Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MEIOSIS
1007
axial cores
of the
homologs
cohesin
transverse
filaments
100 nm
Figure 17–55 Simplified schematic
drawing of a synaptonemal complex.
Each homolog is organized around a
protein axial core, and the synaptonemal
complex forms when these homolog
axes are linked by rod-shaped transverse
filaments. The axial core of each homolog
also interacts with the cohesin complexes
that hold the sister chromatids together
(see Figure 9–35). (Modified from
K. Nasmyth, Annu. Rev. Genet. 35:673–
745, 2001.)
chromatin loops of sister
chromatids of one homolog
along their entire lengths (see Figure 9–35). The pachytene stage can persist for
days or longer, until desynapsis begins at diplotene with the disassembly of the
synaptonemal complexes and the concomitant condensation and shortening of
the chromosomes. It is only at
MBoC6
this stage,
m21.08/17.55
after the complexes have disassembled,
that the individual crossover events between nonsister chromatids can be seen
as inter-homolog connections called chiasmata (singular chiasma), which now
play a crucial part in holding the compact homologs together (Figure 17–57). The
homologs are now ready to begin the process of segregation.
LEPTOTENE
PACHYTENE
paternal
sister
chromatids
chromatid 1
chromatid 2
assembling
synaptonemal
complex
disassembling
synaptonemal
complex
maternal
sister
chromatids
chromatid 3
chromatid 4
(A)
INTERPHASE
ZYGOTENE
DIPLOTENE
FOLLOWED BY DIAKINESIS
(B)
0.1 µm
(C)
(D)
5 µm
Figure 17–56 Homolog synapsis and desynapsis during the different stages of prophase I. (A) A single bivalent is shown schematically. At
leptotene, the two sister chromatids coalesce, and their chromatid loops extend out from a common axial core. Assembly of the synaptonemal
complex begins in early zygotene and is complete in pachytene. The complex disassembles in diplotene. (B) An electron micrograph of a
synaptonemal complex from a meiotic cell at pachytene in a lily flower. (C and D) Immunofluorescence micrographs of prophase I cells of the fungus
Sordaria. Partially synapsed bivalents at zygotene are shown in (C) and fully synapsed bivalents are shown in (D). Red arrowheads in (C) point to
regions where synapsis is still incomplete. (B, courtesy of Brian Wells; C and D, from A. Storlazzi et al., Genes Dev. 17:2675–2687, 2003. With
permission from Cold Spring Harbor Laboratory Press.) MBoC6 m21.09/17.56