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Samenvatting Biology: A Global Approach Global Edition Chapter 12, 13, 14, 15, 23, 47
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Chapter 12 MitosisThe Key Roles of Cell Division
The continuity of life is based on the reproduction of cells, or cell division.
The cell division process is an integral part of the cell cycle, the life of a cell from the time it is
first formed during division of a parent cell until its own division into two daughter cells.
12.1
Most cell division results in genetically identical daughter cells
Cellular Organization of the Genetic Material
Genome -> cell’s genetic information. Prokaryotic genome is often a single DNA molecule,
eukaryotic genomes usually consist of a number of DNA molecules.
The replication and distribution of so much DNA are manageable because the DNA
molecules are packaged into structures, chromosomes. The associated proteins maintain the
structure of the chromosome and help control the activity of the genes. Together, the entire
complex of DNA and proteins that is the building material of chromosomes is referred to as
chromatin.
Every eukaryotic species has a characteristic number of chromosomes in each cell’s
nucleus. The nuclei of human somatic cells (all body cells except the reproductive cells) each
contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent.
Reproductive cells or gametes have half as many chromosomes as somatic cells.
Distribution of Chromosomes During Eukaryotic Cell Division
Each duplicated chromosome consists of two sister chromatids, which are joined copies of
the original chromosome. The two chromatids, each containing an identical DNA molecule,
are typically attached all along their lengths by protein complexes called cohesins; this
attachment is known as sister chromatid cohesion. Each sister chromatid has a centromere,
a region made up of repetitive sequences in the chromosomal DNA where the chromatid is
attached most closely to its sister chromatid. This attachment is mediated by proteins that
recognize and bind to the centromeric DNA; other bound proteins condense the DNA, giving
the duplicated chromosome a narrow “waist”. The portion of a chromatid to either side of the
centromere is referred to as an arm of the chromatid.
Later in cell division process, the two sister chromatids of each duplicated chromosome
separate and move into two new nuclei, one forming at each end of the cell. Once the sister
chromatids separate, they are no longer called sister chromatids but are considered
individual chromosomes; this is the step that essentially doubles the number of
chromosomes during cell division. Thus, each new nucleus receives a collection of
chromosomes identical to that of the parent cell. Mitosis, the division of the genetic material
in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm.
,12.2
The mitotic phase alternates with interphase in the cell cycle
Phases of the Cell Cycle
Mitosis is one part of the cell cycle. The mitotic (M) phase, which includes both mitosis and
cytokinesis, is usually the shortest part of the cell cycle. The mitotic phase alternates with a
much longer stage called interphase, which often accounts for about 90% of the cycle.
Interphase can be divided into three phases:
• G1 phase (“first gap”)
• S phase (“synthesis”)
• G2 phase (“second gap”)
During all three phases a cell grows by producing proteins and cytoplasmic organelles such
as mitochondria and ER. Duplication of the chromosomes occurs entirely during the S
phase.
A cell grows (G1), continues to grow as it copies its chromosomes (S), grows more as it
completes preparations for cell division (G2) and divides (M).
,Mitosis is conventionally broken down into five stages:
• Phophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
Exploring Mitosis in an Animal Cell
, • G2 of Interphase
o Nuclear envelope encloses the nucleus.
o The nucleus contains one or more nucleoli
o Two centrosomes have formed by duplication of a single centrosome.
o Centrosomes are regions in animal cells that organize the microtubules of the
spindle. Each centrosome contains two centrioles.
o Chromosomes, duplicated during S phase, cannot be seen individually
because they have not yet condensed.
• Prophase
o The chromatin fibers become more tightly coiled, condensing into discrete
chromosomes.
o The nucleoli disappear.
o Each duplicated chromosome appears as two identical sister chromatids
joined at their centromeres and all along their arms by cohesins (sister
chromatid cohesion).
o The mitotic spindle begins to form. It is composed of the centrosomes and the
microtubules that extend from them. The radial arrays of shorter microtubules
that extend from the centrosomes are asters.
o The centrosomes move away from each other, propelled partly by the
lengthening microtubules between them.
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