Download MEIOSIS AND SEXUAL LIFE CYCLES CH 13

MEIOSIS AND SEXUAL LIFE CYCLES
CH 13
I. Overview of Reproduction
Asexual reproduction:
produces identical
offspring (budding,
cloning, binary
fission/mitosis)
• Sexual reproduction: produces varied
offspring
• In both forms of reproduction offspring
inherit genes from parent(s)
– Genes: segments of DNA
– Locus: location on a chromosome where
gene is located
– Each gene occupies a particular locus on a
chromosome
II. Meiosis and Fertilization alternate
in sexual life cycles
• A. Homologous Chromosomes
• Somatic cell: nonsex cell
• Gamete: sex cell
• somatic cells have pairs of chromosomes
called homologous pairs
• Homologous pairs are the same size, shape,
and carry the same genes
• Each member of the pair is inherited from
each parent
• Most animals have 1 pair of sex
chromosomes. The other pairs are
autosomes (nonsex)
B. Diploid and Haploid cells
• Somatic cells are diploid and have pairs of
chromosomes (2 sets of chromosomes) and
are 2n where n = # of pairs
• Gametes are haploid and have 1 of each pair
(1 set of chromosomes) and are n.
C. Variety of sexual life cycles
• All sexually reproducing organisms show
alternation between meiosis and fertilization
In animals, special
diploid cells undergo
meiosis to produce
haploid gametes that
fertilize to restore
diploid zygote
• Plants and some algae
undergo alternation of
generation where they
alternate between a
diploid sporophyte and a
haploid gametophyte
• Diploid sporophyte
makes haploid spore by
meiosis which produces
haploid gametophyte by
mitosis
• Gametophyte makes
haploid gametes by
mitosis which fertilize to
form diploid sporophyte
• In fungi, most cells are
haploid
• Haploid adult produces
gametes by mitosis which
fertilize to form diploid
zygote
• Diploid zygote undergoes
meiosis to produces
haploid cells that row into
new adult by mitosis
III. Overview of meiosis
• Produces haploid cells
from diploid cells
• Involves 2 divisions:
meiosis I and meiosis II
• Results in genetically
different haploid cells
The process of meiosis
Interphase: replicates DNA forming sisters
Meiosis I: separates pairs
Meiosis II: separates sisters
IV. Genetic Variation during sexual
reproduction contributes to evolution
• Mutations are the original source of diversity
and created the different alleles
• Reshuffling of alleles during meiosis produces
genetic variation in a population
• Three mechanisms contribute to genetic
variation
– Independent assortment
– Crossover
– Random fertilization
A. Independent Assortment
• Homologous pairs align themselves randomly
in the center of the cell in metaphase I
B. Crossover
Homologous chromosomes
pair in prophase I and
exchange DNA forming
recombinant chromosomes
C. Random fertilization
• Any sperm can fertilize the egg
V. Evolutionary Significance of
Variation
• Mutations creates new alleles
• Independent assortment, crossover, and
random fertilization reshuffles those alleles
• Without the variation, natural selection
could not select for favorable variations
VI. When Meiosis Goes Wrong
Nondisjunction
• Failure of homologous chromosomes or sister
chromatids to separate during meiosis
• Results in gamete with too few or too many
chromosomes
• If this gamete is used in fertilization, the offspring
will have too few or too many chromosomes
• Can determine this from karyotype where DNA is
isolated and chromosomes are paired
• http://www.sumanasinc.com/webcontent/anim
ations/content/mistakesmeiosis/mistakesmeiosis
.swf
Normal karyotype of female:
Normal karyotype of male:
Down’s Syndrome Karyotype: