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Lecture 3, part 2 of 4
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II. Meiosis 1

In a nutshell:  The diploid cell contains two homologues of each chromosome, one parental and one maternal. These replicate in S phase, resulting in two identical sister chromatids. Prophase I begins with pairing of both homologues, resulting in a tetrad containing 4 homologous chromosomes. Crossing over occurs here. As chromosomes continue to condense, the sites of crossing over become visible as chiasmata. Chromosomes begin to separate as they further condense, pushing chiasmata to the chromosomal termini. As in mitosis, chromosomes are aligned between the centrosomes in metaphase I. One kinetochore forms per chromosome pair, rather than one per chromatid. Consequently, chromatid pairs migrate to opposite poles,  such that both chromatids at a given pole are derived from a single parent. In telophase 1, cytokinesis divides the two diploid cells, and in most species, the chromosomes remain condensed and a nucleus does not re-form. In prophase 2, centrosomes divide again, pulling bivalent chromosomes to the center of the cell. Kinetochores divide, and chromosomes migrate to the poles (anaphase 2), followed by telophase 2, in which haploid nuclei form.

Click here for Meiosis summary

A. Premeiotic interphase

Premeiotic interphase can result in an increase in nuclear volume 3 to 4 times that of mitotic nuclei.

B. Meiotic prophase I


Early prophase I

Late Prophase I
Meiotic prophase I in Rye anthers
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.
http://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio13Tuat01-t1-body-d5.html#Bio13Tuat01-fig-Bio13Tuat01_059a
During zygotene, the synaptonemal complex forms between the homologues, apparently aligning them gene by gene.




The observation 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 together.

Click 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 Rhodamine (white).
From Bass lab web site: http://bio.fsu.edu/~bass/images2.html.



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.

From: http://www.sciencedirect.com/science/article/pii/S0006291X06013349
http://dx.doi.org/10.1016/j.bbrc.2006.06.040

 
 
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
 from http://www.lbl.gov/Science-Articles/Archive/LSD-worm-meiosis.html


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
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