Section 10.1 Objectives Analyze how meiosis maintains a constant number of chromosomes within a species Recognize and

1. Section 10.1 Objectives Analyze how meiosis maintains a constant number of chromosomes within a species Recognize and summarize the stages of meiosis Infer how meiosis leads to variation in a species.

2. Genes, Chromosomes, and Numbers  Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes. Typically, a chromosome can contain a thousand or more genes along its length.

3. Diploid and haploid cells In the body cells of animals and most plants, chromosomes occur in pairs. A cell with two of each kind of chromosome is called a diploid cell and is said to contain a diploid, or 2n, number of chromosomes. (46 in humans) Organisms produce gametes that contain one of each kind of chromosome. A cell containing one of each kind of chromosome is called a haploid cell and is said to contain a haploid, or n, number of chromosomes. (23 in humans)

4. A karyotype is a photographic inventory of an individual�s chromosomes Human female karyotype karyotype

5. Human female karyotype

6. Human male karyotype

7. Homologous chromosomes The two chromosomes of each pair in a diploid cell are called homologous chromosomes. Each pair of homologous chromosomes has genes for the same traits.

8. Paired chromosomes Homologous chromosomes both chromosomes of a pair carry �matching� genes control same inherited characters homologous = same information

9. Cell division / Asexual reproduction Mitosis produce cells with same information identical daughter cells exact copies clones same number of chromosomes same genetic information

10. Asexual reproduction Single-celled eukaryotes yeast Paramecium Amoeba Simple multicellular eukaryotes Hydra budding

11. How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? joining of egg + sperm Do we make egg & sperm by mitosis?

12. How do we make sperm & eggs? Must reduce 46 chromosomes ? 23 must half the number of chromosomes haploid

13. Meiosis makes sperm & eggs 46 chromosomes to 23 chromosomes half the number of chromosomes

14. Why meiosis? When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells. Imagine if mitosis were the only means of cell division. IF the parent organism has 14 chromosomes, it would produce gametes that contained a complete set of 14 chromosomes The offspring would have cell nuclei with 28 chromosomes, and the next generation would have cell nuclei with 56 chromosomes

15. Why meiosis? There must be another form of cell division that allows offspring to have the same number of chromosomes as their parents This kind of cell division, which produces gametes containing half the number of chromosomes as a parent�s body cell, is called meiosis. Meiosis reduces the chromosome number from diploid to haploid

16. Why meiosis? Meiosis consists of two separate divisions, known as meiosis I and meiosis II. Meiosis I begins with one diploid (2n) cell. By the end of meiosis II, there are four haploid (n) cells. These haploid cells are called sex cells� gametes. Male gametes are called sperm. Female gametes are called eggs. When a sperm fertilizes an egg, the resulting zygote once again has the diploid number of chromosomes. This pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction

17. The Phases of Meiosis Interphase the cell replicates its chromosomes. After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

18. The Phases of Meiosis Prophase I The chromosomes coil up and a spindle forms. As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

19. The Phases of Meiosis Prophase I The chromatids in a tetrad pair tightly In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

20. The Phases of Meiosis Prophase I Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time. Genetic recombination results from crossing over during prophase I of meiosis This increases variation further

21. The Phases of Meiosis Metaphase I During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber. The spindle fibers pull the tetrads into the middle

22. The Phases of Meiosis Anaphase I begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell. This critical step ensures that each new cell will receive only one chromosome from each homologous pair.

23. The Phases of Meiosis Telophase I Events occur in the reverse order from the events of prophase I. The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells

24. Meiosis 1 overview 1st division of meiosis

25. The Phases of Meiosis The phases of meiosis II The second division in meiosis is simply a mitotic division of the products of meiosis I. During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

26. The Phases of Meiosis Metaphase II The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

27. The Phases of Meiosis Anaphase II begins as the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles.

28. The Phases of Meiosis Telophase II Finally nuclei, reform, the spindles break down, and the cytoplasm divides. At the end of meiosis II, four haploid cells have been formed from one diploid cell. These haploid cells will become gametes, transmitting the genes they contain to offspring.

29. The Phases of Meiosis At the end of meiosis II, four haploid cells have been formed from one diploid cell. These haploid cells will become gametes, transmitting the genes they contain to offspring.

30. 2nd division of meiosis looks like mitosis Meiosis 2 overview

31. Meiosis = reduction division Meiosis special cell division in sexually reproducing organisms reduce number of chromosomes 2n ? 1n diploid ? haploid half makes gametes sperm, eggs

32. Meiosis & mitosis Meiosis to make gametes sperm & egg Mitosis to make copies of cells growth repair development

33. Comparison of Mitosis & Meiosis MITOSIS MEIOSIS

34. Sexual reproduction lifecycle

35. Putting it all together� meiosis ? fertilization ? mitosis + development

36. Meiosis Provides for Genetic Variation Cells that are formed by mitosis are identical to each other and to the parent cell. Crossing over during meiosis I, however, provides a way to rearrange gene combinations. Thus, variability is increased. Reassortment of chromosomes and the genetic information they carry is called genetic recombination.

37. Any Questions??

38. 2006-2007

39. Chromosomal abnormalities Incorrect number of chromosomes nondisjunction chromosomes don�t separate properly during meiosis breakage of chromosomes deletion duplication inversion translocation

40. Human chromosome disorders High frequency in humans most embryos are lost to miscarriage alterations are too disastrous developmental problems result from biochemical problems Certain conditions are tolerated upset the balance less = survivable characteristic set of symptoms = syndrome

41. Nondisjunction The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction

42. Nondisjunction Problems in meiosis cause errors in daughter cells chromosome pairs do not separate properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes

43. Down syndrome Trisomy 21 3 copies of chromosome 21 1 in 700 children born in U.S. Chromosome 21 is the smallest human chromosome but still severe effects Frequency of Down syndrome is related tothe age of the mother Trisomy 13 occurs in about 1 out of every 5,000 live births. It is a syndrome with multiple abnormalities, many of which are not compatible with life. More than 80% of children with trisomy 13 die in the first month. Trisomy 13 is associated with multiple abnormalities, including defects of the brain that lead to seizures, apnea, deafness, and eye abnormalities. The eyes are small with defects in the iris (coloboma ). Most infants have a cleft lip and cleft palate, and low-set ears. Congenital heart disease is present in approximately 80% of affected infants. Hernias and genital abnormalities are common. Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 3,000 live births, and affecting girls more than three times as often as boys. The presence of an extra number 18 chromosome leads to multiple abnormalities. Many of these abnormalities make it hard for infants to live longer than a few months. The cri du chat syndrome is caused by the deletion of information on chromosome 5. It is likely that multiple genes on chromosome 5 are deleted. One deleted gene, called TERT (telomerase reverse transcriptase) is involved in control of cell growth, and may play a role in how some of the features of cri cu chat develop. The cause of this rare chromosomal deletion is not known, but it is expected that the majority of cases are due to spontaneous loss of a piece of chromosome 5 during development of an egg or sperm. A minority of cases result from one parent carrying a rearrangement of chromosome 5 called a translocation. Between 1 in 20,000 and 1 in 50,000 babies are affected. This disease may account for up to 1% of individuals with severe mental retardation. Infants with cri du chat syndrome commonly have a distinctive cat-like cry. They also have an extensive grouping of abnormalities, with severe mental retardation being the most important.Trisomy 13 occurs in about 1 out of every 5,000 live births. It is a syndrome with multiple abnormalities, many of which are not compatible with life. More than 80% of children with trisomy 13 die in the first month. Trisomy 13 is associated with multiple abnormalities, including defects of the brain that lead to seizures, apnea, deafness, and eye abnormalities. The eyes are small with defects in the iris (coloboma ). Most infants have a cleft lip and cleft palate, and low-set ears. Congenital heart disease is present in approximately 80% of affected infants. Hernias and genital abnormalities are common. Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 3,000 live births, and affecting girls more than three times as often as boys. The presence of an extra number 18 chromosome leads to multiple abnormalities. Many of these abnormalities make it hard for infants to live longer than a few months. The cri du chat syndrome is caused by the deletion of information on chromosome 5. It is likely that multiple genes on chromosome 5 are deleted. One deleted gene, called TERT (telomerase reverse transcriptase) is involved in control of cell growth, and may play a role in how some of the features of cri cu chat develop. The cause of this rare chromosomal deletion is not known, but it is expected that the majority of cases are due to spontaneous loss of a piece of chromosome 5 during development of an egg or sperm. A minority of cases result from one parent carrying a rearrangement of chromosome 5 called a translocation. Between 1 in 20,000 and 1 in 50,000 babies are affected. This disease may account for up to 1% of individuals with severe mental retardation. Infants with cri du chat syndrome commonly have a distinctive cat-like cry. They also have an extensive grouping of abnormalities, with severe mental retardation being the most important.

44. Down syndrome & age of mother

45. Nondisjunction Although organisms with extra chromosomes often survive, organisms lacking one or more chromosomes usually do not. When a gamete with a missing chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome This condition is called monosomy A woman with Turner syndrome lacks an X chromosome (XO) A man with Klinefelter syndrome has an extra X chromosome (XXY)

46. Turner syndrome Monosomy X or X0 1 in every 5000 births varied degree of effects webbed neck short stature sterile How many Barr bodies would you expect? How many Barr bodies would you expect?

47. XXY male one in every 2000 live births have male sex organs, but are sterile feminine characteristics some breast development lack of facial hair tall normal intelligence Klinefelter�s syndrome

48. Klinefelter�s syndrome How many Barr bodies would you expect? How many Barr bodies would you expect?

49. Nondisjunction When a gamete with an extra set of chromosomes is fertilized by a normal haploid gamete, the offspring has three sets of chromosomes and is triploid.(3n) The fusion of two gametes, each with an extra set of chromosomes, produces offspring with four sets of chromosomes�a tetraploid. (4n) This is polyploidy.