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