Meiosis Study Guide

Meiosis Study Guide
· Overview
· Stages of Meiosis
· Meiosis Diagrams
· Glossary of Terms
· Quiz
Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.Interphase:
· G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
· S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
· G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
· In the latter part of interphase, the cell still has nucleoli present.
· The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
· In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.
Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.
Ploidy Back to Top
Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of chromosomes are termed polyploid. Chromosomes that carry the same genes are termed homologous chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent.
Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.
Most cells in the human body are produced by mitosis. These are the somatic (or vegetative) line cells. Cells that become gametes are referred to as germ line cells. The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.
Life Cycles Back to Top
Life cycles are a diagrammatic representation of the events in the organism's development and reproduction. When interpreting life cycles, pay close attention to the ploidy level of particular parts of the cycle and where in the life cycle meiosis occurs. For example, animal life cycles have a dominant diploid phase, with the gametic (haploid) phase being a relative few cells. Most of the cells in your body are diploid, germ line diploid cells will undergo meiosis to produce gametes, with fertilization closely following meiosis.
Plant life cycles have two sequential phases that are termed alternation of generations. The sporophyte phase is "diploid", and is that part of the life cycle in which meiosis occurs. However, many plant species are thought to arise by polyploidy, and the use of "diploid" in the last sentence was meant to indicate that the greater number of chromosome sets occur in this phase. The gametophyte phase is "haploid", and is the part of the life cycle in which gametes are produced (by mitosis of haploid cells). In flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is sporophyte, while pollen and ovaries contain the male and female gametophytes, respectively. Plant life cycles differ from animal ones by adding a phase (the haploid gametophyte) after meiosis and before the production of gametes.
Many protists and fungi have a haploid dominated life cycle. The dominant phase is haploid, while the diploid phase is only a few cells (often only the single celled zygote, as in Chlamydomonas ). Many protists reproduce by mitosis until their environment deteriorates, then they undergo sexual reproduction to produce a resting zygotic cyst.
Phases of Meiosis Back to Top
Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I.
The above image is from http://www.biology.uc.edu/vgenetic/meiosis/
Prophase I Back to Top
Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of homologous chromosomes. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. Crossing-over may occur at this point. During crossing-over chromatids break and may be reattached to a different homologous chromosome.
The alleles on this tetrad:
A B C D E F G
A B C D E F G
a b c d e f g
a b c d e f g
will produce the following chromosomes if there is a crossing-over event between the 2nd and 3rd chromosomes from the top:
A B C D E F G
A B c d e f g
a b C D E F G
a b c d e f g
Thus, instead of producing only two types of chromosome (all capital or all lower case), four different chromosomes are produced. This doubles the variability of gamete genotypes. The occurrence of a crossing-over is indicated by a special structure, a chiasma (plural chiasmata) since the recombined inner alleles will align more with others of the same type (e.g. a with a, B with B). Near the end of Prophase I, the homologous chromosomes begin to separate slightly, although they remain attached at chiasmata.
Crossing-over between homologous chromosomes produces chromosomes with new associations of genes and alleles. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Events of Prophase I (save for synapsis and crossing over) are similar to those in Prophase of mitosis: chromatin condenses into chromosomes, the nucleolus dissolves, nuclear membrane is disassembled, and the spindle apparatus forms.
Major events in Prophase I. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Metaphase I Back to Top
Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair. Other metaphase events as in mitosis.
Anaphase I Back to Top
Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact.
Events in prophase and metaphse I. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Telophase I Back to Top
Telophase I is similar to Telophase of mitosis, except that only one set of (replicated) chromosomes is in each "cell". Depending on species, new nuclear envelopes may or may not form. Some animal cells may have division of the centrioles during this phase.
The events of Telophase I. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Prophase II Back to Top
During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis.
The events of Prophase II. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Metaphase II Back to Top
Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region.
Anaphase II Back to Top
During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.
The events of Metaphase II and Anaphase II. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Telophase II Back to Top
Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.
The events of Telophase II. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Comparison of Mitosis and Meiosis Back to Top
Mitosis maintains ploidy level, while meiosis reduces it. Meiosis may be considered a reduction phase followed by a slightly altered mitosis. Meiosis occurs in a relative few cells of a multicellular organism, while mitosis is more common.
Comparison of the events in Mitosis and Meiosis. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Gametogenesis Back to Top
Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line. Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells. Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries. Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop. This all the cytoplasm and organelles go into the egg. Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.
Gametogenesis. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.
Spermatogenesis
Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.
Human Sperm (SEM x5,785). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Oogenesis
The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte. At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
Oogenesis. The above image is from http://www.grad.ttuhsc.edu/courses/histo/notes/female.html.
Links Back to Top
Access Excellence page on Mitosis
Cell Division and the Cell Cycle (University of Alberta): Similar to this page, but with its own glossary and questions.
Amoeba Proteus Mitosis Small photomicrographs of protistan mitosis.
Animated Meiosis Yale University, a simplified series of cartoons about meiosis.
Meiosis Tutorial North Carolina State University, animations and 3-D graphics.
McGill University Mitosis Page Quality site, with photos and downloadable animation and video.
Virtual Meiosis University of Cincinnati, Animated GIF and text/images to explain meiosis.
Definition
The first of the two consecutive divisions of the nucleus of eukaryotic cell during meiosis composed of the following stages: prophase I, metaphase I, anaphase I, and telophase
Scholarship applications for Wikimania 2010 are now open. Apply now!
[Hide] [Help us with translations!]
Consultative Group on International Agricultural Research
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The Consultative Group on International Agricultural Research (CGIAR) was originally created at the initiative of the Rockefeller Foundation, which had sponsored international meetings of agronomists at its Bellagio Conference Center in Lake Como, Italy, from 1968 onwards.
In 1970, foundation officials proposed a worldwide network of agricultural research centers under a permanent secretariat. This was further supported and developed by the World Bank; on May 19, 1971, with the FAO, IFAD and UNDP as co-sponsors, the CGIAR was established. By 1983 there were thirteen research centers around the world under its umbrella.[1] CGIAR now has 64 governmental and nongovernmental members and 15 research centres.
At the time of its establishment there was widespread concern that developing countries would succumb to famine; the successes of the Green Revolution had started in Asia and the Pearson Commission on International Development had urged that the international community undertake "intensive international effort" to support "research specializing in food supplies and tropical agriculture". CGIAR was formed for the coordination of international agricultural research with the goals of poverty reduction and achieving food security in developing countries through agricultural research.

Active CGIAR Centres
Headquarters location
International Center for Tropical Agriculture (CIAT)
Cali, Colombia
Center for International Forestry Research (CIFOR)
Bogor, Indonesia
International Maize and Wheat Improvement Center (CIMMYT)
El Batán, Mexico State, Mexico
International Potato Center (CIP)
Lima, Peru
International Center for Agricultural Research in the Dry Areas (ICARDA)
Aleppo, Syria
WorldFish Center (International Center for Living Aquatic Resources Management, ICLARM)
Penang, Malaysia
World Agroforestry Centre (International Centre for Research in Agroforestry, ICRAF)
Nairobi, Kenya
International Crops Research Institute for the Semi-Arid Tropics (ICRISAT)
Hyderabad, India
International Food Policy Research Institute (IFPRI)
Washington, D.C., United States
International Water Management Institute (IWMI)
Battaramulla, Sri Lanka
International Institute of Tropical Agriculture (IITA)
Ibadan, Nigeria
International Livestock Research Institute (ILRI)
Nairobi, Kenya
Bioversity International
Maccarese, Rome, Italy
International Rice Research Institute (IRRI)
Los Baños, Laguna, Philippines
Africa Rice Center (West Africa Rice Development Association, WARDA)
Bouaké, Côte d'Ivoire / Cotonou, Benin

Defunct CGIAR Centres
Headquarters
Change
International Livestock Centre for Africa (ILCA)
Addis Ababa, Ethiopia
1994: merged with ILRAD to become ILRI
International Laboratory for Research on Animal Diseases (ILRAD)
Nairobi, Kenya
1994: merged with ILCA to become ILRI
International Network for the Improvement of Banana and Plantain (INIBAP)
Montpellier, France
1994: became a programme of Bioversity International
International Service for National Agricultural Research (ISNAR)
The Hague, Netherlands
2004: dissolved, main programmes moved to IFPRI
CGIAR also organises a number of inter-Center initiatives and Systemwide Programmes (SP), and Challenge Programmes (CP). The Initiatives and SPs cover cross-Center issues. The CPs are time-bound, independently-governed programs of high-impact research, executed in a partnership among a wide range of institutions. Currently there are three in operation: the Generation Challenge Programme, Harvest Plus and Water and Food.
[edit] Notes
^ Establishment of CGIAR - see Mark Dowie, American Foundations: An Investigative History, Cambridge, Massachusetts: MIT Press, 2001, (p.114)
[edit] External links
Official website
Generation Challenge Programme
HarvestPlus Challenge Programme
CGIAR Challenge Program on Water and Food
Institutional Learning and Change (ILAC)
Central Advisory Service on Intellectual Property
ICT-KM: the CGIAR program on ICT and Knowledge Management
CGIAR Systemwide Program on Collective Action and Property Rights (CAPRi)
[hide]
v • d • e
Consultative Group on International Agricultural Research (CGIAR) Centers
International Center for Tropical Agriculture (CIAT) · Center for International Forestry Research (CIFOR) · International Maize and Wheat Improvement Center (CIMMYT) · International Potato Center (CIP) · International Center for Agricultural Research in the Dry Areas (ICARDA) · International Center for Living Aquatic Resources Management (ICLARM) · International Centre for Research in Agroforestry (ICRAF) · International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) · International Food Policy Research Institute (IFPRI) · International Water Management Institute (IWMI) · International Institute of Tropical Agriculture (IITA) · International Livestock Research Institute (ILRI) · Bioversity International · International Rice Research Institute (IRRI) · West Africa Rice Development Association (WARDA)
Retrieved from "http://en.wikipedia.org/wiki/Consultative_Group_on_International_Agricultural_Research"
Categories: Agriculture organizations Rockefeller Foundation World Bank Agricultural research institutes
Views
Article
Discussion
Edit this page
History
Personal tools
Try Beta
Log in / create account
Navigation
Main page
Contents
Featured content
Current events
Random article
Search

Interaction
About Wikipedia
Community portal
Recent changes
Contact Wikipedia
Donate to Wikipedia
Help
Toolbox
What links here
Related changes
Upload file
Special pages
Printable version
Permanent link
Cite this page
Languages
Català
Deutsch
Français
日本語
Português
This page was last modified on 22 December 2009 at 12:26.
Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. See Terms of Use for details.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.
Contact us
Privacy policy
About Wikipedia
Disclaimers