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MCAT Content / Nucleotides And Nucleic Acids / Deoxyribonucleic Acid Dna Double Helix

Deoxyribonucleic acid: DNA; double helix

Topic: Nucleotides And Nucleic Acids

Deoxyribonucleic acid (DNA) is formed into a double helix structure that consists of two polymer nucleotide strands that are joined together by hydrogen bonds formed between nitrogenous base pairs.

The structure of DNA is called a double helix, which looks like a twisted staircase. The sugar and phosphate make up the backbone of the nucleotides and are bonded together by phosphodiester bonds. The nitrogenous bases that are bonded to the pentose sugars pair to each other by weak hydrogen bonds. Adenine (A) hydrogen bonds to Thymine (T) and Guanine (G) hydrogen bonds to Cytosine (C) – the bases pair in this way due to the relative sizes and functional groups of the bases  This is known as complementary base pairing and causes the two separate strands of nucleotides to be joined together forming a double strand. The double-strand coils up forming a long double helix molecule of genetic material.

The nucleotide strands are complementary to each other and run in opposite directions. This means that on one strand if the code of nitrogenous base pairs is AATTGGCC then on the opposite nucleotide strand which is running in the opposite direction will contain the bases TTAACCGG, as shown below. This ensures the base pairs match up. Due to the unique complementary nature of a DNA double helix it is called antiparallel as both strands are in the opposite sequence of base pairs to each other. The 5′ to 3′ indicates the direction of the strand. This is commonly called 5 prime to 3 prime. This is used to discuss the direction of the DNA for example in DNA replication as DNA polymerase can only synthesize new strands in the 5′ to 3′ direction.

5'-AATTGGCC-3' 3'-TTAACCGG-5' These two strands are complementary, with each base in one sticking to its partner on the other. The A-T pairs are connected by two hydrogen bonds, while the G-C pairs are connected by three hydrogen bonds.

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Key Points

• DNA has a double helix structure consisting of two nucleotide polymers joined together by hydrogen bonds between their nitrogenous bases.

• The nitrogenous bases bond together in complementary base pairs with hydrogen bonds forming between A and T or G and C.

Key Terms

DNA: Deoxyribonucleic acid, the genetic material found within the cell nucleus

double Helix: the spiral structure of two nucleotide polymer strands joined together by hydrogen bonds between the nitrogenous base pairs

phosphodiester bond: linkage that occurs when two hydroxyl groups in a phosphate molecule react with hydroxyl groups on ribose sugars to form two ester bonds

hydrogen bond: a weak intermolecular attraction between a hydrogen atom and an electronegative element such as oxygen, nitrogen or fluorine

complementary base pairing: the pairing of opposite bases on opposite strands of a DNA double helix, A pairs to T and G pairs to C

antiparallel: the opposite arrangement of nucleotide strands on a DNA double helix with opposite sequences of nitrogenous bases

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