Meiosis and Sexual Reproduction
Bozeman Video—Cell Cycle,
Mitosis, & Meiosis
http://www.youtube.com/watch?v=2aVnN4RePyI
Impacts, Issues: Why Sex
 Asexual reproduction is easier and faster
 Sexual reproduction can be an alternative
adaption in changing environments
Asexual Reproduction
 Single parent produces offspring
 All offspring are genetically identical to one
another and to parent
Sexual Reproduction
 Involves
 Meiosis
 Gamete production
 Fertilization
 Produces genetic variation among offspring
SOMATIC VS GAMETE CELLS
AUTOSOMES VS.
SEX CHROMOSOMES
Homologous Chromosomes
Carry Different Alleles
 Cell has two of each chromosome
 One chromosome in each pair from mother, other
from father
 Paternal and maternal chromosomes carry
different alleles
Homologous
Chromosomes
Fig. 10-2, p.156
Sexual Reproduction
Shuffles Alleles
 Through sexual reproduction, offspring inherit
new combinations of alleles, which leads to
variations in traits
 This variation in traits is the basis for
evolutionary change
Gamete
Formation
 Gametes are sex cells (sperm, eggs)
 Arise from germ cells
ovaries
testes
anther
ovary
Figure 10-3
Page 156
Chromosome Number
 Sum total of chromosomes in a cell
 Germ cells are diploid (2n)
 Gametes are haploid (n)
 Meiosis halves chromosome number
Human
Karyotype
1
2
3
6
7
8
13
14
15
19
20
21
4
9
22
5
10
11
16
17
12
18
XX (or XY)
Fig. 10-4, p.157
Meiosis: Two Divisions
 Two consecutive nuclear divisions
 Meiosis I
 Meiosis II
 DNA is not duplicated between divisions
 Four haploid nuclei form
Meiosis I
Each homologue in the
cell pairs with its partner,
then the partners
separate
p. 158
Meiosis II
 The two sister chromatids of each duplicated
chromosome are separated from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)
p. 158
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I Telophase I
Prophase I
 Each duplicated chromosome
pairs with homologue
 Homologues swap
segments(THIS IS KNOWN
AS CROSSING OVER WHICH
OCCURS AT A SITE CALLED
THE CHIASMATA)
 Each chromosome becomes
attached to spindle
 Longest phase of meiosis
Fig. 10-5, p. 158
Metaphase I
 Tetrads are aligned on
the metaphase plate
 Chromosomes are
pushed and pulled into
the middle of cell
 The spindle is fully
formed
Fig. 10-5, p. 158
Anaphase I
 Homologous
chromosomes segregate
to opposite poles
 The sister chromatids
remain attached
Fig. 10-5, p. 158
Telophase
I
 The chromosomes arrive
at opposite poles
 Usually followed by
cytoplasmic division
 Interkinesis (reforming of
the nuclear
membrane)may occur
before Meiosis II but no
more DNA duplication
Fig. 10-5, p. 158
Prophase II
 Microtubules attach to
the kinetochores of the
duplicated
chromosomes
 If interkinesis
happened, the nuclear
membrane
redisappears
Fig. 10-5, p. 158
Metaphase II
 Duplicated
chromosomes line up
singly at the spindle
equator, midway
between the poles
Fig. 10-5, p. 158
Anaphase II
 Sister chromatids and
their centromeres
separate to become
independent
chromosomes at
opposite poles of each
cell
Fig. 10-5, p. 158
Telophase II and
Cytokinesis
 The chromosomes have
arrived at opposite
ends of the cell
 A nuclear envelope
forms around each set
of chromosomes
 Four haploid cells
Fig. 10-5, p. 158
newly forming spindle
equato
microtubules
r
Prophase I
one pair of
homologous
chromosomes
Metaphase I
Anaphase I
Telophase I
Meiosis I
Stepped Art
Fig. 10-5a, p.158
Prophase II
Metaphase II
Meiosis II
Anaphase II
Telophase
II
Stepped Art
Fig. 10-5b, p.159
Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned
•Nonsister chromosomes
exchange segments
Effect of Crossing Over
 After crossing over, each chromosome contains
both maternal and paternal segments
 Creates new allele combinations in offspring
Random Alignment
 Either the maternal or paternal member of a
homologous pair can end up at either pole
 The chromosomes in a gamete are a mix of
chromosomes from the two parents
Possible Chromosome
Combinations
As a result of random alignment, the number of
possible combinations of chromosomes in a gamete
is:
2n
(n is number of chromosome types)
Bozeman Video--Meiosis
http://www.youtube.com/watch?v=rB_8dTuh73c
ROLES OF MITOSIS/MEIOSIS
IN LIFE CYCLES
Plant Life Cycle
sporophyte
zygote
fertilization
diploid
haploid
meiosis
spores
gametes
gametophytes
Fig. 10-8a, p.162
Animal Life Cycle
multicelled
body
zygote
fertilization
diploid
haploid
meiosis
gametes
Fig. 10-8b, p.162
FUNGAL AND ALGAL LIFE
CYCLE
Fertilization
 Male and female gametes unite and nuclei fuse
 Fusion of two haploid nuclei produces diploid
nucleus in the zygote
 Which two gametes unite is random
 Adds to variation among offspring
Factors Contributing to
Variation among Offspring
 Crossing over during prophase I
 Random alignment of chromosomes at metaphase I (AKA
Law of Independent Assortment of Chromosomes)
 Random combination of gametes at fertilization (1 in 8
million possible egg combinations x 1 in 8 million posssible
sperm combinations = 1 in 64 trillion possible zygote
 Natural Selection-increases frequency of reproductively
favorable traits
Mitosis & Meiosis Compared
Mitosis
 Functions
 Asexual reproduction
 Growth, repair
 Occurs in somatic cells
Meiosis
 Function
 Sexual reproduction
 Occurs in germ cells
 Produces clones
 Produces variable
offspring
Bozeman –Mitosis/Meiosis
Bead Simulation
http://www.youtube.com/watch?v=zGVBAHAsjJM&f
eature=c4overview&playnext=1&list=TLZldufdv0wDU
Prophase vs. Prophase I
 Prophase (Mitosis)
 Homologous pairs do not interact with each other
 Prophase I (Meiosis)
 Homologous pairs become zippered together and
crossing over occurs
Anaphase, Anaphase I, and
Anaphase II
 Anaphase I (Meiosis)
 Homologous chromosomes separate from each other
 Anaphase/Anaphase II (Mitosis/Meiosis)
 Sister chromatids of a chromosome separate from
each other
Comparison of Mitosis and
Meiosis
Meiosis Square Dance Video
http://www.youtube.com/watch?v=eaf4j19_3Zg