The Cell Nucleus and Mitosis

By concentrating the transcription and replication into the nucleus, from which specialized instructions can be sent out into the cell in a controlled manner, a much better regulatory power over the biochemical processes and the organelles was achieved. Expressing only specific genes became particularly important for the development of organs in multicellular organisms. Moreover, a nucleus permits the existence and removal of introns, and prevents unwanted protein synthesis. The function of introns is not yet fully understood, but they are possibly involved in the variation of gene expression and serve as "scratch gene material" which, by mutation, can be employed to modify or create genes. New genes can be created, for instance, by exon shuffling, where an exon from another gene is anomalously inserted into an intron. Putting the genetic material into the nucleus had also the important advantage that it allowed the lengthy DNA to be subdivided into sets of chromosomes. This powerful invention not only sped up the replication process but permitted sexuality, by which genes from different parents could be combined and reshuffled. However, to realize these advantages, very complicated reproduction processes of the nucleus, called mitosis and meiosis, had to be developed.

Mitosis is the division of a parental nucleus in an eukaryotic cell which produces two daughter nuclei with the same complete set of chromosomes. It consists of five phases (see Fig. 7.4). The interphase is the synthesis and growth phase, in which the cell spends most of its time. Just before cell division, the DNA of each chromosome is replicated. Two identical chromosomes for each parent chromosome are formed, held together by a centromere. Outside the nuclear envelope there are the two centrioles (Fig. 7.4). In the prophase, the centrioles move apart to opposite sides of the cell and organize

Centrioles Nucleolus Chromatin threads Nuclear envelope Cytokinesis (cell division) beginning

Telophase

Telophase

Centrioles Nucleolus Chromatin threads Nuclear envelope Cytokinesis (cell division) beginning

Nucleus Nucleolus Folic Acid

Nuclear envelope Chromatin threads Nucleolus Centrioles Cytoplasm

Cell surface membrane

Interphase

Interphase

Chromatids are pulled apart, centromeres first

Anaphase

Chromatids are pulled apart, centromeres first

Anaphase

Nuclear envelope Chromatin threads Nucleolus Centrioles Cytoplasm

Cell surface membrane

Centrioles Spindle Fibers Equator

Aster

Centriole

Nuclear envelope

Nucleolus

Chromosome

Centromere

Metaphase

Centromeres on equator of spindle

Fig. 7.4. The mitosis cycle (clockwise) of cell division in eukaryotic cells (after Taylor et al. 1997)

Metaphase

Aster

Centriole

Nuclear envelope

Nucleolus

Chromosome

Centromere

Centromeres on equator of spindle

Fig. 7.4. The mitosis cycle (clockwise) of cell division in eukaryotic cells (after Taylor et al. 1997)

a division spindle. Later, in the metaphase, the nuclear envelope dissolves and the chromosomes align themselves in the equatorial plane, perpendicular to the spindle. In the anaphase, the copies of the chromosomes split at the centromere and each part is pulled toward its side of the spindle. Finally, in the telophase a new nuclear envelope forms around the chromosomes, the spindle fibers disintegrate, and the centrioles replicate. Subsequently, in cytokinesis the cell splits in two.

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