Mitosis - Cell Division

Mitosis

We all are aware of reproduction process in humans and in the animal kingdom. Have you ever thought about how reproduction takes place in the plant kingdom and in other eukaryotic organisms?

Plants and other eukaryotic cells do reproduce by separating the duplicated chromosomes and this process is known as mitosis. In this topic let's learn about mitosis and its various stages.

The basic mechanism of mitosis was described in early 1880s. The term mitosis was first coined by the scientist named "Fleming" in the year 1882. He studied and observed mitosis both in vivo and in stained preparation to study the process of cell division in unicellular animals.

Mitosis is generally defined as a type of cell division which occurs to form daughter cells, with same number of chromosomes as that of parental cells. This process is seen in all vegetative cells. In case of plants, mitosis is observed in all roots and shoot tip. In case of eukaryotic animals, mitosis is observed in organs related to the production of blood cells, skin cells, wound healing regions, etc.

STAGES OF MITOSIS:

Mitosis is divided in to six stages.

· INTERPHASE:

A cell during this stage of mitosis shows a clearly defined nuclear envelope, nucleolus and chromatin. We can also see the increased size of nucleolus. This phase is called as a resting period as division of cytoplasm and nucleolus does not take place. It is the longest phase of cell cycle which requires one to two days for completion.

· PROPHASE: (In Greek pro-first phase stage)

In this stage, the chromatin material of nucleus gradually condenses into distinct chromatin threads by using water. They gradually become thicker and shorter forming chromosomes. Nucleus and nuclear membrane gradually disappears at the end of prophase. It is the longest phase of cell cycle.

· METAPHASE:

In this stage of mitosis the chromosomes move towards the equator of the cell, by attaching themselves to the spindle fibers with the help of their centromere. At the end of metaphase the centromere of each chromosomes divides to reach each chromatids to have its own centromere. These chromosomes further become shortened and thickened. The spindle fibers formed are of two types, namely chromosomal and polar spindle.

· ANAPHASE:

By the separation of chromatids, daughter chromosomes are formed towards the opposite pole of the cell along with the spindle fibers. At the end of anaphase each set of daughter chromosomes reach their respective pole. It is the shortest phase of mitosis as it lasts for few minutes.

· TELOPHASE: (In Greek telo-end phase stage)

In this stage of mitosis, two groups of chromosomes are formed, one at each pole. These chromosomes undergo uncoiling and become thread like structures. At the end of telophase nucleolus and nuclear membrane reappears and cytokinesis takes place by giving rise to new two cells.

· CYTOKINESIS:

In this stage of mitosis the cytoplasm of a single cell is divided in to two separate daughter cells.

SIGNIFICANCE OF MITOSIS:

Mitosis cell division is the process by which single cells reproduce themselves and multicellular organisms grow. This process is replication or multiplication. Mitosis is also called as an equatorial cell division and it takes place only in unicellular animals.

How To Use A Test Cross

A test cross was defined in a recent article as a way to detect a real nature or ability (known to vary within limits) of a living organism by the way of a cross between that organism and a tester genetically neutral in its contribution to the common offspring. Here the question is to know in which case a test cross can be the best choice for a genetic analysis. And the answer is we rather use this cross when we want:

1. To discover the genotype of an individual with a dominant phenotype. With a single locus for example such an individual may be dominant homozygous or heterozygous and you cannot tell which one is true just by looking at its phenotype.


2. To discover how many types of gametes a double heterozygote produces and then see if genes are linked or not. The expectations are: two types of gametes, both parental, with the same frequency (this means both loci are linked but no cross over occurred during meiosis), four types of gametes with the same frequency (when both genes have an independent behaviour but you do not know if it is because of a real physical independence or because of a linkage on a wide distance between them so that every single meiosis is made with a cross over), or four types of gametes in two groups having different frequencies (which means both loci are linked on a short distance and only some meiosis are associated with a cross over, so that at the end we have parental gametes over numbering recombined gametes).


3. To discover how many types of gametes a triple heterozygote produces and then see if genes are all linked, and if so see the order in which they are linked, or if only two loci out of three are linked and then identify them and compute the genetic distance between them, or if the three genes behave independently. Here we expect the target individual to produce height types of gametes in the same amount each (which shows an independent behaviour, true or faked), height types of gametes in four groups with different frequencies (this is the genetically known sign of a linkage between three loci, with a high frequency for both parental gametes, lower frequencies for the two kinds of simply recombined gametes and a very low frequency for both double recombined gametes), or any other number of gamete types in between, including just two types of parental gametes 50% each (in the case the three genes are linked but some how there is no single cross over during meiosis).

Saving the Tuart Forest

The tall Tuart (Eucalyptus Gomphocephala) forest located in between Busselton and Capel in Western Australia's picturesque south west, is one of the rarest forest ecosystems in the world. The trees are named after the local Wardandi aboriginal people's name Too-art, and the forest is a diverse ecosystem and home to over 80 species of birds, reptiles, frogs, bats and many animals including number of endangered species, such as the Western Ringtail and Western Brushtail Possums, Chuditch or Native Cat and the Quenda or Southern Brown Bandicoot.

Before white settlement of the south west, Tuart woodland stretched 400km from north of present-day Perth to Dunsborough near Cape Naturaliste. Now after 175 years of white settlement, less than 30,000 hectares remain and in that, less than 10% of the original understorey.

The reasons for the decline are many, but 3 in particular seem to dominate:

Clearing. In a settlement of poor soils and only seasonal rainfall, the Tuart forest was at once one of the most fertile and so was cleared for farming and town sites and

Logging. Tuart was highly prized for its hard timber which was widely used for ship building, railway trucks, bridges, cog wheels for mills, flooring, stair treads and so on. By 1904 only 40,000Ha remained

Grazing. Begun as early as the 1830's most of the Tuart forest was leased and fenced by the early 1900's. Clovers and grasses were introduced whilst native plants thought to be poisonous to stock such as Zamias were cleared. Even after the Tuart forest was protected in 1918 in what became State Forest No 1, the grazing of cattle was still carried on under the tall trees. Although cattle have now been removed since the declaration of the 2049Ha National Park in 1987, Western Grey Kangaroos have taken their place multiplying to large numbers. High on their list of menu favourites are young Tuart trees!

Early in the 2000's there was a lot of protesting about a proposed mineral sands mine within the Tuart forest, with the predictable bumper stickers "Save the Tuart forest" and sit-ins, etc. Eventually the mine went ahead, but under strict guidelines and conditions (including re-forestation), one of which was that the miner undertake a comprehensive study to establish a list of, and to re-plant native understorey, as well as replacement Tuart trees. Mining has now been completed and rehabilitation has begun.

An observant visitor to the forest today might notice the absence of young Tuarts and the proliferation of weeds such as Arum Lily, and be tempted to draw the conclusion that once the old trees eventually die off, the Tuart forest would cease to exist.

So both the mining company on its old minesite, and the Department of Conservation (DEC) have begun programmes of replanting not only young Tuart trees, but also the range of understorey plants which the research indicated were endemic to the forest. The programmes would be a waste of resources if that was all there was to it, but the areas of rehabilitation have been fenced off with high fences to keep kangaroos out until the young trees and the other vegetation reach sufficient size to be able to survive the kangaroos appetite.

A new section of this can be seen on the site of what was an old Forests Department pine plantation near Inn the Tuarts Guest Lodge, the only accommodation in the Tuart forest, at the end of Rushleigh Rd, just off the Tuart Tourist Drive, about 7km north-east of Busselton city centre. Another visible rehab site is near the bird hide on the Vasse-Wonnerup estuary, accessed from the Spotlight Possum Walk near historic Wonnerup House, off Layman Rd.The old minesite is visible from Tuart Drive around 12 km from Busselton, but is not accessible to the public at this stage, although inspections may be possible by prior arrangement.

So by removing a pine plantation, clearing and sand mining and excluding the native kangaroos, the future of the Tuart forest is a little more assured.