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Meiosis
Biology
Chapter 11.4
Mr. Hines
Mendel was correct with his approach to
genetics, be he did not know where genes
were located in living things.
Later discoveries determined that genes were
found on chromosomes inside of the
nucleus.
Mendel’s principles required 2 things
1. Each organism must inherit a single copy of
every gene from each parent.
2. When gametes are formed, the genes must
be segregated from each other.
So there must be a process that separates
genes – meiosis was discovered.
Chromosome number
Very important terms
Homologous chromosomes – term used to
refer to the chromosomes that each have a
corresponding chromosome from the
opposite sex parent.
Each parent organism has 2 matching
chromosomes – one from mom and one
from dad – homologous chromosomes.
Diploid – term used to refer to a cell that
contains both sets of homologous
chromosomes.
Sometimes the term diploid is written with the
symbol 2N.
All of the cells in an organism are diploid
except sex cells (gametes)
Haploid – term used to refer to a cell that
contains only a single set of chromosomes
and therefore only single set of genes.
All gametes are haploid – the chromosomes
have been segregated and therefore each
gamete will only have one of each
chromosome (one chromosome from one
parent)
Human Karyotype (photograph of
chromosomes cut out and arranged)
So how do the chromosomes
segregate into gametes?
Meiosis is a process of a reduction division in
which there number of chromosomes per
cell is cut in half through the separation of
homologous chromosomes in a diploid
cell to create a haploid cell.
Meiosis is broken into two distinct
phases
Meiosis I
Meiosis II
Meiosis I contains 4 sub phases
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Prophase I
Metaphase I
Anaphase I
Telophase I
PMAT – just like in Mitosis
Meiosis II contains 4 sub phases
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Prophase II
Metaphase II
Anaphase II
Telophase II
Before we begin Meiosis, we must
explore the concept of
fertilization first
Humans have 23 homologous chromosomes
(total of 46)
This is too complicated to model – we will
only use 2 homologous chromosomes so
that the drawings are more
understandable.
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The following drawings are not in your book.
Pay close attention.
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At this point, we can begin meiosis
as seen in your book on page 276
Prophase 1
1. Each pair of chromosomes pair with their
corresponding homologous chromosome to
form a tetrad.
2. Crossing Over occurs
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Prophase 1
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Metaphase I
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1. Spindle fibers attach to chromosomes
Metaphase I
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Anaphase I
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The fibers pull the homologous
chromosomes to opposite ends of the cell.
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Telophase I
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Nuclear membrane forms and 2 new cells
are a result
Telophase I
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If you have been following the color
sequence, mom and dad’s chromosomes
have now been separated.
Notice that after crossing over, each
homologous chromosome has exchanged
genetic material with the other.
So mom and dad’s chromosomes mixed – just
a little.
After Telophase I
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Prophase II
• Meiosis I results in 2
daughter cells, each
with half the number
of chromosomes from
the original.
Metaphase II
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Chromosomes line up at the center of the
cell – Similar to Metaphase I
Metaphase II
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Anaphase II
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The sister chromotids separate and move
to opposite ends of the cell.
Anaphase II
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Telophase II
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This is the final stage of meiosis
there are now 4 individual haploid
daughter cells
Notice that none of them are the same as
indicated by colors.
These cells can now be called “gametes.”
Telophase II
For males, these will be
sperm
For females, these will
become eggs.
Notice that these are
haploid cells.
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Crossing over in more detail
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Crossing-Over
Section 11-4
Crossing-Over
Section 11-4
Crossing-Over
Section 11-4