 Describe
the result of meiotic division in
terms of sexual reproduction
 Discuss the structure of homologous
chromosomes
 Describe chromosomes in terms of
ploidy
 Distinguish between sexual and asexual
reproduction
 Discuss genetic variation
 Chromosomes
occur in pairs
• Each chromosome may contain ~ 1000 genes
• Diploid cells contain two of each kind of
chromosome (2n)
 Somatic or body cells (46 chromosomes in humans)
 Produced by mitosis
• Haploid cells contain one of each kind of
chromosome (n)
 Gametes or sex cells (23 chromosomes in humans)
 Produced by meiosis

Homologous
Chromosomes
(Homolog's)
• “Homo” Greek word for
the same
• Represents the two
chromosomes of each
pair in a diploid cell
• Each homologous pair
has genes for the same
trait
 Tall or short
• Homolog's are not
always identical
 Why
is meiosis needed to produce
gametes?
• Meiosis – form of cell division which produces ½
the number of chromosomes as a somatic cell
 Meiosis divided into two phases
 Meiosis I begins with one diploid cell
 Meiosis II ends with four haploid gametes
 Gametes must be haploid in order to continually
produce a diploid zygote
 Sperm (n) + Egg (n) = Zygote (2n)
 Sexual
reproduction
• Production and fusion of haploid gametes
• n + n = 2n
• Genetic information is exchanged
 Asexual
reproduction
• Single parent produces one or more identical
offspring by dividing into two cells
• No exchange of genetic material
 Binary Fission, Parthenogenesis
 Budding, Fragmentation
 Interphase
• Cell replicates its
chromosomes
• Each chromosome
consists of two sister
chromatids
 Prophase
I
• DNA coils as the spindle forms
• Two homologous chromosomes line up gene by
gene to form a tetrad
• Tetrad forms so tightly that crossing over occurs
 Exchange of genetic material
 Can occur at any location along the chromosome
 Results in new combinations of alleles
 Metaphase 1
• Centromere of each chromosome becomes
attached to a spindle fiber
• Tetrads move to the equatorial plane of the
spindle
• Unique to Meiosis
 Anaphase I
• Homologous chromosomes separate and move
to opposite ends of the cell
• Centromeres holding sister chromatids together
do not split
 Telophase
1
• Spindle breaks down
• Chromosomes uncoil
• Nucleus Reappears
• Cytoplasm divides
• One more cell division is needed b/c each
chromosome is still doubled
(2 sister chromatids)
 Mitotic
division of products of Meiosis I
 Prophase 2
• Spindle forms in each cell
• Nucleus disappears
• Chromatin coils into chromosomes
• Centrioles migrate to opposite sides of cell

Metaphase 2
• Chromosomes line up on the equatorial plane of the
spindle
• Each centromere connected to two spindle fibers

Anaphase 2
• Centromeres split
• Chromatids separate
• Chromatids migrate to opposite ends of the cell

Telophase 2
•
•
•
•
Nuclei reform
Spindle breaks down
Cytoplasm divides
Chromosomes uncoil into chromatin
 End
result of Meiosis
• Four haploid cells formed from one diploid cell
• Four haploid cells become gametes
 Genetic
Recombination –
re-assortment of
chromosomes and
the genetic
information they
carry
• Independent
segregation and
crossing over increase
genetic variability and
drive evolution
 Genetic
Recombination
• Independent Segregation
 Gene combinations vary depending on how each pair of
homolog's lines up during Metaphase 1
 Random Process
 Number of combinations increases as chromosome
increases
 Each 23 pairs of chromosomes may align independently in
a gamete
 223 = 8 million types of egg or sperm a person can produce
 When fertilization occurs 223 x 223 = 70 trillion possible
zygote combinations
 Genetic
Recombination
• Crossing over
 May occur at any location
when tetrads are formed
 Variation
is the raw
material that forms
the basis of evolution
 Nondisjunction
• Failure of homologous chromosomes to separate
properly during meiosis
• Four basic types
 Trisomy
 One gamete with an extra chromosome
 One gamete missing a chromosome
 n + 2n = 3n
 Trisomy 21 (down syndrome)
 Gamete with an extra chromosome is fertilized by a normal
gamete
 Resulting zygote has 47 chromosomes
 Nondisjunction
• Monosomy
 Gamete missing a chromosome fuses with a normal
gamete
0+n=n
 Most zygotes with monosomy do not survive
 Turner syndrome
 Human females have only one X chromosome instead of two
 Nondisjunction
• Tetraploidy
 Fusion of gametes, each
with a complete set of
chromosomes
 2n + 2n = 4n
 Results from a total lack of
separation of homologous
chromosomes
 Common in plants
 Chrysanthemum
 Nondisjunction
• Polyploidy
 Organisms with more than usual number of
chromosomes
 Rare in animals, usually results in death of zygote
 Frequently occurs in plants
 Errors in meiosis can be beneficial for agriculture
 6n Wheat
 3n Apples
 Resulting plants are usually larger, healthier and more
disease resistant