Chapter 13: Meiosis and Sexual Life Cycles

1. Chapter 13: Meiosis and Sexual Life Cycles

2. Essential Knowledge 3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (13.1-13.3). 3.c.2 – Biological systems have multiple processes that increase genetic variation (13.4).

3. Heredity • The transmission of traits from parents to offspring • Comment - Humans have been aware of heredity for thousands of years • Also known as inheritance

4. Genetics • The scientific study of heredity • Comment - Genetics is only about 150 years old • NEW/MODERN field of science • DNA was only discovered in 1950s • Only 60 years ago!

5. Chromosome Review • Chromosomes (23 pairs=46) • Inherit one chromosome of each pair from each parent • Total of 46 (23 maternal, 23 paternal) • 1 pair/2 = Sex chromosomes • 22 pairs/44 = Autosomes

6. Genes • The DNA for a trait • Locus - the physical location of a gene in a chromosome • Genes program traits • How? Program cells (through protein synthesis) to make specific proteins • These proteins become “visible” to us through outside traits

7. Reproduction • A method of copying genes to pass them on to offspring • Fertilization: male/female gametes unite during reproduction • Results in zygote (fertilized egg) • Two main types: • Asexual reproduction • Sexual reproduction

8. Asexual Reproduction • Parent passes all of its genes to its offspring • Offspring are exact copies of parents • Also known as cloning (clones) • Uses mitosis • Often called budding • Comment - many organisms reproduce this way • Ex: jellyfish, hydra, sponges

9. Asexual Bud

10. Advantages of Asexual • Only need 1 parent • Offspring are identical to the parent • Good genetic traits are conserved and reproduced • Usually requires less energy • Why?

11. Disadvantages of Asexual • No new DNA combinations for evolution to work on • Only genetic differences come from DNA mutations (during DNA replication, etc) • Clones may become extinct if attacked by a disease or pest

12. Sexual Reproduction • Two parents contribute DNA to an offspring • Comment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others

13. Sex Chromosomes • Homologous? • Mostly no! Only a SMALL part of these 2 are homologous • Males: One X chrom, One Y chrom • Y chromosomes are smaller (with more genetic disorders) • Females: Two X chromosomes • X chromosomes are bigger (usually contain more genes)

15. Egg vs. Sperm • Eggs contain ONLY X chromosomes • Sperm contain EITHER X or Y chromosomes • Why? • Because boys (when they are made during fertilization) get an X chromo from mom and a Y chromo from dad – making them a MALE! • If the cell were to get two X chromos, the fertilized cell would become a FEMALE!

17. Who determines sex of baby? • Males determine what sex the offspring will be • Why? • Their sperm have the “option” of having X or Y • X from egg and Y from sperm = XY (male) • X from egg and X from sperm = XX (female)

18. Advantages of Sexual • Offspring has a unique combination of DNA which may be an improvement over both parents • New combination of DNA for evolution to work with

19. Disadvantages of Sexual • Need two parents (requires time and energy to look/find the right mate) • Good gene combinations can be lost • Offspring may not be an improvement over the parents

20. Question ? • Do parents give their whole DNA copy to each offspring? • What would happen to the chromosome number if they did?

21. Chromosome Number • Is usually constant for a species • Two of these are usually sex chromosomes • XY or XX in humans • Examples: • Humans – 46 (pair #23 is sex) • Corn - 20 • Onions - 16 • Dogs - 72

22. Two options for life cycle… 1) Mitotic cell division continues life 2) Meiosis cell division continues life WHICH way does life reproduce?

23. Result of Life cycle: if Mitosis • 46 in egg + 46 in sperm= 92 total chromosomes • Chromosome number would double each generation. • Need a method to reduce the chromosome number.

24. Result of life cycle: if Meiosis • Egg 23 + sperm 23 = 46 total chromosomes • Chromosome number will remain the same with each sexual reproduction event. • Meiosis is used to produce the gametes or sex cells. • 46 reduced to 23 through meiosis YES!!

25. Meiosis - Purpose • To reduce the number of chromosomes by half • 46 to 23 (in humans) • Prevents doubling of chromosome numbers during sexual reproduction

27. Sexual Life Cycle • Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species • Fertilization: 23 + 23 = 46 total • Mitosis: takes original cell made through fertilization and multiplies it to form organs, etc. • Meiosis: 46 reduced to 23 (in sex organs)

29. Life Cycle Variations

30. Diploid • 2 sets of chromosomes • Referred to as 2n • Most common number in body or somatic cells • Humans 2n = 46 • Corn 2n = 20 • Fruit Flies 2n = 8

32. Haploid • 1 set of chromosomes • Number in the gametes or sex cells • Humans n = 23 • Corn n = 10 • Fruit Flies n = 4 • Half of diploid #!!!!!!!

33. Polyploids • Multiple sets of chromosomes • Examples • 3N = triploid • 4N = tetraploid • Common in plants, but usually fatal in animals • Ex: Down Syndrome • Ex: Seedless watermelons

36. Triploid watermelon = no gametes produced (no seeds) Triploid Daffodil = more stem produced per bulb

37. Meiosis/Mitosis Preview of differences • LOOK at pg. 256 • Meiosis • Two cell divisions, not one • Four cells produced, not two • Synapsis and chiasmata will be observed

38. Meiosis/Mitosis Preview of differences • Meiosis, cont. • 1st division separates PAIRS of chromosomes, not duplicate chromatids (of chromosome) • Known as homologous pairs of chromosomes • Interkinesis is present. Mitosis vs. Meiosis Animation

39. Meiosis • Has two cell divisions (I and II) • Steps follow the names for mitosis • A “I” or “II” will be added to label

40. Replicated chromatids(dupl. during Inter)

41. Prophase I • Basic steps same as in prophase of Mitosis. • Synapsis occurs as the chromosomes condense. • Synapsis - homologous chromosomes form bivalents or tetrads • Synaptonemalcomplex - proteins which hold chromosomes together

42. Tetrad

43. Prophase I, cont. • Chiasmataobserved • Spot where chromatids cross-over • Held together by this until Anaphase • Longest phase of division • Cell spends 90% of meiosis in this stage • Chromatin condenses, nucleolus disappears, nuclear envelope dissolves

44. Metaphase I • Tetrads or bivalents align on the metaphase plate • Attached to kinetochores of opposite poles • Centromeres of homologous pairs point toward opposite poles

45. Anaphase I • Homologous PAIRSseparate • Uses spindle apparatus • Duplicate chromosomes are still attached at the centromeres • Maternal and paternal chromosomes are now separated randomly

47. Telophase I • Similar to Mitosis • Chromosomes may or may not unwind to chromatin • Cytokinesis separates cytoplasm and 2 cells are formed • NOW: • Each cell has a haploid set of chromosomes • Each chromosome is STILL comprised of 2 identical sister chromatids!!!

48. Interkinesis • No DNA synthesis/copying occurs • May last for years, or the cell may go immediately into Meiosis II • Can appear similar to Interphase of Mitosis

49. Meiosis II • Steps are the same as in Mitosis • Prophase II = Prophase • Metaphase II = Metaphase • Anaphase II = Anaphase • TelophaseII = Telophase

50. Meiosis - Results Meiosis animation • 4 cells produced • Each cell has ONE sister chromatid • Remember: Started with homologous chromosome PAIR • Chromosome number reduced • Gametes (sex cells) made • Genetic variation increased