DNA Structure : Phosphodiester bonds

DNA is a polymer of deoxyribonucleoside monophosphates (dNMP) covalently linked by 3′→5′-phosphodiester bonds. With the exception of a few viruses that contain single-stranded DNA (ssDNA), DNA exists as a double-stranded molecule (dsDNA), in which the two strands wind around each other, forming a double helix. [Note: The sequence of the linked dNMP is primary structure, whereas the double helix is secondary structure.] In eukaryotic cells, DNA is found associated with various types of proteins (known collectively as nucleoprotein) present in the nucleus, whereas the protein–DNA complex is present in a non–membrane-bound region known as the nucleoid in prokaryotes.
 3′→5′-Phosphodiester bonds
Phosphodiester bonds join the 3′-hydroxyl group of the deoxypentose of one nucleotide to the 5′-hydroxyl group of the deoxypentose of an adjacent nucleotide through a phosphoryl group (Fig. 30.2). The resulting long, unbranched chain has polarity, with both a 5′-end (the end with the free phosphate) and a 3′-end (the end with the free hydroxyl) that are not attached to other nucleotides. By convention, the bases located along the resulting deoxyribose-phosphate backbone are always written in sequence from the 5′-end of the chain to the 3′-end. For example, the sequence of bases in the DNA shown in Figure 1D (5′-TACG-3′) is read “thymine, adenine, cytosine, guanine.” Phosphodiester linkages between nucleotides can be hydrolyzed enzymatically by a family of nucleases, deoxyribonucleases for DNA and ribonucleases for RNA, or cleaved hydrolytically by chemicals. [Note: Only RNA is cleaved by alkali.]

Figure 1: A. DNA with the nucleotide sequence shown written in the 5′→3′ direction. A 3′→5′-phosphodiester bond is shown highlighted in the blue box, and the deoxyribose-phosphate backbone is shaded in yellow. B.DNA written in a more stylized form, emphasizing the deoxyribose-phosphate (p) backbone. C.A simpler representation of the nucleotide sequence. D.The simplest (and most common) representation. [Note: The nucleotide base sequence is assumed to be written in the 5′→3′ direction unless otherwise indicated.]

مواضيع ذات صلة

Protein Phosphorylation is Extremely Versatile

Reversible Covalent Modification Regulates Key Mammalian Proteins

تحويل الكوليسترول إلى البريغنينولون

تصنيف الهرمونات Classification of Hormones

الطبيعة الكيميائية للمستقبلات Chemical Nature

الحقول الوظيفية في المستقبلات Functional Domains

دور المستقبلات Role of Receptors

الخلية الهدفية والمستقبلات Target Cell & Receptors

دينامية المستوى البلازمي للهرمونات

تنوع وظائف الهرمونات وشموليتها

العناصر الزهيدة الأخرى Other trace elements

الفلور Fluorine