Chapter 10.1, Meiosis
Chromosomes and Genes
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Recall from chapter 9, during mitosis, the DNA
(Chromatin) condenses into x-shaped
structures called chromosomes.
The chromosomes are arranged into
segments called genes. Each gene gives
instructions to the cell to create proteins that
produce characteristics called traits.
Traits, such as hair color, eye color and height,
are inherited through genes on chromosomes.
Genes, Chromosomes, and Traits
Chromosome Numbers
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Each organism has a specific number of
chromosomes in each cell.
Humans cells have 46 chromosomes; 23 from
your mother, and 23 from your father.
The 46 chromosomes inherited from your
parents are called homologous, because each
chromosome has a matching chromosome that
is the same length, has the same centromere
position and they have the same genes that
code for particular traits.
Homologous Chromosomes
Haploid vs. Diploid Cells
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All the cells of our body (except sex cells) are called
Diploid cells.
Diploid cells have all the chromosomes inherited from
your mother and father, and they are represented by
the symbol 2n.
The sex cells of your body are called gametes.
Gametes have only half the number of chromosomes
as the rest of your body's cells. Gametes are called
Haploid cells. Haploid cells are represented by the
symbol n.
Haploid vs. Diploid Cells
Where do haploid and diploid cells
come from?
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During sexual reproduction, two gametes
(sperm and egg cells) combine by fertilization.
When two haploid cells combine, a diploid cell
is formed to make a zygote.
A zygote later develops into an embryo.
When an organism becomes sexually mature,
they start to develop gametes.
The process that forms gametes is called
meiosis.
Human Life Cycle and the formation
and union of Gametes
What is Meiosis?
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Meiosis is a type of cell division that produces
gametes.
Meiosis is different from mitosis because the
number of chromosomes are reduced.
Another name for meiosis is reduction division.
Meiosis occurs only in organisms that
reproduce sexually.
Meiosis involves two cell divisions called
meiosis I and meiosis II.
Mitosis and Meiosis Comparison
Meiosis I
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Very similar to mitosis with a few differences,
so only the differences will be discussed.
In Prophase I, homologous chromosomes
(similar pairs from mother and father) pair up
and crossing over occurs.
Crossing over is when segments of
homologous chromosomes are exchanged.
This is one way that genetic variety and
diversity can occur by sexual reproduction.
Crossing Over in Prophase I
Meiosis I
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In Metaphase I, the homologous
chromosomes pair up in random order and
attach to spindle fibers at the equator.
During Anaphase I, the homologous
chromosomes separate rather than the sister
chromatids, so each chromosome still consists
of two sister chromatids after anaphase I.
The sister chromatids might not be identical in
Telophase I because of crossing over.
Overview of Meiosis I
Meiosis II
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After Meiosis I, the two cells are now reduced
from 2n (Diploid) to n (Haploid).
At this point a second division occurs that is
very similar to mitosis, so we will only go over
the major differences.
During Metaphase II, a haploid number of
chromosomes line up at the equator.
During Anaphase II, sister chromatids are
pulled apart (rather than homologous pairs).
Overview of Meiosis II
Overview of Meiosis
What is the end result?
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At the conclusion of Meiosis I and II, the result
is 4 separate haploid (n) daughter cells that
are each unique and different from each other.
Remember from mitosis, the end product is
two identical diploid (2n) daughter cells.
Variety is produced when homologous
chromosomes cross over (during Prophase I),
and when homologous chromosomes randomly
line up at the equator (during Metaphase I).
This is produces Independent Assortment.
Variety from Independent Assortment