Download AP Ch. 13 Genetic Variation

AP BIOLOGY
GENETIC VARIATION
CHAPTER 13: MEIOSIS
Meiosis 1
Meiosis 2
Mitosis vs. Meiosis
Mitosis vs. Meiosis
• Mitosis
– 1 cell division
– daughter cells
genetically identical to
parent cell and each
other
– produces 2 cells
– 2n  2n
diploid diploid
– produces cells for
growth & repair ; Also
produces new
organisms in asexual
reproduction
– no crossing over
• Meiosis
– 2 cell divisions
– daughter cells
genetically different
from parent and each
other
– produces 4 cells
– 2n  1n
diploidhaploid
– produces gametes
– crossing over occurs
Fig. 13-9a
MITOSIS
MEIOSIS
Parent cell
Chromosome
replication
Prophase
Chiasma
Chromosome
replication
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
MEIOSIS I
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
Daughter cells
of mitosis
2n
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
meiosis
23
23
egg
23
46
23
zygote
fertilization
sperm
46
46 46
46 46 46
46
46
46
mitosis
&
mitosis
development
GENETIC VARIATION
• Sexual reproduction produces genetic
variation in three ways:
A. independent assortment of chromosomes
B. crossing over
C. random fertilization
metaphase1
A. Independent assortment of chromosomes
- Occurs during metaphase I in meiosis.
- Each homologous pair of chromosomes
lines up along the metaphase plate
independently of other pairs.
- When pairs separate during anaphase I,
various combinations of chromosomes go
to each pole.
Fig. 13-11-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
-
Gametes from offspring do not have
same combination of genes as gametes
from parents.
• Random assortment in humans produces
223 (8,388,608) different combinations in
gametes
• To calculate the number of possible
independent assortment combinations of
gametes from a parent,
calculate 2n where n = haploid number.
from Mom
from Dad
offspring
new gametes
made by offspring
B. Crossing over
• Crossing over creates completely new
combinations of genes on each chromosome.
– Creates an infinite variety of gametes.
Fig. 13-12-5
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
C. Random fertilization
• Sperm + Egg = ?
– Any 2 parents will produce a zygote with
over 70 trillion (223 x 223) possible diploid
combinations.
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Jonas
Brothers
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
E. Sperm production
Epididymis
Testis
Coiled
seminiferous
tubules
germ cell
(diploid)
primary
spermatocyte
(diploid)
MEIOSIS I
secondary
spermatocytes
(haploid)
Vas deferens
spermatids
(haploid)
spermatozoa
• Spermatogenesis
– continuous
& prolific
process
Cross-section
of
seminiferous tubule
– each ejaculation =
100-600 million sperm
MEIOSIS II
F. Egg production
• Oogenesis
– eggs in ovaries halted
before Anaphase 1
– Meiosis 1 completed
during maturation
– Meiosis 2 completed after
unequal division
fertilization
– 1 egg + 2 polar bodies
Meiosis 1 completed
during egg maturation
Meiosis 2 completed
triggered by fertilization
ovulation