Nucleic acids

– DNA (deoxyribonucleic acid) stores genetic information and is a polymer made of nucleotides. RNA also uses nucleotides and is used by some viruses instead of DNA.

– A nucleotide consists of a pentose sugar, phosphate group, and nitrogenous base. The phosphate is bonded to a specific point on the sugar.

– Nucleotides form phosphodiester bonds (covalent) between the phosphate of one and the sugar of another to create a polymer of DNA or RNA.

– DNA uses four bases: adenine (A), thymine (T), cytosine (C), guanine (G). RNA replaces thymine with uracil (U). Purines = A and G; Pyrimidines = C, T, U.

– DNA is double-stranded, RNA is single-stranded. The sequence of nitrogenous bases (A, T, C, G) determines genetic traits like binary code.

– DNA and RNA strands are formed through condensation reactions (removal of water), enabling phosphodiester bonds.

– DNA strands are antiparallel and joined by hydrogen bonds between complementary bases: A–T (2 bonds) and G–C (3 bonds).

– Structural differences: DNA = double-stranded, deoxyribose sugar, A–T base pairing. RNA = single-stranded, ribose sugar, A–U pairing.

– DNA replication is semi-conservative: each new DNA has one original and one new strand, using complementary base pairing.

– Despite huge genetic diversity from base sequence variation, all organisms share a universal genetic code, supporting the theory of a common ancestor.

DNA strands are anti-parallel, running 5’ to 3’ on one strand and 3’ to 5’ on the other.

• Nucleotides can only be added to the 3’ end, not the 5’.

• This orientation is critical for understanding DNA replication and synthesis.

Complementary base pairing involves purines (A, G) pairing with pyrimidines (T, C).

• This results in equal-length base pairs, contributing to DNA molecule stability.

Nucleosomes stabilize DNA in eukaryotic cells (not prokaryotes).

• DNA wraps twice around an octamer of histone proteins, with H1 histone aiding further compaction.

• Linker DNA connects nucleosomes.

Hershey and Chase experiment showed DNA is the genetic material.

• Used viruses labeled with radioactive sulfur (proteins) and phosphorus (DNA).

• Only phosphorus was found in infected cells, proving DNA is injected and hereditary.

Chargaff’s experiment disproved the tetranucleotide hypothesis (equal amounts of A, T, G, C).

• Found that A = T and G = C within species, but total base compositions differ across species.

• Led to Chargaff’s Rule and showed both unity (base-pair ratios) and diversity (species differences) in DNA.