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Meiosis and Sexual Life Cycles
Asexual
Reproduction
Sexual
Reproduction
One parent,
mitotic
division
2 parents,
meiotic
division
identical
offspring,
clones
Unique
offspring
 Somatic
cell vs. Gametes
Homologous
chromosomes
• 23 Pairs, one
from each
parents
• Carry the same
genes
Sex
chromosomes
Autosomes
• Carry different
genes
• Determine sex
(X and Y)
• Everything
besides the X
and Y
chromosome
 Diploid
vs. Haploid
Cells
 Fertilization – fusion of
nuclei of 2 gametes
• Forms a zygote
• How many chromosomes
does a zygote have?
 Human
life cycle:
meiosis only in
gametes
• Variations found in other
organisms
•Meiosis I: homologous
chromosomes separate, 2 diploid
cells produced
•Meiosis II: sister chromatids
separate, 4 haploid cells produced
•Interphase only occurs before
Meiosis I
Prophase I
• Chromosomes
condense,
homologs pair
by gene,
crossing over
• Centrosomes
move, spindle
fibers form,
envelope
breaks
• Kinetochores
attach
Telophase I and
Cytokinesis
Metaphase I
• Pairs of
homologous
chromosomes line
up at equator
• Both chromatids
are attached to
same microtubule
• Each half has
haploid set of
chromosomes
• Cytokinesis occurs,
forms 2 new cells
Anaphase I
• Homologs
separate to
opposite sides
• Sister
chromatids still
connected
Prophase II
Spindle apparatus forms
Sister chromatids move toward center
Metaphase II
Chromosomes move to equator
Sister chromatids not identical!
Anaphase II
Sister chromatids separate
Move to opposite poles
Telophase II & Cytokinesis
Nuclei form, chromosomes uncoil, cytokinesis occurs
4 haploid cells produced
 If
a haploid cell has 24 chromosomes,
how many does the diploid have?
 The gene for eye color would be found
on which type of chromosome?
 If a genetic disorder is found on the Xchromosome, why might males be more
prone to it?
 Compare and Contrast Mitosis and
Meiosis.
MITOSIS

1 division (PPMAT)
MEIOSIS

• Prophase, prometaphase,
metaphase, anaphase,
telophase




Occurs in somatic cells
2 Diploid cells
Daughter cells genetically
identical to parent
Used for growth, repair,
asexual reproduction
8 stages (2 divisions)
• Meiosis I & II




Occurs in gametes
4 Haploid cells
Daughter cells genetically
unique
Used to reduce # of
chromosomes in cell
 Independent
assortment (2n possibilities)
 Crossing Over
• Homologous genes pair
• Maternal & Paternal genes exchanged
• Chiasma forms
 Random
Fertilization
 Best
variations survive
 Pass on genes
 Genes eventually dominant in a
population = natural selection