MEIOSIS

1. MEIOSIS Mr. Tsigaridis

2. Review • What is the purpose of mitosis? • Growth • Repair

3. Cells Themselves • Every cell has its own life span, all cells eventually die, so all of them need to replicate (go through mitosis) • Blood replaces all its cells in about 3 months • Lungs will replace themselves in 5 years • Neurons, will never

4. Mitosis As a Review • Mitosis looks at growth and repair of a single organism • Also explains replication of single celled organisms

5. An Issue? • What is the problem with having identical cells as organisms? • Do you see a problem with having with having identical organisms in the populations?

6. The Reality of It • Imagine a million of Mr. Paschos • I like him, he is my boss, but is there a problem, • Is he perfect? • Susceptible to anything?

7. The Problem • A single disease can wipe out the entire population • Do we actually find populations that are completely identical to each other?

8. What is the benefit of not being Identical? • For survival, reproduction went from asexual (mitosis), to sexual (which has a great deal of diversity) • Survival of the individual does not matter, but survival of the species does

9. Steps of Mitosis • What are the four steps? What happens in each? • Prophase • Chromosomes become visible • Nuclear Membrane/Envelope Disappears • Spindle Fibers Appear and Centrioles begin to move to the different poles • Metaphase • The chromosomes line up in the middle of the cell

10. Steps of Mitosis Continued… • Anaphase • Chromosomes are pulled by the spindle fibers to opposite ends of the cell • Telophase • Spindle fibers disappear • Nuclear envelope reforms • Cells begin to separate and become 2 different identical cells • Cytokinesis • The 2 daughter cells move apart and the cycle begins again

12. Cell History • Historically, mitosis evolved before meiosis • So how are you created? • Well you need a mom and a dad

13. But…. • How can a male with 46 chromosomes, and a female with 46 chromosomes produce a child with 46 chromosomes • Half must come from the mother, and the other half must come from the father to produce the baby • The baby will then grow using mitosis

14. Lets Get Started • Who can tell me what the purpose of meiosis is? • The production of sperm in male and egg in female, sex cells are called gametes • Lets look at the process of creating the sex cells happening in both males and females

15. A Comparison • What happens to the genetic info before mitosis begins? • It is replicated, two identical chromatids on the chromosomes • Well the same thing happens in meiosis

16. A Comparison • The process of meiosis is a modified process of mitosis

17. The First Step • We have duplicated genetic info going into the first stage of gamete production • What did we call the first stage of mitosis? • Prophase • What do you think we are going to call the first step in meiosis • Prophase as well

18. The First Step… • What happens in prophase of mitosis • The genetic info condenses into visible chromosomes • The nuclear envelope disappears • The Centrioles move apart and start to form spindle fibres • It happens in meiosis as well

19. Starts to Disappear

20. The Second Step • What happens in mitosis during metaphase • Chromosomes line up in the equator of the cell and each chromosome connects to the spindle fibres • Here is where meiosis starts to change • If we are talking about a human, how many spindle fibres do we have in mitosis • 46, one for each duplicated chromosome

21. The Second Step • In meiosis, what are we trying to make? • sex cells, but when fertilization takes place, the genetic material is donated half from mom, and half from dad, so we are trying to make cells with half the genetic material • So how many spindle fibres form in a meiotic cell • 23

22. Lets back track a Bit • Because the spindle is forming with 23 fibres only, and in a human cell there are 46 chromosomes, the 46 need to pair up the chromosomes • They do this with their "look alike partner“ • What do you think i mean by the words “look alike” partner?

24. Why You Look Like That • Every sexually reproducing organism gets 50% of its genetic information from one parent, and 50% from the other • So which parent gives you the information for hair color? • Eye colour? • Height? • In actuality, you get this from both parents equally

25. Why You Look Like That • If that comes from both parents, we need some genetic information from dad, and one from mom, • Does it have to be the same information?

26. Why You Look Like That • Example, what if mom’s genes says blue, and dad’s genes said brown eyes? • The information can be different, but its still information about eye colour. • So the “look alike” partners have information about the same traits, but do not necessarily have the same identical information, they talk about the same words

27. The Look Alikes • Look alike partners actually have a proper name HOMOLOGOUS CHROMOSOMES • What does that mean?

28. What does that Mean? • What does “homo” mean in Greek? • It means the "same“ • What does “logos” mean in Greek? • It means “words” • Homologous = the same words (or information)

29. Homologous Chromosomes • Homologous chromosomes contain similar information • We can think of the genetic material as 23 pairs of chromosomes • Each pair containing the similar information. • 46 is therefore called the diploid number • 23 is called the haploid number • This will be important later on in our discussions

30. Back To Prophase • The “look alike” partners find each other, or the homologous chromosomes pair and they form what is called a tetrad • The process of coming together is called synapsis, so when the process of synapsis takes place, it forms tetrads

31. Time for a Bio Break…for Math • Lets pause for a second we need to do some math • lets take a look at an organisms that has one pair of chromosomes • How many ways can u arrange the 2 chromosomes when they pair up, lets call one mom and one dad? • Two ways first one can be mom, and then dad, the second can be dad and then mom

32. Time for a Bio Break…for Math • Now, lets double the number of chromosomes to two pairs • How many ways can you arrange 2 pairs?

33. Time for a Bio Break…for Math • What about 3 sets of chromosomes • 8 different ways • What is the relationship? • # of combinations = 2n • (n=number of chromosome pairs)

34. Time for a Bio Break…for Math • How many different combinations are there for 23 pairs? • 223 • 223 = 8,388,608 • Well hold on a second, its not a big number. How many people do we have on the planet?

35. The Trick • 6.5 billion, and I will bet, that no two individuals are identical except for identical twins, triplets etc, and there are not that many of them • So variation in the genetic information can not only be due to the way the chromosomes line up, there must be a way to mix up the genetic information further

36. The Trick • We have to look closer at the tetrad • As the chromosomes of the tetrad come together in synapsis, is there are possibility that the 2 chromatids that are on the inside may lie across each other? • YES! That is a possibility

37. The Trick • Well what if identical segments from dad and moms chromosomes were exchanged? • Now you have increased diversity, this event is called a cross over • it is a physical exchange of identical pieces

38. Where it happens • Does it have to happen at the same place?

39. Epiphany • No wonder we have so many different people on the planet with different genetic make ups. • We can have hundreds of thousands of cross over spots which gives us a lot more variation

40. Metaphase • Lets move on to metaphase, • How many spindle fibres did we say will form? • 23, one for each pair of pair homologous chromosomes, • Unlike metaphase of mitosis, the chromosomes line up as homologous pairs, not in single file

41. Anaphase • What happens in anaphase of mitosis? • the individual chromatids are pulled apart by the shortening of the spindle fibres • Think about a rope tug of war between the 2 Centrioles • Homologous pairs lined up in meiosis are pulled apart to opposite ends of the cell

42. Daughter Cells • How many chromosomes does each daughter cell have? • 23 duplicated chromosomes • Remember the diversity, tons of combinations, but what we have done is taken 46 chromosomes and created 2 cells with 23 duplicated chromosomes,

43. The major Step • Since 46 chromosomes became 23 duplicated chromosomes, this particular process is going to be called the reduction division after telophase is completed • Telophase is the same as in mitosis

44. Are We Done? • We have taken the diploid number 46, and reduced it to the haploid number 23 • Do you see any issues with these 2 product cells? • Each of the chromosomes is still duplicated, and contains double the genetic information, therefore we must separate the copies.

45. Another Difference • Meiosis has to go through the process again to separate the chromatids in each chromosome. • All we do is repeat prophase, metaphase, anaphase and telophase the same as mitosis, except with half the number of chromosomes

46. The Second Phase

47. The Second Phase

48. How we distinguish

49. The Final Tally • Mitosis produces two identical daughter cells (diploid), • Meiosis 1 produces two daughter cells with 23 duplicated chromosomes • Meiosis 2 separates the duplicated chromosomes to a total of 4 daughter cells with a haploid # of chromosomes

50. But aren't there 2 Kinds? • All organisms produce 2 types of sex cells, male produces sperm and female, eggs • There are differences in the two processes that produces these 2 types of gametes, lets look at them