Friday, August 20, 2010


Prophase I
  • Homologous chromosomes pair and crossing over, or recombination, occurs--a step unique to meiosis.
  • Chromosomes form structures called synapsis.
  • The paired chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.
  • At this stage, non-sister chromatids may cross-over at points called chiasmata.
  • The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads."
  • During this stage, individual chromosomes begin to condense into long strands within the nucleus.
  • However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.
  • The chromosomes in the leptotene stage show a specific arrangement where the telomeres are oriented towards the nuclear membrane.
  • Hence, this stage is called "bouquet stage".
  • The zygotene stage, also known as zygonema, from Greek words meaning "paired threads," occurs as the chromosomes approximately line up with each other into homologous chromosomes.
  • The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids.
  • The two homologous chromosomes become "zipped" together, forming the synaptonemal complex, in a process known as synapsis.
  • The pachytene stage, also known as pachynema, from Greek words meaning "thick threads," contains the following chromosomal crossover.
  • Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. (Sex chromosomes, however, are not identical, and only exchange information over a small region of homology.)
  • Exchange takes place at sites where recombination nodules or chiasmata (singular: chiasma) have formed.
  • The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process.
  • Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
  • During the diplotene stage, also known as diplonema, from Greek words meaning "two threads," the synaptonemal complex degrades and homologous chromosomes separate from one another a little.
  • The chromosomes themselves uncoil a bit, allowing some transcription of DNA.
  • However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred.
  • The chiasmata remain on the chromosomes until they are severed in Anaphase I.
  • In fetal oogenesis all developing oocytes develop to this stage and stop before birth.
  • This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia exist until meiosis begins at puberty.
  • Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through." This is the first point in meiosis where the four parts of the tetrads are actually visible.
  • Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible.
  • Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Synchronous processes
  • During these stages, centrosomes, each containing a pair of centrioles are migrating to the two poles of the cell.
  • These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, essentially cellular ropes and poles, during crossing over.
  • They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore.
  • The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track.
  • There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.
  • Microtubules that attach to the kinetochores are known as kinetochore microtubules.
  • Other microtubules will interact with microtubules from the opposite centriole. These are also nonkinetochore microtubules.
Metaphase I
  • Homologous pairs move together along the phase plate: as kinetochore microtubules from both centrioles attach to their respective kinetochores
  • The homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes.
  • The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate.
Anaphase I
  • Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart.
  • Since each chromosome only has one functional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets.
  • Each chromosome still contains a pair of sister chromatids.
  • Nonkinetochore microtubules lengthen, pushing the centrioles further a part.
  • The cell elongates in preparation for division down the middle.
Telophase I
  • The last meiotic division effectively ends when the centromeres arrive at the poles.
  • Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids.
  • This effect produces a variety of responses from the neuro-synchromatic enzyme, also known as NSE.
  • The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set.
  • The chromosomes uncoil back into chromatin.
  • Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.
  • Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.telophase I contains no nucleus, two daughter cells, and chromatids remain attached.
Meiosis II
  • Meiosis II is the second part of the meiotic process. Much of the process is similar to mitosis and meiosis I.
  • End result is production of four haploid cells (23,1N) from the two haploid cells (46,1N) produced in meiosis I.
Prophase II
  • Takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids.
  • Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.
Metaphase II
  • The centromeres contain two kinetochores, that attach to spindle fibers from the centrosomes (centrioles) at each pole.
  • The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
Anaphase II,
  • where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart.
  • The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
Telophase II,
  • Which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the microtubules.
  • Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
  • Meiosis is now complete.

  • Meiosis adalah proses seluler yang membelah sel diploid hingga membentuk sel gamet haploid.
  • Meiosis adalah bentuk pembelahan inti yang sangat penting diantara reproduksi seksual organisme.
  • Meiosis terjadi pada organisme ekuariot, yang selnya mengandung jumlah kromosom diploid.
  • Dioploid berarti rangkap, dalam artian bahwa informasi genetik pada salah satu kromosom dapat dijumpai pada bentuk yang sama ( atau termodifikasi) pada kromosom kedua didalam inti.
  • Kedua kromosom membentuk pasangan sedemikian yang dinamakan homolog.
  • Sel diploid manusia mengandung 46 kromosom, atau 23 pasang homolog ke 46 kromosom dari zigote terbentuk dari fertilisasi, yang berasal dari sel sperma dan sel telur yang masing-masing gamet memberikan satu anggotanya dari setiap pasangan homolognya.
Pemembelahan meiosis terdiri atas 2 tahap yaitu:
  1. Meiosis pertama (I)
  2. Meiosis kedua (II).
Masing-masing memiliki empat fase
  1. Profase
  2. Metafase
  3. Anafase
  4. Telofase.
Istirahat antara kedua tahap disebut interkinesis.

Profase meiosis I dibagi atas 5 sub-tahap (LeZy PaDiDi)
  1. Leptoten
  2. Zygoten
  3. Pakhiten
  4. Diploten
  5. Diakinesis.
Pre Meiosis I
  • Meiosis didahului oleh interfase ketika calon sel kelamin ( Sel induk = 2n / Gametogonium : Spermatogonium / Oogonium ) membelah mitosis , dimana setiap kromosom mengalami proses replikasi.
  • Proses ini menyerupai pada replikasi kromososm mitosis.
  • Untuk setiap kromosom, stiap kromatid ( anak) menyerupai sifat genetik yang sama menambat pada sntromer.
  • Ada sepasang sentriol (pada sel hewan) juga mengalami replikasi untuk membentuk dua pasang.
Profase I
Profase meiosis I dibagi atas 5 sub-tahap: Leptoten, Zygoten, Pakhiten, Diploten, dan Diakinesis.
1) Leptoten
  • Kromatin terpilin menjadi kromosom. Terdapat 2 pasang kromosom homolog
2) Zigoten
  • Kromosom homolog mengandeng; sebelah berasal dari kromosom induk (kromosom matroklin) dan sebelah lain dari kromosom bapak (kromosom patroklin).
  • Dibeberapa tempat terjadi persilangan (chiasma; jamak: chiasmata).

3) Pakiten
  • Kromosom homolog mengandeng rapat sepanjang lengannya, dari pangkal ke ujung terbentuk tetrade.
4) Diploten
  • Setiap kromosom membelah longitudinal membentuk dua kromatid, sentromer masih satu terjadi chiasmata pada beberapa tempat natara kromatid homolog; dari chiasmata timbul crossing over.
5) Diakinesis
  • Kromosam (kromatid) mencapai pilinan maksimal, sehingga mencapai besar maksimal pula. Kromosom homolog merenggang, nukleus menghilang, selapu inti hancur, sentriol menganda dan setiap pasang menuju kutub berseberangan.
c. Metafase I
  • Selaput inti menghilang, serat gelondong terbentuk anatara kedua pasang sentriol, yang terdiri dari: mikrotubuli dan mikrofilia. Kromosom (berpasangan homolog) bergerak ke bidang ekuator.
d. Anafase I
  • Sel memanjang dari kutub ke kutub. Kromosom homolog berpisah ke kutub berseberangan dan kromatid belum terbentuk.
e. Telofase I
  • Selaput inti terbentuk kembali. Sepasang sentriol berada dipinggir luar selaput. Cytokinesis terjadi, sehingga sel induk menjadi sel anak. Gametosit I pada akhir meiosis I menjadi gametosit II.
2. Meiosis II
Gambar 7. Proses Miosis II
a. Profase II
  • Masanya pendek sekali. Selaput inti hilang. Sentriol mengganda dan menuju ke kutub berseberangan inti. Kromatid disetiap kromosom belum terpia=sah. Sentromer masih satu.
b. Metafase II
  • Serat gelondong terbentuk antara pasangan sentriol. Kromosom (sepasang kromatid) yang menggatung pada serat gelondong lewat sentromer pindah ke bidang equator.
c. Anafase II
  • Sel memanjang dari kutub ke kutub menurut poros serat gelondong. Sentromer pada setiap pasangan kromatid membelah sehingga kromatid bersaudara lepas. Kromatid berpisah dan bergerak ke kutub berseberangan.
d. Telofase II
  • Kromatid terbuka kembali pilinannya, terlepas-lepas, menjadi jala halus: kromatin. Selaput inti terbentuk kembali. Nucleolus muncul, melekat pada kromatin. Terjadi sitokinesis, sehingga dari dua gametaosit II terbentuk 4 gametid. Gametid mengandung kromosom separuh dari sel induk, dari 2N pada gametosit I, menjadi 1N pada gametid.
  • Dengan proses transformasi gametid nanti akan berubah menjadi gamet, yakni sel benih matang.
Meiosis menghasilkan gamet yang mengandung bahan genetis yang:
  1. Separuh dari bahan gametogonium
  2. Bervariasi, karena terjadinya crossing over pada profase I
Profase Sebentar
  • Tidak ada subfase
  • Hanya sekali Agak lama
  • Dibagi atas subfase pada meiosis I
  • 2x pembelahan
  • Profase II kromatid tidak menggandakan lagi
  • Terbentuknya kromosom Awal profase Pertengahan profase: pakiten
  • Kromosom homolog Tidak bergandeng Bergandengan pada zigoten sampai anafase meiosis I dan diploidTetrad, synapsis
  • Tidak terbentuk Terbentuk pada pakiten dan diploid
  • Sentromer Membagi 2 sehingga kromatid berpisah
  • Metafase I: belum menbagi 2 , Metafase II: membagi 2
  • Kromatid Pindah ke kutub berseberangan
  • Anafase I: kromosomhomolog pindah ke kutub berseberangan , Anafase II: kromatid pindah ke kutub bersebernaga
  • Terbentuk 2 sel anak masing-masing 2 n ( diploid )
  • Telofase I: terbentuk 2 sel anak masing-masing 2n , Telofase II: terbentuk 4 sel anak masing-masing 1n ( haploid )
  • Interkinesis Tidak ada Ada, antara meiosis I dan meiosis II
  • Terjadi pada Jaringan Germinatif sewaktu membentuk sel kelamin ( gamet )
  • OK

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