Nuclear and Extranuclear DNA in Insects - Insect Molecular Genetics - page 82

DNA is thought to act as a buffer zone to protect nearby genes from DNA
loss during DNA replication, to promote the spread of short repeats between
telomeres through recombination, to give rise to telomere-to-telomere asso-
ciations that affect the three-dimensional arrangement of chromosomes in the
interphase nucleus, and to promote meiotic pairing of chromosomal homologs
( Kamnert et al. 1997 ).
Telomeres are crucial for the viability of the cell. If telomeres are damaged,
progressive loss of DNA occurs, and the sticky ends of damaged chromosomes
will bind to other chromosomes with sticky ends, resulting in chromosome
abnormalities such as dicentric chromosomes (chromosomes with two centro-
meres), which can lead to chromosome breakage and loss. Our understanding
of telomeres continues to increase and could continue to provide some surprises
( Greider 1999 ).
3.12 Chromosomes during Mitosis and Meiosis
Chromosomes are visible by light microscopy during the cell divisions called
mitosis and meiosis. The following reviews the basic aspects of these two types
of cell division, both of which are essential for growth, development, and repro-
duction. Mitosis occurs in somatic cells, whereas meiosis occurs only in germ-line
cells in eukaryotic organisms.
3.12.1 Mitosis
Somatic cells divide by mitosis . Mitosis produces two nearly identical daugh-
ter cells, each containing the same number of chromosomes as the original cell
( Figure 3.3 ). Before the onset of mitosis, the chromosomes within the nuclear
membrane are not visible by light microscopy, because they are not condensed.
Cells not actively undergoing mitosis are in the interphase state.
The cell cycle consists of a coordinated set of processes by which a cell repli-
cates all its components and is divided into two nearly identical daughter cells.
The coordination of cell growth and periodic chromosome replication and divi-
sion during the cell cycle has important implications for understanding the
evolution of cells and development ( Edgar and Lehner 1996, Novak et al. 1997,
Dobie et al. 1999, Zachariae 1999, Zhang 1999 ).
The cell cycle consists of four phases: G 1 → S → G 2 → M. DNA synthesis and chro-
mosome duplication take place during the portion of the cell cycle called the
S phase (for synthesis), but they do not occur during the G 1 and G 2 phases of the
cell cycle. The M phase represents mitosis , in which the duplicated chromosomes
and the cytoplasm are divided into two daughter cells. G 1 is the gap between