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Meiosis


Meiosis
Time period: day 0 to adult
Diversity
Cell division by mitosis gives no opportunity for change or diversity, which is ideal for processes like growth and repair. In humans, sex- ual reproduction allows random mingling of maternal and paternal DNA to produce a new, unique individual. This is able to occur because of a different type of cell division called meiosis.

During meiosis a single cell divides twice to form four new cells.
These daughter cells have half the normal number of chromosomes (they are haploid cells). Meiosis is the method of producing spermatozoa and oocytes. When an egg is fertilised by a sperm the chromosomes will combine to form a cell with the normal number of chromosomes.
Meiosis, Human chromosomes, Interphase, Prophase I, Prometaphase, Metaphase, Anaphase, Telophase, Cytokinesis, Homologous recombination,

Human chromosomes
There are 23 pairs of human chromosomes (Figure 7.1) in a nor- mal, diploid cell (from the Greek word diploos, meaning ‘double’). Each chromosome is a length of DNA wrapped into an organised structure (Figure 7.2). Twenty‐two of the pairs of chromosomes are known as autosomes. The remaining pair are known as the sex chromosomes, which hold genes linked to the individual’s sex. When condensed the pairs of autosomes look like X’s (Figures 7.3 and 7.4), and the sex chromosomes look like X’s or Y’s (Figure 7.1). The female sex chromosome pair appears as XX, the male as XY.

Meiosis I
A cell dividing by meiosis divides twice (meiosis I and meiosis II). During meiosis I (Figure 7.5), a cell passes through phases very similar to those of mitosis, but with some significant differences. It begins with 23 pairs of chromosomes (46 chromosomes in total).
            Interphase: the cell goes about its normal, daily business (diploid).
   Prophase I: homologous chromosomes exchange DNA (homologous recombination); chromosomes condense and become visible; centrioles move to opposite ends of the cell and extend microtubules out (mitotic spindle); centromeres extend fibres out from chromosomes (diploid).
  Prometaphase I: the nuclear membrane disappears, microtubules attach centrioles to centromeres and start pulling the chromosomes (diploid).
           Metaphase I: chromosomes are aligned in the middle of the cell (diploid).
   Anaphase I: homologous chromosome pairs split, one of each pair (each pair has two chromatids) moving to either end of the cell (diploid).
     Telophase I: homologous chromosomes reach each end of the cell; new membranes form around the new nuclei for the daughter cells (diploid).
    Cytokinesis: an actin ring around the centre of the cell shrinks and splits the cell in two (haploid).
After meiosis I each cell has 23 chromosomes, and each chromo- some has two chromatids. It is therefore haploid.

Homologous recombination
The key event during meiosis I is the separation of homologous chromosomes, rather than the separation of sister chromatids as occurs during mitosis. But what are homologous chromosomes?
Sister chromatids (Figure 7.4) are identical copies of DNA that are attached to one another by the centromere to form the X‐shaped chromosomes that we recognise. So, when sister chromatids are separated into two new cells by mitosis the new cells will be genetically identical.
Homologous chromosomes (Figure 7.4) are the two chromosomes that make up the ‘pair’ of chromosomes that we talk about in diploid cells. We say that human diploid cells contain 23 pairs of chromosomes. They are homologous in that they are the same chromosome but with subtle differences. One chromosome has been inherited from the father and one from the mother.
Homologous chromosomes contain genes for the same biological features, but the genes may be slightly different. For example, the genes for eye colour would be found on both homologous chromosomes but one chromosome may hold the gene that encodes for blue eyes and the other for green eyes. These are different alleles of the same gene.
During homologous recombination those genes are swapped around randomly between the homologous chromosomes before they are pulled into new cells. Therefore, each new cell could be quite different with many, many genes randomly exchanged. In this way the gametes (eggs, sperm) formed by meiosis become very diverse.
The female sex chromosomes (XX) are homologous, but the male sex chromosomes (XY) are not.

Meiosis II
Without replicating its DNA the cell moves from meiosis I to meiosis
II. Meiosis II is very similar to mitosis.
         Prophase II: chromatids condense and become visible; centrioles move to opposite ends of the cell and extend microtubules out (mitotic spindle); centromeres extend fibres out from chromosomes (haploid).
    Prometaphase II: the nuclear membrane disappears, microtu- bules attach centrioles to centromeres and start pulling the chromo- somes (haploid).
           Metaphase II: chromosomes are aligned in the middle of the cell (haploid).
         Anaphase II: chromosome pairs split (centromeres cut), one of each pair (sister chromatids) moving to either end of the cell (haploid).
    Telophase II: sister chromatids reach opposite ends of the cell; new membranes form around the new nuclei for the daughter cells (haploid).
     Cytokinesis: an actin ring around the centre of the cell shrinks and splits the cell in two (haploid).
The end result is, generally speaking, 4 cells with 23 unpaired chromosomes each (Figure 7.6). We will find out more about this in the gamete chapters (see Chapter 8, spermatogenesis and Chapter 9, oogenesis).

Clinical relevance
Karyotyping and comparing a patient’s chromosomes to the expected normal chromosomal pattern is important in diagnosing a number of chromosomal abnormalities, such as trisomy 21 (Down syndrome), XXY (Klinefelter syndrome) and trisomy 18 (Edwards syndrome).
The homologous recombination of prophase I is an important mechanism of Mendelian inheritance. It is a key tenet of modern genetics and underlies most clinical disorders with a genetic basis.



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