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MCAT Content / Nucleotides And Nucleic Acids / Nucleotide Composition

Nucleotide and Nucleoside Composition

Topic: Nucleotides And Nucleic Acids

Nucleotides are composed of a pentose sugar molecule, a nitrogenous base, and a phosphate group. These make up one nucleotide monomer which, when combined into a polymer, can create nucleic acids, DNA or RNA.

Each nucleotide is made up of three components:

  1. a nitrogenous base
  2. a pentose (five-carbon) sugar
  3. a phosphate group

When multiple nucleotide monomers combine, they create polymers called nucleic acids, such as DNA and RNA.

A molecule with just a nitrogenous base and a sugar is known as a nucleoside. Once at least one phosphate is covalently attached, it is known as a nucleotide

Nitrogenous Bases

The nitrogenous bases are organic molecules that contain carbon and nitrogen. They are bases because they contain an amino group that forms hydrogen bonds within the double-stranded DNA polymer to bases on the adjacent strand (shown in the figure below). There are two families of bases, Purines, and Pyrimidines. Adenine (A) and guanine (G) are classified as purines, and have a primary structure consisting of two carbon-nitrogen rings. Cytosine (C), thymine (T), and uracil (U) are classified as pyrimidines which have a single carbon-nitrogen ring as their primary structure.  Because of the sizes and functional groups of the bases, base pairing is highly specific in DNA: A only pairs with T, and G only pairs with C, as shown below. RNA contains A U C and G.

Uracil is identical to thymine, without the methyl group.

Hydrogen bonding between complementary bases holds DNA strands together in a double helix of antiparallel strands. Thymine forms two hydrogen bonds with adenine, and guanine forms three hydrogen bonds with cytosine.

Five-Carbon Sugar

The pentose sugar in DNA is deoxyribose, and in RNA it is ribose. The difference between the sugars is the presence of the hydroxyl group (-OH), on the second carbon of the ribose, and hydrogen on the second carbon of the deoxyribose.

Phosphate Group

The phosphate residue forms a phosphodiester bond between the ribose sugars attached to each base. The phosphodiester linkage forms the backbone of the double helix structure of DNA and the single strand structure of RNA.

Practice Questions

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Molecular targets of antiretroviral therapies

MCAT Official Prep (AAMC)

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Section Bank C/P Section Question 82

Online Flashcards Biology Question 25

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Biology Question Pack, Vol. 1 Question 83

Biology Question Pack, Vol. 1 Question 119

Biology Question Pack, Vol 2. Passage 15 Question 97

Practice Exam 3 B/B Section Passage 2 Question 7

Sample Test B/B Section Question 46

Key Points

• Nucleotides are comprised of a ribose sugar, a nitrogenous base, and a phosphate group.

• There are two families of nitrogenous bases, the pyrimidines (C, T, and U) and the purines (A and G).

• Bases A T G and C are found in DNA and A U C and G found in RNA.

• The ribose sugar and phosphate form a phosphodiester bond, which links several nucleotide monomers together in a polymer to form a nucleic acid such as RNA (single strand) or DNA (double strand with specifically paired bases).

Key Terms

nucleotide: the monomers which make up DNA or RNA polymer molecules. These consist of a nitrogenous heterocyclic base that can be a purine or pyrimidine, a five-carbon pentose sugar, and a phosphate group

monomer: a relatively small molecule which can be covalently bonded to other monomers to form a polymer

polymer: many monomers joined together to form long chains

nucleic acid: polymers comprised of many nucleotides bonded together, found in cells in two forms, RNA and DNA

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

RNA: ribonucleic acid, codes for proteins at ribosomes

purines: a nitrogenous base consisting of two carbon-nitrogen rings

pyrimidines: a nitrogenous base consisting of a single carbon-nitrogen ring

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

nucleoside: a nitrogenous base attached to a ribose sugar without a phosphate molecule

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