Meiosis is a type of cell division that produces haploid gametes from diploid cells in two stages. In meiosis I, homologous chromosomes pair and undergo crossing over, then separate. This reduces the chromosome number by half to produce haploid cells. Meiosis II then divides the contents of these haploid cells without further combining of homologs, resulting in four haploid gametes. Two gametes combine at fertilization to form a diploid zygote and complete the life cycle.

1. Campbell & Reece, 2010
Chapter 13
UNIT 3: MEIOSIS
REDUCTION DIVISION

2.  In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes and
• one member of each pair from each parent.
 The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic composition.
 The other 22 pairs of chromosomes are
autosomes with the same size and genetic
composition.
1. CHROMOSOMES ARE MATCHED IN
HOMOLOGOUS PAIRS
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3.  Homologous chromosomes are matched in:
• length,
• centromere position, and
• gene locations (locus).
 A locus (plural, loci) is the position of a gene.
 Different versions (alleles) of a gene may be
found at the same locus on maternal and
paternal chromosomes.
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Centromere

4.  Homologous chromosome pair
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Centromere

5.  Humans and most animals and plants have diploid
body cells.
 That means they have two sets of chromosomes
(homologous chromosome pair) one from each
parent.
 Diploid is written 2n.
 It refers to the total number of chromosomes
a cell can have.
2. GAMETES HAVE A SINGLE SET OF
CHROMOSOMES
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6.  Meiosis is a process that converts diploid
nuclei to haploid nuclei.
• Diploid cells have 2 sets of chromosomes.
• Haploid cells have 1 set of chromosomes.
• Meiosis occurs in the sex organs,
producing gametes—sperm and eggs.
 Fertilization is the fusion of a sperm and egg
cell.
 The zygote has a diploid chromosome
number, one set from each parent.
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7. Haploid gametes (n  23)
Egg cell
Sperm cell
Fertilization
n
n
Meiosis
Ovary Testis
Diploid
zygote
(2n  46)
2n
Mitosis
Key
Haploid stage (n)
Diploid stage (2n)
Multicellular diploid
adults (2n  46)
A life cycle

8.  All sexual life cycles include an alternation
between
• a diploid stage and
• a haploid stage.
 Why is meiosis so important? It produces
haploid gametes which prevents the
chromosome number from doubling in every
generation. Produce gametes for fertilization.
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9. 3
 Meiosis is a type of cell division that produces
haploid gametes from diploid cells.
 Two haploid gametes combine in fertilization to
restore the diploid state in the zygote.
3. MEIOSIS

10. 3
SUMMERY OF THE MEIOSIS PROCESS

11. MEIOSIS I consisting of 5 phases:
Interphase I, Prophase I, Metaphase I,
Anaphase I, Telophase I.
MEIOSIS II consisting of 4 phases
Prophase II, Metaphase II, Anaphase II,
Telophase II.
MEIOSIS HAS 2 STAGES:
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12. Cell build up energy
DNA Replication (to make
duplicated chromosomes
Cell doesn’t change
structurally.
MEIOSIS I : INTERPHASE
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13. Events occurring in the nucleus:
• Chromosomes coil and become individual chromo-
somes, nucleolus and nuclear envelope disappear.
• Homologous chromosomes come together as pairs by
synapsis forming a tetrad (Each pair, with four
chromatids)
• Non-sister chromatids exchange genetic material
through the process of crossing over to ensure genetic
variation.
• Centrioli move to opposite poles with spindle fibers
between them.
MEIOSIS I : PROPHASE I

14. MEIOSIS I : PROPHASE I
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15.  Genetic recombination is the production of new
combinations of genes due to crossing over.
 Crossing over is an exchange of genesbetween
separate (non-sister) chromatids on homologous
chromosomes.
• Non-sister chromatids join at a chiasma
(plural, chiasmata), the site of attachment.
• Genetic material are exchanged between
maternal and paternal (nonsister) chromatids.
CROSSING OVER
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16. CROSSING OVER
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17.  Centrioli has reached the
poles.
 Homologous pairs align at
the cell equator.
 The two chromosomes attach
to one spindle fiber by means
of the kinetochore of the
centromere.
.
MEIOSIS I: METAPHASE I
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18.  Spindle fibers contract.
 Duplicated chromosomes
move to opposite poles.
.
MEIOSIS I: ANAPHASE I
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19. • Duplicated chromosomes
have reached the poles.
• A nuclear envelope and
nucleolus re-forms around
chromosomes.
• Each nucleus now has the
haploid number of
chromosomes.
• Cell invaginates forming a
cleavage furrow, which
extends to for 2 separate
haploid cells.
MEIOSIS I: TELOPHASE I
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20.  Follows meiosis I without chromosome
duplication.
 Each of the two haploid products enters
meiosis II.
MEIOSIS II
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21. • Chromosomes coil and
become compact (if
uncoiled after telophase I).
• Nuclear envelope and
nucleolus, if re-formed,
dissappears again.
• Centrioli move to opposite
poles, forming spindle
fibers between them.
MEIOSIS II: PROPHASE II
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22. • Individual duplicated
chromosomes align on the
equator.
• One chromosome per spindle
fiber attached by means of
kinetochore of centromere.
• Centrioli has reached the
poles.
MEIOSIS II: METAPHASE II
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23. • Spindle fibers contract.
• Duplicated chromosomes
split in half (centromere
dividing in 2)
• Daughter chromosomes
move to opposite poles.
MEIOSIS II: ANAPHASE II
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24. • Daughter chromosomes has
reached the poles.
• Two cells invaginate and form 4
daughter haploid cells
(gametes)
• They uncoil and form
chromatin.
• Nuclear envelope and
nucleolus for around chromatin
again.
• Centrioli for centrosome.
MEIOSIS II: TELOPHASE II

25. SUMMERY OF MEIOSIS II
Prophase II Metaphase II Anaphase II
Haploid daughter
cells forming
Telophase II
and Cytokinesis

26.  Mitosis and meiosis both
• begin with diploid parent cells that
• have chromosomes duplicated during the
previous interphase.
 However the end products differ.
• Mitosis produces two genetically identical
diploid somatic daughter cells.
• Meiosis produces four genetically unique
haploid gametes.
4. SIMILARITIES AND DIFFERENCES
BETWEEN MITOSIS AND MEIOSIS

27. • Independent orientation at metaphase
I
• Random fertilization.
• Crossing over of genes during
prophase I
5. GENETIC VARIATION IN GAMETES
RESULTS FROM:
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28. 6. KARYOTYPE
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• A karyotype is an ordered display of
magnified images of an individual’s
chromosomes arranged in pairs.
• Karyotypes allow for the observation of :
 homologous chromosome pairs,
 chromosome number, and
 chromosome structure.

29. SCIENTIST OBSERVING A HUMAN
KARYOTYPE

30. Centromere
Sister
chromatids
Pair of
homologous
chromosomes
Sex chromosomes

31.  An extra copy of chromosome 21 causes
Down syndrome or also known as TRISOMY
21.
 A. Trisomy 21
• involves the inheritance of three copies of
chromosome 21 and
• is the most common human chromosome
abnormality.
7. ALTERATION IN CHROMOSOME NUMBER
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32. Down syndrome

33.  Trisomy 21 produces a characteristic set of symptoms,
which include:
• mental retardation,
• characteristic facial features,
• short stature,
• heart defects,
• susceptibility to respiratory infections, leukemia,
and Alzheimer’s disease, and
• shortened life span.
 The incidence increases with the age of the mother.

34.  Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis. This
can happen during:
• meiosis I, if both members of a homologous pair go
to one pole or
• meiosis II if both sister chromatids go to one pole.
 Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes.
B. ACCIDENTS DURING MEIOSIS CAN
ALTER CHROMOSOME NUMBER

35. Nondisjunction
MEIOSIS I
MEIOSIS II
Normal
meiosis II
Gametes
Number of
chromosomes
Abnormal gametes
n  1 n  1 n  1 n  1

36. Normal
meiosis I
MEIOSIS I
MEIOSIS II
Nondisjunction
Abnormal gametes Normal gametes
n  1 n  1 n n

37. Sex chromosome abnormalities tend to
be less severe, perhaps because of
• the small size of the Y chromosome
or
• X-chromosome inactivation.
C. ABNORMAL NUMBERS OF SEX
CHROMOSOMES
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38.  In general,
• a single Y chromosome is enough to produce
“maleness,” even in combination with several
X chromosomes, and
• the absence of a Y chromosome yields
“femaleness.”
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39.  The following table lists the most common human sex
chromosome abnormalities.
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40.  Errors in mitosis or meiosis may produce
polyploid species, with more than two
chromosome sets.
 .
D. NEW SPECIES CAN ARISE FROM
ERRORS IN CELL DIVISION
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41.  Chromosome breakage can lead to
rearrangements that can produce:
• genetic disorders or,
• if changes occur in somatic cells, cancer.
8. ALTERATIONS OF CHROMOSOME
STRUCTURE
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42. • a deletion, the loss of a chromosome
segment,
• a duplication, the repeat of a chromosome
segment,
• an inversion, the reversal of a chromosome
segment, or
• a translocation, the attachment of a segment
to a nonhomologous chromosome that can be
reciprocal.
THESE REARRANGEMENTS MAY INCLUDE:

43. THESE REARRANGEMENTS MAY INCLUDE:
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Deletion
Duplication
Inversion
Reciprocal translocation
Homologous
chromosomes Nonhomologous
chromosomes