Meiosis and mitosis
Meiotic prophase (diakinesis) in a sporocyte of
Ophioglossum reticulatum, showing about 630 bivalents.
Meiotic chromosome dance
The function of meiosis is to
generate cells that contain
exactly half of the genetic
materials of the parental cells
and that develop into germ cells.
Chromosome rarrangements
could occur during meiosis, get
fixed in populations, and
eventually can contribute to
genetic differentiation and
speciation.
Mitosis vs. Meiosis
I
II
Meiosis I and II
Meiosis II
(equational division)
• prophase
• metaphase
• anaphase
• telophase
Meiosis I
(reductional division)
• prophase
leptotene
zygotene
pachytene
diplotene
diakinesis
• metaphase
• anaphase
• telophase
Prophase I
• leptotene
• zygotene
• pachytene
• diplotene
• diakinesis
(a – f) prophase I
(a) early zygotene
(b - d) early to late pachytene
(e) diplotene
(f) diakinesis
(g, h) metaphase I
(i, j) anaphase I
(k) telophase I
prophaseI
MEIOSISIMEIOSISII
Meiotic divisions I and II in the rye (Secale cereale)
(l) prophase II
(m) metaphase II
(n) anaphase II
(o) telophase II (four haploid
pollen mother cells)
Meiotic divisions I and II in Arabidopsis thaliana
MEIOSISIMEIOSISII
(A - H) prophase I
(A) leptotene
(B) zygotene
(C) pachytene
(D) diplotene
(E) diakinesis
(F) metaphase I
(G) anaphase I
(H) telophase I
(I) prophase II
(J) metaphase II
(K) anaphase II
(L) telophase II
(M) four newly formed nuclei
IV a
IV b
IV a
IV b
Prophase I in Arabidopsis thaliana as revealed by
chromosome painting
NOR
CEN
leptotene zygotene
pachytene
139 clones of a BAC tiling path covering Arabidopsis chromosome 4 were divided into 11 pools of 8-18
BACs. Individual pools were labelled either by biotin-dUTP (red) or digoxigenin-dUTP (green) for
painting of either the long arm (113 BACs) or the entire chromosome (139 BACs).
early prophase I
CEN
Key events of meiosis I
Chromosome recognition and pairing
- the mechanism is not known
Synapsis
- synaptonemal complex (SC)
- the link between synapsis and
recombination is not well understood
Meiotic recombination
- process of formation of doublestrand
breaks (DSBs) and their
subsequent repair
- results in formation of crossover
and non-crossover products
Links between chromosome pairing,
synapsis and recombination are not
well undestood. Recombination plays a
key role in unifying meiotic events in
prophase I.
Homologous chromosome recognition and pairing
Schematic of centromere behavior in early meiotic prophase I in a hypothetical diploid
organism with two pairs of chromosomes
Homologous chromosome pairing facilitated by telomere
and centromere clustering/coupling/pairing
Homologous chromosome pairing facilitated by telomere
and centromere clustering
2n = 28
CENs
TELs
leptotene zygotene
telomere bouquet
(centromere pairing is prior to telomere clustering)
Arabidopsis: telomere clustering around nucleolus
Armstrong et al. 2001, JCS
Prophase I – pachytene (synapsis of homologous chromosomes)
synaptonemal complex in Arabidopsis thaliana
Synaptonemal complex (SC)
- consists of two lateral elements (le) connected by
a central element (ce) [the lateral elements formed
as axial elements (AEs, also called the chromosome
axis) in leptotene]
- the central element assembles following
chromosome pairing during zygotene
- recombinational nodule (rn) – place of crossingover
(genetic exchange between homologues
Recombination (crossing over) and chiasmata
Prophase I: crossing over
• SPO11 protein introduces DSBs into
chromosomal DNA
• DNA ends adjacent to these breaks are later
bound by RAD51 and DMC1 that catalyze singleend
invasion of the broken DNA ends into the
homologous chromosome
Recombination: double-strand breaks (DSBs) and their repair
dHJ: double-Holliday junction; SDSA: synthesis-dependent strand annealing; COs: crossovers; NCOs: noncrossovers
D-loop
Recombination (crossing over) and chiasmata
chiasma formation
Plants – male gametogenesis
Plants – famale gametogenesis
Plants – fertilization
3x endosperm