Perkembangan Hewan Drs. Jodion Siburian, M.Si Program Studi Pendidikan Biologi FKIP Unja E-mail: jdsiburian@gmail.com Telepon: 0741.7089467/ 08127861167

Daur Sel,Mitosis & Meiosis)

Cell Increase and Decrease Pembelahan sel meningkatkan jumlah sel somatik (sel tubuh) dan terdiri dari: Mitosis (pembelahan nukleus) Sitokinesis (pembelahan sitoplasma) Apoptosis (kematian sel) menurunkan jumlah sel. Baik penambahan jumlah sel atau kematian sel terjadi selama perkembangan dan pertumbuhan normal

Daur Sel Daur sel adalah urutan lengkap proses yang terjadi di dalam sel sehingga sebuah sel akan memproduksi dirinya sendiri. Pada organisme multiseluler, reproduksi sel akan menyediakan bahan untuk pertumbuhan, perkembangan dan perbaikan.

(a – c) perkembangan embrio Pembelahan sel mitosis Pembelahan sel mitosis (a) (b) (c) A layer of dead cells form on the surface of skin Pembelahan mitosis menghasilkan sel yang dibutuhkan untuk pertumbuhan, perkembangan dan perbaikan (a – c) perkembangan embrio (d) Perbaikan jaringan yang rusak seperti pada kulit Cel move up Pembelahan sel mitosis terjadi disini (d)

Pada organisme uniseluler seperti bakteri, reproduksi sel terjadi secara fusi binari (pembelahan biner) dengan urutan pertama akan dibentuk duplikat dari DNA Kemudian DNA tersebut akan menempel pada membran plasma. Pertumbuhan membran plasma akan memisahkan dua kromosom duplikat. Tahap berikutnya sel akan mencapai dua kali volume semula, membran akan melekuk diantara dua kromosom dan dinding sel akan terbentuk sehingga dihasilkan dua sel anak

Pembelahan biner (binarry fission) pada bakteri

Siklus Sel Siklus sel merupakan urutan yang terjadi dari sel pertama kali terbentuk sampai terbentuk dua sel baru Bagian terpanjang dalam siklus sel adalah fase interphase. Setelah interfase, tahap berikutnya adalah pembelahan mitosis.

Diagram siklus sel Terjadi pertumbuhan Organel double Mitosis & Sitokinesis Replikasi DNA, duplikasi kromosom Sel siap untuk membelah Diagram siklus sel

Tahap interfase membutuhkan 90% dari seluruh waktu reproduksi sel. Dalam interfase, sel mengadakan pertumbuhan, aktivitas metabolisme, dan pembelahan kromosom. Tahap interfase dibagi menjadi tiga tahap yaitu: G1, S dan G2.

Fase G1: sel aktif tumbuh, ditandai dengan bertambahnya sitoplasma, organel, dan sintesis bahan-bahan yang dibutuhkan untuk fase S Fase S (sintesis): terjadi replikasi DNA Fase G2: benang-benang gelondong (spindel) disintesis dan jumlah DNA sudah berlipat

Structure and main features of a spindle apparatus

Tahap Mitosis Setelah interfase adalah tahap M termasuk mitosis dan sitokinesis. Selama mitosis, sister chromatids (kromosom homolog) terpisah dan akan menjadi nukleus pada sel anak. Siklus sel diakhiri dengan tahap sitokinesis, pembelahan sitoplasme sampai berakhir.

Apoptosis Apoptosis is programmed cell death. Apoptosis occurs because of two sets of enzymes called capsases. The first set, the “initiators” receive a signal to activate the second set, the “executioners”. The second set of capsases activate enzymes that tear apart the cell and its DNA.

Tahapan Pembelahan Mitosis

Interfase akhir Selama mitosis, benang-benang spindel mendistribusikan kromosom ke setiap sel anak. Benang spindel mengandung serabut yang dibentuk oleh mikrotubula. Sentosom yang membelah selama masa interfase, terorganisasi dengan benang spindel. Sentrosom mengandung sentriol dan aster. Mitosis terdiri dari empat tahap: prophase, metaphase, anaphase, and telophase.

Profase Benang-benang kromatin akan berubah menjadi kromosom, membelah menjadi kromatida yang sentromernya masih satu Dinding nukleus dan nukleolus menghilang Pasangan sentriol yang terdapt dalam sentrosom berpisah bergerak menuju kutub berlawanan Benang-benang spindel terbentuk diantara dua kutub pembelahan

Metafase Setiap kromosom yang terdiri dari sepasang kromatida menuju ketengah sel dan berkumpul pada bidang pembelahan (equator) dan menggantung pada serat gelondong melalui sentromernya

Anafase Sentromer dari setiap kromosom akan membelah menjadi dua beserta masing-masing satu kromatida Setiap kromatida berpisah dengan pasangannya dan menuju ke kutub yang berlawanan Pada akhir anafase semua kromatid telah menuju ke kutub yang berlawanan

Telofase Kromatida yang ada pada masing-masing kutub berubah menjadi benang-benang kromatin kembali Terbentuk kembali dinding inti dan nukleolus sehingga terbentuk dua inti baru Serat-serat gelondong menghilang Terjadi pembalahan sitoplasma (sitokinesis) menjadi dua bagian, terjadi membran sel pemisah ditengah bidang pembelahan, sehingga terbentuk 2 sel anak

Sitokinesis pada sel tumbuhan

Sitokinesis pada sel hewan

Pembelahan Meiosis

Meiosis membutuhkan dua kali pembelahan nukleus dan menghasilkan empat nukleus haploid (n). Manusia mempunyai 23 pasang kromosom homolog, atau 46 kromosom. Pada meiosis I, terjadi replikasi DNA. Pada meiosis I, terjadi sinapsis.

Meiosis I memisahkan pasangan kromosom homolog. Sel anak bersifat haploid, tetapi kromosom tetap dalam kondisi duplikasi. Tidak terjadi replikasi DNA diantara dua pembelahan.

Meiosis II memisahkan pasangan kromatid. Pada beberapa siklus hidup organisme, sel anak haploid matang menjadi gamet. Fertilization mengembalikan kromosom menjadi diploid melalui reproduksi seksual.

Meiosis in detail The same four phases seen in mitosis – prophase, metaphase, anaphase, and telophase – occur during both meiosis I and meiosis II. The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs during interkinesis because the DNA is already duplicated.

Meiosis I in an animal cell During prophase I, the spindle appears while the nuclear envelope fragments and the nucleolus disappears. Because DNA replicated during interphase prior to meiosis I, the homologous chromosomes each have two sister chromatids. During synapsis, crossing-over can occur. If so, the sister chromatids of a duplicated chromosome are no longer identical. During metaphase I, homologous pairs are aligned at the metaphase plate.

During anaphase I, homologous chromosomes separate and move to opposite poles of the spindle. Each chromosome still consists of two chromatids. At telophase I, nuclear envelopes may reform and nucleoli may reappear. Cytokinesis may or may not occur at this point. Interkinesis is a stage between the two meiotic divisions when chromosomes still consist of two chromatids.

Meiosis II At the beginning of prophase II, a spindle appears while the nuclear envelope disassembles and the nucleolus disappears. During anaphase II, sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei.

In telophase II, the spindle disappears as nuclear envelopes reform. During the cytokinesis that follows meiosis II, the plasma membrane furrows to give two complete cells, each with the haploid number of chromosomes.

Comparison of Meiosis with Mitosis In both mitosis and meiosis, DNA replication occurs only once during interphase. Mitosis requires one division while meiosis requires two divisions. Two diploid daughter cells result from mitosis; four haploid daughter cells result from meiosis.

Daughter cells from mitosis are genetically identical to parental cells; daughter cells from meiosis are not genetically identical to parental cells. Mitosis occurs in all somatic cells for growth and repair; meiosis occurs only in the reproductive organs for the production of gametes.

Comparison of Meiosis I to Mitosis Prophase I - pairing of homologous chromosomes Metaphase I – homologous pairs line up at metaphase plate Anaphase I – homologous chromosomes separate Telophase I – daughter cells are haploid MITOSIS: Prophase has no such pairing Metaphase – chromosomes align at metaphase plate Anaphase – sister chromatids separate Telophase – diploid cells

Comparison of Meiosis II to Mitosis The events of meiosis II are like those of mitosis except in meiosis II, the nuclei contain the haploid number of chromosomes. At the end of telophase II of meiosis II, there are four haploid daughter cells that are not genetically identical. At the end of mitosis, there are two diploid daughter cells that are identical.

Meiosis compared to mitosis

The Human Life Cycle The human life cycle requires both mitosis and meiosis. In males, meiosis occurs as spermatogenesis and produces sperm. In females, meiosis occurs as oogenesis and produces egg cells. Mitosis is involved in the growth of a child and repair of tissues during life.

Life cycle of humans Meiosis in human males is a part of sperm production, and meiosis in human females is a part of egg production. When a haploid sperm fertilizes a haploid egg, the zygote is diploid, The zygote undergoes mitosis as it develops into a newborn child. Mitosis continues after birth until the individual reaches maturity; then the life cycle begins again.

Spermatogenesis in human males produces four viable haploid sperm. Diploid primary spermatocytes undergo meiosis I to produce haploid secondary spermatocytes. Secondary spermatocytes divide by meiosis II to produce haploid spermatids. Spermatids mature into sperm with 23 chromosomes.

Spermatogenesis Spermatogenesis produces four viable haploid sperm cells.

During oogenesis, a diploid primary oocyte undergoes meiosis I to produce one haploid secondary oocyte and one haploid polar body. The secondary oocyte begins meiosis II but stops at metaphase II and is released at this stage from the ovary. Meiosis II will be completed only if sperm are present. Following meiosis II, there is one haploid egg cell with 23 chromosomes and up to three polar bodies. Polar bodies serve as a dumping ground for extra chromosomes.

Oogenesis Oogenesis produces one egg and two or three polar bodies. The extra cytoplasm of the single egg cell serves as a source of nutrients for the developing embryo.

In humans, both sperm cells and the egg cell have 23 chromosomes each. Following fertilization of the egg cell by a single sperm, the zygote has 46 chromosomes, the diploid number found in human somatic cells. The 46 chromosomes represent 23 pairs of homologous chromosomes.

Genetic Recombination in Humans There are three ways in which meiosis and fertilization ensure that a child has a different combination of genes from that of either parent: Independent assortment of chromosomes during metaphase I Crossing-over during prophase I Upon fertilization, recombination of chromosomes occurs.

Chapter Summary Cell division increases the number of body cells; apoptosis decreases it. Cells goes through a cell cycle that has three control checkpoints. Each species has a characteristic number of chromosomes. Mitosis has four phases and maintains the chromosome number.

Mitosis is used for growth and repair. Meiosis reduces the chromosome number and includes two nuclear divisions. The human life cycle includes both mitosis and meiosis. Meiosis and fertilization in humans and other sexually reproducing organisms result in genetic recombination in the offspring.

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