Meiosis

1. Meiosis Chapter 11

2. Review • Cell has issues when it grows larger in size • Not enough DNA • Nutrients and wastes cannot pass the cell membrane • Cell solves these problems through the process of cell division • Mitosis – the portion of cell division where the nucleus divides

3. Mitosis • Almost every cell of the body uses mitosis to divide the nucleus • Somatic cells – cells that are not sex cells/gametes • Exs) liver cell, bone cell, brain cell etc. AS LONG AS IT IS NOT SPERM OR EGGS IT USES MITOSIS • Cell grows (G1), synthesizes DNA (S), makes molecules and organelles (G2) and then is ready for cell division (mitosis and cytokinesis)

4. Stages of Mitosis PROPHASE • Prophase – chromatin condenses into chromosomes and nuclear envelope breaks down, centrioles move and spindle fibers form • Metaphase – chromosomes line up in middle of cell and spindle fibers attach to centromere

5. Stages of Mitosis • Anaphase – spindle fibers pull at centromere and separate sister chromatids pulling them to opposite ends of the cell • Telophase – chromosomes break down into chromatin and nuclear envelope reforms

6. The end of Mitosis • After telophase, the cells cytoplasm splits by the process of cytokinesis • As a result we are left with… 2 IDENTICAL DAUGHTER CELLS

7. The New Stuff… • There are many studies into the process that makes each one of us different • Gregor Mendel – a monk born in 1822 who did many studies in the field of genetics • Genetics – the study of heredity • Why is it that we have traits (eye color, hair color, etc.) similar to our parents, yet we are not all alike?

8. Genetics • Mendel recognized that the offspring of “parents” were similar and began to investigate why this happens • He came to the conclusion that the parent organisms must pass on traits in their genetic material • These traits are located on their genes (DNA)

9. Mendel’s Predictions • Mendel was correct about the passing of traits and the idea of genes • He wasn’t sure how these events happened but knew • An organism must inherit a single copy of every gene from both its “parents” • When and organism produces its own cells to pass to offspring, there are 2 sets that must separate from each other so that each cell contains just 1 set of genes

10. STOP! What does this mean?!?! • Gametes – the sex cells of an organism • Sperm and eggs • Mitosis deals with non-gametes (somatic cells) • Remember, in mitosis we result in genetically identical cells • WE DO NOT WANT GENETICALLY IDENTICAL OFFSPRING!!!

11. Translation of Mendel’s Idea #2 When and organism produces its own cells to pass to offspring, there are 2 sets that must separate from each other so that each cell contains just 1 set of genes • Every cell of the human body contains a specific number of chromosomes (46) • In order for offspring to maintain that number of 46 and not end up with duplicate (92), the parent gamete (sex cell) must half their number of chromosomes • End result > 23 (mom) + 23 (dad) = 46 offspring chromosomes

12. Translation of Mendel’s Idea #1 An organism must inherit a single copy of every gene from both its “parents” • Remember, each of the 46 chromosomes needs to halved by the parents (to make 23) • Parents will only contribute each of those 23 genes one time to their offspring • (parents do not want to give multiple copies of the same gene) • So, again 23 + 23 = 46 chromosomes total

13. Chromosomes and gametes Remember… • Chromosomes – the structures in the cell that carry the genetic material (genes) from the parent cell to the daughter cell • Gametes – sex cells or germ cells (sperm and egg)

14. Chromosomes • There are many chromosomes in the body that carry information for many different “things” • Examples – eye color, hair color, height, 2nd toe length, etc… (everything that makes you, you!) • When 2 cells come together from 2 parents, the matching chromosomes must come in contact • These matching chromosomes are called homologus • Homologous Chromosomes – same chromosome types between mother and father cells

15. Chromosome Numbers • Haploid – a cell that contains a single set of chromosomes • Remember “hap” sounds like half • Is usually represented as N • Diploid – a cell that contains both sets of homologous chromosomes • Remember “di” means 2 (kinda like “bi” – bicycle) • Is usually represented as 2N • Since the somatic cells of the body are diploid we need the sex cells to be haploid so offspring do not have more chromosomes than necessary

16. What do Diploid/Haploid Numbers Mean? Diploid Barbie 2N = 46 chromosomes Diploid Ken 2N = 46 chromosomes Haploid Ken Cell N = 23 chromosomes Haploid Barbie Cell N = 23 chromosomes + Sperm cell Egg cell Equals Diploid Baby 2N = 46 chromosomes

17. How are Haploid Gametes Produced? • Meiosis • a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell • Involves 2 distinct divisions • Meiosis 1 • Meiosis 2 • Starts with 1 diploid cell and results in 4 haploid daughter cells that are GENETICALLY DIFFERENT from the parent cell

18. Meiosis 1 • Prior to meiosis 1, each chromosome is replicated • Same as S phase of interphase prior to mitosis • Meiosis 1 starts with the cell beginning to divide very similarly to mitosis • Unlike mitosis, meiosis 1 has homologous pairing in each step • Homologous Pairing – the same chromosome types from mom and dad come together (eye color, hair color, etc.)

19. Prophase 1 • Homologous chromosomes pair and form a tetrad • Since 1 chromosome is made of 2 chromatids, there are 4 chromatids in a tetrad • While in their tetrad, homologous chromosomes are able to trade/swap information in a process called crossing over • this process results in the exchange of traits (alleles) between the same chromosomes therefore creating new trait combinations • One reason why you are different from your parents!

20. Remainder of Meiosis 1 • Meiosis utilizes the remainder of the cycles as mitosis did separating homologous chromosomes instead of sister chromatids • Metaphase 1 – spindle fibers attach to homologous chromosomes • Anaphase 1 – spindle fibers separate homologous chromosomes

21. Remainder of Meiosis 1 • Telophase 1/Cytokinesis – the nuclear envelope reforms around chromosomes and the cytoplasm splits • As a result of Meiosis 1, 2 daughter cells are produced tat have half the number of genetically different chromosomes

22. Meiosis 2 • After Meiosis 1, the 2 cells produced proceed to meiosis 2 (there is no chromosome replication) • Each of the daughter cells move through Meiosis 2 much in the same way Mitosis occurs • Prophase 2 – the centriole move to opposite ends of the cell, the nuclear envelope breaks down • Metaphase 2 – chromosomes line up in the middle of the cell and spindle fibers attach to the centromere

23. Meiosis 2 • Anaphase 2 – the spindle fibers pull on the centromere and split the sister chromatids • Telophase 2 & Cytokinesis – the nuclear envelope forms and the cell cytoplasm splits • 4 genetically DIFFERENT cells result

24. Gamete Formation • The male gamete that is produced is called a sperm (spermatocyte) • There are 4 sperm cells that are produced as a result of meiosis • The female gamete that is produced is called an egg (oocyte) • There is 1 egg cell produced as a result of meiosis • The 3 other cells produced are called polar bodies • Polar Bodies are not used in reproduction and are considered to be the trash bags of the egg, but can be useful in genetic testing

25. Comparing Mitosis and Meiosis • Sloppy Copy • Mitosis/Meiosis Picture • Fold paper down middle • Draw Mitosis on left side starting with interphase and ending with cytokinesis beginning with 4 chromosomes (X’s) • Draw Meiosis on right side starting with interphase and ending with cytokinesis 2 beginning with 4 chromosomes (X’s) • Mitosis/Meiosis Compare/Contrast Graphic Organizer • 3 ways they are similar • 3 ways they are different

26. The Work of Gregor Mendel • Every living thing has a set of characteristics inherited from its parent(s) • Genetics – study of heredity • The father of genetics was Gregor Mendel • Was an Austrian monk that studies the passing of traits in pea plants

27. Gregor Mendel’s Pea Plants • He was put in charge of the gardens at the monastery • He knew how the process of fertilization occurred • When the male and female cells join during sexual reproduction • In plants fertilization happens because of pollination

28. Creating Plants • Pea plants are able to self-pollinate • The sperm in the pollen can fertilize the egg cell of the same plant • As a result, a plant can be created from only 1 “parent” and therefore have the same characteristics of that 1 parent • Since plants were self-pollinating, they would also be considered true-breeding • They would produce offspring identical to themselves • Ex) tall plants would make more tall plants • Ex) green seeded plants would make more green seeded plants

29. Mendel’s Gets Started • Even though true-breeding plants are good, Mendel was interested in what would happen if different traits were “crossed” • Mendel manipulated flowers so they could not self-pollinate and then began to cross breed plants with different characteristics • Ex) cross a tall plant with a short plant • Ex) cross a yellow seed plant with a green seed plant

30. Genes and Dominance • Mendel studied contrasting pea plant traits • Trait – a specific characteristic (such as color or height) that varies from one individual to another • Mendel looked at the offspring that he created by crossing parents with different traits • These offspring were called hybrids P – symbol for the parental generation F1 – symbol for the 1st generation

31. What Did the 1st Generation Look Like? • To Mendel’s surprise, the offspring were not a combination of the 2 different traits • One of the traits did not even appear

32. Mendel’s Conclusions • Biological inheritance is determined by factors that are passed from one generation to the next • We know that this is because of genes – chemical factors that determine traits • The different forms of a gene is an allele • Ex) alleles of height – tall & short alleles of color – yellow & green • Some alleles are dominant and others are recessive (principal of dominance) • Dominant traits will be exhibited whenever present • Recessive traits will only be exhibited when it’s the only trait present

33. Segregation • Mendel wanted to know if the recessive trait had completely disappeared in the F1 generation • He allowed the F1 generation plants to cross by self-pollination, creating a F2 generation

34. F1 Cross Results • Mendel discovered the recessive traits reappeared • Mendel concludes that the dominant allele had only masked the recessive allele and that recessive alleles never disappeared • The alleles must separate or segregate from each other • Gametes pick up one allele or another during the segregation process

35. How Did Segregation Work? • Mendel concludes: • When each of the F1 plant flowers produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene • Therefore, each F1 plant produces 2 types of gametes – those with the allele for tallness and those with the allele for shortness

36. Mendel’s Results • Mendel repeated his experiments many times to see what would happen • He realized that for each cross, he got the same basic results • Ex) whenever he crossed Tt for height, ¾ of offspring were tall and ¼ of offspring were short • Mendel realized that he could apply the principals of probability to explain the results of genetic crosses

37. Genetics and Probability • Probability – the likelihood that a particular event will occur • Example – Coin Flip • The chances that a coin will come up heads is 50% (1/2 or 1:2) • The chances that a coin will come up tails is 50% (1/2 or 1:2) • If you flipped a coin and it came up heads, what are the chances that it will come up heads the next time? The next time? • Each event is independent of each other, so each time is a 50% chance

38. Coin Toss Activity • Hypothesis: If I toss a coin 2 times, I would expect to get _______ Heads and ________ Tails. • Toss your coin 2 times and record results • Toss 1 ____ H _____ T Toss 2 ____ H _____ T • Toss your coin 8 more times and record results

39. Actual Coin Toss Results • For first 2 tosses: • Number Heads ______ Number Tails ______ • Total Results • Number Heads ______ Number Tails ______ • Did the actual results for the entire class come closer to your hypothesis of 50/50 chance for heads and tails?

40. How is Coin Flipping Relevant to Genetics? • When we are dealing with probabilities of an event, past outcomes DO NOT affect future ones • Each event is completely separate and therefore has just as equal chances for an outcome as the one before • The segregation of alleles is completely random, just like a coin toss • You randomly get what you get, every single time

41. Punnett Squares • Punnett Square – is a diagram that shows the gene combinations that can result from a genetic cross • Dominant traits are expressed by capital letters (T) • Recessive traits are expressed by lower case letters (t) • Homozygous – organisms that have identical alleles for a particular trait (TT or tt) • Are true breeding (pure-bred) organisms for a trait • Heterozygous – organisms that have different alleles for a particular trait (Tt) • Are hybrid organisms for a particular trait

42. Punnett Square Results • The outcomes of a punnett square can be described by both their “letters” and their “appearances” • Phenotype – the physical characteristics of an organism (the appearance) • Ex) tall, short, yellow, green • Genotype – the genetic makeup of an organism (the letters) • Ex) TT, Tt, tt • Organisms can have the same phenotype even though they have different genotypes • TT – tall & Tt – tall

43. Probability and Segregation • Was Mendel’s model for segregation correct when looking at alleles for height? • YES! (branching tree example) • Are we able to determine the same results by looking at probabilities for height? • YES (punnett square example) • The results are the same no matter how you look at it

44. Probabilities Predict Averages • Probabilities can predict the average outcome of a large number of events (remb. the coin tossing) • They cannot predict the precise outcome of an individual event • The predictions are based on chances for each individual event • When dealing with genetics, the larger the number of offspring, the closer your results will be to the predicted number

45. Independent Assortment • Mendel said that alleles segregate during the formation of gametes • When Mendel was doing his experiments he wanted to know if alleles segregated on their own (independently) or if one allele can affect another allele • Mendel goes back to the pea plant to find the answer

46. 2 Factor Crosses – F1 • Mendel performed his pea plant experiments now looking at 2 different traits at once • Mendel crossed purebred plants round yellow (RRYY) and wrinkled green (rryy) [P] and looked at their offspring [F1] • All offspring were heterozygous round yellow (RrYy) • We are not able to see if the alleles segregate independently with these results…

47. 2 Factor Cross – F2 • Mendel crossed 2 of the F1 offspring to create a F2 generation • Mendel wanted to know if dominant/recessive alleles stay together (RY/ry) • Mendel looked at the F2 results and saw… • 9/16 were round yellow • 3/16 were round green • 3/16 were wrinkled yellow • 1/16 was wrinkled green Remember our P generation genotypes… RRYY – round yellow rryy – wrinkled green

48. F2 Results • Mendel saw in the F2 generation the presence of offspring that did not exist in any parent • Round green and wrinkled yellow • This fact means that alleles are capable of segregate independently • Independent Assortment – genes for different traits are able to separate on their own and do not influence each others inheritance • the alleles for seed shape and color did not influence each other since they were capable of separation

49. Principal of Independent Assortment • This states… • Genes for different traits can segregate independently during the formation of gametes • Along with crossing over, independent assortment is the other process that accounts for genetic variation

50. Summary of Mendel’s Principals • The inheritance of biological characteristics is determined by individual units known as genes. Genes are passes from parents to offspring • In cases in which 2 or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. • In most sexually reproducing organisms, each adult has 2 copies of each gene – one from each parent. These genes are segregated from each other when gametes are formed • The alleles for different genes usually segregate independently of one another.