11.4
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Homologous
chromosomes:
pairs of
chromosomes with
genes for the same
traits, but they can
have different
information about
those traits
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Humans have 46 chromosomes or 23 pairs of
homologous chromosomes.
Somatic (body) cells contain 46
chromosomes.
46 is the diploid number for humans.
Diploid means that a cell has both
homologs for each chromosome
Somatic cells reproduce by mitosis
½
the chromosomes in a cell come
from Mom, ½ the chromosomes in
a cell come from Dad
23 pairs of chromosomes
1
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I
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Gametes(eggs and sperm) are called haploid
because they contain one set of
chromosomes.
Eggs also called Ovum and sperm contain 23
chromosomes.
@Eggs and sperm are produced by meiosis- a
process that reduces chromosomes by half@


A type of cell division where ONE diploid cell
(46 chromosomes) become FOUR haploid
cells(23 chromosomes each).
The cells produced by meiosis are gametes
(sex cells)
46
23
23
23
23
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
Plants
Fungi
Animals (including you)
◦ Males start meiosis at puberty.
◦ Females start meiosis in utero.

In your gonads
◦ Females- the ovary
◦ Males-the testicles

Genetic Variation
◦ Mixes genes from Mom and Dad to create a unique
baby

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@Reduces the chromosome number by half@
Ensures that each cell produced has one copy
from each homologous pair of chromosomes
23 in sperm + 23 in egg unite (fertilization)
leads to a 46 zygote(baby)


In two stages called Meiosis I and Meiosis II.
Each stage contains 4 distinct phases
prophase
metaphase
anaphase
teleophase
telophase is followed by the division of
cytoplasm known as cytokinesis.

Homologous
chromosomes join to
form a tetrad
-chromosomes in the
tetrad cross over and
exchange genes

@Crossing over produces new
genetic combinations@
- It is another way to increase
genetic diversity

During Metaphase 1, the
tetrads (groups of 4 homologous
chromosomes) line up down the
center of the cell
- they assort independently
-Chromosomes from each
parent line up
independently of each other
Lesson Overview
Meiosis
During anaphase I, spindle fibers pull each homologous
chromosome pair toward opposite ends of the cell.
.
Lesson Overview
Meiosis
During telophase I, a nuclear membrane forms around
each cluster of chromosomes.
Cytokinesis follows telophase I, forming two new cells.
Lesson Overview
Meiosis
@The
two cells produced by
meiosis I now enter a second
meiotic division. @
Unlike the first division, neither
cell goes through a round of
chromosome replication before
entering meiosis II.
Lesson Overview
Meiosis
As the cells enter prophase II,
their chromosomes—each
consisting of two
chromatids—become visible.
The chromosomes do not pair
to form tetrads, because the
homologous pairs were
already separated during
meiosis I.
Lesson Overview
Meiosis
During metaphase of meiosis
II, chromosomes line up in the
middle of each cell.
Lesson Overview
Meiosis
As the cell enters anaphase,
the paired chromatids
separate.
Lesson Overview
Meiosis
These four daughter cells now contain the
haploid number (N)—just two chromosomes
each.

@Independent assortment increases genetic diversity@
- There are 23 pairs of homologous chromosomes
in humans
- independent assortment ensures that there are
223 possible combinations
223= 8,388,608 combinations
- When two gametes join to form the zygote in
fertilization the possible number of combinations is
223 X 223 = 64 trillion

Crossing over

23 chromosomes
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Mitosis creates 2 daughter cells with same
amount of chromosomes
Meiosis creates 4 daughter cells with half
amount of chromosomes
Meiosis goes through 2 cycles
Mitosis occurs in somatic cells, Meiosis in sex
cells