Sexual Reproduction
and Meiosis
Chapter 11
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.
2
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.
3
4
5
6
7
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.
8
9
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.
10
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.
11
12
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
13
14
15
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
16
17
18
19
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
20
21
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
22
23
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
24
25
26
27
28
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.
29
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.
30
31
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
Monosomy zygote
Trisomy 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
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
Analysis of fetal cells
to determine sex
Fetal cells
are removed
from the
solution
Cells are
grown in an
incubator
Karyotype analysis
p. 46
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