Meiosis

1. Meiosis A reduction division

2. Meiosis • Multi-celled organisms • produce gametes (cells) through the process of meiosis. • In humans and many other organisms, these cells are called ova(eggs) and sperm. • When an ovum and a sperm combine, a new organism is formed with a full set of chromosomes, half of which came from the father, and half of which came from the mother.

3. So what is a chromosome anyway?

4. CHROMOSOME the DNA-containing structures that carry the organism’s genetic information

5. Looking at Chromosomes Chromosomes are arranged in matching pairs – Homologous pairs A normal set of chromosomes will have 23 pairs The first 22 are completely homologous – Autosomes The 23rd pair are sex chromosomes

6. Karyotype

7. Meiosis is a type of cell division where one cell divides to form new cells, each of which have half the number of chromosomes as the original (ex – Humans have ? normally?). • This condition is called haploid. • Cells with the original number of chromosomes are called diploid. • Another way to say this is that each new daughter cell gets half of the parent cell’s DNA.

8. Humans have 23 pairs of chromosomes, for a total of 46. • In each pair, one chromosome comes from mom and one comes from dad. • Both members of the chromosome pair are the same size, shape, and carry the same genetic information.

9. There are two main stages of meiosis: Meiosis I and Meiosis II. • Each of these is divided into four stages: Prophase, Metaphase, Anaphase and Telophase • Before the cell starts dividing, it has to complete Interphase – What happens during this very important time?

10. homologous pair tetrad Meiosis • We refer to the 23 pairs as homologous pairs. Why? • During the earliest stage of meiosis, the members of each pair will temporarily attach together to form a structure called a tetrad. Why are there four instead of two?

11. Prophase I • Very similar to Prophase of mitosis. • What happens during Prophase? • Chromatin does what? • Nucleus does what? • Spindle fibers? • Chromosomes (tetrads) do what? • Centrioles do what? • There are a few extras to add! • The tetrads actually exchange some of their DNA in a process called crossing-over (Swapping spit – Disgusting!!)

12. Metaphase I • ______ , not individual chromosomes, line up on the equator of the cell. • _______ fibers attach to the centromeres of chromosomes. Tetrads Spindle

13. Anaphase I • Spindle fibers contract and pull ______ apart. • Entire double stranded chromosome pulled to opposite ends of the cell. • Chromosome number is now half of original (haploid). Tetrads

14. Cell plate (plant) or cleavage furrow (animal) divides the _________. Cell usually goes directly into Prophase II, but if not, chromosomes relax, nuclei reform, and spindle fibers break down. Each daughter cell has _____ of the number of chromosomes as the original. Telophase I cytoplasm half Chromosomes not to scale

15. Summary of Meiosis II identical • The mechanics of Meiosis II are _______ to Mitosis. • Each haploid daughter cell produced by Meiosis I divides again, so that four haploid cells are produced altogether. • In _____________, all four cells survive to produce viable sperm. • In _________, only one cell survives to produce a viable ovum. spermatogenesis oogenesis

16. Overview • Tetrads form (Pro I) • Crossing over can occur (Pro I) • Tetrads line up and are separated (Meta I) • Daughter cells are haploid • Produces gametes (ova and sperm)

17. How is mitosisdifferent from meiosis?

19. How is mitosis different from meiosis? Mitosis makes two identical daughter cells which have the same number of chromosomes as parent cell. Makes body cells. Meiosis makes four different daughter cells which have half the number of chromosomes as parent cell. Makes sex cells (or gametes).

20. Compare • Critical differences between Mitosis and Meiosis: • Tetrads form (Pro I) • Crossing over can occur (Pro I) • Tetrads line up and are separated (Meta I) • Daughter cells are haploid • Only occurs in germ cells • Produces gametes (ova and sperm)

21. Mitosis vs. Meiosis I Left: Metaphase of MITOSIS: no pairing!!!! Right: Metaphase I of Meiosis: homologous chromosomes Pair UP!

22. What are the two gametes?Egg cell – ovaSperm cell

23. How many chromosomes do human gametes have? How many chromosomes do human body cells have? (haploid - 23) (diploid - 46)

25. What is the difference between diploid and haploid? How are these related to mitosis, meiosis and number of chromosomes?

26. Associated Terms.. Fertilization – Zygote - Homologous pair – Sister Chromatids - A haploid sperm cell fuses with a haploid egg cell The resulting fertilized egg Two chromosomes, with matching size and genes An identical copy of the chromosome

31. Independent Assortment

32. Independent Assortment Results of alternative arrangements of chromosomes at metaphase of meiosis I. The positioning of each homologous pair of chromosomes is random; the two red chromosomes can be on the same side (possibility 1) or on opposite sides (possibility 2).

33. The arrangement of chromosomes at metaphase I determines which chromosomes will be packaged together in the haploid gametes. Because possibilities 1 and 2 are equally likely, the four possible types of gametes will be made in approximately equal numbers.

34. MEIOSIS Round 2

35. How do do we get variety? The tetrads actually exchange some of their DNA in a process called crossing-over. During Prophase I homologous chromosomes line up gene-by-gene and may exchange segments.

36. Crossing Over The exchange of segments allows for genetic variability. The resulting chromosomes are called “recombinant”

37. When Meiosis goes WRONG?

38. Chromosome Mutations are spontaneous changes involving an entire chromosome or pieces of a chromosome Remember Mutations?

39. Can occur during meiosis Some harmful, some not Bigger effect than gene mutations 2 types: Structural mutations and Numerical Mutations Chromosome Mutations

40. change in arrangement of genes on a chromosome For example, Deletion- one or more genes are lost Structural Mutations

41. Another example…. Inversion- piece of a chromosome breaks off and reattaches itself backwards Structural Mutations

42. Another example, Duplication- extra piece duplicated genes Structural Mutations

43. Another example, Translocation- piece of a chromosome breaks off of one & attaches to another different chromosome Structural Mutations

44. QRSTUV to QRUTSV Inversion UVWXY to ABCDUVWXY Insertion/Addition LMNOP to LMNLMNOP Duplication ABCDEF to ABEF Deletion What TYPE of mutations are these? Structural - change in arrangement or # of genes What causes them? Problems with crossing over What’s the Mutation?

45. changes in the number of chromosomes in a zygote Usually results from nondisjunction- failure of chromosomes to separate Can occur in Meiosis I or II Chromosome pair fail to separate in Anaphase Numerical Mutations

46. Numerical Mutations Monosomy- zygote receives only 1 of a particular chromosome EX: female being XO or getting 1 # 6 chromosome

47. Trisomy- zygote receives 3 chromosomes of a particular kind Down’s Syndrome - extra 21st chromosome Numerical Mutations

48. XXY - 47 Males The most common sexchromosome disorder. ~1/1000 males Males are usually infertile. Klinefelter’s Syndrome

49. Polyploidy- none of the chromosomes separate and the zygote gets 3 or more sets of chromosomes EX: instead of 23 pairs, you get 46 pairs of chromosomes…this means 92 chromosomes!!! Numerical Changes

50. 46 (23 pairs) What are pairs 1-22 called? Autosomes What is the 23rd pair called? Sex chromosomes Male? Female? XY XX How many chromosomes does a human have?