Dr. Israa ayoub alwan Lec – 2- ) )

1. AL-Ma’moon University CollegeMedical Laberatory techniques Department Molecular biology/ Second stage Dr. Israaayoubalwan Lec– 2-))

2. DNA Structure

3. 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.

4. 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.

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7. 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.

8. 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) .

9. 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.”

10. 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.

11. 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.

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