3.3 Meiosis
3.3 Meiosis – Understandings:
One diploid nucleus divides by
meiosis to produce 4 haploid
nuclei
The halving of the chromo #
allows a sexual life cycle with
fusion of gametes
DNA is replicated before meiosis
so that all chromos consist of 2
sister chromatids
Early stages of meiosis involve
pairing of homo chromos and
crossing over followed by
condensation
Orientation of pairs of homo
chromos prior to separation is
random
Separation of pairs of homo
chromos in the 1st division of
meiosis halves the chromo
number
Crossing over and random
orientation promotes genetic
variation
Fusion of gametes from
different parents promotes
genetic variation
One diploid nucleus divides by meiosis to
produce 4 haploid nuclei.
• Diploid (2n) = containing 2 sets of chromosomes, 1 from each parent
• Haploid (n) = containing a single set of chromosomes
The halving of the chromosome number allows a
sexual life cycle with fusion of gametes.
• Fertilization = fusion of gametes
• Gametes = sex cells = egg & sperm
• Sexual reproduction involves fertilization.
• Fertilization doubles the # of chromos each time it occurs
• Meiosis is important because it halves the chromo # so that
fertilization results in a cell with the diploid # (otherwise the chromo
# would double with every generation!)
DNA is replicated before meiosis so that all
chromos consist of 2 sister chromatids.
• DNA is replicated during interphase before Meiosis I.
• DNA is not replicated again before Meiosis II.
The early stages of meiosis involve pairing of homo
chromos & crossing over followed by condensation.
Crossing over
• Occurs during Prophase I of Meiosis only
• Synapsis = coming together of 2 homo chromos (“bivalents”)
• Tetrad = 4 chromatids in a row
• Cross-over of non-sister chromatids
• Chiasma (chiasmata, plural) = point of crossover and exchange of
pieces
• Crossing over increases genetic variation!
Crossing Over
Can have multiple cross-overs
Positive correlation of distance between
genes and % cross-over
Orientation of pairs of homo chromos prior to
separation is random.
• Orientation = which way chromo is facing
• Random orientation = each pair of chromos has equal chance of
attaching to either spindle fiber
• Independent assortment = the orientation of one pair of chromos has
no affect on the orientation of other pairs of chromos
Possible orientations of homo chromos = 2n
(n=haploid #)
Separation of pairs of homo chromos in the
st
1 division of meiosis halves the chromo #.
• Disjunction = separation of homo chromos
• = dividing # of chromos in half  occurs in Meiosis I
Crossing over & random orientation promotes
genetic variation.
2 processes in meiosis that cause diversity (genetic variation) even
within a family:
• Random orientation
 each egg has 2n possible chromosome combinations
 each sperm has 2n possible chromosome combinations
• Crossing over
Allows linked genes (genes on same chromo) to be reshuffled onto
different chromos, producing infinitely more combinations!
Fertilization of gametes from different
parents promotes genetic variation.
Fertilization = fusion of gametes (egg and sperm) to produce zygote:
• Start of the life of a new individual
• Allows alleles from 2 different individuals to be combined into one
• Combo of alleles is unlikely ever to have existed before!
• Fusion of gametes promotes genetic variation in a species
• Genetic variation is essential for evolution
In Summary…
4 processes that promote genetic variation:
In Meiosis:
• CO of NS HC in PO of M
• RO of HC in MO of M
Not in Meiosis:
• RF of E by S
•M
Nature of Science: making careful observations:
Meiosis was discovered by microscope examination of dividing
germ-line cells.
• 1800’s – improved microscopes allowed us to see inside cells; dye used to
stain nucleus
• 1880’s – German biologists observed dividing nuclei
• Preparation of slides showing meiosis is challenging
• Key observation made: Horse threadworm – fertilized egg has 4
chromosomes, whereas egg and sperm have 2
• Hypothesis made: a special nuclear division halves the chromo #
• Sequence of meiosis events observed in rabbit eggs (0-28 day old rabbits)
• Advantage of observing rabbits: meiosis begins at birth & occurs slowly
over many days
Skill: Draw diagram showing stages of meiosis
resulting in formation of 4 haploid cells.
Difference between oogenesis &
spermatogenesis
Application: Non-disjunction can cause Down syndrome and other
chromosome abnormalities. Studies showing age of parents influences
chances of non-disjunction.
• Disjunction = separation of chromosomes
• Non-disjunction = failure of chromos to separate - Can occur in MI or MII
Trisomy 21 (Down’s Syndrome)
Incidence of Down’s syndrome vs mom’s age
Application: Methods used to obtain cells for
karyotype analysis e.g. CVS and amniocentesis and
associated risks.
Chorionic Villus Sampling
Risk of miscarriage = 2%
10-13 weeks
Amniocentesis
Risk of miscarriage = 1%
15-22 weeks
Couples may choose to have certain prenatal tests
for different reasons, including to:
• Allow for possible medical interventions that may be needed
• Begin planning for a child with special needs
• Identify support groups and resources
• Make a decision about whether to continue the pregnancy