|previous page||PLNT3140 Introductory Cytogenetics
Lecture 3, part 2 of 4
Click here for Meiosis summary
Premeiotic interphase can result in an increase in nuclear volume 3 to 4 times that of mitotic nuclei.
Early prophase I
Late Prophase I
|Meiotic prophase I in
Meiotic prophase I sets up the mechanisms for most of what makes meiosis unique
Images from Dept. of Plant Science, University of Manitoba
Click here for a figure summarizing meiotic prophase I, from Hank Bass' Lab at Florida State University
1. Leptotene (leptonema) -
During leptotene, there are many independent coiling events occurring simultaneously along the length of each chromosome. The chromosomes appear distinct as long slender threads with
many bead-like structures scattered along the length. These
bead-like chromomeres are localized aspects of late
leptotene coiling that later spreads along the length of the
pachytene chromosomes. These are the result of many independent coiling events happening simultaneously along the entire length of the chromosome.
Remember that prior to prophase I, DNA replication has already occurred. That means that each thin 'thread' contains two identical chromatids, coiled together. The individual chromatids cannot be resolved at this point.Chromosomal termini can be seen to be attached to the inner surface of the nuclear membrane at "attachment plaques". Through a "homology searching" mechanism that is still unknown, the ends of homologous chromosomes migrate together on the nuclear membrane, making it possible for synapsis to begin at the termini.
2. Zygotene (zygonema) - Think of zippering. Zygotene is characterized by the synapsis of homologous chromosomes. (Remember, each chromosome has two identical chromatids, so at synapsis there are 4 homologous chromatids.)right - Zygotene in pollen mother cell meiosis of Lilium regale. Arrows - sites of synapsis
from Rickards GK (1965) The Cell Nucleus. Tuatara 13:43.
During zygotene, the synaptonemal complex forms between the homologues, apparently aligning them gene by gene.
that telomeres appear to cluster together at the
beginning of meiotic prophase I (ie. leptotene) suggests
that synapsis may be physically coordinated for all
chromosomes at a single location. One simple hypothesis
is that by bringing all chromosome ends together into a
small bundle, it is easier for the homology-searching
mechanism to bring both homologues of each chromosome
here for a Quicktime
movie showing a single zygotene nucleus from maize, in
which telomeres hybridized with FITC dye (green) appear
to cluster together in a "bouquet" formation.
Chromosomal DNA is visualized using DNA-specific DAPI
stain (red), and heterochromatic knobs stained with
3. Pachytene (pachynema) -Pachytene is defined as the phase at which chromosome pairing is complete.
During pachytene the chromosomes
become shorter and thicker than previously, about 1/5 the length
as at leptotene.
| Human chromosomes at pachytene.
Note that the X and Y chromosomes remain unsynapsed except
at one terminus. Chromosome painting probes highlight
chromatin from the X (magenta) and Y (chromosomes). PAR -
Pseudoautosomal region, which is shared by both X and Y
chromosomes. Pairing can only take place at the PAR,
because only those sequences are homologous, between the X
and Y chromosomes.
Structure of synaptonemal complex of the nematode worm, C. elegans.:One role of the Pairing Centers in C. elegans is to promote synapsis, which holds matching chromosomes together during recombination. Fluorescent stains make the proteins of the synaptic "zipper" visible under the microscope
Meiosis can proceed in the absence of the synaptonemal complex but there is no recombination. The synaptonemal complex brings paired chromosomes into stable alignment during which crossing over can occur.
Pachytene is the stage at which
crossing-over produces genetic recombination. Crossing over
appears to occur within recombination nodules.
There is a stable association between homologues, pairing is
completed and bivalents formed. When crossing over occurs there
is a breakage and reunion of chromatin strands. DNA synthesis
inhibitors cause an increase in chromosome breakage, implying
that DNA repair is necessary during crossing over.
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Lecture 3, part 2 of 4