scienceclarified

1. Cellular Basis of Reproduction and InheritanceLecture 9, Meiosis http://www.scienceclarified.com

2. Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

3. Outline • Karyotypes • Sexual lifecycle • Fertilization • In vitro fertilization • Meiosis • Genetic variation • Chromosomal disorders • Sexual differentiation • Polyploid organisms • Words and terms to know • Possible test items

4. A Time of Wonder http://nmhm.washingtondc.museum

5. Sexual Reproduction http://about.biology.com Last week we discussed asexual reproduction—this week we cover some of the aspects of sexual reproduction.

6. Homologous Chromosomes • Almost all chromosomes have a ‘twin’ that matches in shape, size, and bands. • The pair are known as homologous since each chromosome carries the same sequence of genes controlling an inherited characteristic. • The genes for eye color, for example, are found at identical locations in homologous pairs. • The instructions in each matching gene are different since they are inherited from each parent.

7. Pairings • In both sexes, one chromosome from each homologous pair is inherited from the mother and the father. • The 46 chromosomes in a human female can be organized into an array of 23 homologous pairs. • In the human male, one of the 46 pairs does not match—the unmatched pair is the male’s sex chromosomes.

8. Karyotype • A typical body cell in humans, called a somatic cell, has 46 chromosomes • If the cell is opened during mitosis, a micrograph of the chromosomes can be made. • The individual chromosomes are arranged in an array called a karyotype.

9. Unordered Chromosomes http:www.biotechnologyonline.gov Micrograph of chromosomes during mitosis.

10. Karyotype http://www.ucl.ac.uk An ordered array of chromosomes.

11. Sex Chromosomes and Autosomes • This 23rd pair—the sex chromosomes—determines the genetic sex of the human. • Genetic females usually, but not always, have two X chromosomes. • Genetic males usually, but not always, have X and Y chromosomes. • The remaining 22 pairs, common to both females and males, are the autosomes.

12. Sexual Life Cycle • The sexual lifecycle is the sequence of biological stages from the adults of one generation to the adults of the next generation. • Paired chromosomes—one inherited from each parent—are are found in all species that reproduce sexually.

13. Diploid • Humans, and most animals and plants, are diploid organisms because all body cells contain paired sets of homologous chromosomes. • The number of pairs is represented by n (in humans, n = 23 ). • The number of chromosomes (46) is the diploid number, 2n. • Exceptions to this rule are egg and sperm cells, known as gametes.

14. Haploid • Gametes, formed by meiosis in ovaries and testes, contain one member of each homologous chromosome pair. • Gametes are haploid since they contain one-half the number of chromo-somes found in body cells. • The total number of chromosomes in human gametes (23) is the haploid number, n. Spermatozoa (sperm) http://zoology.unh.edu

15. Fertilization • A sperm cell fuses with an egg cell, known as an ovum, through the process of fertilization. • Each gamete is haploid, and the fertilized egg, known as a zygote, is diploid • The two sets of homologous chromosomes—one member of each pair is contributed by each parent. • Mitotic cell division assures all body cells receive a complete copy of the 46 chromosomes. • Every one of 60+ trillion cells in the human body can be traced to a single zygote.

16. Alternating Stages • The sexual lifecycles over generations involve alternation between diploid and haploid stages. • The process of meiosis keeps the number of chromosomes from doubling each generation. http://iep.water.ca

17. Path of the Egg Fallopian tube Uterus Ovary http://www.ehd.org Cilia help move the egg down the fallopian tube once it is released from one of the ovaries.

18. Sperm Abnormal shapes http://spermomax.net Apparently healthy sperm http://www.cit.astate.edu

19. Sperm and Egg http://nmhm.washingtondc.museum Many sperm are present, but only one can fertilize the egg.

20. Two-Cell Stage http://www.midwesttiv.com The first day after fertilization—mitoticcell division has begun.

21. Eight-Cell Stage http://fig.cox.miami.edu Three-day-old human embryo

22. Embryonic Growth Eight-week-old human embryo http://nmhm.washingtondc.museum http://library.thinkquest.org Five-week-old human embryo

23. In Vitro Fertilization • In vitrofertilization is a medical technique for fertilizing an egg outside of the woman’s reproductive tract. • IVF can be used when a couple cannot conceive through sexual inter-course.

24. IVF Procedures • IVF involves stimulating the ovaries with hormones to release ova, harvesting them, and fertilizing them with sperm in a fluid medium. • The embryois transferred to the mother once the uterus has been readied hormonally. • Precision techniques are sometimes used to insert the sperm into the ovum.

25. IVF Procedures http://www.ivi.es Traditional IVF method is shown.

26. IVF Microtechnique A more recent, precision approach to IVF http://www.tylermedicalclinic.com http:www.stoeltingco.com

27. Success Rates • The success rate of IVF is about 33 percent, although it can be lower for older women. • Multiple eggs, to improve the likelihood of pregnancy, may result in more than one embryo. • A woman may provide her own eggs or rely on a donor if one is available.

28. Ethical Considerations The use of donor eggs can involve ethical and legal considerations. http://www.chass.ncsu.edu Branching tree of decision possibilities—some medical situations are ethically complex.

29. Meiosis • Meiosis—the basis of sexual reproduction—resembles mitosis, but it has two additional features: • Halving of the number of chromosomes (2n is reduced to n). • Exchange, or crossing-over, of genetic material between the homologous pairs of chromosomes. • The gametes undergo two consecutive divisions in meiosis I and II. • Four daughter cells result, each with one-half as many chromosomes (n) as the starting cell (2n). Meiosis takes place in the testes and ovaries—mitosis occurs in body cells.

30. Importance • Meiosis I is the basis of sexual reproduction in eukaryotic organisms (animals, plants, and fungi). • Each offspring inherits a unique combination of genes from the two parents. • Unlike asexual reproduction, offspring can show substantial genetic variation. In vitro fertilization http://medicineworld.org

31. Homologous Pairs • The two chromosomes in a homologous pair are the individual chromo-somes inherited from each of the parents. • Each homologous pair appears alike under a light microscope, although they have different versions of some of their genes.

32. Sister Chromatids • During the interphase, before meiosis begins, each chromosome in a homologous pair replicates to form sister chromatids of identical genetic content. • The sister chromatids remain together until the end of meiosis. • Before crossing-over, sister chromatids are identical and carry the same versions of all their genes.

33. Interphase and Meiosis I http://www.mun.ca

34. Interphase • Meiosis is preceded by interphase when homologous chromosomes are duplicated, just as in mitosis. • Each duplicated chromosome consists of two identical sister chroma-tids. http://www.sinauer.com

35. Prophase I • Specialized proteins keep the homologous chromosomes together in pairs as the chromatin condenses. • The resulting structure of four chromatids is known as a tetrad. • Within each tetrad, the chromatids exchange segments in a process known as crossing-over. http://www.uic.edu

36. Prophase I • Crossing-over is key to genetic variation from generation-to-generation, and between siblings. • The process rearranges genetic information from the two parents, as we will discuss. • As prophase I continues, a spindle of microtubules form, and the tetrads are moved toward the cell’s equator. http://www.uic.edu

37. Meiosis I (continued) http://www.mun.ca

38. Metaphase I • The sister chromatids in the tetrad are attached at their centromeres, or waists. • The tetrads are aligned on the cell’s equator by the spindle anchored to the opposite poles of the cell. • The spindle is arranged so that the homologous chromosomes of each tetrad are posed to move to the opposite poles of the cell. http://www.uic.edu

39. Anaphase I • The microtubules in the spindle move the chromosomes toward the opposite poles of the cell. • Unlike mitosis, sister chromatids migrate as pairs rather than splitting up. • The sister chromatids are not separated from each other, but from their homologous partners. http://www.uic.edu

40. Telophase I and Cytokinesis • The sister chromatids reach the poles as a haploid set—the chromo-somes are still in duplicate form. • Cytokinesis forms two haploid daughter cells during Telophase I. • Depending on the species, the nuclei may or may not return to an interphase state. • No further chromosome duplication occurs in the subsequent stages of meiosis II. http://www.uic.edu

41. Meiosis II http://www.mun.ca

42. Meiosis II • Meiosis II is similar to mitosis, but it starts with a haploid cell (n) rather than a diploid cell (2n). • The processes of prophase II, metaphase II, anaphase II, telophase II, and cytokinesis are very similar to what was discussed in the lecture on mitosis. • Meiosis I results in two haploid daughter cells, and meiosis II doubles the number to four haploid daughter cells.

43. Mitosis versus Meiosis • Mitosis enables growth, tissue repair, and asexual reproduction by the production of daughter cells that are genetically-identical to the parent cell. • In comparison, meiosis enables sexual reproduction by the production of genetically-unique daughter cells known as zygotes (i.e., eggs and sperm). • In both mitosis and meiosis I, chromosomes duplicate only once during the interphase.

44. Mitosis versus Meiosis (continued) • Mitosis has one division of the cell nucleus and cytoplasm to produce two diploid daughter cells. • In comparison meiosis I and II have two divisions of the cell nucleus and cytoplasm to produce four haploid daughter cells. • All events unique to meiosis (that is, not occurring in mitosis), happen in the meiosis I stage.

45. Sources of Genetic Variation • Independence • Random fertilization • Crossing-over Each offspring will have substantial genetic variation from her/his parents and all siblings except an identical (monozygotic) twin.

46. Independence • The orientation of homologous chromosomes during meiosis I is a matter of chance similar to flipping a coin. • Each pair of chromosomes orients itself independently during metaphase I. • The total number of unique chromosome combinations in a gamete is 2n where n is the haploid number. • Since n = 23 in humans, over 223 combinations of pairings are possible. • Each gamete—egg or sperm—is one of over eight million (8 x 106) com-binations.

47. Random Fertilization • A human egg (8 x 106 possibilities) when fertilized by a sperm (8 x 106 possibilities) will produce one of over 6.4 x 1013 possible combinations. • The fertilization process adds a high degree of genetic variability to the offspring. 6.4 x 1013 = 64,000,000,000,000 possible combinations.

48. Crossing-Over Homologous chromosomes Tetrads Gametes http://regentsprep.org The processes of crossing-over and recombination provides even more possibilities for genetic variation.

49. Crossing-Over • In the prophase of meiosis I, the homologous chromosomes position themselves very precisely along their lengths in a gene-by-gene align-ment. • Crossing-over sites are X-shaped regions in homologous chromosomes. • The exchange of DNA segments at the crossing-over sites contributes to genetic variation. • Multiple cross-overs can occur in a tetrad, leading to even more genetic variation.

50. Genetic Recombination • Chromosomes resulting from the process of crossing-over are known as recombinant. • The genetic recombinations are different from the parent chromosomes. • A single cross-over can affect many genes because most chromosomes contain thousands of genes.