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Meiosis and Heredity

Meiosis and Heredity. Honors Biology - rest of Chapter 8. Heredity. Passing along the traits of parents to the offspring How ? Through sexual reproduction (meiosis ). Terms. Chromosome – rod-shaped structures made of DNA and proteins (histones)

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Meiosis and Heredity

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  1. Meiosis and Heredity Honors Biology - rest of Chapter 8

  2. Heredity • Passing along the traits of parents to the offspring • How? Through sexual reproduction (meiosis)

  3. Terms • Chromosome – rod-shaped structures made of DNA and proteins (histones) • Histones – in eukayrotes; helps chromosome keep it’s shape

  4. Chromosomes • Sex chromosomes (X or Y in humans) • Autosomes – all other chromosomes • Humans - 46 total (occur in pairs – 23 pairs) Females - XX Males - XY

  5. Chromosomes • Every cell has 2 copies of each autosome (1 from dad, 1 from mom) • Homologous chromosomes - (or pairs) • Are same size, shape, carry genes for same traits

  6. Karyotype • Used to look at a person’s chromosomes to detect problems • Shows the 46 human chromosomes as 22 homologous pairs of autosomes + 2 sex chromosomes

  7. Cell Types • Diploid cells (2n) - cells with 2 sets of chromosomes (all human cells except...) • Haploid cells (1n) - cells with 1 set (sperm and eggs)

  8. Haploid cells • Have only 23 total chromosomes • Haploid + haploid = a diploid cell results • Haploid cells are the products of meiosis

  9. Cell Division • Mitosis - most body cells (asexual reproduction) • Meiosis - (sexual reproduction) - occurs when gametes form • Gametes - Egg and sperm

  10. Preparing for meiosis • Cells go through interphase just like before mitosis • DNA is duplicated • Only difference - this happens in reproductive cells (ovary and testes)

  11. Meiosis • Diploid cells in ovaries and testes start dividing • Haploid cells (sperm and eggs) are made

  12. Meiosis Stages (pg. 162-163) • Meiosis 1 - similar to mitosis; 1 cell duplicates DNA then goes through 4 phases • Meiosis 2 - Cells continue dividing to make 4 haploid cells

  13. Meiosis 1 • Prophase 1 – duplicated chromosomes line up next to it’s homologue (stage is called synapsis) • Each pair called a tetrad • Mitosis - same things happen here (except pairing up is unique to meiosis) Tetrad 2n

  14. Prophase 1 • During synapsis, chromatids twist around each other and may exchange portions with adjacent chromatids • Results - new combination of genes • Genetic recombination – of maternal and paternal DNA Crossing-over 2n

  15. Metaphase 1 • Tetrads line up along equator • Spindle fibers attach to centromere of one homologous pair 2n

  16. Anaphase 1 • Each homologous pair is pulled toward each pole (pair still attached by centromere) • Independent assortment – random separation of pairs = genetic variation 2n

  17. Telophase 1 & Cytokinesis 1 • Chromosomes reach opposite poles of cell • New cells are now haploid = half total # of original chromosomes • But...chromosomes present still as a pair 1n

  18. Meiosis 2 • Cells from Meiosis 1 continue dividing • Results in formation of gametes

  19. Prophase 2 • Spindle fibers form and nuclear membrane dissolves (in some species it never forms at end of Meiosis 1) • DNA is NOT duplicated again 1n

  20. Metaphase 2 • Chromosome pair moves to equator • Each chromatid faces opposite poles; still attached centromeres 1n

  21. Anaphase 2 • Chromatids now separate and move toward poles 1n

  22. Telophase 2 & Cytokinesis 2 • Each of the two cells now separate, forming 4 new cells • New cells are haploid and chromosomes are no longer in pairs 1n

  23. Meiosis in males • Reproductive cells form four haploid cells called spermatids. • Each spermatid develops into mature sperm cells = spermatogenesis

  24. Meiosis in females • Oogenesis = Cells make only 1 mature cell out of 4 (ovum) • Cytokinesis 1 and 2 - cytoplasm divides unequally; only 1 cell matures to form an egg • Other 3 cells called polar bodies - later dissolve

  25. Sexual reproduction • Union of sperm and egg to form offspring • Offspring - genetically different from parents (as result of crossing over and random separation of chromosome pairs)

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