Meiosis and Sexual Reproduction
Most eukaryotic species reproduce sexually at some point in
their life cycles
Gametes (both sperm and egg cell) of opposite sexes unite to
form a zygote – starting a new generation of offspring
Gametes are formed from a special kind of cell division called
Meiosis
Meiosis is one key factor that generates genetic diversity in a
species population and produces the particular genetic mixes that
contribute to evolution
For sexual reproduction,
Meiosis has to occur first!
Notice the
condition of
each
chromosome!
Meiosis reduces the number of chromosomes
• Early on it became clear that gamete formation
must reduce the number of chromosomes
• Or else, with each successive generation,
chromosome number would double!
• Gametes (sex cells) revealed half the chromosomes of somatic (body) cells
• The fusion of gametes to form a zygote is called
fertilization or syngamy
The life cycles of higher life forms show a pattern
of both diploid and haploid
stages
a. Some Algae and fungi – reveal a
Zygotic life cycle
b. Animals – reveal a Gametic Life
cycle
c. Plants and some algae – reveal a
Sporic Life Cycle
Humans exhibit what type of Life Cycle?
“Germ line” cells are
specialized cells which
may undergo Meiosis to
produce gametes
Features of Meiosis
• Meiosis exhibits 2 separate division processes:
– Meiosis I = Reduction division; number of chromosomes
divides in half
– Meiosis II = Equational division; double-stranded chromosomes
separate (as in mitosis)
• Homologous chromosomes (homologues) pair during
meiosis I in a process called synapsis
• During synapsis, homologues exchange pieces of
chromosomes in a process called crossing over
Synapsis
and
Crossing over
*Synapsis and crossing over occurs during
Prophase of Meiosis I (Prophase I)
Alignment of chromosomes differs between
Meiosis I and Mitosis
The process of
Meiosis I
The process of
Meiosis II
Meiosis vs Mitosis
• For either cell division process, chromosomes
must replicate to form DS chromosomes
• In meiosis:
»
»
»
»
»
Homologous chromosomes come together (synapsis)
Crossing over occurs
Meiosis I divides number of chromosomes in half
2N, DS  1N, DS
Meiosis II separates each of the DS chromosomes in each
cell
» 1N, DS  1N, SS
» Due to crossing over between homologues, the resultant
cells are not identical, genetically