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| Image/Video Creator | James R LaFountain Jr (University at Buffalo, Buffalo, NY), Rudolf Oldenbourg (Marine Biological Laboratory, Woods Hole, MA) |
| Important Concept | During meiosis, a type of cell division in eukaryotes specific to sexual reproduction, a diploid precursor cell (a cell containing two sets of homologous chromosomes and hence two copies of each gene) divides twice with a single round of DNA replication, resulting in four daughter cells each of which has half the number of chromosomes of the precursor.
During the first division (meiosis I), replicated homologous chromosomes (bivalents, duplicated chromosomes paired with their homologous duplicated chromosomes) become paired at the beginning of meiosis during crossover events (a process whereby two homologous chromosomes break at corresponding sites and rejoin to produce two recombined chromosomes) between paternal and maternal chromatids (a chromatid is a chromosome following DNA replication in which the copies, or sisters, remain joined at the centromere). The crossover events result in genetic recombinations between the paternal and maternal chromatids. |
| What should I look for? | The video begins during the terminal stages of prophase I, the DNA having been replicated during the preceding S phase (period in cell cycle during which DNA is synthesized). Synapsis (pairing of two homologous chromosomes during meiosis) occurs during prophase I when replicated homologous chromosomes pair due to crossover events between paternal and maternal chromatids, resulting in exchange of paternal and maternal genetic sequences. Microtubules, emanating from centrosomes form star-shaped asters at opposite sides of the nucleus. The transition to prometaphase I is characterized by nuclear envelope breakdown and the penetration of microtubules into the space once delineated by the nucleus. The microtubules then attach to kinetochores (protein structures in eukaryotes that link the chromosomes to the spindle apparatus) on chromosomes, forming highly structured, intensely birefringent (an optical phenomenon that occurs from the splitting in two of a ray of light) bundles of microtubules termed kinetochore microtubules. Homologous chromosomes associate with microtubules emanating from opposite poles. As a result, the chromosomes (consisting of sister chromatids or dyads) are pulled to opposite sides of the cell during anaphase I. The resulting cells each contain 23 dyad chromosomes in humans. In contrast, during mitosis, sister chromatids associate with microtubules from opposite poles and are separated at anaphase, resulting in daughter cells with 46 individual chromosomes. Meiosis I concludes with telophase and cytokinesis – a nice contractile ring can be seen pinching off the two cells.
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| How was it done? | These cells were among approximately 10, 000 such cells that were contained within one of the testes of a crane fly larva (Nephrotoma suturalis). Following removal from the larva, the testis was ruptured under a droplet of inert oil on a glass coverslip, which was then mounted onto a slide and put onto a microscope stage for viewing in polarized light. This living cell preparation contains no stains or labels.
These images were captured with a polarizing light microscope augmented by a liquid crystal (LC) compensator, electronic imaging and digital processing tools to build a birefringence imaging system known as the LC-PolScope. In contrast to the traditional polarizing microscope, which uses linearly polarized light, the LC-PolScope uses nearly circularly polarized light for illuminating the specimen. Images created by the LC-PolScope depict birefringence in the sample independent of orientation.
See 'Meiosis II: Second Meiotic Division' at http://cellimages.ascb.org/u?/p4041coll12, 213 for an explanation of why the scientists used crane flies in their studies.
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| Why is this important? | Meiosis results in the reassortment and recombination of genetic material between parental DNA molecules such that every germ cell (a sperm or an egg) generated contains a unique combination of genes. Upon fertilization, this guarantees that every individual of a species will have a unique genetic composition – a key factor in assuring the genetic diversity of the species. The recombination of maternal and paternal genetic elements takes place during meiosis I. |
| Points to ponder | Go to http://www.pbs.org/wgbh/nova/baby/divide.html# for a step-by-step comparison between meiosis and mitosis.
What are the implications of the crossover events that take place during prophase I?
During prophase I, homologous chromosomes, rather than sister chromatids (as in mitosis), segregate to the opposite sides of a cell. How does this happen? What proteins specific to meiosis are involved? Get started by reading the paper by Yoshinori Watanabe: http://jcs.biologists.org/cgi/reprint/117/18/4017 |
| Advanced Material | LaFountain JR Jr, Oldenbourg R. Meiosis in a living crane fly spermatocyte: meiosis I still images. ASCB Image & Video Library. February 2007:NUC-7. Available at: http://cellimages.ascb.org/u?/p4041coll12,98
LaFountain JR Jr, Oldenbourg R. Meiosis I in a living crane fly spermatocyte: time lapse movie. ASCB Image & Video Library. February 2007:VID-13. Available at: http://cellimages.ascb.org/u?/p4041coll12,105
LaFountain JR Jr, Oldenbourg R. Meiosis in a living crane fly spermatocyte: meiosis II time lapse movie. ASCB Image & Video Library. February 2007:VID-14. Available at: http://cellimages.ascb.org/u?/p4041coll12,106
Definitions (hit your browser's back button to return to the IVL):
birefringent: http://en.wikipedia.org/wiki/Birefringent
bivalents: http://en.wikipedia.org/wiki/Bivalent_%28genetics%29
centromere: http://en.wikipedia.org/wiki/Centromere
centrosome: http://en.wikipedia.org/wiki/Centrosome
chromatid: http://en.wikipedia.org/wiki/Chromatid
chromosome: http://en.wikipedia.org/wiki/Chromosome
crossover events: http://en.wikipedia.org/wiki/Chromosomal_crossover
cytokinesis: http://en.wikipedia.org/wiki/Cytokinesis
diploid: http://en.wikipedia.org/wiki/Ploidy
DNA replication: http://en.wikipedia.org/wiki/DNA_replication
dyad: http://en.wikipedia.org/wiki/Dyad_%28biology%29
kinetochore: http://en.wikipedia.org/wiki/Kinetochore
microtubule: http://en.wikipedia.org/wiki/Microtubule and http://cellbio.utmb.edu/cellbio/microtub.htm
meiosis: http://en.wikipedia.org/wiki/Meiosis
Nephrotoma suturalis: http://en.wikipedia.org/wiki/Crane_fly
polarizing microscope: http://www.microscopyu.com/articles/polarized/polarizedintro.html
S phase: http://en.wikipedia.org/wiki/Synthesis_phase
Synapsis: http://en.wikipedia.org/wiki/Synapsis
spindle apparatus: http://en.wikipedia.org/wiki/Spindle_apparatus
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| Title | 1. Teaching Item. Meiosis I: First Meiotic Division |
| Annotator(s) | David L Ennist (The American Society for Cell Biology, Bethesda, MD) |
| Citation | Ennist DL. Meiosis I: First Meiotic Division. ASCB Image & Video Library. 2007;EDU-6. Available at: http://cellimages.ascb.org/u?/p4041coll12,209 |
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| Publication Date | Published in October 2007. |
| License Details | Terms for non-commercial use: http://cellimages.ascb.org/cdm4/terms.php
For commercial use contact contact us at: cellimages@ascb.org. |
| Publisher | The American Society for Cell Biology |
