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LECTURE 10A: MEIOSIS

LECTURE 10A: MEIOSIS. TOPICS OF DISCUSSION. INTRODUCTION MEIOSIS vs MITOSIS B. INTRODUCTION. Meiosis Definition Meiosis is the production of gametes , which is a reduction division which means a diploid gamete produces haploid gametes - from a full complement of chromosomes to half…

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LECTURE 10A: MEIOSIS

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  1. LECTURE 10A: MEIOSIS

  2. TOPICS OF DISCUSSION • INTRODUCTION • MEIOSIS vs MITOSIS • B

  3. INTRODUCTION Meiosis Definition Meiosis is the production of gametes, which is a reduction division which means a diploid gamete produces haploid gametes - from a full complement of chromosomes to half… Two new cells, produced during mitosis, each have a complete set of chromosomes (a diploid cell) Meiosis reduces the number of chromosomes by half and produces two haploid cells from a diploid cell. A gamete is a cell that is involved in reproduction Fertilization restores the diploid (two) chromosome number by combining the haploid gametes to the fertilized zygote (A zygote is a diploid cell that results from the union of two haploid cells). At the end of meiosis gametes are produced, four cells - each with half a set of chromosomes - haploid cells One set of chromosomes from each parent

  4. Gametes Sex chromosomes (X & Y) Autosomes (non-sex chromsomes) Somatic cells have 2 sets of autosomes Gametes have one set of sex chromosomes and one set of autosomes

  5. Gametes are Haploid Gametes must have half the genetic material of a normal cell If the genetic material in the gametes was not halved, when they combined the zygote would have more genetic material than the parents Meiosis is specialized cell division resulting in cells with half the genetic material of the parents Gametes have exactly one set of chromosomes, this state is called haploid (1n) Regular cells have two sets of chromosomes, this state is called diploid (2n)

  6. How it works... The chromosomes are duplicated in the S phase of interphase (just like mitosis) In Meiosis there are 2 cell divisions In mitosis the chromosomes line up individually, but in meiosis they line up in homologous pairs, next to each other

  7. Terminology p-arm ( short ) <= centromere q-arm ( long ) autosomes NOR, ==> Acrocentic chromosomes 13, 14, 15 21, 22 Xp21 Sex chromosomes Ideogram of the human chromosomes

  8. A little more basic terminology

  9. The

  10. INTRODUCTION Meiosis is the production of gametes, which is a reduction division which means a diploid gamete produces haploid gametes - from a full complement of chromosomes to half… Meiosis reduces the number of chromosomes by half and produces two haploid cells from a diploid cell. A gamete is a cell that is involved in reproduction Fertilization restores the diploid (two) chromosome number to the zygote (A zygote is a diploid cell that results from the union of two haploid cells).

  11. INTRODUCTION Chromosomes are paired – one set are maternal chromosomes, the others are paternal  Members of the pair are homologous chromosomes  There are two divisions in meiosis: meiosis I and meiosis II. Each division may include a prophase, metapahse, anaphase and telophase  As in mitosis the chromosomes are duplicated in the S phase of interphase. In meiosis I, the members of each homologous pair of chromosomes are separated into two daughter cells

  12. INTRODUCTION • The chromosomes were duplicated prior to meiosis I so that there are two chromotids in each daughter cell. • In meiosis II, chromotids separate into individual chromosomes and enter different haploid daughter cells • In Prophase I, homologous chromosomes are paired, one of them is from the mother and one from the father. Remember each chromosome is comprised of two chromatids so that there are four chromatids present at this time. • The complex of four chromatids is called a tetrad. The fastening together is called the synapsis. In humans there are 23 tetrads (or 92 chromatids). The number of tetrads is equal to the number of haploid chromosomes.

  13. INTRODUCTION All genes located on a particular chromosome are linked and will tend to be inherited together in in linkage groups. Sometimes this doensn’t happen, during synapsis genetic material may be exchanged bwetween hpomologous chromatids by crossing-over. Crossing-over produces new combinations of of genetic materials. Plant

  14. MEIOSIS vs MITOSIS In mitosis there is one cell division resulting in two daughters with the identical set of chromosomes as the parent cell. In meiosis there are two cell divisions, producing four daughters. Each cell has the haploid number of chromosomes that is one member of each homolgous pair (one set from each parent) So that…

  15. MEIOSIS vs MITOSIS So that… Mitosis - diploid somatic cells yield diploid somatic cells …one cell divides into 2 duplicates of itself Meiosis - Diploid gamete producing cells divide into haploid gametes…one cell divides into four cells but they are a mix of the original Meiosis has two divisions. Similar interphase as in mitosis.

  16. MEIOSIS vs MITOSIS Prophase I there are two homologous pairs, half of each pair is maternal and half is paternal. Each chromosome consists of two chromatids at this time Synapsis or the pairing of homologous pairs results in the coming together of four (two maternal and two paternal) chromatids instead of just two. This is called a tetrad. The number of tetrads equals the haploid number of chromosomes (half). Since humans have 46 chromosomes, there are 23 tetrads and a total of 92 chromatids.

  17. MEIOSIS vs MITOSIS Each cell duplicates the haploid (half of chromosomes) which combines with the other haploids into tetrads. Instead of one homologous pair there are two. They separate and one of each pair move to ends of the cell and then the cell separates, another interphase occurs and the second series of meiosis begins.

  18. Stages Of Meiosis

  19. Overview

  20. Steps are similar to mitosis Duplicated chromosomes (chromatids) are present after interphase The chromatids become paired One of each pair is contributed by the organism’s mother, the other by the father Because the pairing is of chromatids (2n) there are actually 4 chromosomes present in what is called a tetrad

  21. Stages Of Meiosis Meiosis resembles mitosis except that it is actually two divisions not one These divisions are called Meiosis I and Meiosis II Meiosis I results in haploid cells with chromosomes made up of two chromatids Meiosis II is essentially mitosis on haploid cells Stages of meiosis resemble mitosis with two critical differences: the first in prophase I and the second in Metaphase I

  22. The number of tetrads is the haploid number of chromosomes (humans have 46 chromosomes, so in prophase 1 there are 92 chromatids and 23 tetrads) Combination process is called synapsis Sometimes during synapsis, genetic material is exchanged between chromosomes in process called crossing over All genes located on a particular linkage group and tend to be inherited together except for crossing over

  23. Stages Of Meiosis - Meiosis I Prophase I - The beginning phase DNA which was unraveled and spread all over the nucleus is condensed and packaged Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids) Crossing over, in which chromatids within tetrads exchange genetic material, occurs

  24. Stages Of Meiosis - Meiosis I Metaphase I - Middle stage Tetrads line up along the equator of the cell and sister chromatids orient toward opposite poles Anaphase 1 Tetrads separate and move toward poles but chromatids still connected by centromere! One copy of each chromosome still composed of two chromatids moves to each pole of the cell

  25. Stages Of Meiosis - Meiosis I Telophase I - End stage New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells At the end of telophase 1 there are two doubled chromosomes at each end of cell.

  26. Meiosis

  27. Meiosis

  28. Stages Of Meiosis - Meiosis II Prophase II Cells do not typically go into interphase between meiosis I and II, thus chromosomes are already condensed Metaphase II Chromosomes line up at the equator of the two haploid cells produced in meiosis I Because homologous pairs line up in Metaphase 1 there is a 50/50 chance of which one of each pair will go to each pole In humans with 23 chromosomes any couple would have 223 x 223 chance of different possible children (about 1 in 70 trillion and this doesn’t account for crossing over!)

  29. Stages Of Meiosis - Meiosis II Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells. Centromeres split in anaphase 2 and chromatids move to poles Telophase II - Cytokinesis and reformation of the nuclear membrane in haploid cells each with one set of chromosomes made of one chromatid. At the end of telophase there is one chromatid at each end (haploid)

  30. Meiosis

  31. Meiosis

  32. Meiosis

  33. Stages Of Meiosis: Meiosis I Mother cell Interphase Prophase I: Condensing Chromosomes Prophase I: Tetrad formation/ crossing over Metaphase I Meiosis II Anaphase I Telophase I

  34. Stages Of Meiosis: Meiosis II Metaphase II Anaphase II Telophase II Telophase I Prophase II The products of meiosis are 4 haploid cells each with a unique set of chromosomes. The products of mitosis are 2 diploid cells with identical chromosomes.

  35. Crossing Over Prophase I: Tetrad formation/ crossing over Metaphase I Telophase I Telophase II Anaphase I Because of crossing over, every gamete receives a unique set of genetic information.

  36. Chromosomes, Chromatids and Centromeres Chromatid Identical chromatid Replication Anaphase A packaged chromosome Chromosome arm Two identical chromosomes Centromere Chromosome arm

  37. In The Beginning Two Humans and many other complex multi-celled organisms incorporate genetic recombination in their reproduction Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops Meiosis is the process of producing sperm and eggs (gametes)

  38. Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Sperm 2n Haploid nucleus

  39. Fertilization Results In A Diploid Zygote Egg 1n Haploid nucleus Sperm 2n Haploid nucleus

  40. Fertilization Results In A Diploid Zygote Haploid nucleus Egg 1n Haploid nucleus Sperm 2n

  41. Fertilization Results In A Diploid Zygote Haploid nucleus Egg 1n Haploid nucleus Sperm 2n

  42. From Zygote to Embryo Zygote 2n Zygote 2n

  43. From Zygote to Embryo Cleavage

  44. From Zygote to Embryo Cleavage

  45. From Zygote to Embryo Cleavage

  46. From Zygote to Embryo Cleavage

  47. From Zygote to Embryo Morula

  48. The End

  49. Meiosis

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