AP Biology Unit 1 (Chemistry of Life): Learning Nucleic Acids from the Ground Up

Nucleic acids

Biological macromolecules that store, transmit, and help express genetic information; the information is encoded in the sequence of bases.

DNA (deoxyribonucleic acid)

A nucleic acid used primarily for long-term information storage; typically double-stranded and forms a double helix.

RNA (ribonucleic acid)

A nucleic acid often involved in information transfer and functional roles; typically single-stranded and able to fold into complex shapes.

Nucleotide

The monomer of nucleic acids; composed of a phosphate group, a five-carbon sugar, and a nitrogenous base.

Nucleoside

A sugar + nitrogenous base (no phosphate group); differs from a nucleotide by lacking the phosphate.

Nitrogenous base

A ring-shaped, nitrogen-containing part of a nucleotide that carries genetic information; its order (sequence) encodes information.

Purines

Nitrogenous bases with two rings (larger); adenine (A) and guanine (G).

Pyrimidines

Nitrogenous bases with one ring (smaller); cytosine (C), thymine (T, in DNA), and uracil (U, in RNA).

Thymine (T) vs. Uracil (U)

Thymine is used in DNA and uracil is used in RNA; in RNA base pairing, adenine (A) pairs with uracil (U) instead of thymine (T).

Sugar-phosphate backbone

The repeating structural framework of a nucleic acid strand made of alternating sugars and phosphates; bases attach to the sugars and stick out from the backbone.

Phosphodiester bond

The covalent bond that links nucleotides in a strand by connecting the phosphate of one nucleotide to the 3′ carbon (OH) of the next nucleotide’s sugar.

5′ and 3′ ends

The two chemically distinct ends of a nucleic acid strand: the 5′ end has a free phosphate, and the 3′ end has a free hydroxyl (-OH) on the sugar.

Directionality (polarity)

The property that nucleic acid strands have a 5′ end and a 3′ end; many enzymes add nucleotides only to the 3′ end.

Dehydration synthesis (condensation reaction)

A polymer-forming reaction where a covalent bond forms and water is produced; nucleotides polymerize to build the sugar-phosphate backbone.

Complementary base pairing

Specific pairing between bases in double-stranded regions: A pairs with T in DNA (or with U in RNA), and G pairs with C; supports accurate copying.

Hydrogen bonds (in base pairing)

Weak bonds that hold complementary bases together across strands; A–T (or A–U) forms 2 hydrogen bonds, and G–C forms 3.

Antiparallel

Describes the orientation of two strands in double-stranded DNA (and many paired regions): one runs 5′→3′ while the other runs 3′→5′.

GC content

The proportion of G–C base pairs in a DNA region; higher GC content often correlates with greater thermal stability because G–C pairs form three hydrogen bonds.

Deoxyribose vs. ribose

Deoxyribose (in DNA) has an H at the 2′ carbon, while ribose (in RNA) has an OH at the 2′ carbon; the 2′ OH makes RNA generally less chemically stable.

Double helix

The typical shape of DNA: two antiparallel strands twisted together, with sugar-phosphate backbones on the outside and stacked bases inside.

Template strand

A DNA strand used to build a complementary nucleic acid strand (e.g., during RNA synthesis); the new strand is complementary and antiparallel to the template.

mRNA (messenger RNA)

An RNA type that carries a copy of a gene’s information from DNA to ribosomes for protein production.

rRNA (ribosomal RNA)

An RNA type that is a major structural and functional component of ribosomes involved in protein synthesis.

tRNA (transfer RNA)

An RNA type that delivers amino acids to the ribosome during protein synthesis; its shape helps it match codons to the correct amino acids.

Central dogma (DNA → RNA → protein)

A common framework for information flow in cells: DNA stores information, RNA acts as a complementary copy/functional intermediary, and proteins are produced based on that information.