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Learn about the components of DNA, such as nucleotides, nucleosides, and nucleotides, and how they form the double helix structure. Explore the significance of base pairing and hydrogen bonds in DNA stability.
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AL-Ma’moon University CollegeMedical Laberatory techniques Department Molecular biology/ Second stage Dr. Israaayoubalwan Lec– 2-))
DNA Structure:- • The structure of DNA is easiest to understand if we begin with the smallest components. A single building block of DNA is a nucleotide. It consists of one deoxyribose sugar, one phosphate group (a phosphorus atom bonded to four oxygen atoms), and one nitrogenous base Adenine (A) and guanine (G) are purines, which have a two-ring structure. Cytosine (C) and thymine (T) are pyrimidines, which have a single-ring structure.
The bases are the information-containing parts of DNA because they form sequences. DNA sequences are measured in numbers of base pairs. The terms kilobase (kb) and megabase (mb) are used to abbreviate a thousand and a million DNA bases, respectively. A particular gene, for example, may be “1,400 base pairs long.” Nucleotides join into long chains when chemical bonds form between the deoxyribose sugars and the phosphates. This creates a continuous sugar-phosphate backbone. Two such chains of nucleotides align head-to-toe, depicts.
Nucleoside:- A nucleoside consists of a base covalently bonded to the 1-position of a pentose sugar molecule . in RNA the sugar is ribose and the compound are ribonucleoside , whereas in DNA it is 2- deoxy ribose, and the nucleosides are named 2-deoxyribonucleoside. • (Base + Sugar =nucleoside) • The bond between the bases and the sugars is the glycosylic (or glycosidic ) bond. • Nucleotides :- are nucleosides with one or more phosphate groups covalently bound to the 3 ,5 or in ribonucleotides, the 2- position . • (Base + Sugar + phosphate = nucleotide) • The nucleoside 5- triphosphates (NTPs or dNTPs) are respectively the building blocks of polymeric RNA and DNA.
Phosphodiester bonds :-in nucleic acid polymers, the ribose or deoxyribose sugars are linked by a phosphate bound between the 5-position of one sugar and 3- position the next, forming a 3 , 5 - Phosphodiester bond. Nucleic acids consist of a directional sugar – phosphate backbone with a base attached to the 1-position of each sugar. The repeat unit is a nucleotide. Nucleic acids are highly charged polymers with a negative charge on each phosphate. • DNA / RNA sequence:- the nucleic acid sequence is the sequence of bases A,C,G,T/U in the DNA or RNA chain. the sequence is conventionally written from the free 5- to the free 3- end of the molecule, for example 5-ATAAGCTC-3 (DNA) or 5-AUAGCUUGA-3 (RNA) .
The opposing orientation of the two nucleotide chains in a DNA molecule is called antiparallelism. It derives from the structure of the sugar-phosphate backbone. Antiparallelism becomes evident when the carbons of the sugars are assigned numbers to indicate their positions in the molecule . • The carbons are numbered from 1 to 5, starting with the first carbon moving clockwise from the oxygen in each sugar in .One chain runs from the 5 carbon (top of the figure) to the 3 carbon, but the chain aligned with it runs from the 3 to the 5 carbon. These ends are depicted as “5 ′ ” and “3 ′ ”, pronounced “5 prime” and “3 prime.”
The symmetrical DNA double helix forms when nucleotides containing A pair with those containing T, and nucleotides containing G pair with those carrying C. Because purines have two rings and pyrimidines one, the consistent pairing of a purine with a pyrimidine ensures that the double helix has the same width throughout, as Watson discovered using cardboard cutouts. These specific purine-pyrimidine couples are called complementary base pairs.
Chemical attractions called hydrogen bonds hold the base pairs together. They are weak individually, but over the many bases of a DNA molecule impart great strength. Two hydrogen bonds join A and T, and three hydrogen bonds join G and C, DNA forms a double helix when the antiparallel, base-paired strands twist about one another in a regular fashion. • The double-stranded, helical structure of DNA gives it great strength—50 times the strength of single-stranded DNA, which would not form a helix.
