Meiosis Gamete Production

Occurs in specialized cells of the reproductive organs Mitosis vs. Meiosis Occurs in all cells Occurs in specialized cells of the reproductive organs All new cells are identical (have the original number of chromosomes) New cells are NOT identical  have half the original number of chromosomes 2 sets of chromosomes (eg. humans have 2 sets of 23) 1 set of chromosomes (sperm & egg have 1 set of 23) Used in somatic (body) cells for growth, maintenance, repair Used in gonads (ovaries and testicles) to produce gametes

Gonad (Ovary or Testes) Cells Gametes (Sperm and Eggs) Mitosis vs. Meiosis PMAT (mitosis) P1M1A1T1 + meiosis P2M2A2T2 2 sets of chromosomes (2N) 1 set (N) Somatic (body) Cells Gonad (Ovary or Testes) Cells Gametes (Sperm and Eggs)

Meiosis - Terminology Meiosis: a specialized form of cellular reproduction that is used to create reproductive cells called ‘gametes’ Gametes: reproductive cells; each contains one ‘set’ of hereditary material (chromosomes) eg. sperm/egg in animals or pollen/egg in plants Fertilization: joining of a male (♂) gamete and a female (♀) gamete to form a ‘zygote’ Zygote: a fertilized egg

Meiosis - Terminology Zygote (a fertilized egg) the first cell of the new individual contains 2 ‘sets’ of chromosomes (2N), one set from each parent (paternal + maternal chromosomes) Zygote Set Fertilization Mitosis etc Male Gamete Female Gamete

Meiosis - Terminology Haploid #: Number of chromosomes in one set  also called ‘N’ Since gametes contain one set of chromosomes, they are said to be ‘haploid’ Diploid #: Number of chromosomes in two sets  also called ‘2N’ Since zygotes contain 2 sets of chromosomes, they are said to be ‘diploid’ Haploid and diploid numbers are species specific

Haploid and diploid numbers are species specific For humans: diploid number (2N) = 46 chromosomes haploid number (N) = ____ chromosomes For chickens: diploid number (2N) = 78 chromosomes For rice: diploid number (2N) = 24 chromosomes

Haploid-N & Diploid-2N Male Gamete Fertilization N Mitosis 2N etc Zygote N 2N Fertilization Mitosis etc Male Gamete Female Gamete

Meiosis - Terminology Homologous chromosomes each chromosome in the paternal set has a similar (but not identical) “partner” in the maternal set the “partner” chromosomes have a similar physical appearance and carry genes for the same proteins the genes (‘recipes’) for the proteins may not be identical since the chromosomes are from different individuals maternal homologue + paternal homologue = a homologous pair of chromosomes

Human Chromosomes

Meiosis involves 2 successive cell divisions results in the formation of 4 haploid (N) cells called gametes in animals, the gametes are ova (eggs) and sperm ova are produced in the ovaries in a process called oogenesis sperm are produced in the testes in a process called spermatogenesis occurs in 2 phases: Meiosis I and Meiosis II

Spermatogenesis & Oogenesis Sperm formation Egg formation

Interphase before Meiosis I During the S phase of interphase, the DNA replicates and the identical sister chromatids stay attached at the centromere

Prophase I: Synapsis the pairs of duplicated homologous chromosomes line up next to each other to form “tetrads” the homologous chromosomes in each pair are similar to each other, but not identical: Sister chromatids Homologous Pair of Chromosomes Duplicated paternal chromosome Duplicated maternal chromosome

Prophase I: Crossing Over During Prophase I, homologous chromosomes pair up in the cytoplasm to form tetrads Areas of homologous chromosomes connect at areas called chiasmata Crossing over happens at the chiasmata

Prophase I: Crossing Over Segments of homologous chromosomes break and re-attach at similar locations Results in new genetic combinations in the offspring This is the main advantage of sexual reproduction

Prophase I: Crossing Over at Chiasmata

Prophase I: Results of Crossing Over Crossing over of non-sister chromatids allows exchange of maternal and paternal genes and creates four different chromatids instead of two, increasing genetic diversity homologous pairs of replicated chromosomes line up to form tetrads. This is called ‘synapsis’. chromatids cross over at chiasmata chromatids “break” and rebond at specific locations to create two new combinations of genes

Metaphase I after crossing-over is complete, the homologous chromosomes remain close together as tetrads spindle fibers line the tetrads up along the equator, with one homologous pair on each side of the equator the tetrads arrange themselves randomly – with a mixture of paternal and maternal homologous chromosomes on each side of the equator this random arrangement is called “random segregation” and further “mixes up” the maternal and paternal genes

Metaphase I Independent assortment introduces many possible combinations of maternal and paternal genes, increasing the diversity of the offspring For humans, there are (2)23 possible combinations.

Anaphase I During Anaphase I, one half of each tetrad (one homologous pair) is pulled to each pole of the cell. Unlike mitosis, the centromeres DO NOT BREAK, so the sister chromatids stay together Each pole only gets half of the original chromosomes Telophase I and cytokinesis I may or may not occur completely

Meiosis II There is no interphase between Meiosis I and Meiosis II so the DNA does NOT replicate Meiosis II follows the same steps as regular mitosis During Anaphase II, the centromeres between the sister chromatids break and one sister chromatid is pulled to each pole of the cell Nuclei reform and cytokinesis usually occurs (although it may be unequal). Sister chromatids

Overview of Meiosis

Benefits of Meiosis In mitosis (asexual reproduction), all offspring are virtually identical to the parents. If there is a change in the environment, all offspring are equally “fit” or “unfit” to cope. In meiosis (sexual reproduction), each offspring has significantly different combinations of genes from its parents and its siblings. This genetic diversity allows some individuals to cope better with challenges and change, in the hope that some individuals will survive and keep the species from going extinct.

Animation of Meiosis http://www.cellsalive.com/meiosis.htm

Meiosis video http://www.learn360.com/ShowVideo.aspx?ID=754670&SearchText=meiosis&lid=13840772