Meiosis and Sexual Reproduction. Bozeman Video—Cell Cycle, Mitosis, & Meiosis .
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Transcript of Meiosis and Sexual Reproduction. Bozeman Video—Cell Cycle, Mitosis, & Meiosis .
Bozeman Video—Cell Cycle, Mitosis, & Meiosishttp://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 ReproductionInvolves
Meiosis
Gamete production
FertilizationProduces genetic variation among offspring
Homologous Chromosomes Carry Different AllelesCell has two of each chromosome
One chromosome in each pair from mother,
other from father
Paternal and maternal chromosomes carry
different alleles
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 FormationGametes are sex cells (sperm, eggs)Arise from germ cells
testes
ovaries
anther ovary
Figure 10-3Page 156
Chromosome NumberSum 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 DivisionsTwo consecutive nuclear divisions
Meiosis I
Meiosis II
DNA is not duplicated between divisions
Four haploid nuclei form
Meiosis IIThe two sister chromatids of each duplicated
chromosome are separated from each other
one chromosome (duplicated)
two chromosomes (unduplicated)
p. 158
Prophase IEach 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 ITetrads 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 IHomologous
chromosomes
segregate to opposite
poles
The sister chromatids
remain attached
Fig. 10-5, p. 158
Telophase IThe 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 IISister 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 IAnaphase IMetaphase I
spindle equator
one pair of homologous chromosomes
newly forming microtubules
Prophase I
Meiosis I
Fig. 10-5a, p.158
Stepped Art
Crossing Over
•Each chromosome
becomes zippered to
its homologue
•All four chromatids are
closely aligned
•Nonsister
chromosomes
exchange segments
Effect of Crossing OverAfter crossing over, each chromosome
contains both maternal and paternal
segments
Creates new allele combinations in offspring
Random AlignmentEither 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--Meiosishttp://www.youtube.com/watch?v=rB_8dTuh73c
sporophyte
meiosisdiploid
fertilization
zygote
gametes
gametophytes
spores
haploid
Fig. 10-8a, p.162
Plant Life Cycle
multicelledbody
meiosisdiploid
fertilization
zygote
gametes
haploid
Fig. 10-8b, p.162
Animal Life Cycle
FertilizationMale 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 ComparedMitosis
FunctionsAsexual
reproductionGrowth, repair
Occurs in somatic cells
Produces clones
MeiosisFunction
Sexual reproduction
Occurs in germ cells
Produces variable offspring
Bozeman –Mitosis/Meiosis Bead Simulationhttp://www.youtube.com/watch?v=zGVBAHAsjJM&feature=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 IIAnaphase I (Meiosis)
Homologous chromosomes separate from each
other
Anaphase/Anaphase II (Mitosis/Meiosis)
Sister chromatids of a chromosome separate
from each other
Meiosis Square Dance Videohttp://www.youtube.com/watch?v=eaf4j19_3Zg