesis does not always occur with mitosis.
Reproduction Some animals reproduce asexually. Example below are species that are all female. Mexican Whiptail Lizards Amazon Molly Fish Marbled Crayfish Some capable of reproducing if males are not available: Komodo Dragon Most animals reproduce by sexual reproduction. https://www.treehugger.com/animals-that-reproduce-asexually-5112566
Meiosis Sexual reproduction requires that meiosis occurs. Chromosome numbers of somatic cells are the same for each species of organisms and except for gametes that will have half that number. For example, the somatic cells of humans have 46 chromosomes, the diploid (2n) number, When meiosis occur in sex cells, i.e., eggs and sperms, their chromosome number is reduced by half, the haploid (1n), because of the separation of homologous chromosomes. When fertilization occurs, the sperm and eggs fuse, the zygote , the first 2n cell of the next generation is formed that reestablishes the 2n number of 46 chromosomes.
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Meiosis What if that didn ’ t happen and mitosis gave rise to eggs and sperms?
Meiosis creates gametes Meiosis occurs only in the sex organs to give rise to eggs and sperms. Gametes are the only 1n cells in human lifecycle.
If a frog has 16 chromosomes in an egg cell, how many chromosomes would be in a somatic cell? A. 4 B. 8 C. 16 D. 32
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The Process of Meiosis Meiosis is also known as reduction division because there are two divisions: Meiosis I : The chromosome number is reduced from 2n to 1n and homologous chromosomes are separated into two cells during cytokinesis. Cell division occurs and each cell contains one set of homologous chromosome, but each chromosome is still composed of two sister chromatids. Preceded by interphase , followed by prophase I , metaphase I , anaphase I , telophase I and cytokinesis .
The Process of Meiosis Meiosis is also known as reduction division because there are two divisions: Meiosis II : The sister chromatids are separated into four separate cells and chromatids can now properly be called chromosomes. Begins with interkinesis , which is similar to interphase, but does not have an S phase (No chromosomes duplicated), before the start of meiosis II. Has the same phases as meiosis I, but with “II” after each phase, prophase II , metaphase II , anaphase II , telophase II and cytokinesis . Note that your textbook includes prometaphase I and II, but I have omitted this phase for meiosis.
Interphase and Prophase I Interphase before prophase I is like interphase before mitosis. Divided into G1, S and G2 phases. Sister chromatids held together by cohesion protein at centromere. Prophase I : Chromosomes are visible because of shortening and thickening due to condensing and can be seen to be paired as homologous chromosomes. Nuclear envelope breaks down and nucleolus disappears. Homologous chromosomes are green and purple. Each composed of sister chromatids.
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Interphase and Prophase I Prophase I : Synapsis, the tight pairing of homologous chromosomes by synaptonemal complex , a lattice of protein that joins chromosomes along their entire length. Genes of homologous chromosomes aligned precisely with each other at this time. Crossing over, an exchange of segments of non-sister chromatids can occur at this time. Chiasma Chiasma, an x-shaped area where segments of non-sister chromatids can exchange genetic information. homologous chromosomes crossing over Figure 11.3
Interphase and Prophase I Prophase I : Synapsis, the tight pairing of homologous chromosomes by synaptonemal complex , a lattice of protein that joins chromosomes along their entire length. Non -sister chromatids exchange genetic information at their tips forming recombinant chromatids. Non-recombinant chromatids recombinant chromatids The recombinants now carry some DNA from both parents, forming a combination of genes that had not previously existed. The centrosomes also moves to the poles of the cell and microtubules are forming at this time. Figure 11.3
Metaphase I Metaphase I : The microtubules attach to kinetochores of each sister chromatids from opposite poles and are moved to the equator of the cell. Kinetochore of each sister chromatids are facing towards opposite poles. Homologous chromosomes are all lined up at the equator of the cell, randomly. This provides another means creating more genetic variation.
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Metaphase I Metaphase I : The microtubules attach to kinetochores of each sister chromatids from opposite poles and are moved to the equator of the cell. Random assortment of all homologous chromosomes. Random assortment and crossing over are means of increasing genetic variability during meiosis.
Anaphase I Anaphase I : The microtubules of homologous chromosomes are pulled apart, pulling each chromosome to opposite poles of the cell. Sister chromatids are still tightly bound together at their centromeres.
Telophase I Telophase I : The homologous chromosomes are now at opposite poles of the cell. The chromosomes decondense and become long and stringy again. The nuclear envelop begins to reform around the chromosomes. Cytokinesis occurs and two daughter cells will form. The two cells are haploid because there is just one chromosome of each homologous pair in each cell. Each chromosome still consists of two sister chromatids.
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At the end of Meiosis I, the following has occurred: A. There are two haploid cells. B. The homologous chromosomes are in two different cells. C. Crossing over has occurred. D. All of the above are correct.
Meiosis II Interkinesis : Essentially the cell cycle without the S phase. All the DNA that is required for the rest of meiosis is already present. Prophase II : The chromatin becomes shortened and thickened again and are once again visible as chromosomes. Each chromosome is composed of two sister chromatids. The nuclear envelop will once again break down. The centrosomes move to opposite poles of the cell and new spindles are formed.
Meiosis II Metaphase II : The kinetochores are formed on the sister chromatids. Each sister chromatid is attached to microtubules originating from opposite poles of the cell. Sister chromatids are lined up at the equator of the cell. Recall that in metaphase I the homologous chromosomes were randomly arranged at the cells equator and crossing over occurred in prophase I.
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Meiosis II Metaphase II : Random arrangement of chromatids provides more genetic variation.
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Meiosis II Anaphase II : The sister chromatids are pulled apart by the kinetochore microtubules to opposite poles of the cell. Nonkinetochore microtubules elongate the cell. Cell begins to furrow.
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Meiosis II Telophase II : Chromosomes are at opposite poles of cell and nuclear envelope begins to reform around the chromosomes. Chromosomes decondense and become long stringy. Cell begins to furrow. Cytokinesis will separate the four nuclei into four haploid cells that are genetically different.
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The following are sources of genetic recombination: A. Mitotic crossing over. B. Random assortment of homologous chromosomes. C. Crossing over during prophase I of meiosis. D. Both C and D are correct.
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Meiosis II Meiosis in sex cells : Spermatogenesis : Meiosis produce four sperms. Oogenesis : Meiosis produce four haploid cells, but only one will be an egg. The other three products are polar bodies . Polar bodies give up their cytoplasm to nourish the one egg. Egg polar bodies Sperms
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Comparing Mitosis and Meiosis
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Summary Meiosis reduces the chromosome number by 50% Recombination happens in Prophase I Creates genetic diversity by exchanging alleles between homologous chromosomes, i.e., crossing over. Independent assortment happens in Metaphase I Creates genetic diversity by randomly allocating the two members of a homologous pair of chromosomes into separate gametes Meiosis I – separation of homologous chromosomes Cells becomes haploid during this phase. Meiosis II – separation of sister chromatids
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