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Cell Division: Eukaryotes
Mitosis
The Players

All cells arise from the growth and division of existing cells. This process, called asexual reproduction, is the way in which every eukaryotic cell, in every organism (including humans) has been created and come into being.

From the instant of conception, all the trillions of cells in a human body, or the hundreds of cells in the tiny, microscopic nematodes living in the soil, have been produced by the growth and then division of a pre-existing cell. Since the first cell came to life on earth 3 to 4 billion years ago, this has been so.

Asexual reproduction of cells creates new amoeba, paramecia and, from a fertilized egg, new whales, new flowers and all the myriad of single and multicelled creatures that inhabit the earth today.

Watching it happen

With the advent of powerful microscopes and the sophisticated techniques of cell biology, it is now possible to investigate the sequence of events that gives rise to a new, eukaryotic, cell. Although the details may vary along with the many different types of cells, the same steps and stages can usually be detected in every cell cycle.

Asexual cell reproduction in almost all eukaryotic cells takes place in two major stages:

  • karyokinesis
    this is the stage in which the nucleus of the cell, and all its contents, are duplicated. This stage is often called "mitosis".

  • cytokinesis
    this is the stage where the cytoplasm and the whole contents of the cell divides into two parts. Each part receives one of the duplicated nuclei.

The nucleus

One of the major compartments in a eukaryotic cell is the nucleus. Despite its size and prominence, this organelle is one of the least understood. This is in part due to it's critical role as the place where genetic and hereditary functions take place, and partly because the nucleus is a dynamic organelle that changes, (and even vanishes) at different times in the cell cycle.

At high magnification, however, and at times when the nucleus is not replicating or dividing, it is possible to see some of the main features.

One prominent feature is the complex, double membrane that separates the nuclear contents from the cytoplasm. The nuclear envelope consists of an outer lipid bi-layer (or membrane), a narrow perinuclear space, and a second, inner lipid bi-layer (or membrane).

This envelope is penetrated by a series of 70 nm pores that connect the cytoplasm with the nuclear contents. These pores, however, are not just simple holes in the nuclear membrane, but have a large central granule (or plug) and a ring or annulus of eight symmetrical granules around the edges.

The outer nuclear membrane is also connected, and interconnected, with elaborate membranes, organelles and tubules that run though out the cytoplasm.

Use of stains or dyes shows that the inner contents of the nucleus of non-dividing cells consists of a vast mesh of DNA molecules complexed with various proteins to form a heterogeneous substance called chromatin. This is where the genes and are located, and from where the RNA is synthesized.

Many nuclei also have one or more smaller, darker staining bodies of variable size and form called nucleoli (singular = "nucleolus"). These bodies contain large amounts of RNA, and is probably where the RNA for ribosomes is made by copying appropriate parts of the DNA genes.

and the chromosomes

Chromosomes are dynamic, constantly changing structures found within the nucleus. DNA is the main element in every chromosome, but in almost every state, these important carriers of genetic information are complexed with a large variety of proteins.

  • histones
    rich in basic amino acids, there are five kinds of histones, H1, H2A and H2B, H3, H4 all of which differ in their constituent amino acids, but all of which bind to the acidic DNA molecules to form higher level structures.

  • nonhistone proteins
    a wide variety of proteins playing a wide variety of roles.

A 200 base pair length of DNA loops around an octomer (8) of histone units to form a nucleosome, the smallest recognizable structure of chromatin and chromosomes, which is about 10 nm in diameter.

More complex fibers are formed when a type of folding of the polynucleosome strings arrange creates a solenoid shaped chromatin fiber. Which in turn loops, and then loops again into higher and higher levels of complexity.

Looking at chromosomes

During one critical stage of cell division, all chromosomes are fully packed with proteins and are fully condensed into their most compact form within the nucleus.

At this point in cell division it is possible to kill the cell, fix the chromosomes (so they don't untangle or breakdown) and then stain them with a variety of colorful dyes. This makes it possible to count the total number of chromosomes in the cell (and organism), and also sort out different types of chromosomes and chromosome patterns.

This visualization of chromosomes is called a karyotype.

In diploid organisms, like ourselves, each recognizable chromosome has an equivalent partner, or homologue, which carries an almost identical set of genes and information.

In haploid cells only one copy of each chromosome is present.

In polyploid cells there may be many multiples of each chromosome.

During asexual reproduction of eukaryotic cells, the type and number of chromosomes in the nucleus is preserved and conserved.


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© 2001, Professor John Blamire