1) Meiosis is a cell division process that produces haploid gametes from diploid cells for sexual reproduction. It involves two rounds of division called Meiosis I and Meiosis II. 2) During meiosis, homologous chromosomes pair up and may exchange genetic material through crossing over, resulting in genetic variation. The chromosomes then segregate so that each gamete receives one chromosome of each type. 3) Errors in meiosis can result in aneuploidy, where a gamete gains or loses a whole chromosome, such as occurs in Down syndrome where a third copy of chromosome 21 is present.

1. BY L.N
MASIA
LIFE SCIENCES

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.

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.

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.

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.