lab 4: mitosis and meiosis - linn–benton community...
TRANSCRIPT
Meiosis
Chapter 11
04/25/2016
Introduction to Meiosis: the Big Picture
Starts with 1 original cell
Homologous chromosomes cross over
Ends with 4 non-identical cellsEach has half the DNA of the original cell
Meiosis is Essential for Sexual Reproduction
meioticcell division
fertilization
diploidparentalcells
diploidfertilizedegg
haploidgametes
2n
2n
2n n
n
With Sexual Reproduction, Offspring Get Half of the DNA
from Each Parent
• 23 chromosomes from maternal (mom)
• 23 chromosome from paternal (dad)
Meiosis Allow For Genetic Recombination
• Genetic recombination (during formation of sperm and eggs) is why most siblings are not identical
– Exception: identical twins
Twins
http://www.embryology.ch/anglais/fplacenta/gemell01.html
Genetic Recombination by Crossing Over of Homologous Chromosomes
Figure 13.8a
Prophase I Metaphase I Anaphase I Telophase I andCytokinesis
Centrosome(with centriole pair)
Sisterchromatids
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs.
Sister chromatidsremain attached
Homologouschromosomesseparate
Each pair of homologous chromosomes separates.
Cleavagefurrow
Two haploid cells form; each chromosomestill consists of two sister chromatids.
Figure 13.8b
Prophase II Metaphase II Anaphase IITelophase II and
Cytokinesis
Sister chromatidsseparate
Haploid daughtercells forming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
Spermatogenesis Oogenesis
46
92
46 46
46
92
46
2323 23 23 23
Chromosome Non-Disjunction
• Chromosomes do not properly separate into daughter cells
• Can happen in mitosis; much more likely in meiosis
Chromosome Non-Disjunction
• Abnormal gamete can participate in fertilization with a normal gamete
• Offspring either…– Have one copy of a chromosome
• Monosomy
• Nearly always fatal
• Exception: Turner Syndrome (XO)
– Have 3 copies of a chromosome • Trisomy
• Usually fatal, some exceptions
Chromosome Non-Disjunction
• Trisomy 13 – Patau Syndrome
– 80% fatal in first year
• Trisomy 18 – Edward’s Syndrome
– 50% fatal in first year
• Trisomy 21 – Down Syndrome
– Survival to adulthood common
Is this karyotype normal?
Summary
Mitosis Meiosis
Where it occurs Body cells Reproductive organs
Why it occurs Cell repair, growth, asexual reproduction
Sexual reproduction
Number of cells produced 2 4
Number of divisions 1 2
Number of chromosomes in daughter cells
Same as mother cell(diploid)
Half that of mother cell (haploid)
Amount of variation in daughter cells None Lots
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The human life cycle
meiotic cell
division in
testes
meiotic cell
division in
ovaries
adults (2n)
egg (n)zygote (2n)
haploid (n)
diploid (2n)fusion of gametes
sperm (n)
embryo (2n)
baby (2n)
mitotic cell division,
differentiation, and growth
mitotic
cell division,
differentiation,
and growth
mitotic cell division,
differentiation,
and growth
Figure 9-19
Lifecycle of moss