MEIOSIS: IN HUMANS
Meiosis is the process in which sexually reproducing organisms produce gametes (which is much needed for the normal structure of the embryo) as a method of reproduction. Meiosis occurs in eukaryotic organism which means that they have a true nucleus which contains DNA and is separated from other cellular structures. Humans rely on meiosis for many reasons including:
- Variety of genetic make-up
- Creation of a proper environment in order to go through the process of reproduction and it be successful.
There are two stages of meiosis, those stages being MEIOSIS I and MEIOSIS II. Within those stages there are phases of meiosis, those phases being INTERPHASE, PROPHASE I/II, METAPHASE I/II, ANAPHASE I/II and TELOPHASE I/II (Cytokinesis).
MEIOSIS I
INTERPHASE I
Interphase is preparation for PROPHASE. This is the phase in which the cell replicates its chromosomes, each chromosome having two sister CHROMATIDS which are held together by the centromere. Two pairs of centrioles are formed from the replication of one pair are situated outside the nucleus. Once interphase has been complete, the cell begins the next phase of meiosis, PROPHASE I.
PROPHASE I
The Chromosomes begin to condense and start pairing up. Spindles are also formed. Each of the chromosomes match up with their homolog partner and form a TETRAD in order to have a gene from one homolog line up with the matching gene from another homolog. Special proteins then go into the process of breaking the DNA in the same place on each homolog and other proteins then reattach the DNA in a somewhat criss-cross formation in order to exchange specific portions of their DNA with each other. CHIASMATA keeps the chromosomes held together. This crossing over occurs at least once during the process of meiosis although it is not uncommon for this to happen twice or three times. The only exception to this crossing over is the X and Y chromosomes of a male which are technically not true homologs but are equipped with small matching areas near the tip which enable them to undergo the crossing over.
METAPHASE I
Once the crossing over has been completed, the spindles start to capture the chromosomes and pull them towards the equator. Each chromosome now attaches to microtubules from one pole of the spindle and the two homologs of a pair are captured by MICROTUBULES from opposite poles. Homolog pairs line up on the equator under tension from the opposite spindle poles.
ANAPHASE I
The chiasmata holding the chromosomes together breaks which allows the homologs to split apart to opposite ends of the cell. An important part of this is that the COHESIN proteins at the centromere are not destroyed so that the sister chromatids of each chromosome remain attached together.
TELOPHASE I
The chromosomes arrive at each opposite pole of the cell. CYTOKINESIS generally occurs at the same time as telophase which results in the forming of two haploid daughter cells. The chromosomes in the cells formed have two sister chromatids each but the chromatids are no longer classified as identical as a result of crossing over.
MEIOSIS II
The two cells that enter meiosis II are a product of meiosis II and both cells are haploid although their chromosomes still consist of two sister chromatids.
PROPHASE II
Spindle forms again and the chromosomes begin to attach to the microtubules. The microtubules from opposite spindle poles capture the two sister chromatids of each chromosome.
METAPHASE II
The chromosomes are then pulled into the centre of the cell and are lined up individually along the equator of the cell. An important thing to remember is that the chromosomes are still double stranded.
ANAPHASE II
The COHESIN proteins that are holding the chromatids together that can now only be found on the centromere are destroyed. This destroying allows for the sister chromatids to separate and the now single stranded chromosomes are pulled towards opposite ends of the cell.
TELOPHASE II
A nuclear membrane then re-forms around each set of chromosomes and the chromosomes begin to de-condense. Cytokinesis takes place when the chromosome sets split into new cells which results in the formation of the final products of meiosis which consist of:
- Four HAPLOID cells in which each chromosomes has just one chromatid
- In humans, the result of meiosis is GAMETES (SPERM and/or OVUM).
TO MITOSIS