Meiosis
Lifeline Week 12 Follow Along Sheet
REMINDER: Moodle Critique and Survey due Friday, April 4th for Monday-Tuesday classes
Sunday, April 6th for Wednesday-Friday classes
Word Origin:
“_________” is Latin for the word “less” and Portuguese for the word “half.”
Review: Haploid vs. Diploid
• All organisms have 1 set of chromosomes, at least.
• Unpaired chromosomes would be ________ set.
• Paired chromosomes would be_________ sets.
• ___________ organisms have unpaired chromosomes and one complete set of DNA.
• ___________ organisms have paired chromosomes and two complete sets of DNA.
Meiosis Summary
The special process of cell division in ______________________ organisms that reduces the
number of chromosomes in reproductive cells from diploid (2n) to ______________.
- __________ successive divisions take place
- End result = ____________ haploid (n) daughter cells with ____________ the number of
chromosomes of the original parent cell = GAMETES
Goals:
-produce gametes in animals, _____________ in plants
-separate paired chromosomes
-create genetic variation among individuals
Meiosis is for SEXUAL reproduction.
Where do each set of chromosomes in a diploid organism come from?
If haploid organisms want to have sex, do they need to undergo Meiosis?
Key Terms
_______________ __ __________________
• A pair of the same type of chromosomes
• Each has a layout of genes that code for the same products, but different versions. =
VARIATION
____________ _____ ___________________
• Two identical copies of the same chromosome connected at the centromere
Note: Somewhere on the centromere is a kinetochore for microtubule attachment.
Remember: _________________________________________________
A human karyotype represents _________________ of chromosomes.
• 22 pairs of autosomes
• 1 pair of sex-determining chromosomes
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Chromosomes pair up based on the _________ and ______________ position of each
chromosome.
Humans have a total of:
• 23 homologous pairs
• 46 chromosomes
Where does Meiosis take place?
• In the ___________ CELLS…
How many cell divisions occur?
• 2
What is the purpose?
• ______________ __________________
• _____________ the # of chromosomes
The final products:
• Four daughter cells whose chromosome number is _______________
But why??
Because during fertilization, the diploid state will be restored.
Meiotic Cell Division
• Interphase
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Prophase I
Metaphase I
Anaphase I
Telophase I / Cytokinesis
Meiosis I:
Goal: Separation of ______________
________________
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Prophase II
Metaphase II
Anaphase II
Telophase II / Cytokinesis
Meiosis II:
Goal: Separation of ______________
________________
Interphase
• ______ a stage of meiosis (or mitosis), but comes before each process
• 3 Phases
• _______ – Cell grows
• _______ – DNA Replication
• _______ – Necessary proteins synthesized
* Checkpoints ensure that everything goes smoothly and that there are no errors!
Meiosis I
The main purpose:
• Separation of Homologous Chromosomes
The Stages:
Prophase I:
• Chromosomes begin to condense.
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Homologous chromosomes pair up (called ______________) to form a ___________,
held together by a synaptonemal complex.
Formation of ______________ apparatus
Breakdown of nuclear envelope
Microtubules attach to ________________ at centromeric regions.
Crossing over occurs at ________________.
• An exchange of genetic information between a homologous pair of chromosomes
(NOT SISTER CHROMATIDS!)
• to increase genetic variation
Metaphase I
• _______________ become arranged on the metaphase plate.
• Each chromosome of a tetrad is attached to a microtubule from opposite centrosomes.
• *_______________ ________________ *
• The random alignment of chromosomes on the metaphase plate causes a plethora of
different, possible “assortments.”
Anaphase I
• ______________ _______________ are pulled apart toward opposite poles by the
spindle apparatus.
• _____________ ________________ remain attached at the centromere and move as a
single unit towards a pole.
• Homologous pairs officially separated
-Each side of the cell now has a mixture of maternal and paternal chromosomes.
Telophase I and Cytokinesis
• Each half of the cell has a ______________ set of chromosomes.
• Nuclear envelope may re-form; chromosomes may relax to chromatin.
• _________________ creates two daughter cells.
After Meiosis I, the total number of chromosomes is __________________.
• A ____________ cell will have become two ___________ cells.
2n  n
Meiosis II
The main purpose:
• Separation of the _____________ _____________
The Stages:
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II / Cytokinesis
Notice that the stages are the same as Meiosis I, but the purpose is different!
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Similar to _____________ in that you end up with the same number of chromosomes
with which you started
nn
Allows for direct separation of the sister chromatids and the production of ______
haploid daughter cells (GAMETES!)
An _____________ ______________of each of the two cells produced in Meiosis I,
which is why ________ ______________ daughter cells are produced
Meiosis vs. Mitosis
Meiosis
• Two successive divisions of a diploid (2n) cell
• Results in four haploid (n) daughter cells, each with half of the genetic material of the
original cell.
• Through independent assortment and crossing-over, creates genetic variation between the
daughter cells and parent cells.
Mitosis
• Singular division of diploid OR haploid cell
• Produces 2 daughter cells identical to the parent cell
“What you start with is what you end with.”