Kamis, 19 April 2012

STRUKTUR DNA SEL EUKARYOTIK PART 2




Struktur DNA Eukaryotik
              Pada jasad eukaryotik molekul DNA terdiri dari 2 rantai polinukleotida yang membentuk double heliks. Kedua rantai berjalan anti paralel artinya ikatan ester fosfat pada atom C nomer 5 dan atom C nomer 3 berjalan kearah yang berlawanan. Kedua rantai polinukleotida satu sama lain dihubungkan oleh ikatan hidrogen dengan perantara basanya. Pasangan basa nitrogen yang dapat dihubungkan oleh ikatan hidrogen bersifat khusus artinya basa pirimidin hanya dapat dihubungkan dengan basa purin. Ikatan hidrogen menghubungkan adenin dengan timin dan sitosin dengan guanin. Hubungan adenin dengan timin dilaksanakan dengan dua ikatan hidrogen sedangkan sitosin dengan guanin oleh tiga ikatan hidrogen. Basa purin dan basa pirimidin yang dihubungkan dengan ikatan hidrogen yang demikian disebut komplementer. Konsep komplementer sangat penting artinya karena dapat digunakan untuk menjelaskan mengapa molekul DNA yang panjang dapat mengadakan replikasi dengan cara yang cepat. Didalam suatu molekul DNA urutan nukleotida pada rantai yang satu tidak dapat ditentukan, tetapi harus komplementer dengan urutan nukleotida pada rantai pasangannya. Dalam setiap jarak 34 A, heliks ganda membentuk satu putaran lengkap. Karena jarak antara dua monomer nukleotida 3,4 A, maka setiap putaran lengkap terdiri atas 10 nukleotida. Molekul DNA panjang dan tak bercabang, pada sel-sel eukaryotik molekul DNA linier dan bersenyawa dengan protein sedang pada sel prokaryotik molekul DNA tidak bersenyawa dengan protein dan bentuknya melingkar. Jumlah DNA pada eukaryotik jauh lebih besar karena DNA terdapat dalam paket-paket yang disebut kromosom. Setiap kromosom terdiri dari benang DNA yang membelit protein histon. Tidak seluruh DNA mengkode protein tetapi ada bagian yang tidak mengkode 
                   Bagian yang mengkode protein disebut ekson sedangkan yang tidak mengkode protein disebut intron. Histon adalah protein yang relatif kecil, bermuatan positif pada rentangan PH yang sesuai dengan sel hidup.Protein tersebut mudah dipisahkan dari DNA dengan menempatkan kromatin dalam larutan garam. Dengan perlakuan tersebut dapat dipisahkan lima jenis protein histon yaitu H1, H2, H2B, H3 dan H4. Pada sel-sel eukaryotik yang derajatnya lebih tinggi jenis-jenis histon tidak banyak menunjukan variasi urutan asam amino. Adanya konservasi histon yang demikian menunjukan bahwa pada hampir semua mahluk hidup protein histon sangat penting baik untuk struktur maupun untuk fungsi kromatin. Pada jasad eukaryotik , DNA membelit protein histon membentuk nukleosom. Menurut hipotesis Roger D. Konberg setiap nukleoplasma mempunyai struktur berikut :
  • Histon H2A, H2B, H3, dan H4 berpasangan membentuk oktamer
  • Setiap oktamer dibungkus oleh DNA yang melilit sebanyak 1,5-1,75 kali putaran
  • Nukleosom yang satu dengan yang lain dihubungkan oleh DNA penghubung pendek yang bersenyawa dengan H1.
Pada nukleus berbagai protein non histon lebih bervariasi dari pada protein histon. Dalam campuran larutan garam yang konsentrasinya sama dengan nukleus pada sel yang hidup, nukleosom dapat terbentuk dengan mencampurkan molekul DNA dengan lima protein histon, sedang protein non histon tidak perlu ada. Ini menunjukan bahwa protein histon tidak berperan dalam pembentukan struktur kromatin, melainkan berupa protein fungsional antara lain berupa enzim yang membantu proses transkripsi dan replikasi molekul DNA. Duplikasi DNA terjadi selama interfase yaitu periode ketika kromosom tidak dapat dilihat dengan mikroskop cahaya. Pada saat sel akan membelah, kromosom menebal sehingga dapat dilihat dengan mikroskop cahaya. Jadi kromosom dapat dilihat selama mitosis dan meiosis.
 
Siklus sel eukaryotik 
 
 
 
               Sel yang sedang tumbuh selalu mengalami siklus sel yang merupakan serangkaian proses yang berlangsung sejak pembentukannya sampai dengan mulai membelah. Siklus sel terdiri atas dua fase yaitu : interfase dan fase pembelahan. Pada sel eukaryotik, pembelahan sel ada dua macam yaitu mitosis dan meiosis. Mitosis berfungsi mengadakan pertumbuhan termasuk mengganti sel-sel yang rusak atau mati sedangkan meiosis bertujuan untuk membentuk sel-sel perkembangbiakan. Pada interfase sebenarnya berlangsung kegiatan yang intensif  antara lain biosintesis DNA dan pembagian komponen-komponen kromosom menjadi dua bagian yang sama. Sementara itu ukuran sel pun bertambah menjadi kurang lebih dua kali semula. Beberapa sel yang telah berdeferensiasi jarang sekali membelah, misalnya limfosit sedangkan pada mamalia, sel saraf tidak pernah membelah setelah individu lahir. Dengan demikian, selama hidupnya neuron tersebut tetap dalam keadaan interfase. Pada interfase berlangsung serangkaian proses yang komplek sebagai persiapan untuk membagikan materi yang terdapat dalam sel kepada anak, dengan jumlah yang sama. Sebelum membelah, komponen-komponen molekuler yang penting dalam sel yang telah digandakan menjadi dua kali lipat sehingga pembelahan sel merupakan fase akhir dari perubahan-perubahan tingkat molekul yang telah berlangsung.
                  Siklus sel eukaryotik terdiri dari fase gap, sintesis, G2, mitosis dan sitokinesis. Pada fase gap tidak ada aktivitas kromosom. Selama fase sintesis terjadi replikasi DNA. Pada fase G2 tidak ada fase kromosom dan fase mitosis merupakan proses yang diikuti oleh fase sitokinesis (pembelahan sel). Lamanya siklus sel sangat bervariasi. Pada umumnya periode sintesis, G2 dan mitosis pada sel suatu organisme relatif tetap sedangkan periode gap mungkin berlangsung beberapa hari, beberapa bulan bahkan beberapa tahun. Pada jaringan-jaringan yang selnya secara normal tidak pernah membelah atau sel-sel yang jarang membelah terdapat sejumlah DNA yang tetap sama dengan periode gap. Pengaturan tentang lamanya siklus sel tergantung kepada terhentinya siklus pada tahap tertentu dalam periode gap. Sel yang berada dalam kondisi demikian dikatakan dalam periode gap0 dan siklusnya dianggap mundur. jika kondisi sudah berubah dan pertumbuhan sudah mulai, maka sel tersebut kembali lagi ke dalam periode gap.
 
Sintesis protein pada eukaryotik 
Sintesis protein pada eukaryotik lebih rumit dibandingkan prokaryotik. Transkripsi terjadi dalam nukleus dan mRNA yang dihasilkan di modifikasi dahulu sebelum ditranslasikan didalam sitoplasma. bagian intron akan dilepas dan bagian ekson akan disambung sehingga terbentuklah mRNA yang siap ditranslasikan. Ribosom pada sel eukaryotik yang mentranslasikan mRNA mempunyai tipe 80S terdiri atas sub unit 40S dan 60S. 

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