The initiation of DNA synthesis (Figure 1) requires priming by a short length of RNA, about 10 to 200 nucleotides long. In E. coli primer formation is catalyzed by dnaG (primase), in eukaryotes DNA Pol α synthesizes these RNA primers. Once complete DNA synthesis commences on this short RNA primer. The priming process involves nucleophilic attack by the 3′-hydroxyl group of the RNA primer on the phosphate of the first entering deoxynucleoside triphosphate (N in Figure 1) with the splitting off of pyrophosphate; this transition to DNA synthesis is catalyzed by the appropriate DNA polymerases (DNA pol III in E. coli; DNA pol δ and ε in eukaryotes). The 3′-hydroxyl group of the recently attached deoxyribonucleoside monophosphate is then free to carry out a nucleophilic attack on the next entering deoxy ribonucleoside triphosphate (N + 1 in Figure 1), again at its α phosphate moiety, with the splitting off of pyrophosphate. Of course, selection of the proper deoxyribonucleotide whose terminal 3′-hydroxyl group is to be attacked is dependent on proper base pairing with the other strand of the DNA molecule according to Watson and Crick base pairing rules (Figure 2). When an adenine deoxyribonucleoside monophosphoryl moiety is in the template position, a thymidine triphosphate will interact with the dXTP binding site of the DNA polymerase and its α phosphate will be attacked by the 3′-hydroxyl group of the deoxyribonucleoside monophosphoryl most recently added to the polymer. By this stepwise process, the template dictates which deoxyribonucleoside triphosphate is complementary, and by hydrogen bonding, holds it in place while the 3′-hydroxyl group of the growing strand attacks and incorporates the new nucleotide into the polymer. These segments of DNA attached to an RNA primer component are the Okazaki fragments (Figure 3). In both bacteria and mammals, after many Okazaki fragments are generated, the replication complex begins to remove the RNA primers, to fill in the gaps left by their removal with the proper base-paired deoxynucleotide, and then to seal the fragments of newly synthesized DNA by enzymes referred to as DNA ligases.

Fig1. The initiation of DNA synthesis upon a primer of RNA and the subsequent attachment of the second deoxyribonucleoside triphosphate. Note the blue highlighting of the 2’-H moiety of deoxyribose and the yellow highlighting of the 2′-OH moiety within the RNA primer.

Fig2. The RNA-primed synthesis of DNA demonstrating the template function of the complementary strand of parental DNA.

Fig3. The discontinuous polymerization of deoxyribonucleotides on the lagging strand; formation of Okazaki fragments during lagging strand DNA synthesis is illustrated. Okazaki fragments are 100 to 250 nucleotides long in eukaryotes, 1000 to 2000 nucleotides in prokaryotes.

اخر الاخبار

اخبار العتبة العباسية المقدسة

سماحة السيد الصافي يُشيد بجهود إنقاذ مكتبة جامعة الموصل ويؤكّد دعم العتبة العباسية المقدسة للكفاءات العراقية

المجمع العلمي ينظم ندوة في بابل بمناسبة مرور 1500 عام على ولادة النبي الأعظم (صلى الله عليه وآله)

قسم الشؤون الفكرية ينظم ندوة عن صناعة التأثير في المحتوى الإعلامي في ذي قار

شعبة فاطمة بنت أسد تطلق العام الدراسي الجديد في مدرستها القرآنية

المزيد

الآخبار الصحية

هل يتنبأ الذكاء الاصطناعي بأمراضنا قبل حدوثها؟

وداعا للنظارات.. قطرة عين واحدة قد تغير كل شيء ؟

دراسة تكشف تأثير "تيك توك" وتطبيقات الفيديو على سلوك الأطفال

"أسرار خفية" في الشاي الأخضر.. تغير قواعد الصحة والوزن

المزيد

الآخبار التكنلوجية

أول قطار يعمل بالهيدروجين في أميركا يدخل الخدمة رسميًا

أجهزة أبل القادمة.. "حصن منيع" في عصر الهجمات الرقمية !

روسيا: تطلق قمرين اصطناعيين بواسطة الصاروخ سويوز 2.1

ميزة ثورية.. أبل تدخل عالم قياس ضغط الدم

المزيد

مواضيع ذات صلة

DNA Synthesis Occurs During the S Phase of the Cell Cycle

Formation of Replication Bubbles

The Exact Function of Much of the Mammalian Genome is Not Well Understood

DNA is organized into Chromosomes

Some Regions of Chromatin are “Active” & Others are “Inactive”

The Nucleosome Contains Histone & DNA

Histones Are the Most Abundant Chromatin Proteins

Specific Nucleases Digest Nucleic Acids

Directions for Future Research in Molecular Biology

The ras-fos-jun Pathway

Recessive Oncogenic Mutations, Tumor Suppressors

Small RNA