The chromosomes first become visible as long tangled treads, which gradually shorten and widen. The centrioles migrate to opposite ends of poles of the cell. Microtubules develop and form a star-shaped structure called the spindle. The nucleolus and the nuclear envelope disappear leaving the chromosomes.
Prophase in Meiosis 1 is similar to prophase in mitosis, as the chromosomes become visible, shorten and widen but it different because they associate in their homologous pairs. Organisms consist of two sets of chromosomes where any two chromosomes, which decide the same characteristic, are called a homologous pair.
In Meiosis these homologous pairs come together by a process called synapsis and each pair is called a bivalent. The chromosomes are made of two chromatids, which wrap around each other. The chromatids of the pair partially repel each other but stay joined at the point called chiasma. At this point the chromatids may break and recombine with different chromatids, this is called crossing over.
Metaphase
In Mitosis Metaphase stage the chromosomes go to the centre/equator of the spindle. The chromosomes get attached to spindle fibres at the centromere, when these contract; the individual chromatids get pulled apart slightly.
In Meiosis Metaphase 1 is similar to Mitosis stage one but differs as chromosomes form a double row at the equator of the spindle instead of a single row.
Anaphase
In Mitosis and Meiosis Anaphase; the centromere splits as a result the spindle contract further and the two chromatids of each chromosome separate and migrate to opposite ends. The spindle fibres are made of tubulin molecules and they shorten through the removal process. The energy for this process is provided by mitochondria, which surround the spindle fibres.
Telophase
In Mitosis and Meiosis Telophase the chromatids reach their respective poles and a new nuclear envelope forms around each group. The chromatids uncoil and lengthen, becoming invisible again. The spindle fibres disappear and a nucleolus forms in each new nucleus.
Comparing Mitosis and Meiosis with reference to Chromosomes
Comparing Mitosis and Meiosis with reference to Biological significance
The significance of mitosis
The significance of mitosis is its ability to produce daughter cells which are exactly the same as the parent cell. It is important for three reasons…
1. Growth
If a tissue wants to get bigger by growth needs new cells that are identical to the existing ones. Cells division must therefore be by mitosis.
2. Repair
Damaged cells have to be replaced by exact copies of the organism so that it repairs the tissues to their former condition. Mitosis is the means by which this is achieved.
3. Asexual reproduction
If a species is good at colonizing a habitat, there might be no point, in producing offspring which are different from the parents, because they might be less effective at survival. Therefore it might be better, in the short term, to make a colony which is similar to the parents. In simple animals and most plants this is achieved by mitotic division.
Significance of Meiosis
The long term survival of a species depends on its ability to adapt to a changing environment. To do this the offspring need to be different from their parents and each other.
These are three ways in which variety occurs because of meiosis.
- Production and fusion of haploid gametes
The variety of offspring is increased by mixing the genotype of one parent with that of the other. It involves the production of special sex cells, called gametes, which fuse together to produce a new organism. Each gamete contains half the number of chromosomes of the adult. It is important that meiosis, which halves the number of chromosomes in daughter cells, happens at some stage in the life cycle of a sexually reproducing organism. Therefore Meiosis is important in order for variety in organisms, and allowing them to evolve.
- The creation of genetic variety by the random distribution of chromosomes during metaphase 1
When the pairs of homologous chromosomes arrange themselves on the equator of the spindle during metaphase 1 of meiosis, they do it randomly. Even though each one of the pair determines the same general features, they’re detail of the feature is different. The randomness of this distribution and independent assortment of these chromosomes produces new genetic combinations.
c. The creation of genetic variety by crossing over between homologous chromosomes
During prophase 1 of meiosis, equal portions of homologous chromosomes may be swapped. In this way new genetic combinations are made and linked genes separated.
The variety which meiosis brings vital for to the process of evolution. By providing a varied stock of individuals it allows the natural selection of those best suited to the existing conditions and makes sure that species constantly change and adapt when these conditions change. This is the main biological significance of meiosis.
