Meiosis is a type of cell division that produces gametes, such as sperm or egg cells, with half the number of chromosomes. It involves two cell divisions: Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and undergo crossing over, then separate so each daughter cell gets one chromosome of each pair. Meiosis II separates the sister chromatids, resulting in four haploid daughter cells with half the original number of chromosomes. This ensures genetic variation between gametes and offspring through independent assortment and crossing over.

1. Reduction-Division
Genetic Recombination
1

2.  The form of cell divisioncell division by which
GAMETESGAMETES,, with HALFHALF the
number of CHROMOSOMESCHROMOSOMES,, are
produced.
 DIPLOIDDIPLOID (2n pair of(2n pair of
chromosomes)chromosomes) →→ HAPLOIDHAPLOID (n half(n half
a pair of chromosomes)a pair of chromosomes)
 MeiosisMeiosis is SEXUALSEXUAL reproduction.reproduction.
 TWOTWO divisionsdivisions (MEIOSIS IMEIOSIS I and
MEIOSIS IIMEIOSIS II).
2

3.  Sex cells divide to produce GAMETESGAMETES
(sperm or egg)(sperm or egg).
 Gametes have HALFHALF the # of
chromosomes.
 Occurs only in GONADSGONADS (testes or(testes or
ovaries).ovaries).
 Male:Male: SPERMATOGENESIS -SPERMATOGENESIS -spermsperm
 Female:Female: OOGENESIS -OOGENESIS - egg or ovaegg or ova
3

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

5. 2n=46
human
sex cell
diploid (2n)
n=23
n=23
Meiosis I
n=23
egg
Haploid
(1n)
Meiosis II
5
Polar
Bodies
(die)

6.  Similar to mitosismitosis interphase.
 CHROMOSOMESCHROMOSOMES (DNA)(DNA) replicate in the SS
phasephase
 Each duplicated chromosomechromosome consist of twotwo
identicalidentical SISTER CHROMATIDSSISTER CHROMATIDS attached
at their CENTROMERESCENTROMERES.
 CENTRIOLECENTRIOLE pairs also replicate.
6

7.  Nucleus and nucleolus become
visiblevisible.
Nucleus
nucleolus
cell
membrane
chromatin
7

8.  Cell divisionCell division that reduces the
chromosome numberchromosome number by one-half.one-half.
 Four phasesFour phases:
a.a. Prophase IProphase I
b.b. Metaphase IMetaphase I
c.c. Anaphase IAnaphase I
d.d. Telophase ITelophase I
8
Prophase IProphase I

9.  LongestLongest andand most complexmost complex
phase (90%).phase (90%).
 ChromosomesChromosomes condense.
 SynapsisSynapsis occurs - HomologousHomologous
chromosomeschromosomes come together
to form a tetradtetrad.
 TetradTetrad is two chromosomestwo chromosomes
or four chromatidsfour chromatids (sister and
non-sister chromatids). 9

10. 10
•Homologs contain DNA
that codes for the same
genes , but different
versions of those genes
•Genes occur at the same
loci

11. Homologous chromosomes
sister chromatids sister chromatids
Tetrad 11

12.  Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal
) that are similar in shape and sizesimilar in shape and size..
 Homologous pairs ((tetradstetrads) carry GENES) carry GENES
controlling the SAME inherited traitscontrolling the SAME inherited traits.
 Each locuslocus (position of a gene)(position of a gene) is in the
same position on homologues.
 HumansHumans have 23 pairs23 pairs of homologoushomologous
chromosomes:chromosomes:
a. First 22 pairs22 pairs of autosomesautosomes
b. Last pair of sex chromsex chromosomesosomes
12
LOCILOCILOCILOCI
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13. Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus
13

14. 14
Chromosomes exist in homologous pairs inChromosomes exist in homologous pairs in diploiddiploid
(2n) cells.(2n) cells.
Exception:Exception: Sex chromosomesSex chromosomes (X, Y).(X, Y).
Other chromosomes, known asOther chromosomes, known as autosomesautosomes, they, they
have homologues.have homologues.

15.  Crossing overCrossing over may occur between non-
sister chromatidschromatids at sites called chiasmatachiasmata.
 Crossing overCrossing over: segments of nonsister
chromatidschromatids break and reattach to the other
chromatidchromatid.
 Chiasmata (chiasma)Chiasmata (chiasma) are where
chromosomes touch each other and
exchange genesexchange genes (crossing overcrossing over.)
 Causes Genetic RecombinationGenetic Recombination
15

16. nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad
16

17. 17

18. XX chromosome - female XY chromosome - male
18

19. HomologsHomologs
separateseparate
19
Meiosis IMeiosis I

20. • Nucleus & NucleolusNucleus & Nucleolus disappear
• SpindleSpindle forms
• Chromosomes coilcoil & SynapsisSynapsis (pairing)
occurs
• TetradsTetrads form & Crossing overCrossing over Occurs
centrioles
spindle
fiber
aster
fibers
20TETRADTETRAD

21.  Shortest phase
 Tetrads align on the equator.
 Independent assortment occurs – chromosomes
separate randomly causing GENETIC
RECOMBINATION
21

22. Homologs line up at equator or metaphasemetaphase
plate
OR
22

23.  Formula: 2n
 Example:Example:2n = 42n = 4
thenthen 1n = 21n = 2
thusthus 2222
= 4= 4
combinationscombinations
23

24.  In terms ofIn terms of IndependentIndependent
AssortmentAssortment -how many different-how many different
combinations of sperm could acombinations of sperm could a
humanhuman malemale produce?produce?
24

25.  Formula: 2Formula: 2nn
 Human chromosomes:Human chromosomes: 2n =2n =
4646
 n = 23n = 23
 222323
= ~8 million combinations= ~8 million combinations
25

26.  Homologous chromosomesHomologous chromosomes separate
and move towards the poles.
 Sister chromatidsSister chromatids remain
attached at their centromerescentromeres.
26

27. 27
HomologsHomologs separateseparate

28.  Each pole now has haploidhaploid (1n) set
of chromosomeschromosomes.
 CytokinesisCytokinesis occurs and two haploid
daughter cells are formed.
28

29. 29
cytokinesis

30. SisterSister
ChromatidsChromatids
SeparateSeparate
30
Meiosis IIMeiosis II

31.  No Interphase IINo Interphase II or very shortor very short
 No DNA ReplicationNo DNA Replication
 Remember: Meiosis II is similarRemember: Meiosis II is similar
to mitosisto mitosis
31

32.  Same asSame as ProphaseProphase in mitosisin mitosis
 Nucleus & nucleolus disappearNucleus & nucleolus disappear
 Chromosomes condenseChromosomes condense
 Spindle formsSpindle forms
32

33.  Same as Metaphase in mitosis
Chromosomes (not homologs) line up at equator
33

34.  Same as AnaphaseAnaphase in mitosismitosis
 SISTER CHROMATIDSSISTER CHROMATIDS
separateseparate
34

35.  Same asSame as TelophaseTelophase in mitosis.in mitosis.
 NucleiNuclei andand NucleoliNucleoli reform, spindlereform, spindle
disappearsdisappears
 CYTOKINESISCYTOKINESIS occurs.occurs.
 Remember:Remember: FOUR HAPLOIDFOUR HAPLOID DAUGHTERDAUGHTER
cells are produced.cells are produced.
 CalledCalled GAMETESGAMETES (eggs and sperm)(eggs and sperm)
35
1n Sperm cell
fertilizes 1n egg
to form 2n zygote

36. 36

37. 37

38.  Also known asAlso known as GENETICGENETIC
RECOMBINATIONRECOMBINATION
 Important to population as the rawImportant to population as the raw
material formaterial for NATURAL SELECTION.NATURAL SELECTION.
 All organisms areAll organisms are NOTNOT alikealike
 StrongestStrongest ““most fit”most fit” survivesurvive toto
reproducereproduce && pass onpass on traitstraits
38

39.  What are theWhat are the 33 sources ofsources of
genetic recombinationgenetic recombination oror
variation?variation?
39

40. 1.1. CROSSING OVERCROSSING OVER (prophase I)(prophase I)
2.2. INDEPENDENT ASSORTMENTINDEPENDENT ASSORTMENT
(metaphase I)(metaphase I)
3.3. RANDOM FERTILIZATIONRANDOM FERTILIZATION
40

41.  A cell containing 20 chromosomes20 chromosomes
(diploid)(diploid) at the beginning of
meiosis would, at its completion,
produce cells containing how many
chromosomeschromosomes?
41

42.  1010 chromosomes (haploid orchromosomes (haploid or
1n)1n)
42

43.  AAn organizedorganized
picturepicture of the
chromosomes of a
human arranged inarranged in
pairs by sizepairs by size from
largest to smallest.
 PPairs 1-221-22 called
AUTOSOMESAUTOSOMES
 LLast pair are SEXSEX
CHROMOSOMESCHROMOSOMES
43
Male - XYMale - XY

44. Here is a human male karyotypeHere is a human male karyotype
showingshowing duplicated homologousduplicated homologous
chromosomes.chromosomes.
Here is a human male karyotypeHere is a human male karyotype
of a human spermof a human sperm.
44

45. 45
Female - XXFemale - XX

46. 46
Female - XXFemale - XXDown Syndrome – Trisomy 21

47.  The fusion of a spermsperm and
eggegg to form a zygotezygote.
 A zygote is a FERTILIZEDFERTILIZED
EGGEGG
n=23
egg
sperm
n=23
2n=46
zygote
47

48.  A cell containing 40 chromatids40 chromatids at
the beginning of meiosis would, at
its completion, produce cells
containing how many chromosomeschromosomes?
48

49.  10 chromosomes10 chromosomes
49

50. 50

51. 51

52. 52