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Meiosis Chapter 13. Sexual Life Cycles. Ppt courtesy of Tracy Jackson http://home.att.net/~tljackson/neville.html. Introduction to Heredity. Heredity is the transmission of traits from one generation to the next. Variation – differences in characteristics seen in parents and offspring.

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MeiosisChapter 13

Sexual Life Cycles

Ppt courtesy of Tracy Jackson http://home.att.net/~tljackson/neville.html


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Introduction to Heredity

  • Heredity is the transmission of traits from one generation to the next.

  • Variation – differences in characteristics seen in parents and offspring.

  • Genetics is the study of heredity and hereditary variations.


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Heredity

  • Genes are the hereditary units of coded information (for protein synthesis) passed from parent to offspring.

  • A genome is all of the genes of an individual or species, etc.

  • Genes are segments of DNA and the information is contained in the sequence of nucleotides in the DNA – A, T, C, and G.


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Reproduction

Reproduction can be asexual and sexual.

  • Sexual reproduction is when gametes come together to form a fertilized egg. This form of reproduction imparts genes from both parents and results in much variation.

  • Asexual reproduction is when a single individual divides, buds, or undergoes fission to form a new individual and passes an exact copy of its genes to the new offspring.


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Life Cycles

  • A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.


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The Human Life Cycle

  • Somatic cells are cells of the body other than the sex cells (sperm and egg).

  • Somatic cells have 46 chromosomes.

  • There are 22 pairs of autosomes (non-sex chromosomes) and 1 pair of sex chromosomes.

  • For sex chromosomes, XX = female and XY equals male.


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Chromosomes

  • Chromosomes are in pairs. The pairs that are alike are called homologous chromosomes.

  • A karyotype is a picture of the chromosomes – arranged by pairs.

  • Sex chromosomes are not homologous for human males. While females are homologous with XX, males have XY.




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Chromosome Number

  • Haploid and diploid are terms referring to the number of sets of chromosomes in a cell.

  • Gregor Mendel determined his peas had two sets of alleles, one from each parent.


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Diploid

  • Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid.

  • We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes.


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Haploid

  • Haploid organisms/cells have only one set of chromosomes, abbreviated as n.

  • Organisms with more than two sets of chromosomes are termed polyploid.

  • Chromosomes that carry the same genes are termed homologous chromosomes.

  • The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals.

  • Organisms (normally) receive one set of homologous chromosomes from each parent.


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Meiosis

  • Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete".

  • Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.).

  • Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination occurs the ploidy of the parents will be reestablished.


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Phases of Meiosis

  • Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division).

  • Meiosis produces 4 haploid cells. (Mitosis produces 2 diploid cells.)

  • Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division).


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Prophase I

  • Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of homologous chromosomes.

  • Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure.


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Crossing Over

  • Crossing-over may occur at this point. During crossing-over chromatids break and may be reattached to a different homologous chromosome.


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Metaphase and Anaphase I

  • Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair. Other metaphase events as in mitosis.

  • Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact.


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Telophase I

  • Telophase I is similar to Telophase of mitosis, except that only one set of (replicated) chromosomes is in each "cell". Depending on species, new nuclear envelopes may or may not form. Some animal cells may have division of the centrioles during this phase.


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Prophase II

  • During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis.


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Metaphase and Anaphase II

  • Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region.

  • During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.


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Telophase II

  • Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.


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Meiosis Animations

Frame-by-frame animation

  • http://www.csuchico.edu/~jbell/Biol207/animations/meiosis.html

    Continuous animation – no words

  • http://www.people.virginia.edu/~rjh9u/meioanim.html

    Continuous frame-by-frame

  • http://www.trentu.ca/biology/101/14.html


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Gametogenesis

  • Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line.

  • Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells.


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  • Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries.

  • Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis places most of the cytoplasm into the large egg.

  • The other cells, the polar bodies, do not develop. Thus, all the cytoplasm and organelles go into the egg. Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.


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