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BY L.N
MASIA
LIFE SCIENCES
 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.
.
 Homologous chromosomes are

matched in:
 length,
Centromere
 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.
 Homologous chromosome pair

Centromere
 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.
 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
Haploid gametes (n  23)

A life cycle
n
Egg cell
n
Sperm cell
Meiosis

Ovary

Fertilization

Testis
Diploid
zygote
(2n  46)

2n

Key
Multicellular diploid
adults (2n  46)

Mitosis

Haploid stage (n)
Diploid stage (2n)
 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.
3. MEIOSIS
 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
SUMMERY OF THE MEIOSIS
PROCESS

3
 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.
 Cell build up energy
 DNA Replication (to

make duplicated
chromosomes
 Cell doesn’t change
structurally.
 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 materi
through the process of crossing over to ensure
genetic variation.
 Centrioli move to opposite poles with spindle
fibers between them.
Unit 3 meiosis
 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)
© 2012 Pearson Education, Inc.
 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.
.
 Spindle fibers

contract.
 Duplicated
chromosomes move
to opposite poles.
.
 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.
 Follows meiosis I without chromosome

duplication.
 Each of the two haploid products enters
meiosis II.
 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.
 Individual duplicated

chromosomes align on
the equator.
 One chromosome per
spindle fiber attached by
means of kinetochore of
centromere.
 Centrioli has reached the
poles.
 Spindle fibers contract.
 Duplicated

chromosomes split in
half (centromere
dividing in 2)
 Daughter
chromosomes move to
opposite poles.
 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.
Prophase II

Metaphase II

Anaphase II

Telophase II
and Cytokinesis

Haploid daughter
cells forming
 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.
 Independent orientation at

metaphase I
 Random fertilization.

 Crossing over of genes during

prophase I

© 2012 Pearson Education, Inc.
6. KARYOTYPE
• 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.
© 2012 Pearson Education, Inc.
Unit 3 meiosis
Centromere

Sister
chromatids
Pair of
homologous
chromosomes

Sex chromosomes
 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.
Down syndrome
 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.
 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.
MEIOSIS I

Nondisjunction

MEIOSIS II

Normal
meiosis II

Gametes
Number of
chromosomes

n1

n1

n1

Abnormal gametes

n1
MEIOSIS I

Normal
meiosis I

MEIOSIS II

Nondisjunction

n1

n1

Abnormal gametes

n

n

Normal gametes
 Sex chromosome abnormalities

tend to be less severe, perhaps
because of
 the small size of the Y
chromosome or
 X-chromosome inactivation.
 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.”
 Chromosome breakage can lead to

rearrangements that can produce:
 genetic disorders or,

 if changes occur in somatic cells,

cancer.
 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.

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Unit 3 meiosis

  • 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. .
  • 3.  Homologous chromosomes are matched in:  length, Centromere  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.
  • 4.  Homologous chromosome pair 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.
  • 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
  • 7. Haploid gametes (n  23) A life cycle n Egg cell n Sperm cell Meiosis Ovary Fertilization Testis Diploid zygote (2n  46) 2n Key Multicellular diploid adults (2n  46) Mitosis Haploid stage (n) Diploid stage (2n)
  • 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.
  • 9. 3. MEIOSIS  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
  • 10. SUMMERY OF THE MEIOSIS PROCESS 3
  • 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.
  • 12.  Cell build up energy  DNA Replication (to make duplicated chromosomes  Cell doesn’t change structurally.
  • 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 materi through the process of crossing over to ensure genetic variation.  Centrioli move to opposite poles with spindle fibers between them.
  • 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)
  • 16. © 2012 Pearson Education, Inc.
  • 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. .
  • 18.  Spindle fibers contract.  Duplicated chromosomes move to opposite poles. .
  • 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.
  • 20.  Follows meiosis I without chromosome duplication.  Each of the two haploid products enters meiosis II.
  • 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.
  • 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.
  • 23.  Spindle fibers contract.  Duplicated chromosomes split in half (centromere dividing in 2)  Daughter chromosomes move to opposite poles.
  • 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.
  • 25. Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Haploid daughter cells forming
  • 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.
  • 27.  Independent orientation at metaphase I  Random fertilization.  Crossing over of genes during prophase I © 2012 Pearson Education, Inc.
  • 28. 6. KARYOTYPE • 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. © 2012 Pearson Education, Inc.
  • 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.
  • 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.
  • 35. MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Number of chromosomes n1 n1 n1 Abnormal gametes n1
  • 36. MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n1 n1 Abnormal gametes n n Normal gametes
  • 37.  Sex chromosome abnormalities tend to be less severe, perhaps because of  the small size of the Y chromosome or  X-chromosome inactivation.
  • 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.”
  • 39.  Chromosome breakage can lead to rearrangements that can produce:  genetic disorders or,  if changes occur in somatic cells, cancer.
  • 40.  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.