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Meiosis
Page 275
Meiosis
• The most important things to remember in
distinguishing Mitosis from Meiosis are:
- Mitosis deals in body cells. (SOMATIC CELLS)
- Meiosis deals with sex cells. (SEX CELLS)
- Mitosis results in the production of 2 genetically
identical diploid cells.
- Meiosis produces 4 genetically different haploid
cells.
Meiosis
• Every organism must inherit a single copy of every
gene from each of its parents. Humans get 23
chromosomes from our mothers and and 23 from
our fathers: 46 total.
• When gametes (sex cells) are produced, 2 sets of
genes have to be separated from each other so
that each gamete contains just one set of genes.
Meiosis
• A cell that contains both sets of homologous
chromosomes is said to be diploid.
• This means “two sets”.
2N = 46
for humans
• Diploid cells contain 2 complete sets of
chromosomes therefore 2 complete sets of genes.
Meiosis
• In contrast, haploid cells, gametes of sexually
reproducing organisms, contain only a single set of
chromosomes, and only a single set of genes.
N = 23
for humans
Meiosis
• Let’s start with an easier number and separate it.
A fruit fly has 8 chromosomes total. 4 from mom
and 4 from dad. These chromosomes are
homologous, meaning that each of the 4
chromosomes that came from the male parent
has a corresponding chromosome from the female
parent.
Chromosome 1 from mom matches up with
Chromosome 1 from dad. This makes a homologous pair.
Each chromosome in a homologous pair is:
 the same size
 the same shape
 the same gene arrangement (generally
genes that code for the same trait)
but
 they are not identical because the actual
code is different. (have different alleles
which means copies of the gene)
For example, both chromosomes in a
homologous pair might code for “curling of
your tongue” but one might let you curl your
tongue and the other might not.
Meiosis
How are haploid (N) gamete cells produced from
diploid (2N) cells?
By meiosis
Meiosis is a process of reduction division in which
the number of chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell.
Meiosis
Meiosis consists of two separate processes called
Meiosis I and Meiosis II.
The phases have the same names as the phases
in mitosis except the name is followed by “I” or
“II”, depending on whether the phase is in
meiosis I or meiosis II.
Before you go any further, watch the Crash
Course video on meiosis.
http://www.youtube.com/watch?v=qCLmR9YY7o
Meiosis I
• Before Meiosis I, the chromosomes are replicated.
(Just like before mitosis)
• In Prophase I of Meiosis I, each homologous
chromosome pair joins with its corresponding
homologous pair to form a tetrad.
• See Figure 11-16 page 276 in your book!
• Synapsis is the process in which homologous pairs of
chromosomes pair up during Prophase I to form
what is called a tetrad.
• During synapsis parts of the chromatids in a
tetrad may twist around each other, break off
and attach to another chromatid in a process
called crossing over.
• This results in exchange of genetic
material between the maternal and
paternal chromosomes. This process
is called crossing over. The result of
crossing over is genetic
recombination.
• The point at which the chromosomes
cross over is called the chiasma.
Crossing-Over
Section 11-4
Go to
Section:
Tetrad
• Remember, these are the
homologous chromosomes
together.
• There are 4 sister chromatids in
a tetrad.
Prophase I of Meiosis I is where crossing over takes place!
Crossing-Over
Section 11-4
Go to
Section:
Crossing-Over
Section 11-4
Go to
Section:
Meiosis I
Interphase I
Cells undergo a round of
DNA replication, forming
duplicate Chromosomes.
Go to
Section:
Prophase I
Each chromosome pairs with
its corresponding
homologous chromosome to
form a tetrad.
Metaphase I
Spindle fibers attach to the
chromosomes.
Anaphase I
The fibers pull the
homologous chromosomes
toward the opposite ends of
the cell.
Law of Independent Assortment
• During metaphase I, as the homologous pairs line up,
the orientation of maternal and paternal
chromosomes is random
Not all maternal go to one side, and paternal the
other….it is mixed.
As the pairs of chromosomes are separated in
anaphase I, the maternal and paternal chromosomes
have random separation which results in genetic
variation.
Meiosis II
• The 2 cells produced in Meiosis 1 then go through
another meiotic division.
• There is no DNA replication between Meiosis I and
Meiosis II.
• The end result of Meiosis II is 4 cells with 2 sister
chromatids in them.
Meiosis II
Prophase II
Meiosis I results in two
haploid (N) daughter cells,
each with half the number of
Go to
chromosomes as the
Section:
original.
Metaphase II
The chromosomes line
up in a similar way to the
metaphase stage of
mitosis.
Anaphase II
The sister chromatids
separate and move
toward opposite ends
of the cell.
Telophase II
Meiosis II results in
four haploid (N)
daughter cells.
Chromosomal Disorders
• Non-disjunction: when this occurs,
abnormal numbers of chromosomes may
find their way into gametes, and a disorder
of chromosome numbers may result.
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Go to
Section:
Meiosis I:
Nondisjunction
Meiosis II
Review – If you understand this, you understand
meiosis.
• http://www.youtube.com/watch?v=iCL6d0Ow
Kt8