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Meiosis Animation
Meiosis
• A second type of cell division designed to make
haploid cells called gametes
• Gametes include sperm and egg
• Meiosis occurs in 2 parts
• Meiosis 1 and Meiosis 2
• Meiosis 1 is a reductive division where
homologous chromosomes line up together on a
spindle fiber
• Therefore, # spindle fibers required = half of
diploid number = haploid number
Meiosis Tutorial: The Biology Project!
• Structure formed when chromosomes come
together is called a tetrad
• A tetrad consists of 2 replicated chromosomes or
2 chromatids
• Chromatids are chromosomes that are still
attached to their sister strand via a centromere
• When centromere eventually splits and
chromatids are free of their sister chromatid,
both are called chromosomes….
• this does not happen until anaphase 2
Excellent Narrated Movie of Meiosis
Main Difference between
Mitosis and Meiosis
• During prophase 1, homologous (similar)
chromosomes come together
• This process is known as synapsis
• During this time the chromosomes are so close
together that crossing over can occur
• This leads to genetic variation since sections of the
DNA are exchanged mixing maternal and
paternal genes
Prophase 1
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Nuclear membrane disintegrates
Spindle fibers form
Chromosomes become distinct
Homologous (similar) chromosomes come
together
• This process is called
synapsis
Stages of Meiosis
Always ask yourself
“What’s going on here?”
Stages of Meiosis
Metaphase 1
• Homologous replicated chromosomes line up on
the same spindle fiber
• Individual chromatids in the replicated
chromosomes do not separate yet
• Not until Anaphase 2
• In human cell, since 2n = 46
• 23 pairs line up
• Therefore 23 spindle fibers are needed
• If 2n = 10, 5 spindle fibers are required for this
stage
Stages of Meiosis
Anaphase 1
• Homologous replicated chromosomes
separate to opposite poles
• Centromeres do not split yet
Stages of Meiosis
Telophase 1
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Chromosomes arrive at opposite poles
Cytoplasm pinches in
Cytokinesis occurs in animal cells
Equatorial plate forms in plant cells
2 haploid cells form
Chromosomes may become chromatin
and enter an interphase 2 in some cells
Stages of Meiosis
Prophase 2
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New spindle fibers form
Nuclear membrane dissolves
Chromosomes become visible
If 2n = 10, then only five spindle fibers
needed…why?
• Half of the chromosomes are in the other
cell
Stages of Meiosis
Metaphase 2
• Chromosome line up on new spindle fibers
• Chromosomes here are made up of 2
chromatids
• If 2n = 46, 23 chromosomes are in each cell
produced from telophase 1
• This means that 23 spindle fibers must be
present
• If 2n = 8, 4 spindle fibers needed now
Stages of Meiosis
Anaphase 2
• Centromere splits
• Chromatids (now called chromosomes)
migrate to opposite poles of the cell
Stages of Meiosis
Telophase 2
• Chromatids (now called chromosomes)
arrive at poles
• Cytoplasm starts to pinch in
• Cytokinesis occurs
• End result is 4 haploid cells
• 2 from each of the 2 cells produced at the
end of telophase 1
Stages of Meiosis
Notice the difference
Stages of Meiosis
End result of
Overview of Meiosis 1
Overview of Meiosis 1
Overview of Meiosis II
Meiosis in males and females
• In males called
Spermatogenesis
• Involves production of 4
equal sized cells each with
same amount of
cytoplasmic organelles
• In females called
Oogenesis
• First division unequal
One cell is a polar body
and smaller
• Second division produces
another smaller cell called
a polar body and then the
Egg
• Egg contains the majority
of cytoplasmic organelles
Oogenesis
Spermatogenesis
Meiosis Produces Sperm Cells
Structure of Human Sperm
Abnormal Meiosis
• When homologous chromosomes fail to separate
during anaphase of meiosis…
• Abnormal gametes form with abnormal numbers of
chromosomes (trisomy 21 etc..)
• Process called non-disjunction
• Karyotyping used to detect disorders
• Chromosomes are arranged according to 3
characteristics
– Centromere position
– Banding pattern
– Size
Non-disjunction animation
Sources of Variation
1. Gamete success
2. Crossing over
depends on distance between genes and
length of chromosome
3. Independent Assortment
the way one homologous pair lines up has nothing to
do with the way another homologous pair lines up!
Animation of Crossing Over
Another great animation!!
Homologous pair of chromosomes
Independent Assortment
Detecting genetic Disorders
• Ultrasound locates position of fetus
• Amniocentesis uses a needle to withdraw some
amniotic fluid from around fetus
• Fluid is in uterus
• Fetal cells can be analysed
• Chorionic Villus Sampling (CVS) draws cells
from outer membrane of embryo
• CVS can be used as early as 8 weeks of
pregnancy
Animal Life Cycle
Plant Life Cycle
Alternation of
Generations
Sexual Stage
Alternates with
Asexual
Plant and Animal Life Cycle
Meiosis
Oogenesis
Spermato-genesis
Sperm
Egg (Ovum)
Zygote
Mitosis
Embryo
Fetus
Child
Moss
Life
Cycle
Demonstrates
Alternation
Of
Generations
Alternation of Generations
• Moss Plant
• Fern Plant
• Sexual stage alternates with
an asexual stage
• Sexual stage involves
fertilization by union of 2
gametes (needs water)
• Asexual stage involves spore
production
• One cannot exist without the
other
• Mitosis & Meiosis occurs
• Sexual & Asexual stage
separate
• Same
• Same
• Same
• Same
• Sexual & Asexual stage on
same plant
Fern Life Cycle
Alternation of
generations
Alternation of Generations in Fern Plants
Cell Division Tutorials/Questions