Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the number of chromosomes as the original parent cell. It involves two cell divisions: Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and separate, reducing the chromosome number by half. Meiosis II then separates the sister chromatids, resulting in four haploid daughter cells each containing a single set of chromosomes. Meiosis introduces genetic variation through independent assortment and crossing over.

1. Reduction-Division
Genetic Recombination
1

2. The form of cell division by which
GAMETES, with HALF the numbe
r of CHROMOSOMES, are produc
ed.
DIPLOID (2n)  HAPLOID (n)
Meiosis is SEXUAL reproduction.
TWO divisions (MEIOSIS I and
MEIOSIS II).
2

3. Sex cells divide to produce GAMET
ES (sperm or egg).
Gametes have HALF the # of chro
mosomes.
Occurs only in GONADS (testes or
ovaries).
 Male: SPERMATOGENESIS -sperm
 Female: OOGENESIS - egg 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 mitosis interphase.
 CHROMOSOMES (DNA) replicate in the S ph
ase
 Each duplicated chromosome consist of two
identical SISTER CHROMATIDS attached
at their CENTROMERES.
 CENTRIOLE pairs also replicate.
6

7. Nucleus and nucleolus visible.
Nucleus
nucleolus
cell
membrane
chromatin
7

8. Cell division that reduces the c
hromosome number by one-hal
f.
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
8
Prophase I

9. Longest and most complex pha
se (90%).
Chromosomes condense.
Synapsis occurs - Homologous
chromosomes come together
to form a tetrad.
Tetrad is two chromosomes
or four chromatids (sister and
non-sister chromatids).
9

10. 10
•Homologs contain DNA tha
t codes for the same genes
, but different versions of
those genes
•Genes occur at the same l
oci

11. Homologous chromosomes
sister chromatids sister chromatids
Tetrad
11

12.  Pair of chromosomes (maternal and patern
al) that are similar in shape and size.
 Homologous pairs (tetrads) carry GENES c
ontrolling the SAME inherited traits.
 Each locus (position of a gene) is in the sa
me position on homologues.
 Humans have 23 pairs of homologous chro
mosomes:
a. First 22 pairs of autosomes
b. Last pair of sex chromosomes
12
LOCI

13. Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus
13

14. Crossing over may occur between non
-sister chromatids at sites called chi
asmata.
Crossing over: segments of nonsiste
r chromatids break and reattach to t
he other chromatid.
Chiasmata (chiasma) are where chro
mosomes touch each other and excha
nge genes (crossing over.)
Causes Genetic Recombination
14

15. nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad
15

16. 16

17. XX chromosome - female XY chromosome - male
17

18. Homologs separat
e
18
Meiosis I

19. • Nucleus & Nucleolus disappear
• Spindle forms
• Chromosomes coil & Synapsis (pairing) oc
curs
• Tetrads form & Crossing over Occurs
centrioles
spindle fiber
aster
fibers
19
TETRAD

20.  Shortest phase
 Tetrads align on the equator.
 Independent assortment occurs – chromosomes s
eparate randomly causing GENETIC RECOMBIN
ATION
20

21. Homologs line up at equator or metaphase
plate
OR
21

22. Formula: 2n
 Example:2n = 4
then 1n = 2
thus 22 = 4
combinations
22

23. In terms of Independent Ass
ortment -how many different
combinations of sperm could a
human male produce?
23

24. Formula: 2n
Human chromosomes: 2n = 4
6
n = 23
223 = ~8 million combinations
24

25. Homologous chromosomes separate
and move towards the poles.
Sister chromatids remain attache
d at their centromeres.
25

26. 26
Homologs separate

27. Each pole now has haploid (1n) set o
f chromosomes.
Cytokinesis occurs and two haploid
daughter cells are formed.
27

28. 28
cytokinesis

29. Sister Chromatid
s Separate
29
Meiosis II

30.  No Interphase II or very short
 No DNA Replication
Remember: Meiosis II is similar t
o mitosis
30

31. Same as Prophase in mitosis
Nucleus & nucleolus disappear
Chromosomes condense
Spindle forms
31

32. Same as Metaphase in mitosis
Chromosomes (not homologs) line up at equator
32

33. Same as Anaphase in mitosis
SISTER CHROMATIDS separ
ate
33

34.  Same as Telophase in mitosis.
 Nuclei and Nucleoli reform, spindle disa
ppears
 CYTOKINESIS occurs.
 Remember: FOUR HAPLOID
DAUGHTER cells are produced.
 Called GAMETES (eggs and sperm)
34
1n Sperm cell fer
tilizes 1n egg to f
orm 2n zygote

35. 35

36. 36

37.  Also known as GENETIC RECOMBINATI
ON
 Important to population as the raw mate
rial for NATURAL SELECTION.
 All organisms are NOT alike
 Strongest “most fit” survive to repro
duce & pass on traits
37

38. What are the 3 sources of ge
netic recombination or variati
on?
38

39. 1. CROSSING OVER (prophase I)
2. INDEPENDENT ASSORTMENT (
metaphase I)
3. RANDOM FERTILIZATION
39

40. A cell containing 20 chromosomes (
diploid) at the beginning of meiosis
would, at its completion, produce c
ells containing how many chromoso
mes?
40

41. 10 chromosomes (haploid or 1
n)
41

42.  An organized pictur
e of the chromoso
mes of a human arr
anged in pairs by si
ze from largest to
smallest.
 Pairs 1-22 called A
UTOSOMES
 Last pair are SEX
CHROMOSOMES
42
Male - XY

43. 43
Female - XX

44. 44
Female - XX
Down Syndrome – Trisomy 21

45. The fusion of a sperm and egg to
form a zygote.
A zygote is a FERTILIZED EGG
n=23
egg
sperm
n=23
2n=46
zygote
45

46. A cell containing 40 chromatids at
the beginning of meiosis would, at
its completion, produce cells conta
ining how many chromosomes?
46

47. 10 chromosomes
47

48. 48