1. Sexual Reproduction and Meiosis
Chapter 11

2. Overview of Meiosis
Meiosis is a form of cell division that leads to the production of gametes.
gametes: egg cells and sperm cells
-contain half the number of chromosomes of an adult body cell
Adult body cells (somatic cells) are diploid, containing 2 sets of chromosomes.
Gametes are haploid, containing only 1 set of chromosomes.

3. Overview of Meiosis
Sexual reproduction includes the fusion of gametes (fertilization) to produce a diploid zygote.
Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages.
Some life cycles include longer diploid phases, some include longer haploid phases.

8. Features of Meiosis
Meiosis includes two rounds of division – meiosis I and meiosis II.
During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis.
Proteins between the homologues hold them in a synaptonemal complex.

10. Features of Meiosis
Crossing over: genetic recombination between non-sister chromatids
-physical exchange of regions of the chromatids
chiasmata: sites of crossing over
The homologues are separated from each other in anaphase I.

11. Features of Meiosis
Meiosis involves two successive cell divisions with no replication of genetic material between them.
This results in a reduction of the chromosome number from diploid to haploid.

13. The Process of Meiosis Prophase I: -chromosomes coil tighter
-nuclear envelope dissolves
-homologues become closely associated in synapsis
-crossing over occurs between non-sister chromatids

16. The Process of Meiosis Metaphase I:
-terminal chiasmata hold homologues together following crossing over
-microtubules from opposite poles attach to each homologue, not each sister chromatid
-homologues are aligned at the metaphase plate side-by-side
-the orientation of each pair of homologues on the spindle is random

20. The Process of Meiosis Anaphase I:
-microtubules of the spindle shorten
-homologues are separated from each other
-sister chromatids remain attached to each other at their centromeres

22. The Process of Meiosis Telophase I:
-nuclear envelopes form around each set of chromosomes
-each new nucleus is now haploid
-sister chromatids are no longer identical because of crossing over

24. The Process of Meiosis Meiosis II resembles a mitotic division:
-prophase II: nuclear envelopes dissolve and spindle apparatus forms
-metaphase II: chromosomes align on metaphase plate
-anaphase II: sister chromatids are separated from each other
-telophase II: nuclear envelope re-forms; cytokinesis follows

29. Meiosis vs. Mitosis Meiosis is characterized by 4 features:
1. Synapsis and crossing over
2. Sister chromatids remain joined at their centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach to the same pole in meiosis I
4. DNA replication is suppressed between meiosis I and meiosis II.

30. Meiosis vs. Mitosis
Meiosis produces haploid cells that are not identical to each other.
Genetic differences in these cells arise from:
-crossing over
-random alignment of homologues in metaphase I (independent assortment)
Mitosis produces 2 cells identical to each other.

32. Nondisjunction Chromosomes fail to separate
Results in gametes and zygote with an abnormal chromosome number
Aneuploidy is variations in chromosome number that involve one or more chromosomes
Most aneuploidy from errors in meiosis

33. Meiosis: The Creations of Gametes

34. Non-Disjunction During Meiosis
Non-disjunction in Meiosis 1
Non-disjunction in Meiosis 2

35. Monosomy zygote
Trisomy zygote

36. Aneuploidy Effects vary by chromosomal condition
Many cause early miscarriages
Leading cause of mental retardation

37. ID of Chromosomal Abnormalities
Two tests:
Amniocentesis (> 16 weeks)
Collects amniotic fluid
Fetal cells grown and karyotype produced
Chorionic villus sampling (CVS) (10–12 weeks)
Rapidly dividing cells
Karyotype within few days

38. Fetal cells are removed from the solution
Removal of about 20 ml of amniotic fluid containing suspended cells that were sloughed off from the fetus
Biochemical analysis of the amniotic fluid after the fetal cells are separated out
Centrifugation
Fetal cells are removed from the solution
Analysis of fetal cells to determine sex
Cells are grown in an incubator
Karyotype analysis
p. 46

39. Amniocentesis Only Used in Certain Conditions
Risks for miscarriage; typically only done under one of following circumstances:
Mother > 35
History of child with chromosomal abnormalities
Parent has abnormal chromosomes
Mother carries a X-linked disorder
History of infertility or multiple miscarriages

40. Chorionic Villus Sampling (CVS)

41. Karyotype

42. Other Chromosomal Variations
Haploid: one copy of each chromosome
Diploid: normal two copies of each chromosome
Polyploidy: multiple sets of chromosomes
Aneuploid: A variation in chromosome number, but not involving all of the chromosomes
Trisomy: three copies of one chromosome
Monosomy: only one copy of a chromosome
Structural changes: duplication, deletion, inversion, translocation

43. Duplication

44. Deletion

45. Karyotype of Deletion on Chromosome 16

46. Inversion

47. Translocation

48. Translocation Karyotype

49. Effects of Changes in Chromosomes
Vary by chromosome and type of variation
May cause birth defects or fetal death
Monosomy of any autosome is fatal
Only a few trisomies result in live births

50. Patau Syndrome

51. Trisomy 13: Patau Syndrome (47,+13)
1/15,000
Survival: 1–2 months
Facial, eye, finger, toe, brain, heart, and nervous system malformations

52. Trisomy 18: Edwards Syndrome (47,+18)
1/11,000, 80% females
Survival: 2–4 months
Small, mental disabilities, clenched fists, heart, finger, and foot malformations
Die from heart failure or pneumonia

53. Down Syndrome

54. Trisomy 21: Down Syndrome (47,+21)
1/800 (changes with age of mother)
Survival up to age 50
Leading cause of childhood mental retardation and heart defects
Wide, flat skulls; large tongues; physical, mental, development retardation

55. Maternal Age and Down Syndrome

56. Aneuploidy and Sex Chromosomes
More common than in autosomes
Turner syndrome (45,X): monosomy of X chromosome
Klinefelter syndrome (47,XXY)
Jacobs syndrome (47,XYY)

57. Turner Syndrome

58. Turner Syndrome (45,X) Survival to adulthood
Female, short, wide-chested, undeveloped ovaries, possible narrowing of aorta
Normal intelligence
1/10,000 female births, 95–99% of 45,X conceptions die before birth

59. Klinefelter Syndrome

60. Klinefelter Syndrome (47,XXY)
Survival to adulthood
Features do not develop until puberty, usually sterile, may have learning disabilities
1/1,000 males

61. XYY Syndrome

62. XYY or Jacobs Syndrome (47,XYY)
Survival to adulthood
Average height, thin, personality disorders, some form of mental disabilities, and adolescent acne
Some may have very mild symptoms
1/1,000 male births

63. Ways to Evaluate Risks
Genetic counselors are part of the health care team
They assist understanding of:
Risks
Diagnosis
Progression
Possible treatments
Management of disorder
Possible recurrence

64. Counseling Recommendations \
Pregnant women or those who are planning pregnancy, after age 35
Couples with a child with:
Mental retardation
A genetic disorder
A birth defect

65. Counseling Recommendations
Couples from certain ethnic groups
Couples that are closely related
Individuals with jobs, lifestyles, or medical history that may pose a risk to a pregnancy
Women who have had two or more miscarriages or babies who died in infancy

66. Genetic Counseling Most see a genetic counselor: Counselor
After a prenatal test;
After the birth of a child; or
To determine their risk
Counselor
Constructs a detailed family history and pedigree
Shares information that allows an individual or a couple to make informed decisions

67. Future of Genetic Counseling
Human Genome Project (HGP) changed medical care and genetic testing
Genetic counselor will become more important
Evaluate reproductive risks and other conditions
Allow at-risk individuals to make informed choices about lifestyle, children, and medical care