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Meiosis

Meiosis is a cell division process that occurs in gametes, reducing the number of chromosomes in half. This process ensures that genetic information is passed on from one generation to another and is used to make a wide range of proteins. This article explores the reasons for reducing chromosomes and explains the role of meiosis in sexual reproduction and genetic variation.

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Meiosis

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  1. Meiosis Cell Division that occurs in gametes - reducingthe number of chromosomes in half

  2. Chromosomes contain Inherited Information • Each of our cells contain chromosomes • GeneticInformationpassed on from one generation to another • This information is used to make a wide range of proteins (genetic blueprints) • Proteinshelp determine individual characteristics

  3. WhyReduce Chromosomes? • 1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? • 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Would it be of the same species? • 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

  4. It’s All About Chromosomes • DiploidNumber (2n) – Refers to the number of chromosomes that exist in all body cells (this number variesfrom species to species) • Humans have 2n= 46 (or 2X 23) • Each individual resulting from sexual reproduction receives half of their diploidnumber from each parent • 23 from mom + 23from dad = 46 total

  5. Fertilization Restores the Diploid Number n + n = 2n • When gametes unite, their chromosomes add together to provide the offspring with the correct diploid number ofchromosomes specific for that species. 23 + 23 = 46 n Sperm (23) Egg (23) n

  6. Zygote (2n Restored) • At fertilization, the DNA of a single sperm and ovum merge to create the genetic blueprint for a new, unique individual. Once the DNA has combined and the single-celled ovum begins to divide, it is called a zygote. Zygote (2n)

  7. Each species has a specific number of chromosomes Chromosome number seem unrelated to complexity

  8. Sexual Reproduction and Meiosis • For most animals and plants, it takes two to reproduce. Sexual Reproduction: - Helps us understand how variations occur among living things • Plays a Significant role in process of Evolution • To understand genetic variations, we need to closely examine the chromosomes during Meiosis, Fertilization, and Inheritance

  9. Meiosis I2n-- n Diploid number = 4 Metaphase I Interphase Prophase I Anaphase I Homologous Pairs form “tetrads” and Exchange genes “crossing-over” Chromosomes replicate Double-stranded Homologous pairs move to Opposite poles

  10. Crossing-over: Mixing-up Gene Combinations During ProphaseI Double-stranded Homologous Pairs Homologues Crossing-over Genes are exchanged Genetic Combinations Have been changed Homologues “find each other” and form A tetrad

  11. Meiosis I… Summed Up • Chromosomes are double-stranded • Homologous Chromosomes form tetrads • Crossing –over occurs during Prophase • Genesare exchanged • Original genetic make-up changes • Diploid number reduced to Haploid • Chromosomes still DOUBLE-STRANDED • Homologous Chromosomesrandomly separate (Disjunction) to opposite poles

  12. Meiosis II Two Daughter cells enter Second Stage of Cell Division Prophase II Anaphase II Metaphase II Double-stranded Chromosomes line-up Each cell has One set of chromosomes (n) Double-stranded chromosomes Separate

  13. Meiosis I & II … summed up • Daughter cells produced from Meiosis I have haploidnumber of chromosomes • Each cell has only one set of chromosomes (n) • Chromosomes still Double-stranded • Double-stranded chromosomes separate (anaphase II) • Total of Fourhaploid cells produced at the end of Meiosis II (single-stranded)

  14. Prophase I (up close) • Note: double-stranded chromosomes • Homologous pairs in Tetrads • Crossing-over

  15. Metaphase I • Homologous pairs Line-up, side-by-side • NOT one under the other • NO particular arrangement

  16. Anaphase I • Double-stranded Homologous Pairs Separate • Called Disjunction Each cell Having only One set

  17. Teleophase I • Two daughter cells with haploid number of chromosomes (but still double-stranded)

  18. Metaphase II • DS line-up, one under the other

  19. Anaphase II Double-strands separate

  20. Four haploid Cells, each Having one set Of single-stranded chromosomes

  21. All Together Now …. • One diploid parent (DS)---4 Haploid (SS

  22. Problems During Meiosis • Errors during Prophase I • Crossing-over while in tetrad formation can result in uneven exchange of genes between homologous pairs (Recall: chromosomal mutations) • inversions • additions • deletions

  23. Problems cont… • Nondisjunction( when chromosomes fail to separate during Anaphase) • Abnormal Number of chromosomes enter gametes. Some have too many,Others have too few. • Examples: • Down Syndrome (trisomy 21)- nondisjunction of chromosome # 21 during anaphase II • Turner’s Syndrome -have only one X (sterile females) • Klinefelter’s Syndrome – XXY genotype (sterile males)

  24. NOW .. Let’s Talk Gametogeneis • As we now know, gametes are our sex cells, sperm and egg (ovum) • Meiosis occurs in gametes (reducing the Diploid# to Haploid# of chromosomes). • BUT, there’s more to process of making gametes than just reducing the number of chromosomes!!

  25. Spermatogenesis – Sperm production • During meiosis of spermatogenesis, the cytoplasm divides EQUALLY; FORMING 4 SPERM OF EQUAL SIZE. • Occurs in the Testes

  26. Oogenesis – producing mature ovum (egg) • During oogenesis, the Cytoplasm is NOT divided Equally between the cells During Meiosis • Only one ovum made • Three Polar Bodies • Occurs in the Ovaries http://www.sumanasinc.com/webcontent/animations/content/meiosis.html

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