Meiosis Notes

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Meiosis Notes

  1. 1. Meiosis
  2. 2. Chromosome Number <ul><li>Organisms have tens of thousands of genes that determine individual traits. The more closely related two organisms are, the more genes they’ll have in common </li></ul><ul><li>Genes are lined up on chromosomes that can hold thousands of genes. </li></ul><ul><li>In body cells of animals and most plants, chromosomes occur in pairs . </li></ul><ul><li>One chromosome in the pair came from the male parent and one came from the female parent. </li></ul><ul><li>These pairs are called homologous chromosomes – each pair has genes for the same traits </li></ul><ul><li>A cell with two of each kind of chromosome is called diploid ( 2n ). </li></ul>
  3. 3. Chromosome Number <ul><li>Organisms produce gametes (sex cells) that contain one of each kind of chromosome. </li></ul><ul><li>A cell with only one of each kind of chromosome is called haploid ( n ). </li></ul><ul><li>Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during fertilization), the resulting cell is diploid . </li></ul><ul><li>Each species has a specific number of chromosomes. </li></ul><ul><ul><li>Humans have 23 pairs ( 46 total) </li></ul></ul><ul><ul><li>Fruit Flies have 4 pairs ( 8 total) </li></ul></ul><ul><ul><li>Dogs have 39 pairs ( 78 total) </li></ul></ul>
  4. 4. Human Chromosomes <ul><li>Humans have 23 pairs of chromosomes (46 total) </li></ul><ul><li>22 pairs of autosomes </li></ul><ul><li>1 pair of sex chromosomes </li></ul><ul><li>Half of each pair came from one parent and half came from the other parent </li></ul>
  5. 5. Meiosis: Why? <ul><li>Mitosis divides one diploid cell to form two diploid cells </li></ul><ul><ul><li>For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes. </li></ul></ul><ul><li>If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each chromosome </li></ul><ul><ul><li>46 chromosomes from each parent would yield a 92 chromosome offspring </li></ul></ul><ul><li>Meiosis allows for two divisions to divide a one diploid cell into four haploid cells. </li></ul>
  6. 6. Meiosis: Where and Who? <ul><li>Meiosis takes place in the gonads (sexual organs) </li></ul><ul><ul><li>For humans , these are the ovaries and testes </li></ul></ul><ul><ul><li>The process of meiosis produces egg and sperm cells </li></ul></ul><ul><li>Two gametes come together by fertilization </li></ul><ul><ul><li>The haploid sperm and egg join to form a diploid zygote </li></ul></ul>
  7. 7. Meiosis Phases <ul><li>Meiosis I </li></ul><ul><ul><li>Prophase I </li></ul></ul><ul><ul><li>Metaphase I </li></ul></ul><ul><ul><li>Anaphase I </li></ul></ul><ul><ul><li>Telophase I </li></ul></ul><ul><li>Meiosis II </li></ul><ul><ul><li>Prophase II </li></ul></ul><ul><ul><li>Metaphase II </li></ul></ul><ul><ul><li>Anaphase II </li></ul></ul><ul><ul><li>Telophase II </li></ul></ul>
  8. 8. Interphase <ul><li>Before Meiosis (just like before Mitosis) the cell must prepare for division: </li></ul><ul><ul><li>Cells increase in size </li></ul></ul><ul><ul><li>DNA is replicated </li></ul></ul><ul><ul><li>Necessary proteins and RNA are synthesized </li></ul></ul><ul><li>During this phase, chromosomes are not yet visible . </li></ul>
  9. 9. Meiosis: Prophase I <ul><li>Chromosomes become visible </li></ul><ul><li>Nuclear envelope disappears </li></ul><ul><li>Centrioles head to opposite poles and spindle forms </li></ul><ul><li>Homologous chromosomes (one pair of sister chromatids from the mother and one from the father ) pair up to form a tetrad </li></ul><ul><li>The tetrad pairs up so tightly that crossing over occurs </li></ul>
  10. 10. Crossing Over
  11. 11. Meiosis: Metaphase I <ul><li>Spindle fibers attach to the centromeres </li></ul><ul><li>Tetrads line up along the equator (or middle of the cell) </li></ul><ul><li>Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they lined up independently to one another. </li></ul>
  12. 12. Meiosis: Anaphase I <ul><li>Homologous chromosomes separate and head to opposite ends of the cell </li></ul><ul><li>Centromeres DO NOT split – Sister chromatids will stay together until the next division </li></ul>
  13. 13. Meiosis: Telophase I and Cytokinesis <ul><li>Spindle is broken down </li></ul><ul><li>Chromosomes uncoil </li></ul><ul><li>Cytoplasm divides into two cells </li></ul>
  14. 14. Meiosis: Prophase II <ul><li>Chromosomes become visible </li></ul><ul><li>Spindle forms </li></ul><ul><li>If nuclear membrane reformed after Telophase I, it will break down now </li></ul>
  15. 15. Meiosis: Metaphase II <ul><li>Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random order (just as they did during Mitosis) </li></ul>
  16. 16. Meiosis: Anaphase II <ul><li>Centromere of each sister chromosome splits and each sister chromatid heads for an opposite pole </li></ul>
  17. 17. Meiosis: Telophase II and Cytokinesis <ul><li>Nuclei reform (nuclear envelope reappears) </li></ul><ul><li>Spindle breaks down </li></ul><ul><li>Chromosomes uncoil </li></ul><ul><li>Cytoplasm divides into a total of four haploid cells that will become gametes </li></ul><ul><li>Each cell contains one chromosome from each homologous pair </li></ul>Let’s See it!
  18. 18. Variability <ul><li>Meiosis has a large role in maintaining variability in a species. </li></ul><ul><li>Through sexual reproduction , offspring are not simply replicas of one organism but a genetic combination of two organisms </li></ul><ul><li>Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it reproduces, creating a variety of offspring. </li></ul>
  19. 19. Chromosomal Mutations <ul><li>Chromosomal mutations can happen when chromosomes break and do not repair correctly. </li></ul>pg. 306
  20. 20. Chromosomal Mutations <ul><li>Errors can also occur during Meiosis . </li></ul><ul><li>Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction </li></ul><ul><li>This results in gametes with either an extra copy of a chromosome or no copy at all. </li></ul>Normal Example Nondisjunction Examples
  21. 21. Types of Nondisjunction <ul><li>Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent resulting in one pair of each type of chromosome (humans: 23 pairs) </li></ul><ul><li>Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one chromosome </li></ul><ul><ul><li>Most zygotes with monosomy do not survive </li></ul></ul><ul><ul><li>One exception is the case of Turner’s Syndrome </li></ul></ul><ul><ul><ul><li>Females have only one X chromosome instead of two </li></ul></ul></ul><ul><ul><ul><li>These people will still have female sexual characteristics but they will generally be underdeveloped </li></ul></ul></ul>
  22. 22. Types of Nondisjunction <ul><li>Trisomy – In this case, the zygote gets one copy of a chromosome from one parent and two copies from the other parent resulting in three copies rather than the normal two copies. </li></ul><ul><ul><li>Down Syndrome (Trisomy 21) – This person has three copies of the 21 st chromosome. This can lead to mental retardation, susceptibility to certain illness or diseases, and a shorter life span </li></ul></ul><ul><ul><li>Klinefelter's syndrome ( XXY ) – This person has two copies of the X chromosome as well as a copy of the Y chromosome. This person will be male but may suffer from underdeveloped testicles and infertility. </li></ul></ul>
  23. 23. Identifying Chromosomal Disorders <ul><li>To determine whether or not an organism has the proper number of each chromosome, one can look at a karyotype </li></ul><ul><li>To make a karyotype a photograph is taken of the paired chromosomes during metaphase </li></ul><ul><li>These pairs are cut out and arranged in a chart according to length and location of centromere </li></ul><ul><li>Once arranged, it is easy to see if there are any extra or missing chromosomes </li></ul>This individual has an extra Y chromosome
  24. 24. Cell Development <ul><li>In the development of most multicellular organisms, a single cell ( fertilized egg ) gives rise to many different types of cells, each with a different structure and corresponding function . </li></ul><ul><li>The fertilized egg gives rise to a large number of cells through cell division , but the process of cell division alone could only lead to increasing numbers of identical cells. </li></ul><ul><li>As cell division proceeds, the cells not only increase in number but also undergo differentiation becoming specialized in structure and function. </li></ul><ul><li>The various types of cells (such as blood, muscle, or epithelial cells) arrange into tissues which are organized into organs , and, ultimately, into organ systems . </li></ul>
  25. 25. Differentiation <ul><li>Nearly all of the cells of a multicellular organism have exactly the same chromosomes and DNA . </li></ul><ul><li>During the process of differentiation, only specific parts of the DNA are activated; the parts of the DNA that are activated determine the function and specialized structure of a cell. </li></ul><ul><li>Because all cells contain the same DNA, all cells initially have the potential to become any type of cell . </li></ul><ul><li>Once a cell differentiates, the process can not be reversed . </li></ul>
  26. 26. Stem Cells <ul><li>Stem cells are unspecialized cells that continually reproduce themselves and have, under appropriate conditions, the ability to differentiate into one or more types of specialized cells. </li></ul><ul><ul><li>Embryonic cells, which have not yet differentiated into various cell types, are called embryonic stem cells . </li></ul></ul><ul><ul><li>Stem cells found in adult organisms, for instance in bone marrow, are called adult stem cells . </li></ul></ul><ul><li>Scientists have recently demonstrated that stem cells, both embryonic and adult , with the right laboratory culture conditions, differentiate into specialized cells . </li></ul>

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