Chapter 8 – Section 3
Bell Work
 What would children be like if humans reproduced
using the process of mitosis?
 List some reasons why it is good for a species overall
for every individual to be a little bit different.
MEIOSIS
Chromosomes are matched in homologous pairs
Chromosomes
 Human cells have
46 chromosomes,
making up 23 pairs
of homologous
Centromere
chromosomes
Sister chromatids
Figure 8.12
Gametes have a single set of chromosomes
Cells with two sets of chromosomes are
said to be diploid (2n = 46 for humans)
Gametes are haploid, with only one set
of chromosomes (n = 23 for humans)
Sexual Life Cycle
• At fertilization, a sperm fuses with an egg,
forming a diploid zygote
• Repeated mitotic divisions lead to the
development of a mature adult
• The adult makes haploid gametes by meiosis
• All of these processes make up the sexual life
cycle of organisms
Haploid gametes (n = 23)
The human life
cycle
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
Figure 8.13
 Meiosis reduces the chromosome number
from diploid to haploid
 Meiosis, like mitosis, is preceded by
chromosome duplication (during
interphase)
 However, in meiosis the cell divides twice to
form four daughter cells, each of which is
haploid (n = 23).
Meiosis I
 In the first division, meiosis I, homologous
chromosomes are paired
 While they are paired, they cross over and exchange
genetic information
 The homologous pairs are then separated, and two
daughter cells are produced, which at this point are
haploid (n = 23).
 But because each chromosome has double the genetic
info (2 sister chromatids), another division is
necessary.
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
Figure 8.14, part 1
PROPHASE I
METAPHASE I
Microtubules
attached to
Spindle kinetochore
Sites of crossing over
Chromatin
Sister
chromatids
Tetrad
Metaphase
plate
Centromere
(with kinetochore)
ANAPHASE I
Sister chromatids
remain attached
Homologous
chromosomes separate
Meiosis II
 Meiosis II is essentially the same as
mitosis
 The sister chromatids of each
chromosome separate
 The result is four haploid daughter cells,
each of which are haploid (n = 23).
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Figure 8.14, part 2
Haploid
daughter cells
forming
Review: A comparison of mitosis and meiosis
For both processes, chromosomes
replicate only once, during interphase
MITOSIS
MEIOSIS
PARENT CELL
(before chromosome replication)
Site of
crossing over
PROPHASE I
Tetrad formed
by synapsis of
homologous
chromosomes
PROPHASE
Duplicated
chromosome
(two sister chromatids)
METAPHASE
ANAPHASE
TELOPHASE
2n = 4
Daughter cells
of mitosis
Figure 8.15
Chromosome
replication
Chromosome
replication
2n = 4
Chromosomes
align at the
metaphase plate
Tetrads
align at the
Metaphase plate
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate
during
anaphase I;
sister
chromatids
remain together
2n = 4
MEIOSIS I
METAPHASE I
ANAPHASE I
TELOPHASE I
Haploid
n=2
Daughter
cells of
meiosis I
No further
MEIOSIS II
chromosomal
replication; sister
chromatids
Haploid
separate during
n=2
anaphase II
n
n
n
n
Daughter cells of meiosis II
 Independent orientation of chromosomes in
meiosis and random fertilization lead to varied
offspring
 Each chromosome of a homologous pair comes
from a different parent
 Each chromosome thus differs at many points
from the other member of the pair
 The large number of possible arrangements
of chromosome pairs at metaphase I of
meiosis leads to many different
combinations of chromosomes in gametes
(Independent Assortment)
• Random fertilization also increases
variation in offspring (Which sperm will
fertilize the egg?)
Independent Assortment
POSSIBILITY 1
POSSIBILITY 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Figure 8.16
Combination 2
Combination 3
Combination 4
Crossing over further increases genetic
variability
 Crossing over is the exchange of
corresponding segments between two
homologous chromosomes
 Genetic recombination results from crossing
over during prophase I of meiosis
 This increases variation further
What leads to variability/diversity?
 Why are we not all identical?
 Independent assortment
 Crossing over
 Random fertilization