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Sexual Reproductionand Meiosis
Chapter 11
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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 cellAdult body cells (somatic cells) are diploid,
containing 2 sets of chromosomes.Gametes are haploid, containing only 1 set
of chromosomes.
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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.
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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.
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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.
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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
Meiosis: The Creations of Gametes
Meiosis 1
Meiosis 2
Non-Disjunction During Meiosis
Non-disjunction in Meiosis 1 Non-disjunction in Meiosis 2
Trisomy zygoteMonosomy zygote
Aneuploidy
• Effects vary by chromosomal condition
• Many cause early miscarriages
• Leading cause of mental retardation
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
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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
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
Chorionic Villus Sampling (CVS)
Karyotype
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
Duplication
Deletion
Karyotype of Deletion on Chromosome 16
Inversion
Translocation
Translocation Karyotype
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
Patau Syndrome
Trisomy 13: Patau Syndrome (47,+13)
• 1/15,000
• Survival: 1–2 months
• Facial, eye, finger, toe, brain, heart, and nervous system malformations
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
Down Syndrome
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
Maternal Age and Down Syndrome
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)
Turner Syndrome
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
Klinefelter Syndrome
Klinefelter Syndrome (47,XXY)
• Survival to adulthood
• Features do not develop until puberty, usually sterile, may have learning disabilities
• 1/1,000 males
XYY Syndrome
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
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
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
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
Genetic Counseling
• Most see a genetic 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
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