Structure of DNA

Purines are double-ringed; pairing two purines would exceed the 2 nm diameter, while pairing two pyrimidines would make the helix too narrow.

With 10 base pairs per complete helical turn of 3.4 nm, the step distance between consecutive base pairs is 3.4/10=0.34 nm.

G-C pairs are linked by three hydrogen bonds, which require more thermal energy to disrupt than the two hydrogen bonds holding A-T pairs together.

Introns are intervening, non-translated sequences within a gene that are transcribed into pre-mRNA but removed by splicing.

Transcription generates an RNA transcript that is complementary and anti-parallel to the DNA template strand, swapping thymine for uracil.

Chargaff’s rules dictate that [A]=[T] and [G]=[C]; therefore, the sum of purines (A+G) must equal the sum of pyrimidines (T+C).

DNA polymerases require a free 3'-OH group to form a phosphodiester bond with the 5' phosphate group of an incoming dNTP.

One complete helical turn spans a longitudinal length of 3.4 nm (34 A˚) and contains approximately 10 base pairs.

Human genes contain introns, which bacteria cannot slice out because they lack spliceosomes, leading to a flawed, non-functional protein.

A codon is a triplet of bases that specifies a particular amino acid or a stop signal during protein translation.

Cytosine and thymine are single-ring pyrimidines, whereas adenine and guanine are double-ring purines.

Structural genes carry the specific nucleotide blueprints required to assemble amino acids into a polypeptide chain on a ribosome.

The 3' end of a nucleic acid strand terminates at the third carbon of the sugar ring, which carries a free hydroxyl group.

While hydrogen bonds hold the pairs together horizontally, vertical base-stacking interactions insulate the hydrophobic bases from water.

One strand runs in the 5' to 3' direction while its complementary partner runs in the 3' to 5' direction.

Guanine pairs with cytosine via three hydrogen bonds, creating a more thermally stable bond than the adenine-thymine pair.

A gene is the fundamental structural and functional unit of heredity, consisting of a nucleotide sequence that carries the code for a protein.

The 5' end of a DNA or RNA strand terminates in a phosphate group attached to the 5' carbon of the pentose sugar.

Adenine (a purine) always pairs with thymine (a pyrimidine) using two hydrogen bonds in standard Watson-Crick base pairing.