Meiosis
Time
period: day 0 to adult
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.
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.
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.
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.
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).
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.
