Ch 13 Sexual Life Cycles and Meiosis
Reproduction and the Transmission of Traits
• Living organisms
Have ability to reproduce
• Heredity
Transmission of traits from one generation to the next
• Variation
• Offspring often differ in appearance from parents and
siblings
• Environmental and genetic influences
• Genetics
Scientific study of heredity and variation
Progeny (or offspring)
-Inherit chromosomes from parents
-DNA in chromosomes encode the genes that
specify characteristics
For example
eye color, blood type, predisposition to disease
What are the mechanisms by which chromosomes
are passed onto (eukaryotic) offspring?
Asexual reproduction
Sexual reproduction
LE 13-2
Asexual reproduction
Bud forms by mitosis of parent cells
Bud genetically identical to parent
Parent
Bud
0.5 mm
Sexual Reproduction
1. Meiosis: formation of haploid gametes (eggs, sperm) in
parents
2. Fertilization: fusion of gametes from opposite parents-->
diploid zygote-->mitotic growth-->embryo-->adult
LE 13-5
Human
Sexual
Reproduction
Key
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
cell (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Offspring
genetically unique
from parents
How many chromosomes are inherited by human zygotes?
46 chromosomes
or
2n= 46
n= the number of chromosomes in a set
23 from mother, maternal chromosomes
23 from father, paternal
“
LE 13-3
Human Karyotype
Stained metaphase
chromosomes
from human
somatic cell
Why stain metaphase
chromosomes?
Compact &
easier to see compared to interphase
5 µm
LE 13-3
Pair of homologous
chromosomes
Centromere
Sister
chromatids
5 µm
Can you tell the difference between a human
male and female by the karyotype?
Humans have
22 pairs of autosomes (non-sex chromosomes)
1 pair of sex chromosomes
Female sex ch.: XX
Male sex ch.: XY
LE 13-3
Human Karyotype
Male or female?
5 µm
If somatic cells have two sets of chromosomes (diploid),
how do gametes end up with only one set (haploid)?
Diploid precursors to the egg and sperm undergo meiosis in
the testis and ovary.
LE 13-7
Overview of meiosis
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Diploid
Chromosomes
replicate
aka tetrad
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
Sister chromatids
separate
Haploid cells with unreplicated chromosomes
Haploid
LE 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
LE 13-8ab
MEIOSIS I: Separates homologous chromosomes
METAPHASE I
PROPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Sister
chromatids
Chiasmata
Metaphase
plate
Spindle
Tetrad
Microtubule
attached to
kinetochore
Homologous chromosomes
(red and blue) pair and
exchange segments
Homologous recombination (crossing-over)
Tetrads line up
Homologous
chromosomes
separate
Pairs of homologous
chromosomes split up
LE 13-8b
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase
Prophase I
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
2n = 6
Metaphase
Chromosomes
positioned at the
metaphase plate
Anaphase
Telophase
Cytokinesis
Sister chromatids
separate during
anaphase
2n
Tetrad formed by
synapsis of homologous
chromosomes
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II
n
Mitosis produces
two genetically identical diploid cells
Meiosis produces
four genetically distinct haploid cells
Mechanisms of Genetic Variation Among Offspring
Caused by Sexual Reproduction
1. Crossing over (homologous recombination)
(Prophase I)
2. Independent assortment of chromosomes
(Metaphase I & II)
3. Random fertilization (post-meiosis)
LE 13-11
Recombination
Nonsister
chromatids
Prophase I
of meiosis
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
Chiasmata, pl
LE 13-10
Independent Assortment
Key
Maternal set of
chromosomes
Possibility 2
Possibility 1
Paternal set of
chromosomes
Metaphase I
Two equally probable
arrangements of
chromosomes at
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Random Fertilization
• Any sperm can fuse with any ovum
(unfertilized egg)
• a (human) zygote has about 64 trillion
diploid combinations
• Each zygote has unique genetic identity
I think they were talking
about us. I hope they had
it right!
If not maybe we can
answer questions.
See meiosis animation
What is missing?