SBI 3U
Meiosis
Mitosis Recap
 Mitosis is a process that occurs in the somatic cells of your body (hair, skin, eyes, teeth,
blood, bones, etc.). The process of mitosis produces cells that have ________________
sets of chromosomes.
 Somatic cells (cells that reproduce by mitosis only), will always have 46 chromosomes
(Diploid number: _____________)
Why We Need Meiosis
 Meiosis is the process by which sex cells, or _________________, are formed (also known
as _______________________). These cells are found in the
ovaries and testes. The purpose of meiosis is to produce cells with
half the normal set of chromosomes. In order for this to happen,
cells of the ovary and testes undergo two sets of divisions.
 A cell with only half of a set of chromosomes is called a
________________ CELL, n. For humans, n=23
 During _______________________, a haploid sperm cell and a
haploid egg cell join together to form a zygote with a full set of
chromosomes. This zygote is called a ________________ cell. It
will become an embryo if it implants in the woman’s uterus.
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What may surprise you is that you receive a nearly complete set of
genetic material from each of your parents! Each parent
contributes a copy of virtually every gene!
Each of the 23 chromosomes you receive from your
_____________ is matched by 23 chromosomes from your
_____________. The paired chromosomes are called
______________________ chromosomes.
These pairs are similar, but not identical. They are made up of the
same genes, but may have different forms of genes called
____________ -i.e. the allele for eye colour on the chromosome
inherited from your mother codes for green eyes, but the allele
from on the chromosome from your father codes for brown
Stages of Meiosis
Meiosis has two stages, Meiosis I and Meiosis II. Meiosis I is similar to Mitosis. Meiosis II is
exactly like Mitosis. All of the phases of each stage have the same names as the stages of
Mitosis.
(_________________, ________________, _________________ and ________________).
What is the name of the phase that is the precursor to Mitosis?____________________
This is the same with meiosis. Interphase must occur before meiosis can begin. (This
means what? chromosomes _________________ and the cell prepares for ____________).
SBI 3U
Meiosis I – The First Cell Division
PROPHASE I
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Duplicated chromosomes shorten and thicken, nuclear
membrane starts to dissolve, centrioles move to opposite poles,
spindle fibres form.
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Sister chromatids pair up to form a _________________
Each sister chromatid intertwines with a sister chromatid from its
matching homologous chromosome, a process called
____________________.
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As prophase I continues, it is common for the intertwined
chromatids from different chromosomes to break and reattach to
each other - exchanging sections in a process called
__________________ ______________.
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Crossing over results in the recombination (mixing) of genetic
information (genetic variation).
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These chromosomes now share characteristics from the maternal
chromosomes and the paternal chromosomes.
METAPHASE I
 The homologous chromosomes line up in pairs (23 prs for humans) at the ______________.
 This is different from mitosis where they line up in a single file line (46 for humans)
ANAPHASE I
 The homologous chromosomes separate and move to opposite poles of the cell. This is
called ____________________________ or segregation.
 Notice that the centromere does not split as in mitosis at this phase.
TELOPHASE I
 A nuclear membrane forms around the chromosomes in each daughter cell.
 Each daughter nucleus receives only one member of each original chromosome pair. These
cells are now _______________ - containing only n chromosomes - and are ready to begin
the second stage of meiosis.
SBI 3U
Meiosis II – The Second Cell Division
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Meiosis II usually begins immediately after telophase I and the first cell division.
The chromosomes still consist of two sister chromatids
Meiosis II is exactly like mitosis, except- This cell division leads to 4 cells
The chromosome number is half that of the original cell
The genetic information itself is slightly different in each cell as a result of crossing over
Cells will not replicate their DNA before entering into Meiosis II. This stage of Meiosis is
identical to Mitosis. In Meiosis II, each centromere is split in half, which leads to a total of
four cells with an unreplicated set of DNA in each after a second cytokinesis occurs.
PROPHASE II
 Centrioles duplicate and move to opposite poles, new spindle fibres form.
 Chromosomes become attached to the spindle
METAPHASE II
 Chromatids line up at the equatorial plate with aid of attached spindle fibres.
 Chromatids remain attached at centromere
ANAPHASE II
 Sister chromatids separate at the centromere and move to opposite poles. They can be
called chromosomes.
TELOPHASE II
 A second cytokinesis occurs at the end of this phase, leaving four haploid cells. The
chromosome number has been reduced by half. These cells may become gametes.
 Nuclear membrane, nucleolus and chromatin also reform
Random Assortment of Homologous Chromosomes
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During meiosis I, homologous chromosomes pair up during prophase I and metaphase I.
They are then separated from each other during anaphase I. Each daughter cell is now
haploid, having received only one chromosome from each pair. When this happens, the
chromosomes in each pair are ______________________ _________________________.
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In other words, a daughter cell can receive either chromosome of each homologous pair—
this is called ___________________________________.
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The number of possible combinations of chromosomes depends on the number of
chromosome ________________.
For any diploid (2n) organism, the number of combinations is 2n.
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With three pairs of chromosomes, the number of possible combinations is 23 = 8.
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If there are 23 pairs of chromosomes, as in humans, what would the number of possible
combinations be? ____________________________________________
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This variation among sex cells does not include the added variation that results from
crossing over. If you have ever wondered why no two people are exactly alike - even two
siblings - you now have your answer!
WORK: Read p.112-115 and answer: #1-4 (p.112) and #1, 2 (p.114-115)
SBI 3U