Nucleic Acids, DNA Structure, and Transcription

$260\,\text{nm}$ UV Absorption

The standard wavelength of ultraviolet light used to measure the quantity of DNA in a sample, enabled by the aromatic delocalized pi systems of nucleotide bases.

Pyrimidines

The class of single-ring nucleobases highlighted in the lecture, consisting specifically of cytosine, thymine, and uracil.

Purines

The larger, double-ring nucleobases involved in genetic information storage, consisting of adenine and guanine.

N-glycosidic Bond

The linkage that joins the anomeric carbon of a pentose sugar in the beta configuration to a nitrogen atom on a purine or pyrimidine base.

Thymine Dimers

A type of DNA lesion caused by UV light absorption in which adjacent thymines dimerize into a central cyclic structure, potentially causing errors in repair and replication.

Ames Test

A biological assay used to assess whether a chemical compound is a mutagen by measuring the reversion of amino-acid-deficient mutant bacteria when cultured on media lacking leucine or lysine.

B-DNA

The standard double-helical form of DNA characterized by major and minor grooves, a width of approximately $20\,\text{\AA}$, a rise of about $3.4\,\text{\AA}$ between base pairs, and about $36\,\text{\AA}$ per helical turn.

A-DNA

A squashed and wider structural form of DNA in which the base pairs are tilted at an angle of roughly $20^\circ$, compared to the approximately $6^\circ$ tilt observed in B-DNA.

Z-DNA

A left-handed helical conformation of DNA that can arise under low water conditions or significant negative supercoiling in large chromosomal regions.

Histones

Specialized proteins that associate into nucleosomes to package and compactly wind DNA inside eukaryotic cells, spooling segments so they remain organized until unspooled for transcription.

Coding Strand

The DNA strand oriented in the $5'$ to $3'$ direction whose sequence matches the codons present in the transcribed messenger RNA, with thymine in place of uracil.

Template Strand

The non-coding DNA strand read by RNA polymerase to synthesize a complementary strand of messenger RNA during transcription.

Wobble Position

The third nucleotide position within a codon where base variation does not alter the encoded amino acid