The 5′-phosphoryl group of a mononucleotide can esterify a second hydroxyl group, forming a phosphodiester. Most commonly, this second hydroxyl group is the 3′-OH of the pentose of a second nucleotide. This forms a dinucleotide in which the pentose moieties are linked by a 3′,5′-phosphodiester bond to form the “backbone” of RNA and DNA. The formation of a dinucleotide may be represented as the elimination of water between two mononucleotides. Biologic formation of dinucleotides does not, however, occur in this way because the reverse reaction, hydrolysis of the phosphodiester bond, is strongly favored on thermodynamic grounds. However, despite an extremely favorable ΔG, in the absence of catalysis by phosphodiesterases hydrolysis of the phosphodiester bonds of DNA occurs only over long periods of time. DNA therefore persists for considerable periods, and has been detected even in fossils. RNAs are far less stable than DNA since their 2′-hydroxyl groups (absent from DNA) can function as nucleophiles for the chemical hydrolysis of 3′,5′-phosphodiester bonds.

Posttranslational modification of preformed polynucleotides can generate additional structures such as pseudouridine, a nucleoside in which d-ribose is linked to C-5 of uracil by a carbon-to-carbon bond rather than by the usual β-N glycosidic bond. The nucleotide pseudouridylic acid (ψ) arises by rearrangement of a UMP of a preformed tRNA. Similarly, methylation by S-adenosylmethionine of a UMP of preformed tRNA forms TMP (thymidine monophosphate), which contains ribose rather than deoxyribose.

Polynucleotides Are Directional Macromolecules

 Directional 3′ → 5′ phosphodiester bonds link the monomers of polynucleotides. Since each end of a polynucleotide thus is distinct, we refer to the “5′-end” or the “3′-end” of a poly nucleotide. Since the phosphodiester bonds all are 3′ → 5′, the representation pGpGpApTpCpA indicates that the terminal 5′-hydroxyl is phosphorylated. More concisely, the representation GGATC, which shows only the base sequence, is by convention written with the 5′-base (G) at the left and the 3′-base (C) at the right.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مع وصول الزيارة ذروتها.. العتبة العباسية المقدسة تستنفر ملاكاتها لخدمة زائري مرقدي الإمامين العسكريين (عليهما السلام)

قسم الشؤون الطبية: الخطة الخدمية في سامراء تضمنت مشاركة نحو ٥٠ متطوعًا من ثلاث محافظات

عبر محطات الإجابة عن الاستفتاءات الشرعية مشروع التبليغ الحوزوي يقدّم خدماته لزائري مدينة سامراء

قسم حفظ النظام يشارك في تأمين زيارة الإمام الحسن العسكري (عليه السلام) بمدينة سامراء

المزيد

الآخبار الصحية

دون آثار جانبية.. زيت عشبة طبيعية قد يعالج القلق

"ليكانيماب".. علاج الزهايمر يطرح رسميا في السوق الألمانية

متى يجب تنظيف الأسنان في الصباح.. قبل أم بعد الإفطار؟

ارتفاع الوفيات.. منظمة الصحة تدق جرس الإنذار بشأن الكوليرا

المزيد

الآخبار التكنلوجية

أكبر مشروع طاقة رياح في نصف الكرة الجنوبي يحقق إنجازين جديدين

5 أمور لا تفعلها أثناء تشغيل مكيف سيارتك

ضوء إدارة المحرك: أسئلة مهمة لفهم عمله ومعانيه

"واتساب" يطلق خاصية جديدة لكتابة الرسائل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

The Deoxyribonucleosides of Uracil & Cytosine are Salvaged

Biosynthesis of Pyrimidine Nucleotides

Hepatic Purine Biosynthesis is Stringently Regulated

Inosine Monophosphate (IMP) is Synthesized From Amphibolic Intermediates

Synthetic Nucleotide Analogs are Used in Chemotherapy

Chemistry of Purines, Pyrimidines, Nucleosides, & Nucleotides

Disorders of Bilirubin Metabolism

Catabolism of HEME Produces Bilirubin

Disorders of Heme Biosynthesis

Porphyrins are Colored & Fluoresce

Heme is Synthesized From Succinyl-CoA & Glycine

NON–α-AMINO ACIDS