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Meiosis
Chromosomes
• The human body has 23
pairs of chromosomes
• Cells that have 46
chromosomes are called
somatic cells
• In these cells there is 23
pairs of matching
chromosomes
• These chromosomes are a
set and can be referred to
as homologous
chromosomes
Chromosomes
• In certain instances
during a cell’s life cycle
they may alternate the
number of chromosomes
that are present
• A diploid cell contains
pairs of homologous
chromosomes
• This is often abbreviated
as 2n
• That means two times the
normal chromosome
number
Chromosomes
• A haploid cell Is a cell
that contains one copy
of each chromosome
• This is often
abbreviated as n
• This means that these
cells only carry one
copy of each
chromosome
Gametes
• Why might a cell only
want to carry half the
chromosomes needed
for life?
• Normally it is because
these cells are gametes
• Gametes are cells that
can combine to create
offspring during sexual
reproduction
Gametes
• That means that the
gametes are combining
to create offspring with
non identical
chromosomes
• One chromosome will
be donated from the
male
• One chromosome will
be donated from the
female
Gametes
• In a human life cycle
this means that a sperm
(n male gamete) and an
egg (n female gamete)
must combine to create
an offspring
• The initial 2n cell that is
created from the
combination is called a
zygote
Alternating Stages
• This means that all
sexual life cycles
alternate between n
and 2n stages
• During most phases the
life cycle is 2n
• However the
combination of haploid
cells that are n must
happen for there to be
sexual reproduction
Meiosis
• Mitosis creates two
identical daughter cells
• However, if a cell is to
go from 2n to n the has
to be a process to
create a different type
of cell
• Meiosis is a type of cell
division that creates
haploid cells in a diploid
organism
Meiosis
• Meiosis and mitosis are
very similar
• They both contain the
stages prophase,
metaphase, anaphase
and telophase
• However, meiosis
performs two distinct cell
divisions
• This means that cells that
undergo meiosis will
undergo PMAT twice
Meiosis
• During Meiosis I cells
undergo prophase I,
metaphase I, anaphase I
and telophase I
• Immediately after the end
of the first cytokinesis
there is a second round of
PMAT
• This means the cells enter
prophase II, metaphase II,
anaphase II and telophase
II
Meiosis I
• We can call the first
PMAT Meiosis I and the
second PMAT Meiosis II
• During Meiosis I there
two main differences
from Mitosis
• All of the other steps
are very similar to
mitosis
Meiosis I
• During prophase I there is
one main distinct
difference
• Crossing over is a period
in prophase I where
sections of homologous
chromosomes will switch
• This means the
chromosomes do not
remain the same during
meiosis I
Meiosis I
• During metaphase I
chromosomes do not
line up directly in the
center of cells
• The chromosomes line
up in pairs at the center
of the cell
• This means that during
anaphase I there is no
separation of
centromeres
Meiosis II
• Meiosis II is very similar
to mitosis
• The steps in PMAT apply
to most of the steps of
PMAT II
• However, remember that
there is already 2 cells
• The result of these steps
will be 4 non identical
cells