3B Scientific Cell Division II Chart, Meiosis - Manual (Page 5)

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5
Meiosis
specific order. Therefore, prophase I is split up into four subsections (leptotene, zygotene, pachytene and
diplotene with diakinesis). In contrast to the mitotic prophase, which lasts several hours, meiotic prophase
I can take days, weeks, months or years.
Leptotene
At the beginning of prophase I (leptotene), the nucleolus (1) and the nuclear membrane (2) can be seen.
The chromosomes (3) are now visible as individual, long, thin threads. Its ends are attached within the
nuclear membrane. Each chromosome has already been replicated (duplicated) during the interphase and
is made up of two sister chromatids, which however are so close to each other that they cannot be diffe-
rentiated. The centrioles were also duplicated in the interphase. Both pairs (4) begin to move in opposite
directions towards the two cell poles. Between them the so-called central spindle (5) begins to build up,
which consists of many microtubules.
3. Zygotene and Pachytene
One maternal (1) and one paternal homologue (2) (consisting of two sister chromatids) of one chromosome
pair are shown in different colours to represent the other chromosomes (2 x 23 in total).
Zygotene
The zygotene phase is initiated as soon as the homologous chromosomes begin to line up side by side to
form the synaptonemal complex (3) (parallel arrangement of the homologous partners). This process usual-
ly begins at the ends of the chromosomes and continues down to the other end, similar to a zipper. The
chromosome pairing (synapsis) occurs with high precision, so that the matching genes of the homologous
chromosomes face each other directly. This is an important requirement for the recombinant exchange of
gene sections (crossing over). The homologous chromosome pairs in meiotic prophase I are usually referred
to as bivalent, but since each homologous chromosome consists of the closely arranged sister chromatids,
they can also be referred to as tetrads.
Pachytene
As soon as all synaptonemal complexes are fully developed, i.e. the homologous chromosomes have all
lined up, the pachytene phase begins. From now on, recombination nodes (4) become visible at intervals
on the synaptonemal complexes, where the exchange of gene sections occurs.
4. Diplotene
After some gene sections have been exchanged, the homologous chromosomes (1) disjoin more and more,
remaining connected at one or more points of crossing over (chiasma bridges or chiasmata) (2). The chias-
ma bridges are the places where the genetic recombination (exchange of maternal and paternal genetic
information) occurred earlier. Egg cells can persist in the diplotene phase for months or even years.
5. Diakinesis
The end of meiotic prophase I begins when the chromosomes detach from the nuclear membrane (1). The
chromosomes are condensed and the sister chromatids, joined by centromeres (short DNA sequences with
a high AT level) (2), become visible. The non-sister-chromatids in which an exchange of gene sections occur-
red remain connected via chiasma bridges (3).
The phase following prophase I is metaphase I. The meiotic phases remaining at this point take up less
than 10% of the total time required for a complete meiosis.
6. Metaphase I
During the transition from prophase I into metaphase I, the centriole pairs (1) have reached the two oppo-
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