Chapter 11-4:
Meiosis
• Genes are lined up
on chromosomes
• Chromosomes
occur in pairs in
body (somatic) cells
– 1 pair from
mother
– 1 pair from father
– diploid cell (2n)
• cell with 2 of
each kind of
chromosome
• Organisms produce gametes (sex cells)
– contain 1 of each kind of chromosome
– haploid cell (n)
• one of each kind of chromosome
Homologous Chromosomes
• Paired chromosomes
• 2 chromosomes of
each pair in a diploid
cell
• each have a
corresponding
chromosome from the
opposite sex parent
Meiosis
• # of chromosomes per cell is cut in half
through separation of homologous
chromosomes
• cell division that produces gametes (sex
cells)
– has ½ the number of chromosomes as
the parent
• involves 2 cell divisions (I and II)
• Haploid cells produced are sex cells or
gametes
–Sperm: male gametes (n)
• 4 equal-sized
–Egg: female gametes (n)
• 1 large egg cell; 3 small polar bodies
• Sperm + egg = fertilization (n + n = 2n)
– Fertilized egg called a zygote
• Zygote goes through mitosis to develop into a
multicellular organism
• Meiosis produces genetic variation
– Genetic recombination
• Reassortment of chromosomes
– Basis for evolution (change through time)
Meiosis I: Interphase
• Begins with a
diploid cell
• cell replicates its
chromosomes
–Consists of 2
identical sister
chromatids
Prophase I
• DNA coils up and spindle forms
– Homologous chromosomes line up with
each other to form a tetrad
– Tetrad: consists of 2 homologous
chromosomes, each made up of 2 sister
chromatids
• Pair up tightly (very tightly)
Crossing-Over
• Non-sister chromatids from
homologous chromosomes can break
and exchange genetic material
• Can occur anywhere on a
chromosome and at several locations
at the same time
• Exchange of genetic material
• Crossing-over provides genetic variation
– Increases variability of organisms
• Sometimes chromosomes fail to separate
correctly
– Called nondisjunction
– Can lead to extra chromosomes or not enough
chromosomes
• Metaphase I
– Spindle fibers pull tetrads to middle of a cell and
line up together
• Anaphase I
– Homologous chromosomes separate and move
to opposite ends of the cell
• Telophase I
– Chromosomes uncoil and cytoplasm divides to
make 2 cells
Phases of Meiosis II
• Second division
• Just like mitosis
– Prophase, Metaphase, Anaphase,
Telophase
– No chromosomes are replicated before
starting meiosis II
• Result of meiosis II: 4 genetically different
haploid (n) cells formed
– Become gametes (sex cells)
Meiosis vs. Mitosis
Number of cells
produced
Are cells produced
genetically identical or
different?
Mitosis
Meiosis
2
4
identical
different
Type of cells produced Diploid Diploid
Diploid  Haploid
2n  2n
2n  n
Somatic/body cells Sex cells
Type of reproduction
Number of divisions
Asexual
reproduction
1
Sexual
reproduction
2
Asexual Reproduction
• Involves one parent
• Binary fission
– Replicates DNA and divides in half  cell
splits into 2
• Budding
– Breaks off from parent and grows into
new organism
• Spore formation  fungi
• Vegetative propagation
– Making new plants from runners, stems,
or roots
• Regeneration
– Making new organism from body parts
Sexual Reproduction
• Union of 2 cells
Bell Ringer:
1. If a diploid cell containing 28 chromosomes
undergoes meiosis, how many chromosomes will
each daughter cell have?
2. In what process do male and female reproductive
cells join during sexual reproduction?
a. segregation
c. self-pollination
b. fertilization
d. cross-pollination
3. What type of cell has 2 sets of chromosomes?
a. diploid
b. haploid
c. tetrad
4. There are four chromatids in a ___.
a. polygenic trait
c. gamete
b. tetrad
d. genotype
d. gene