D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 20
CELL REPRODUCTION – MEIOSIS
1.
Role of meiosis in sexual reproduction: special nuclear division that is
necessary in sexually reproducing species – alternates with fertilization in life
cycle; preserves species chromosome # from organismal generation to generation (fig. 13 –
5 & ppt. 1)
• reduces chromosome # from 2n (diploid) to n (haploid) in formation of
gametes (gametogenesis)
• fertilization: union of two gametes combines genetic material of two cells;
to maintain diploid constitution (= 2n chromosomes, or n pairs = n paternal and n
maternal homologues) (fig. 13 – 4)
• karyotypes: display of the chromosomal constitution of a species, e.g.
human, prepared from blood cells (fig. 13 – 3, ppts. 2 & 3)
• sexual cycles & alternation of generations: varies according to relative
timing of meiosis and fertilization within life cycle (fig 13 – 6 & ppt. 4):
- animals: meiosis produces gametes (haploid); gametes engage in
fertilization that produces multicellular organism (diploid)
- fungi & some algae: meiosis produces spores (haploid) that germinate
into multicellular organism; organisms produce gametes by mitosis (haploid);
fertilization produces zygote (diploid) that undergoes meiosis again
Bio 102
lec. 20 - p. 2
- plants (ferns) & some algae: haploid generation (gametophyte) produces
gametes; fertilization produces diploid generation (sporophyte); sporophyte
produces spores through meiosis; spores germinate to produce gametophyte
Bio 102
2.
lec. 20 - p. 3
Meiotic cell divisions:
• two cell divisions: meiosis I (reduction division) & meiosis II (equation
division), with no intervening duplication of genetic material (no S period);
chromosome number per cell is reduced from 2n to n (figs. 13 – 7, 13 – 8, ppts. 5 – 7)
- prophase I: duplicated chromatin condenses into double-stranded
chromosomes that cluster at one edge of the nucleus
- synapsis – chromosomes pair up with homologous counterparts assisted by
special proteins that align homologues in perfect register (four chromatids = tetrad)
- crossing over – exchanges of identical parts of non-sister chromatids
between homologues (fig. 13 – 11 & ppt. 8)
- disjunction of tetrads; chromosomes remain attached where crossovers
occurred (chiasmata)
metaphase I: homologues (each double-stranded) line up side by side at
equator; one kinetochore per homologue; each kinetochore in a homologous pair is
directed to opposite pole from the other kinetochore of the pair
anaphase I: double-stranded homologues are drawn to opposite poles (no
centromere division occurs)
telophase I: much like mitotic telophase; cytokinesis & brief interkinesis (no
interphase S period) follow before meiosis II begins
Bio 102
lec. 20 - p. 4
meiosis II: similar to mitosis but beginning with haploid (n) number of
double-stranded chromosomes
• final result: four daughter cells, each with haploid # of single-stranded
chromosomes; recall mitosis produces diploid # of single-stranded chromosomes
(fig. 13 – 9 & ppt. 9)
separation of homologues leads to segregation of maternal & paternal
counterparts; contributes to genetic variation (fig. 13 – 10 & ppt. 10)
crossing over increases genetic variation (fig. 13 – 11 & ppt. 11)