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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
Fig. 10-2, p.156
Homologous
Chromosomes
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
testes
ovaries
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
Fig. 10-4, p.157
1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18
19 20 21 22 XX (or XY)
Human
Karyotype
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
one chromosome
(duplicated)
two chromosomes
(unduplicated)
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
Telophase I Anaphase I Metaphase I
spindle equator
one pair of homologous chromosomes
newly forming microtubules
Prophase I
Meiosis I
Fig. 10-5a, p.158
Stepped Art
Meiosis II
Prophase II Metaphase II Anaphase II Telophase
II
Fig. 10-5b, p.159
Stepped Art
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)
Activator:
Discuss how unique you are as
an individual (in terms of
meiosis).
Bozeman Video--Meiosis http://www.youtube.com/watch?v=rB_8dTuh73c
ROLES OF MITOSIS/MEIOSIS
IN LIFE CYCLES
sporophyte
meiosis diploid
fertilization
zygote
gametes
gametophytes
spores
haploid
Fig. 10-8a, p.162
Plant Life Cycle
multicelled
body
meiosis diploid
fertilization
zygote
gametes
haploid
Fig. 10-8b, p.162
Animal Life Cycle
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
Produces clones
(2 diploid daughter cells)
Meiosis Function
Sexual reproduction
Occurs in germ cells
Produces variable offspring
(4 haploid daughter cells)
Bozeman –Mitosis/Meiosis
Bead Simulation http://www.youtube.com/watch?v=zGVBAHAsjJM&f
eature=c4-
overview&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