Double Helix structure of DNA ~ University Notes

A Double-Helix Structure

DNA has a double-helix structure, with sugar and phosphate on the outside of the helix, forming the sugar-phosphate backbone of the DNA. The nitrogenous bases are stacked in the interior in pairs, like the steps of a staircase; the pairs are bound to each other by hydrogen bonds. The two strands of the helix run in opposite directions, so that the 5′ carbon end of one strand faces the 3′ carbon end of its matching strand. This antiparallel orientation is important to DNA replication and in many nucleic acid interactions.

DNA is a Double Helix

Native DNA is an antiparallel double helix. The phosphate backbone (indicated by the curvy lines) is on the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base from the opposing strand.

Base Pairs

Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain pyrimidine. This means Adenine pair with Thymine, and Guanine pairs with Cytosine. This is known as the base complementary rule because the DNA strands are complementary to each other. If the sequence of one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG.

Antiparallel Strands

In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5′ to 3′ and the other 3′ to 5′. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine.

Source: Boundless. “Amino Acids.” Boundless Chemistry. Boundless, 08 Aug. 2016. Retrieved 15 Aug. 2016 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/polymers-24/proteins-170/amino-acids-649-11436/

Source: Boundless. “The DNA Double Helix.” Boundless Chemistry. Boundless, 26 May. 2016. Retrieved 15 Aug. 2016 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/polymers-24/nucleic-acids-172/the-dna-double-helix-654-11440/