Meiosis Cell division for the formation of egg

Meiosis Cell division for the formation of egg cells and sperm cells

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Mitosis occurs to help organisms grow, heal or for asexual reproduction. It makes more somatic cells. Daughter cells are clones of each other And of the original parent

Somatic cells are the cells that make up your body; they are body cells. Cells in ovaries and testes will become reproductive cells; these are called germ cells.

After Interphase, somatic cells will divide by mitosis; germ cells will divide by meiosis. ells: c c i t Soma osis Mit Germ cell s: Meiosis Interphase

Meiosis is a specialized type of cell division that occurs in germ cells and forms egg and sperm cells. Daughter cells are unique to each other And have half the DNA of the original parent

Why must reproductive cells have half the DNA of normal body cells?

Because sperm and egg cells only have one set (half) of the genetic material, they are called haploid (n) cells. In humans, one set of chromosomes consists of 23 chromosomes. The haploid number (n) equals 23.

When a haploid egg cell is joined by a haploid sperm cell, the result is a diploid (2 n) cell. This is because it has two complete sets of chromosomes.

Each chromosome from your mother corresponds to a chromosome from your father. Corresponding chromosomes are called homolgous chromosomes.

Karyotype Photograph of chromosomes grouped in order of pairs

Interphase Just like before Mitosis, Interphase includes Growth, Protein Synthesis and DNA Replication.

DNA Replication

Tetrads form when homologous chromosomes have paired up in the cell. (4 sister chromatids lined up)

Crossing Over: When segments of chromosomes break off and reattach to homologous chromosomes: exchanges genetic material.

Interphase Meiosis II

Meiosis Prophase I Tetrads form Crossing over occurs Nuclear envelope breaks down Spindle fibers form Metaphase I I Anaphase I Tetrads line up Homologous in the middle chromosomes Spindle fibers separate connect to Spindle fibers centromere of shorten chromosomes Telophase I Homologous chromosomes are forming new nuclei Cytokinesis forms 2 Haploid cells Meiosis II

Meiosis II Prophase II Metaphase II Anaphase II Nuclear envelope breaks down Spindle fibers form Chromosomes line up in the middle Spindle fibers connect to centromere of chromosomes Sister Chromatids separate Spindle fibers shorten Telophase II Chromosomes are forming new nuclei Cytokinesis forms 4 Haploid cells Spermatogenesis

Meiosis Overview: ØStart: Diploid germ cell with homologous chromosomes (2 n=46) ØReplicate DNA: still diploid, now 2 identical copies of DNA ØMeiosis I: homologous chromosomes separate, now haploid (n=23), sister chromatids attached ØMeiosis II: haploid (n=23), sister chromatids separate

See it in action…

Gamete Formation: Spermatogenesis 1 st Meiotic division (haploid) (diploid male germ cell) After DNA Replication (diploid) 2 nd Meiotic division (haploid) Finished Sperm cells (haploid)

Gamete Formation: Oogenesis first polar body (haploid) (diploid female germ cell) three polar bodies (haploid) After DNA Replication (diploid) 1 st Meiotic Division (haploid) 2 nd Meiotic Division (haploid) Genetic Variation

Meiosis increases genetic Fac t variation in offspring with or thes e in Cr oss Independent assortment ing O › Each gamete you produce contains one of roughly 8 million possible combinations of chromosomes inherited from your mother and father. Random fertilization › The random fusion of a single sperm with a single ovum during fertilization will produce a zygote with any of about 64 trillion (8 million × 8 million) combinations of chromosomes! ver

1 2 3 Two types of gametes Independent assortment: Chromosomes line up during metaphase I independently of each other. …the resulting gametes have a different assortment of chromosomes

Random fertilization further increases genetic variation

Mitosis and Meiosis work together in the human life cycle

Mitosis vs. Meiosis Mitosis Meiosis What is produced? More Cells New Organism Cell Similarity Identical Cells Similar Cells Cell Type Somatic ‘body’ Cells # of Divisions 1 Division Gametes- sex cells Egg, Sperm 2 Divisions Diploid/Haploid Cells 2 Diploid Cells 4 Haploid Cells Chromosome # 46 chromosomes/cell N? 2 N- 2 copies 23 Chromosomes/cell N- 1 copy

Mitosis and Meiosis are two different types of Cell Division In Mitosis Cell division makes two clones of the parent cell For growth, healing and asexual reproduction Maintains the same number of chromosomes In Meiosis Cell division makes four unique daughter cells With half of the DNA of the parent cell To make sperm cells and egg cells (gametes) Reduces the number of chromosomes by half

If Things go Wrong with Meiosis The cause of chromosomal disorders

Karyotype

Nondisjunction Spindles do not attach properly during metaphase, and chromosomes or chromatids do not separate. Results in too many or not enough chromosomes in gametes

Klinefelter’s Syndrome: XXY Turner’s Syndrome: X Down’s Syndrome: Trisomy 21

Structure Alterations occur because of mistakes in Crossing Over Deletion When part of the chromosome breaks off Duplication When part of the chromosome repeats itself Inversion When two parts of a chromosome flip Translocation when one part of a chromosome breaks off, then reattaches in the wrong place

Can you identify the chromosomal alterations?

Cri du Chat Syndrome: Deletion from Chromosome 5 Emanuel Syndrome: Translocation of 11 and 22 Chromosome 15 q Duplication Syndrome: Diagnosed with Autism

In Conclusion… Meiosis occurs to form gametes for sexual reproduction It involves forming 4 haploid daughter cells Because of Crossing over, Independent Assortment and Random Fertilization, it increases genetic variability Nondisjunction can result in the incorrect number of chromosomes.

Vocabulary to pay special attention to: Somatic cells Gametes Haploid (n) Diploid (2 n) Homologous chromosomes Tetrad Crossing over