MEIOSIS AND
SEXUAL
REPRODUCTION
CHAPTER OUTLINE
I. OVERVIEW
II. HAPLOID AND DIPLOID
III. MEIOSIS, LIFE CYCLE AND DIPLOIDY
IV. THE PROCESS OF MEIOSIS
V. POST MEIOTIC EVENTS
A. OVERVIEW
B. MEGASPOROGENESIS
C. MICROSPOROGENESIS
D. OOGENESIS
E. SPERMATOGENESIS
VI. ASEXUAL REPRODUCTION
Haploid and Diploid
Generations
Most eukaryotic
organisms have an
alternation of
haploid and diploid
generations. In
animals, the haploid
generation is onecelled. In plants, it
usually is many
celled.
Example, fungi and algae such
as Chlamydomonas
Example: Humans
Plants
MITOSIS AND MEIOSIS
MITOSIS
•Somatic Cells
•Daughter cells identical to
parent cells
•No Genetic recombination
2n=2
MEIOSIS
•Gametes or spores
•Daughter cells have half of
the chromosomes of the
parent
•Genetic recombination
• Reassortment
2n=2
2n=2
n=1
MITOSIS
MEIOSIS
Chromosome number is reduced
The Process of Meiosis
Interphase I
Chromosomes duplicate to form two chromatids during S phase
A
a
AA
aa
Each chromosome is composed of two chromatids
PROPHASE I
1. Synapsis of homologous chromosomes
a. the synaptonemal complex
b. gene for gene pairing
2. Crossing over (Recombination)
3. Chromosome condensation
4. Spindle forms
5. Nuclear membrane disappears
The stages of chromosome condensation in prophase
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
Prophase I
Homologous chromosomes synapsed
Blue = 2 chromatids, yellow = 2 chromatids
Crossing
over occurs
RECOMBINATION
Chromosomes showing the
characteristic X-shaped chiasma that
result from crossing over between
homologous chromosomes
Synaptonemal complexes in Hyalophora cecropia (silk moth).
The normal male chromosome number is 62 giving 31 pairs. In
the individual shown here, one chromosome is present in three
copies (center). The dark spot is the nucleolus (site of ribosomal
RNA synthesis)
Synaptonemal complex
(dark lines)
Loops of chromosomal
material
Nucleolus
Unpaired chromosome in Lilium tyrinum - note
central core and lateral loops of chromosome
material
Fig 9-8 a,b
Synaptonemal complex in Lilium tyrinum. Note
synaptonemal complex and chromosome loops on
the sides
Recombination is a source of
genetic variability. Materials on a
chromosome are re-arranged with
respect to each other
Maternal
Grandparents
Paternal
Grandparents
Mother
Father
You
METAPHASE I
AA
aa
Metaphase in Lilium regale
METAPHASE I
THE LAW OF INDEPENDENT ASSORTMENT
A
a
B
b
or
A
b
a
B
Segregation of homologous chromosome pair 1 is independent of
segregation of homologous pair 2
This independent assortment results in mixing of chromosome
sets from the mother (black) and father (red) = reassortment
Figure 9-10
MEIOSIS PRODUCES GENETIC
VARIABILITY
• In prophase as a result of crossing over
• In metaphase I as a result of the way
chromosomes line up on the metaphase plate
(reassortment)
MEIOSIS UNDERGOES TWO CELL
DIVISIONS AND REDUCES THE
CHROMOSOME NUMBER TO THE
HAPLOID OR N NUMBER
METAPHASE I. Homologous chromosomes line up on the
Metaphase plate
Metaphase plate seen from
the top
Metaphase plate seen from
the side
ANAPHASE I
Early anaphase
A
A
a
a
Late anaphase
ANAPHASE I - Homologous chromosomes separate
(reduction division)
Telophase I, Interphase I and Prophase II may
or may not occur, depending on the organism
Telophase I
Interphase I
Prophase II
METAPHASE II
A
A
aa
Anaphase I
METAPHASE II • Spindle reforms at right
angles to the original
spindles
• Chromosomes (each
composed of 2 sister
chromatids) line up on the
metaphase plate
ANAPHASE II
A
A
a
a
ANAPHASE II -
Sister chromatids separate
TELOPHASE II
A
A
a
a
TELOPHASE II - New nuclear membranes form
around the 4 daughter cells
POST-MEIOTIC
MODIFICATIONS
Microspore development in higher plants
(formation of the male gametophyte)
Microspores are the product of meiosis before
differentiation into pollen
Microspores undergo 1 round of mitotic division to
produce a pollen cell with two nuclei
Pollen released from anthers
HAPLOID
OOGENESIS
SPERMATOGENESIS
Asexual
reproduction
Stolons
Vegetative propagules are
genetically identical to the parent
END MEIOSIS