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Meiosis and Sexual Reproduction
Chapter 10
10.1 Introducing Alleles
 Asexual reproduction produces genetically
identical copies of a parent (mitosis)
 Sexual reproduction introduces variation in the
combinations of traits among offspring (meiosis)
Meiosis and Homologous Chromosomes
 Meiosis
 A nuclear division mechanism that precedes
cytoplasmic division of immature reproductive cells
in sexually-reproducing eukaryotic species
 If meiosis did not occur, the chromosome number
would continue to double with each new generation.
 Homologous Chromosomes
 Same length, shape, centromere in the same location,
are in pairs (one from the mother and one from the
father), separate from each other during meiosis I, have
alleles for the same characteristics even though the
gene expression may not be the same.
Genes and Alleles
 Genes are regions in an organism’s DNA that
encode information about heritable traits
• In sexual reproduction, pairs of genes are
inherited on pairs of chromosomes
 Alleles are different or alternate forms of the
same gene
• Offspring of sexual reproducers inherit new
combinations of alleles (ex. Eye color = hazel
eyes (mother), blue eyes (father)
• Study Figure 10.2
Halving the Diploid Number
 A diploid cell has two nonidentical copies of every
chromosome (except XY sex chromosomes)
• Humans have 23 pairs of homologous
chromosomes
 Meiosis in germ cells halves the diploid number
of chromosomes (2n) to the haploid number (n),
producing haploid gametes aka sex cells (egg,
sperm, spores, gametophytes)
Restoring the Diploid Number
 Human gametes (eggs and sperm) have 23
chromosomes – one of each type of
homologous chromosomes.
 The diploid number (23 pairs or 46
chromosomes) is restored at fertilization, when
two haploid gametes fuse and form a diploid
zygote
Two Divisions, Not One
 In meiosis, DNA is replicated once and divided
twice (meiosis I and meiosis II), forming four
haploid cells (four daughter cells)
 In meiosis I, each duplicated homologous
chromosome is separated from its partner
 In meiosis II, sister chromatids are separated
Meiosis I
newly forming
microtubules
plasma
membrane of the spindle
breakup
of nuclear
envelope
one pair of
homologous
chromosomes
centrosome with
a pair of centrioles, moving to
opposite sides of nucleus
A Prophase I =
crossing over
B Metaphase I
C Anaphase I
D Telophase I
Stepped Art
Fig. 10-5a, p. 158
Meiosis II
There is
no DNA
replication
between the
two nuclear
divisions.
E Prophase II
F Metaphase II
G Anaphase II
H Telophase II
Stepped Art
Fig. 10-5b, p. 159
Animation: Meiosis step-by-step
10.4 How Meiosis
Introduces Variation in Traits
 Crossovers
 the random sorting or alignment of
chromosomes
 genetic recombination of alleles
 fertilization in meiosis introduce novel
combinations of alleles into gametes (egg
and sperm), resulting in new combinations of
traits among offspring
Crossing Over in Prophase I
 Crossing over
• The process by which a chromosome and its
homologous partner (two of the same
chromosomes; one from the mother and one from
the father) exchange heritable information in
corresponding segments
• Homo = same
Animation: Crossing over
Segregation of
Chromosomes into Gametes
 Random assortment (independent assortment)
produces (8,388,608) possible combinations of
homologous chromosomes
 Gametes (egg and sperm; 223 )
 Fertilization (fusion or combining of the egg and
sperm; (223)2 = Over 70 trillion different zygotes
– the first cell that is formed after fertilization) are
possible.
Animation: Random alignment
Fig. 10-8b, p. 162
Gamete Formation in Animals
 Males
• Meiosis of primary spermatocytes produces four
haploid spermatids
 Females
• Meiosis of a primary oocyte forms cells of
different sizes; the secondary oocyte gets most of
the cytoplasm and matures into an ovum ; other
three cells (polar bodies – have no function,
are dumping places for excess genetic
material, are produced only during meiosis.)
• Study figure 10.10
Sperm Formation in Animals
Egg Formation in Animals
More Shufflings at Fertilization
 Chance combinations of maternal and paternal
chromosomes through fertilization produce a
unique combination of genetic information
 Fertilization
• The fusion of two haploid gametes (sperm and
mature ova - egg) resulting in a diploid zygote
(the first cell of a new individual with 46
chromosomes.
Fig. 10-11c, p. 164
Fig. 10-11d, p. 165
Animation: Comparing mitosis and
meiosis
Animation: Generalized life cycles
Animation: Meiosis
Video: Why sex?