11/9/10
Meiosis and Sexual Life Cycles
Thank you to Mr. Chris Hilvert
Genes
•  Genes are the units of heredity
•  made up of segments of DNA
•  passed to the next generation through
reproductive cells called gametes (sperm and
eggs)
•  locus: the location on a
•  one set of chromosomes is inherited from each
parent
Heredity
  Heredity: the transmission of traits
from one generation to the next
  Asexual reproduction: clones
(genetically identical individuals from
the same parent)
  Sexual reproduction: variation
  Human life cycle:
–  23 pairs of homologous chromosomes
•  1 pair of sex and 22 pairs of
autosomes
–  karyotype
• 
gametes are haploid (1N)/ all other
cells are diploid (2N)
•  fertilization (syngamy) results in a
zygote
  Meiosis: cell division to produce
haploid gametes
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Normal Human
Karyotypes
• A karyotype is an ordered display of
the pairs of chromosomes from a cell
• Human somatic cells (any cell other
than a gamete) have 23 pairs of
chromosomes
• The two chromosomes in each pair are
called homologous chromosomes, or
homologs
• Chromosomes in a homologous pair
are the same length and carry genes
controlling the same inherited
characters
Some Terminology
• 
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The sex chromosomes are called X and Y
•  Human females have a homologous pair of X chromosomes (XX)
•  Human males have one X and one Y chromosome
•  The 22 pairs of chromosomes that do not determine sex are called
autosomes
Each pair of homologous chromosomes includes one chromosome from
each parent
•  The 46 chromosomes in a human somatic cell are two sets of 23: one
from the mother and one from the father
A diploid cell (2n) has two sets of chromosomes
•  For humans, the diploid number is 46
In a cell in which DNA synthesis has occurred,
each chromosome is replicated; consists of
two identical sister chromatids
Gametes
•  A gamete (sperm or egg) contains a single set of
chromosomes, and is haploid (n)
•  For humans, the haploid number is 23 (n = 23)
•  Each set of 23 consists of 22 autosomes and a single
sex chromosome
•  In an unfertilized egg (ovum), the sex chromosome is X
•  In a sperm cell, the sex chromosome may be either X or
Y
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Meiosis and Fertilization
Maintain Chromosome Number
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At sexual maturity, the ovaries and
testes produce haploid gametes
Gametes are the only types of human
cells produced by meiosis
• 
one set of chromosomes in each gamete
• 
Fertilization and meiosis alternate in
sexual life cycles to maintain
chromosome number
• 
The fertilized egg is called a zygote
• 
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Fertilization is the union of gametes (the
sperm and the egg)
one set of chromosomes from each parent
The zygote produces somatic cells by mitosis
and develops into an adult
Meiosis
 Preceded by
chromosome
replication (remember
interphase?!), but is
followed by 2 cell
divisions (Meiosis I &
Meiosis II)
 4 daughter cells; 1/2
chromosome number
(1N); variation
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Events Unique to Meiosis
 All in meiosis I
–  prophase I
•  Homologous chromosomes physically connect and
exchange genetic information (synapsis and crossing
over)
–  metaphase plate
•  paired homologous chromosomes (tetrads) not
individual replicated chromosomes
–  anaphase I
•  homologous chromosomes, instead of sister
chromatids, that separate
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Fig. 13-9b
SUMMARY
Mitosis
Property
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase, anaphase, and
telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability among the gametes
Origins of Genetic Variation I
  Independent assortment:
homologous pair of
chromosomes position and
orient randomly
(metaphase I) and
nonidentical sister
chromatids during meiosis
II
–  Combinations possible:
2^n; with n the haploid
number of the organism
Independent Assortment, in Detail
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Independent Assortment, in Detail
Independent Assortment, in Detail
Origins of Genetic Variation II
  Crossing over (prophase I):
–  the reciprocal exchange of genetic
material between nonsister
chromatids during synapsis of
meiosis I (recombinant
chromosomes)
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Crossing Over, in Detail
Crossing Over, in Detail
Crossing Over, in Detail
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Crossing Over, in Detail
Crossing Over, in Detail
Origins of Genetic Variation III
 Random fertilization:
–  1 sperm (1 of 8 million
possible chromosome
combinations) x 1
ovum (1 of 8 million
different possibilities)
= 64 trillion diploid
combinations!
Genetic
Variation
Review
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Meiosis vs. mitosis
  Synapsis/tetrad/chiasmata
(prophase I)
  Homologous vs.
individual chromosomes
(metaphase I)
  Sister chromatids do not
separate (anaphase I)
  Meiosis I separates
homologous pairs of
chromosomes, not sister
chromatids of individual
chromosomes.
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