Chromosome Mechanics.pdf
需积分: 5 45 浏览量
2023-07-09
19:41:45
上传
评论
收藏 512KB PDF 举报
Chromosome Mechanics
Charleen M Moore,
University of Texas Health Science Center at San Antonio,
San Antonio, Texas, USA
Robert G Best,
University of South Carolina School of Medicine, Columbia, South Carolina, USA
Chromosome mechanics describes the processes of chromosome replication and
behaviour as they relate to somatic cell division and gametogenesis. Mitosis produces
identical diploid somatic cells, whereas meiosis produces haploid germ cells with
recombination of parental alleles. Aberrations occur in these processes that include
chromosomal nondisjunction leading to numerical abnormalities and unrepaired DNA
damage resulting in chromosomal breakage and structural rearrangements.
Introduction
Cell division in humans occurs in two fundamentally
different modes: mitosis and meiosis. Mitosis is cell
division of the somatic (body) cells by which two identical
diploid daughter cells are produced through DNA
replication and cytoplasmic division. Meiosis is a specia-
lized process of the germline (sperm and eggs) that involves
one round of DNA replication followed by two cell
divisions to produce four haploid germ cells. Male meiosis
results in the production of four equally sized, functional
spermatozoa. In contrast, female meiosis results in a single
large functional ovum, and three small nonfunctional
polar bodies. Abnormalities in these processes include
chromosomal nondisjunction, which results in the loss or
gain of one or more chromosomes, and chromosomal
breakage due to unrepaired DNA damage, which results in
the formation of abnormal chromosomes and an increased
risk for neoplasia.
Mitosis
The cell cycle can be divided into two periods: mitosis, in
which cell division occurs, and interphase, the remainder of
the cell cycle in which the cell is not dividing (
Figure 1
). An
elaborate system of genes regulates the cell cycle by
producing proteins and enzymes that control progression
from interphase to mitosis. Interphase is the time in the cell
cycle that is characterized by intense cytoplasmic activity,
while in the nucleus, the chromosomes are engaged in
messenger RNA synthesis and the darkly staining nucleoli
are producing ribosomal RNA. The chromosomes are
fully extended at this stage and cannot be recognized
individually, but rather are seen as a granular network of
chromatin in the nucleus.
Interphase is generally the largest segment of the cell
cycle and is composed of three phases: G
1
, S and G
2
. The
G
1
phase (Gap 1) is the period from the end of cell division
to the beginning of chromosome replication and is the time
when cells carry out their specialized functions. Cells that
are nondividing enter a resting phase at the end of G
1
,
which is called G
0
, and may remain in this phase of the cell
cycle indefinitely. The length of time spent in G
1
varies
from one cell type to another. For human lymphocytes
cultured in vitro, this period is typically between 12 and 24
hours in length. The S phase (synthesis) is the period in
which DNA replication occurs and generally takes about 7
hours to complete. During G
1
, cells are diploid (2n) with
one copy (1c) of each chromosome. Both homologues of
each chromosome contain a single molecule of double-
stranded DNA. By the end of the S phase, although the
cells remain diploid, each chromosome is composed of two
identical, or sister, chromatids (2c) connected at the
centromere, each chromatid containing a single DNA
molecule. The S phase is followed by G
2
(Gap 2), which is
the period between the end of replication and the start of
cell division. During G
2
some DNA repair takes place, but
otherwise, replication has essentially finished. The G
2
period lasts 4–6 hours and is followed immediately by
mitosis. Cell division is completed fairly quickly with
duration typically less than one hour. Mitosis is divided
into four stages: prophase, metaphase, anaphase and
telophase.
Prophase
The beginning stage of mitosis, prophase, is marked by the
condensation of the chromosomes into visible independent
units. The ends of the chromosomes, and perhaps other
segments as well, are attached to the nuclear membrane at
this time.
Two centrioles are present in the cytoplasm near the
nuclear membrane. These begin to separate and move to
opposite sides of the nucleus, forming a spindle of
microtubules between them. In late prophase, the chromo-
somes become shorter through condensation; individual
chromosomes are clearly visible, while the nucleoli
Article Contents
Introductory article
.
Introduction
.
Mitosis
.
Meiosis
.
Meiotic Pairing
.
Nondisjunction
.
Chromosome Breakage
.
Conclusions
1
ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net
资源评论
