Tiana Hobbs and Brandon Douglas
4.2.3, 10.1-10.1.5
Assessment Statement
 4.2.3,10.1.1,10.1.3
 Outline the process of meiosis, including pairing of
homologous chromosomes and crossing over, followed
by two divisions, which results in four haploid cells.
 Describe the behavior of the chromosomes in the
phases of meiosis.
 Explain how meiosis results in an effectively infinite
genetic variety in gametes through crossing over in
prophase I and random orientation in metaphaseI.
Overview
 Meiosis
 Involves two divisions
 1st division called Meiosis I
 Interphase stage before Meiosis I which includes G1, S,
and G2 phases; similar to Interphase stages before
Mitosis.
 Short Interphase stage between Meiosis I and Meiosis II
Phases of Meiosis
 Prophase I- within this stage, the chromosomes are becoming visible
 The homologous chromosomes pair up and crossing over occurs
 The nuclear membrane separates and the centrioles move to the poles of the cell
 Metaphase I
 Homologous chromosome pairs line up at equator of cell
 Spindle fibers attach to chromosomes so that one
chromosome from each pair is moved to each pole.
 Anaphase I
 Homologous chromosomes separated and pulled to
opposite poles
 Unique number of chromosomes reduced to 23 from 46,
although chromosomes are still composed of 2 sister
chromatids.
 Cell prepares for Cytokinesis
 Telophase I
 Each chromosome from each homologous pair are
located at opposite poles
 Nuclear membrane reforms around each daughter
nucleus
 Cell divides through cytokinesis
 Meiosis II
 Starts with two daughter haploid cells
 Ends in the formation of four daughter cells, each with a
haploid set of chromosomes.
 Prophase II
 Two daughter haploid cells present.
 Nuclear membrane begins to break down
 Spindle fibers form
 Metaphase II
 Chromosomes in each cell line up at equate of each
respective cell
 Spindle fibers attaches to the centromeres of each
chromosome
 Anaphase II
 Chromosomes are divided at centromere as a result of
the spindle fibers pulling each sister chromatid to
opposite poles.
 Sister chromatids then become chromosomes
 Telophase II
 Nuclear membranes form around each of the four sets of
chromosomes
 Cell divides through cytokinesis
 Results in four daughter cells each with a haploid set of
chromosomes.
Crossing Over
 Crossing Over
 Prophase I
 Forms chromatids with new combinations of alleles
 Recombinants are chromatids with different
combinations of alleles than either parent chromatid
 Crossing over occurs at a random point
 More than one chiasma can form per homologous pair.
Random Orientation
 Random Orientation
 Metaphase I
 Homologous chromosomes line up at the equator in a random
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order
Maternal or Paternal homologue can be pulled to either pole
To calculate all variations you raise the number 2 to however
many chromosomes you are dealing with.
For a human, the total variations are over 8 million.
Combining that with the fact of crossing over, means that
there are almost an infinite number of genetic variations
cause my meiosis.
Assessment Statement
 10.1.1
 Describe the behavior of the chromosomes in the phases
of meiosis.
The Chiasmata
 Before the chiasmata can appear, the DNA of one
chromatid bonds together with the DNA of a non sister
chromatid
 Then, both the genetic information is transferred from
both the father and mother (maternal and paternal
chromosomes) this process is called crossing over
 After the homologous chromosomes are no longer tightly
connected, the chiasmata is formed and it links the
homologous chromosomes pairs together until late
metaphase I
Assessment Statement
 10.1.4
 State Mendel’s law of independent assortment
Mendel’s law of Independent
Assortment
 the transmission of traits to the offspring are independent to
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one another
When the chromosomes move to the opposite ends of the pole
(of the equator) either chromosome can end up at a specific end
However, the movement of one chromosome to a pole does not
affect the placement of the other
This connects to the law of independent assortment (Mendel’s)
because when the homologous chromosome pairs separate, it
allows the formation of daughter cells
Contributes to Genetic Variation
Works Cited
 "Animation: Random Orientation of Chromosomes
During Meiosis." Your Page Title. N.p., n.d. Web.
17 Feb. 2012. <http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapte
r28/animation__random_orientation_of_chro
 "IB Biology Notes - Meiosis HL." IB Guides - Free
International Baccalaureate study guides, notes,
videos and powerpoints.. N.p., n.d. Web. 17 Feb.
2012.
<http://www.ibguides.com/biology/notes/meiosi
s->.