Cell Division III) Meiosis A) Introduction

Cell Division III) Meiosis Meiosis a type of cell division results in the formation of sex cells, or gametes. in humans it occurs in the testis and ovaries. involves two stages of cell division  both have similarities to the phases in mitosis the chromosome number of the daughter cells is half that of the parent cell (haploid number or n) A) Introduction

Cell Division III) Meiosis involves two stages of cell division  both have similarities to the phases in mitosis the chromosome number of the daughter cells is half that of the parent cell (haploid number or n) each parent provides half the genetic information to their offspring.  each of the 23 chromosomes you receive from your father is matched up with 23 chromosomes from your mother. the paired chromosomes are called homologous chromosomes.  there are 22 pairs of homologous chromosomes  the 23 rd pair are the sex chromosomes and are only partially homologous. A) Introduction

Cell Division III) Meiosis the paired chromosomes are called homologous chromosomes.  there are 23 pairs of homologous chromosoms  the 23 rd pair are the sex chromosomes and are only partially homologous. during fertilization, a haploid (n = 23) sperm cell unites with a haploid (n = 23) egg cell to produce a diploid (2n = 46) zygote.  the zygote will begin dividing by mitosis and become a multicellular organism. A) Introduction

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis

meiosis  involves two nuclear division. Meiosis I Meiosis II  produces four haploid cells. meiosis I is called reduction division because the diploid (2n) chromosome number is reduced to the haploid (n) chromosome number meiosis II is marked by the separation of the two chromatids. DNA synthesis occurs prior to the two nuclear divisions.* B) Stages

Cell Division III) Meiosis

meiosis II is marked by the separation of the two chromatids. DNA synthesis occurs prior to the two nuclear divisions. Meiosis I Stages:  Prophase I  Metaphase I  Anaphase I  Telophase I Prophase I  nuclear membrane begins to dissolve  centriole splits, parts move to opposite poles and spindle fibres form. B) Stages

Cell Division III) Meiosis Prophase I  nuclear membrane begins to dissolve  centriole splits, parts move to opposite poles and spindle fibres form.  chromosomes come together in homologous pairs. each chromosome of the pair is a homologue and is composed of a pair of sister chromatids.  the whole structure is referred to as a tetrad (because there are four chromatids)

Cell Division III) Meiosis each chromosome of the pair is a homologue and is composed of a pair of sister chromatids.  the whole structure is referred to as a tetrad (because there is four chromatids)  each pair of homologous chromosomes align side by side (non-sister chromatid to another) this aligning is called synapsis. as the chromosomes synapse they often intertwine  the intertwined chromatids from different homologous break and exchange segments in a process called crossing over. B) Stages

Cell Division III) Meiosis this aligning is called synapsis. as the chromosomes synapse they often intertwine  the intertwined chromatids from different homologous break and exchange segments in a process called crossing over. B) Stages

Cell Division III) Meiosis this aligning is called synapsis. as the chromosomes synapse they often intertwine  the intertwined chromatids from different homologous break and exchange segments in a process called crossing over.  DNA information is exchanged during the crossing over event.  promotes variation within the species. B) Stages

Cell Division III) Meiosis Metaphase I  homologous chromosomes attach themselves to the spindle fibres and line up along the equatorial plate B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis Metaphase I  homologous chromosomes attach themselves to the spindle fibres and line up along the equatorial plate Anaphase I  the homologous chromosomes move toward opposite poles. this is called segregation  reduction division occurs One member of each homologous pair will be found in each of the new cells. B) Stages

Cell Division III) Meiosis this is called segregation  reduction division occurs One member of each homologous pair will be found in each of the new cells. Telophase I  a membrane begins to form around each nucleus. the chromosomes in the nuclei are not identical because each of the daughter nuclei contains one member of the homologous chromosome pair.  homologous chromosomes are similar but not identical. B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis

the chromosomes in the nuclei are not identical because each of the daughter nuclei contains one member of the homologous chromosome pair.  homologous chromosomes are similar but not identical Meiosis II Stages:  Prophase II  Metaphase II  Anaphase II  Telophase II the stages occur at approximately the same time for each of the haploid daughter cells. there is no replication of chromosomes prior to meiosis II. B) Stages

Cell Division III) Meiosis the stages occur at approximately the same time for each of the haploid daughter cells. there is no replication of chromosomes prior to meiosis II. Prophase II  the nuclear membrane dissolves and spindle fibres form. B) Stages

Cell Division III) Meiosis Prophase II  the nuclear membrane dissolves and spindle fibres form. Metaphase II  arrangement of each chromosome, each with two chromatids, along the equatorial plate. chromatids are held together by the centromere B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis Prophase II  the nuclear membrane dissolves and spindle fibres form. Metaphase II  arrangement of each chromosome, each with two chromatids, along the equatorial plate. chromatids are held together by the centromere Anaphase II  breaking of the attachment between the two chromatids.  migration of chromatids (now called chromosomes) to opposite poles. B) Stages

Cell Division III) Meiosis Prophase II  the nuclear membrane dissolves and spindle fibres form. Metaphase II  arrangement of each chromosome, each with two chromatids, along the equatorial plate. chromatids are held together by the centromere Anaphase II  breaking of the attachment between the two chromatids.  migration of chromatids (now called chromosomes) to opposite poles. Telophase II  second nuclear division is completed  second division of cytoplasm occurs (cytokenesis)  four haploid daughter cells are produced B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis PRINT ME B) Stages

Cell Division III) Meiosis MITOSISMEIOSIS MEIOSIS I Prophase I Chiasma Homologous chromosome pair Chromosome replication Parent cell 2n = 6 Chromosome replication Replicated chromosome Prophase Metaphase Metaphase I Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I Anaphase Telophase 2n2n 2n2n Daughter cells of mitosis nn n n MEIOSIS II Daughter cells of meiosis II SUMMARY Meiosis Occurs during interphase before meiosis I begins Two, each including prophase, metaphase, anaphase, and telophase Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Produces gametes; reduces number of chromosomes by half and introduces genetic variability amoung the gametes Mitosis Occurs during interphase before mitosis begins One, including prophase, metaphase, anahase, and telophase Does not occur Two, each diploid (2n) and genetically identical to the parent cell Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Property DNA replication Number of divisions Synapsis of homologous chromosomes Number of daughter cells and genetic composition Role in the animal body B) Stages

Cell Division III) Meiosis B) Stages

Cell Division III) Meiosis Cell Division III) Meiosis C) Genetic Recombination

Cell Division III) Meiosis Genetic Recombination the formation of new combinations of genes comes about by  independent assortment  crossing over C) Genetic Recombination

Cell Division III) Meiosis comes about by  independent assortment  crossing over Independent Assortment  during metaphase I chromosome arrange in homologous pairs along the equator of the cell. the chromosome of maternal origin is orientated toward one pole of the cell while the chromosome of paternal origin is oriented towards the other pole this orientation is “independent” of other homologous pairs it results in gametes having different combinations of parental chromosomes. C) Genetic Recombination

Cell Division III) Meiosis Independent Assortment  during metaphase I chromosome arrange in homologous pairs along the equator of the cell. the chromosome of maternal origin is orientated toward one pole of the cell while the chromosome of paternal origin is oriented towards the other pole this orientation is “independent” of other homologous pairs it results in gametes having different combinations of parental chromosomes. tent/independentassortment.html C) Genetic Recombination

Cell Division III) Meiosis Independent Assortment  during metaphase I chromosome arrange in homologous pairs along the equator of the cell. the chromosome of maternal origin is orientated toward one pole of the cell while the chromosome of paternal origin is oriented towards the other pole this orientation is “independent” of other homologous pairs it results in gametes having different combinations of parental chromosomes. C) Genetic Recombination

Cell Division III) Meiosis Crossing Over  occurs when chromosomes synapse, or pair up during prophase I.  Non-sister chromatids exchange pieces of chromosome. the section that is crossed over may contain hundreds or even thousands of genes.  can create individual chromosomes with both maternal and paternal genes.  can occur at several points along the non-sister chromatids. C) Genetic Recombination

Cell Division III) Meiosis Crossing Over  occurs when chromosomes synapse, or pair up during prophase I.  non-sister chromatids exchange pieces of chromosome. the section that is crossed over may contain hundreds or even thousands of genes.  can create individual chromosomes with both maternal and paternal genes.  can occur at several points along the non-sister chromatids.

Cell Division III) Meiosis Cell Division III) Meiosis D) Gamete Development

Cell Division III) Meiosis Gametogenesis the formation of sex cells during meiosis. both females and males follow the same process of meiosis to develop gametes but there is differences in the final products.  the cytoplasm of the female gamete does not divide equally after each nuclear division. this results:  in one cell, called the ootid, receiving most of the cytoplasm.  three polar bodies that die and are absorbed by the body D) Gamete Development

Cell Division III) Meiosis this results:  in one cell, called the ootid, receiving most of the cytoplasm.  three polar bodies that die and are absorbed by the body D) Gamete Development

Spermatogenesis the process starts with a diploid germ cell called a spermatogonium.  starting at puberty mitosis forms two spermatogonium daughter cells. one cell replenishes the spermatogonia one cell develops into a primary spermatocyte  the primary spermatocyte undergoes meiosis I to form two secondary spermatocytes.  the secondary spermatocytes undergo meiosis II to form four spermatids. D) Gamete Development

 the primary spermatocyte undergoes meiosis I to form two secondary spermatocytes.  the secondary spermatocytes undergo meiosis II to form four spermatids. PRINT ME

Oogenesis oogenesis starts with the diploid germ cell called an oogonium.  each oogonium undergoes mitosis to form two primary oocytes.  three months after conception two million primary oocytes can be found in the ovaries arrested in prophase I awaiting puberty.

 every month after puberty one primary oocyte undergoes meiosis. unequal division of cytoplasm called asymmetrical cytokinesis occurs. the cell that receives the most cytoplasm is called the secondary oocyte, the other cell is called the first polar body.  the first polar body may or may not go through a second division to produce a pair of polar bodies. D) Gamete Development

the cell that receives the most cytoplasm is called the secondary oocyte, the other cell is called the first polar body.  the first polar body may or may not go through a second division to produce a pair of polar bodies.  when sperm penetrates the secondary oocyte it undergoes meiosis II.  cytoplasm divides unequally again.  the cell with the most cytoplasm becomes the mature egg.  the cell with the least cytoplasm becomes the second polar body. D) Gamete Development

 cytoplasm divides unequally again.  the cell with the most cytoplasm becomes the mature egg.  the cell with the least cytoplasm becomes the second polar body.

cool facts about oogenesis  asymmetrical cytokinesis allows for one egg to have a large amount of nutrients (cytoplasm) for the zygote to use prior to implantation  meiosis I and II is not continuous primary oocytes begin meiosis I before birth but stalls in prophase I until puberty. secondary oocytes stalls at metaphase II until fertilization occcurs.  if fertilization does not occur meiosis II will not be completed. D) Gamete Development