- About DNA: A Hairpin Loop From a Pre-mRNA - Vossman
DNA and RNA consist of base-containing nucleotides connected by phosphates. The order of the bases determines one's genetic code.
The molecular structure of DNA is quite similar to that of RNA. They both contain phosphate molecules that connect nucleotides. The nucleotides are bonded to bases that make up the genetic code. Two strands of nucleic acid bond together to form a double helix.
Deoxyribonucleic acid (DNA) is made up of nucleotides. If DNA were a brick wall, the nucleotides would be the bricks. Ribonucleic acid (RNA) shares this characteristic with DNA.
Difference between Ribose and Deoxyribose
Deoxyribose and ribose are sugars. The difference between the two is that deoxyribose has one less oxygen atom than ribose. How they connect to other molecules is the same.
The carbon atoms in a sugar are numbered in a clockwise direction using the prime symbol. A carbon that is numbered 3' would be called "three prime." A base connects to 1', and a phosphate connects to 5'.
The sugar is a single ring arranged in a pentagon with an oxygen atom in the 12 o'clock position and carbons completing the other four points of the ring. The 5' carbon is not in the ring and branches off of the 4' carbon. The structural difference between DNA and RNA is that a hydroxyl group (OH) branches off of the 2' carbon in RNA, whereas two hydrogen atoms (H) branch off of the 2' carbon in DNA.
Each DNA Nucleotide Connects to a Phosphate
In a nucleotide, the phosphate consists of a phosphorous atom doubled-bonded to an oxygen atom (O=P), single-bonded to two negatively charged oxygen atoms (O- –P–O-), and single-bonded to a methyl group (CH2, one carbon bonded to two hydrogen atoms) on the sugar. The carbon in the methyl group is the 5' carbon.
A single strand of nucleic acid, both DNA and RNA, consists of alternating phosphate and sugar molecules. The beginning of the strand begins with a phosphate group linked to the 5' carbon, and ends with a hydroxyl group linked to the 3' carbon. The "direction" of a strand goes from 5' to 3'.
Base Pairing of DNA and RNA
Bases make up a genetic sequence that is the blueprint of a gene. There are five bases: adenine (A), guanine (G), thymine (T), cytosine (C), and uracil (U). Adenine and guanine are purines, meaning that they consist of two rings. Thymine, cytosine, and uracil consist of a single ring and are called pyrimidines.
DNA consists of adenine, guanine, thymine, and cytosine. RNA substitutes uracil for thymine. When two nucleic acid strands bond together to form a double helix, these bases pair together. Adenine always pairs with thymine in DNA, or uracil in RNA. To remember this, think of the word "at," or the chemical symbol "AU" for gold. Guanine always pairs with cytosine.
The sugars and phosphates are covalently bonded, meaning that they share electrons. This strong bond greatly decreases the likelihood that the base sequence will change, or mutate.
The sequence of a nucleic acid may be abbreviated by noting the directionality and sequence of the bases. Listing the nucleotides in a hypothetical nucleic acid consisting of five bases would look like this: 5'–GCATA–3', for example.
In 5'–GCATA–3', the nucleic acid would begin with a phosphate group and alternate between sugars and phosphate groups until after the final adenine, which would terminate with a hydroxyl group. The sugars are deoxyribose because this sequence contains thymine, so it is DNA. If it were RNA, it would contain uracil, instead. The sugars are connected to the bases guanine, cytosine, adenine, thymine, and adenine, respectively. This is the structure of this strand of nucleic acid.
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