MEISOSIS GRADE 12

1. MEIOSI
S
GRADE 12
EDITED BY
KHANYISILE
MASIKANE

2. MEIOSIS
 The form of cell division
by which gametes, with
half the number of
chromosomes also known
as haploid, are produced.
 Diploid (2n)
(n)
→ haploid
 Meiosis is sexual
reproduction.

3. MEIOSIS
cont..
 Sex cells divide to produce
gametes (sperm or egg).
 Gametes have half the # of
chromosomes.
 Occurs only in gonads (testes or
ovaries).
Male: spermatogenesis
Female: oogenesis
 Meiosis is similar to mitosis with
some chromosomal differences.

4. Spermatogenesis
n=23
human
sex cell
n=23
sperm
n=23
2n=46
haploid (n)
diploid (2n)
n=23
n=23
n=23
Meiosis I
Meiosis II

5. Interphase I
 Similar to mitosis Interphase.
 Chromosomes replicate (S phase).
 Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
centromeres
 Centriole pairs also replicate, INSIDE THE
CENTROSOME

6. Interphase I
 Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus

7. Meiosis I
(four phases)
 Cell division that reduces the
chromosome number by one-half.
a.
b.
c.
d.
Four phases:
phases
Prophase I
Metaphase I
Anaphase I
Telophase I

8. PROPHASE I
 Longest and most complex phase (90%).
 Chromosomes condense.
 Synapsis occurs: homologous chromosomes come
together to form a BIVALENT TETRAD.
TETRAD
 Tetrad= TWO chromosomes of THREE OR
Tetrad
FOUR chromatids (sister and nonsister chromatids).
 IN THIS PHASE THE PROCESS CROSSING
OVER TAKES PLACE- EXCHANGE OF
GENETIC MATERIAL.

9. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids

10. HOMOLOGOUS
CHROMOSOMES
 PaIr Of chrOMOSOMES (MatErnal
and PatErnal) that arE SIMIlar In
ShaPE and SIzE.
 hOMOlOgOuS PaIrS (tEtradS) carry
gEnES cOntrOllIng thE SaME
InhErItEd traItS.
 Each lOcuS (POSItIOn Of a gEnE) IS In
thE SaME POSItIOn On hOMOlOguES.
 huManS havE 23 PaIrS Of
hOMOlOgOuS chrOMOSOMES.
a. 22 PaIrS Of autOSOMES
b. 01 PaIr Of SEx chrOMOSOMES

11. Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal

12. Crossing
Over
????????????
Crossing over (variation) may occur between nonsister
chromatids at the chiasmata.
chiasmata
Crossing over: segments of nonsister chromatids break and
over
reattach to the other chromatid.
chromatid
Chiasmata (chiasma) are the sites of crossing over.
over

13. Crossing Over nonsister chromatids
chiasmata: site
of crossing over
vARIATION
Tetrad
variation

14. Sex
Chromosomes
XX chromosome - female
XY chromosome - male

15. Prophase I
spindle fiber
aster
fibers
centrioles

16. Metaphase I
 Shortest phase
 Tetrads align on the metaphase plate.
plate
 INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is
random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations

17. Metaphase I
OR
metaphase plate
metaphase plate

18. Anaphase I
Homologous chromosomes
separate and move towards the
poles.
Sister chromatids remain
attached at their centromeres.
centromeres

19. Anaphase I

20. Telophase I
 Each pole now has haploid set of chromosomes.
chromosomes
 Cytokinesis occurs and two haploid daughter cells
are formed.

21. Telophase I

22. Meiosis II
 No Interphase II
(or very short - no more DNA replication)
replication
 Remember: Meiosis II is similar to
Mitosis

23. Prophase
II
 Same as prophase in mitosis

24. Metaphase II
 Same as Metaphase in Mitosis
metaphase plate
metaphase plate

25. Anaphase II
 Same as Anaphase in Mitosis
 Sister chromatids separate

26. Telophase II
Same as Telophase in Mitosis.
Mitosis
Nuclei form.
Cytokinesis occurs.
Remember: four haploid daughter
cells
produced.
gametes = sperm or egg

27. Telophase II

28. Meiosis
sex cell
n=2
sperm
n=2
n=2
2n=4
haploid (n)
diploid (2n)
n=2
n=2
n=2
meiosis I
meiosis II

29. SUMMARY OF MEIOSIS

31. Variation
 Important to population as the raw material for
natural selection.
Question: What are the three
sexual sources Of genetic

32. Answer:
1. Crossing over (Prophase I)
2. Independent assortment (Metaphase
I)
3. Random fertilization
Remember: VaRiAtIoN
iS gOoD!

33. Fertilization

The fusion of a sperm and egg to
form a zygote.
zygote
A zygote is a fertilized egg

34. WOW
ALL OF THAT
FOR ME! WOW

35. 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.

36. SCIENTIST OBSERVING A HUMAN
KARYOTYPE

37. Centromere
Sister
chromatids
Pair of
homologous
chromosomes
Sex chromosomes

38. 7. Alteration in chromosome number
 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.
© 2012 Pearson Education, Inc.

39. Down syndrome

40.  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.

41. B. Accidents during meiosis can alter
chromosome number
 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.

42. MEIOSIS I
Nondisjunction
MEIOSIS II
Normal
meiosis II
Gametes
Number of
chromosomes
n+ 1
n+ 1
n− 1
Abnormal gametes
n−1

43. MEIOSIS I
Normal
meiosis I
MEIOSIS II
Nondisjunction
n+ 1
n−1
Abnormal gametes
n
n
Normal gametes

44. C. Abnormal numbers of sex
chromosomes
 Sex chromosome abnormalities tend
to be less severe, perhaps because of
• the small size of the Y
chromosome or
• X-chromosome inactivation.
© 2012 Pearson Education, Inc.

45. In general,
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.”
© 2012 Pearson Education, Inc.

46.  THE FOLLOWING TABLE LISTS THE
MOST COMMON HUMAN SEX
CHROMOSOME ABNORMALITIES.
© 2012 Pearson Education, Inc.

47. D. New species can arise from errors in
cell division
 Errors in mitosis or meiosis may produce
polyploidy species, with more than two
chromosome sets.
.
© 2012 Pearson Education, Inc.

48. 8. Alterations of chromosome structure
 Chromosome breakage can lead to
rearrangements that can produce:
• genetic disorders or,
• If changes occur in somatic cells,
cancer.
© 2012 Pearson Education, Inc.

49. THESE REARRANGEMENTS MAY
INCLUDE:
• 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 non-homologous
chromosome that can be reciprocal.

50. THESE REARRANGEMENTS MAY
INCLUDE:
Deletion
Inversion
Duplication
Reciprocal translocation
Homologous
chromosomes
© 2012 Pearson Education, Inc.
Nonhomologous
chromosomes

51. PRESENTED and EDITED BY
KHANYISILE MASIKANE
ACKNOWLEDGEMENTS TO
Hasimah Azit
AND
Janice Williamson
Images taken from:
Campbell, N. Reece, J.2007.
BIOLOGY 8th edition. Pearson
Education.