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4.2: Meiosis

4.2: Meiosis. ★State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. ★Define homologous chromosomes. ★Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in 4 haploid cells.

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4.2: Meiosis

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  1. 4.2: Meiosis ★State that meiosis is a reduction division of a diploid nucleus to form haploid nuclei. ★Define homologous chromosomes. ★Outline the process of meiosis, including pairing of homologous chromosomes and crossing over, followed by two divisions, which results in 4 haploid cells.

  2. 4.2: Meiosis ★Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21) ★State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure.

  3. 4.2: Meiosis ★State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities. ★Analyze a human karyotype to determine gender and whether non-disjunction has occurred.

  4. 4.2: Meiosis • Review: • Mitosis is cell division that produces body cells, all identical to each other, all have 46 chromosomes in humans. • New: • Meiosis: cell division that produces gametes. • Gametes: sex cells.

  5. 4.2: Meiosis • In any organism, each cell that is produced as a result of meiosis has half the number of chromosomes of other cells in the body. • Mitosis = 46 chromosomes in every cell • Meiosis = 23 chromosomes in every egg and sperm • Why? When an egg and sperm meet to make a baby, that makes the first fertilized egg = 46 chromosomes!

  6. 4.2: Meiosis • These chromosome #'s have names! • Diploid (2n): two copies of every chromosome, in humans our diploid number is 46. • Haploid (n): one copy of every chromosome, in humans our haploid number is 23 • Mango diploid # = 40, haploid # = 20 • Camel diploid # = 70, haploid # = 35 • How many chromosomes does a camel sperm or egg have? • 35

  7. 4.2: Meiosis • During gamete formation, meiosis reduces the diploid number to the haploid number. • At the time of fertilization (egg meets sperm), the diploid number is restored.

  8. Meiosis is called a reduction division because the number of chromosomes has been reduced. • Living organisms who reproduce sexually have to 1/2 their chromosome number at some stage in the life cycle because the fusion of gametes during fertilization doubles it.

  9. 4.2: Meiosis • Homologous Chromosomes: • In a diploid human cell, the 46 chromosomes can be grouped into 23 pairs or chromosomes called homologous chromosomes. • Homologous = similar in shape and size, 2 chromosomes carry the same genes • With a pair of homologous chromosomes, one coms from the mother, one from the father.

  10. 4.2: Meiosis • Although a pair of homologous chromosomes carry the same genes, they are not completely identical • Homologous chromosomes have the same genes as each other, in the same sequence, but not necessarily the same alleles of those genes. • EX: A pair of homologous chromosomes (one from mom/one from dad) have the same genes (code for proteins) but some bases are different causing different traits. • AGGTTA = brown hair • AGGTTG = blonde hair

  11. 4.2: Meiosis • Phases of Meiosis: • Meiosis is the process by which a diploid parent cell produces 4 haploid daughter cells. • Before meiosis, DNA replication occurs (making a complete DNA copy) during interphase. • In order to produce 4 cells, the original/parent cell must divide 2 times. • First division makes 2 cells • Second division makes 4 cells (final product)

  12. 4.2: Meiosis • Prophase I: • 1. Chromosomes become visible/super coil/condense after interphase • 2. Homologous chromosomes pair up (one from mom, one from dad) • 3. Crossing over occurs (structures crossed over homologous chromosomes formed called tetrads) • 4. Spindle fibers (made from microtubules) form

  13. 4.2: Meiosis • What is the purpose of crossing over? • The biggest event that makes meiosis different from mitosis is crossing over. • This is the exchange of genetic material between non-sister chromatids. • This trading of segments of genes happens when sections of the two homologous chromatids break at the same point, twist around each other, and swap DNA. • This allows DNA from the maternal chromosome to mix with DNA from the paternal chromosome.

  14. 4.2: Meiosis • Crossing over is the reason why, at the end of meiosis, all 4 sperm or egg are genetically different from one another. • If your parents have 10 kids, all kids will be different because each egg your mom produces, and sperm your dad produces, are different! • That means millions and millions of combinations!

  15. 4.2: Meiosis • Metaphase I: • 1. Homologous chromosomes (also can be called bivalents) line up across cell's equator/middle • 2. Nuclear membrane disappears

  16. 4.2: Meiosis • Anaphase I: • 1. Spindle fibers from the poles attach to chromosomes and pull them to opposite poles of the cell. • 2. This is the reduction division because chromosome number is halved from diploid to haploid.

  17. 4.2: Meiosis • Telophase I: • 1. Spindle fibers disintegrate/disappear • 2. Usually, chromosomes uncoil and new nuclear membranes form. • At the end if meiosis 1, cytokinesis happens; cell splits into 2 separate cells. • Cells are haploid because they contain only 1 chromosome of each pair (sister chromatids still attached). No Interphase/S phase necessary because we don't want to double chromosome number.

  18. 4.2: Meiosis • Prophase II: • 1. In each of the 2 cells after meiosis I, new spindle fibers start to form. • 2. Chromosomes recoil/condense again • 3. Nuclear envelopes break down again

  19. 4.2: Meiosis • Metaphase II: • 1. Nuclear envelopes completely broken down • 2. Individual chromosomes line up at equator/middle of cell • 3. Spindle fibers travel to opposite poles and attached to the centromere of each chromosome.

  20. 4.2: Meiosis • Anaphase II: • 1. Sister chromatids separate at the centromere • 2. Spindle fibers pull the chromatids to opposite sides of the cell.

  21. 4.2: Meiosis • Telophase II: • 1. Nuclear envelopes re-form • 2. Chromosomes uncoil, soon to be chromatin • 3. A second cytokinesis follows • 4. FINAL PRODUCT: 4 haploid/genetically different/sperm or egg

  22. 4.2: Meiosis • Major differences between mitosis and meiosis:

  23. 4.2: Meiosis • Homework Questions • 1. How are the cells at the end of meiosis different from the cells at the beginning of meiosis? • 2. If the diploid number of goldfish is 90, what is its haploid number? Which type of cells in the goldfish are diploid, and which ones are haploid? • 3. While examining a cell in prophase I of meiosis, you observe a pair of homologous chromosomes pairing tightly. What is the significance of the places at which the chromosomes are joined and what is this "event" called? • 4. Draw and label the stages of meiosis I and meiosis II. Start with 6 chromosomes.

  24. 4.2: Meiosis • Non-disjunction: failure of chromosomes to separate properly. • If this occurs in meiosis I, some homologous chromosomes don't separate. • If this occurs in meiosis II, some sister chromatids don't separate properly. • Consequence: changes in chromosome number • Any more or less than 23 in each sperm and egg can result in chromosomal disorders.

  25. 4.2: Meiosis • Down Syndrome: Chromosomal disorder due to non-disjunction • Non-disjunction happens in the 21st pair of chromosomes. • Child receives 3 chromosomes instead of 2. • Total #chromosomes in ALL CELLS = 47. • If one chromosome pair has 3 chromosomes, this is called trisomy. • Down Syndrome = Trisomy 21

  26. 4.2: Meiosis • People with Down Syndrome have distinct facial features, are short, have heart defects and mental disability (often in special education programs). • Down Syndrome most common chromosomal disorder - Approximately 1 out of 800 children born in US have Trisomy 21. • Older women have a greater chance of having a child with Down Syndrome. WHY? • The "best" eggs are released first during a woman's lifetime (1 egg every month/menstrual cycle) • Older women have older eggs, more defects

  27. 4.2: Meiosis • Non-disjunction can happen with other chromosomes, and all of them will have a major impact on a child's development. • Some chromosomal disorders are so severe that the fetus may not survive beyond a few weeks or months in the womb. • Some miscarriages (not all) are from non-disjunction.

  28. 4.2: Meiosis • Other non-disjunction disorders: • Trisomy 18/Edwards Syndrome • 95% die in utero, heart/kidney malfunctions • Trisomy 13/Patau Syndrome • Mental/motor challenged, heart/eye defects, polydactyly (extra digits) • Monosomy X/Turner Syndrome • One sex chromosome instead of 2 • Looks female, infertile, ovaries don't develop

  29. 4.2: Meiosis • Karyotyping: • Karyotype: photograph of the chromosomes found in a cell. • Chromosomes are arranged according to their size and shape. • When during the cell cycle do you think chromosomes are photographed for a karyotype?

  30. 4.2: Meiosis • Why karyotype? • From a karyotype, the gender of a person can be determined and chromosome abnormalities can be detected. • The most useful time to do this is before birth. • In order to do this, cells from the fetus (inside the mother's womb) but be taken.

  31. 4.2: Meiosis • Once fetal cells have been obtained, they are mixed with chemicals that stimulate them to divide by mitosis. • What mitosis phase do you think is best to capture a picture of the chromosomes for a karyotype? • Another chemical is used which stops mitosis in metaphase of mitosis.

  32. 4.2: Meiosis • How a karyotype is made: • 1. The cells are stained and prepared on a glass slide to see the chromosomes under a light microscope. • 2. Photomicrograph images are obtained of the chromosomes during mitosis/metaphase • 3. The images are cut out and separated using computer software. • 4. The images of each pair of chromosomes are placed in order by size and the position of the centromere.

  33. 4.2: Meiosis • 2 ways to take fetus cells: • 1. Amniocentesis • A sample of amniotic fluid is removed from the amniotic sac around the fetus (when "her water breaks" right before birth, this is the sac that breaks) • To do this, a hypodermic needle is inserted through the wall of the mother's abdomen and wall of the uterus.

  34. 4.2: Meiosis • Amniotic fluid is drawn out into a syringe. • This fluid contains cells form the fetus.

  35. 4.2: Meiosis

  36. 4.2: Meiosis • 2. Chorionic villus sampling • Cells are removed from fetal tissues in the placenta called chorionic villi. • As with amniocentesis, a hypodermic needle, inserted through the mother's abdomen and uterus wall, is used to obtain the cells. • Different from amniocentesis, because rather than cells taken from fluid, cells are taken from tissues surrounding the baby.

  37. 4.2: Meiosis • Debate the pro's and con's of amniocentesis and chorionic villus sampling. • Since the extraction of cells from an unborn baby is an invasive and expensive procedure, there is a risk that it may harm the child or even cause a miscarriage. Doctors and parents-to-be must consider which is more important, finding out if the baby has a chromosomal disorder or providing safe conditions for its development without extracting fetal cells.

  38. 4.2: Meiosis • Who should decide whether to keep or to abort the babies which present chromosomal disorders? • The doctors? Future parents? Both? • If you found our your future child had a chromosome abnormality that would make him or her very different from other children, what would you do? • The severity of the chromosomal disorder cannot be determined from the information on the karyotype, does this affect your answer?

  39. 4.2: Meiosis • How to analyze a human karyotype for gender or disorders from non-disjunction: • Female: XX • Male: Xy • Normal karyotype: 22 pairs (44) of autosomes (regular chromosomes) and 2 sex chromosomes

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