The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 30

the inward drawing of the membrane ( Fig. 1.1 F). Cleavage is further aided by con-
traction of a thin ring of actin and myosin filaments arranged in an overlapping fash-
ion. The ring assembles in anaphase and is composed of circumferentially oriented
filaments. The sliding action between actin and myosin generates the force for cleav-
age. The cells loosen their adhesion with the extracellular matrix during the initiation
of the M phase, and after cytokinesis again flatten and redevelop the bond with the
extracellular matrix [-] .
1.8.3 Cell Division—Meiosis
In general, eukaryotic cells contain two sets of chromosomes, one from the mater-
nal side and the other from the paternal side. These pairs of chromosomes are called
homologous chromosomes. Meiosis is a specialized cell division in which chromo-
some content is halved to produce haploid cells, and is important for sexual repro-
duction. It allows for DNA recombination when two haploid cells fuse to form a
diploid cell. Such haploid cells, specific for sexual reproduction, are called gametes.
Gametes contain half of the genome, that is, one chromosome from the homol-
ogous pair, so that each cell inherits either a maternal or a paternal chromosome.
Meiosis is strikingly different from mitosis, because it can be assumed as a mitotic
cycle without an S phase that duplicates the chromosomes. But meiosis is a complex
process that can be divided into two stages: meiosis I and meiosis II.
Meiosis I is further divided into an elaborate prophase I comprised of five sub-
phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. Following pro-
phase I are metaphase I, anaphase I, and telophase I. Then the cell enters meiosis II,
which is quite similar to mitosis.
Meiosis starts with the synthetic phase where the chromosomes are duplicated,
but they exist as a pair of sister chromatids joined together tightly by groups of pro-
teins. The replicated homologues come together and form a bivalent. The synthetic
phase is followed by an elaborate prophase I. Prophase I initiates with the leptotene
stage in which the sister chromatids from the S phase condense into discreet struc-
tures and the pair of homologous chromosomes come together. Leptotene advances
into zygotene, where the homologues are brought closer together into a structure
called synapsis; this is held by a protein scaffold called synaptonemal complex and is
assembled between the homologues. But the end of pachytene is marked by the dis-
assembly of synaptonemal complex, and the chromosomes assume a somewhat dif-
fused state. The chromosomes recondense in the diplotene, and chiasmata are visible,
which are sites for crossover and allow for genetic recombination between homolo-
gous chromosomes. The paired chromosome homologues form a bivalent that con-
tains four chromatids, and genetic recombination occurs by breakage and crossover
of the DNA strand. Pieces of DNA are exchanged between the paternal and maternal
sister chromatids during the process. Diplotene is followed by diakinesis, in which
spindle formation starts with the disappearance of the nuclear membrane and pro-
gression into the further phases of meiosis.
Subsequent to crossover, the chromosomes line up on the spindle fibers, and the
two sister chromatids joined together behave as a single unit. Hence, during anaphase