Lecture 16- Introduction to Nucleic Acids

nucleic acids

-are polymers of nucleotides can be viewed in essentially the same way as proteins structurally

-arguable the most important carriers of information in a cell; DNA acts as the entire blueprint of an organism

-nucleic acids have directionality (5’ and 3’ ends)

primary structure of nucleic acids

-determined by its order of the bases in the polynucleotide sequence

secondary structure of nucleic acids

-is the three-dimensional conformation of the backbone

tertiary structure of nucleic acids

-the supercoiling of the molecule

the 2 principal types of nucleic acids encountered in living systems

-deoxyribonucleic acid (DNA)

-ribonucleic acid (RNA)

deoxyribonucleic acid (DNA)

-polymer composed of 2 nucleotide chains that coil around each other to form a double helix

-contains no 2’ hydroxyl group on the ribose sugar

-much more stable compared to RNA, but can only occupy a few different structural configurations and is primarily used as a blueprint for RNA transcription and DNA replication

ribonucleic acid (RNA)

-polymer composed of a single nucleotide chain (typically)

-contain a 2’ hydroxyl group on the ribose sugar

-presence of a 2’ hydroxyl group has enormous implications for nucleotide molecules

-comparatively less stable, but can carry out a vast array of different function (catalysis, protein translation, trafficking, gene slicing)

nucleotide monomer

-the smallest single unit of a polynucleotide or nucleic acid

-individual nucleotides consist of 3 components that are covalently bonded: nitrogenous base, ribose sugar, phosphoric acid

nitrogenous base

-these are planer heterocycles made from pyrimidine or purine foundational structures

ribose sugar

-this is a furanose ring that is stabilized by substituted anomeric carbons

phosphoric acid

-there can be 1 to 3 phosphates within the nucleotide that are linked to the sugar via a phosphodiester bond through the 5’ or 3’ carbon

nucleic acid bases

-are purines (2 ring) or pyrimidines (1 ring); these aromatic structures are flat and planar

-the base is the nitrogenous aromatic purine and pyrimidines, not an alkaline compound

3 pyrimidine bases

-thymine

-cytosine

-uracil

2 purine bases

-adenine

-guanine

nucleic acid base

-purine or pyrimidine ring structure (no sugar or phosphate)

nucleoside

-nucleic acid base with a ribose sugar attached (no phosphate)

nucleotide

-full structure of nucleoside with phosphate ester

ribonucleotides

-contain a hydroxyl (-OH) at position C2’; this is called a “2 prime hydroxyl”

deoxyribonucleotides

-do not contain a hydroxyl (-OH) at position C2’; rather, these have a hydrogen (-H) at this position

glycosidic linkages

-almost always in the β-configuration at the C1’ of the ribose ring

linking nucleotide monomers through phosphodiesters bonds

-allows for the generation of nucleic acids

-nucleotides are linked from the C5’ on 1 deoxyribose to the C3’ of the next deoxyribose

-RNA contains ribose and uracil (U); DNA contains deoxyribose and thymine (T)

structure of DNA

-first published in 1953 by Watson and Crick

-DNA double helix is formed when 2 complementary polynucleotide strands are hydrogen bonded and wrap around each other in opposite directions

Chargaff’s rule

-A pairs with T and G pairs with C

-requires a balance of pyrimidines and purines during nucleotide biosynthesis

-A and Ts bond with 2 hydrogen bonds

-Cs and Gs bond with 3 hydrogen bonds

-DNA/RNA strands with more Cs and Gs have a higher melting point

Structure of the double helix suggests

-a mechanism for copying genetic information to potential progeny (daughter strand)

-copying of the genetic information requires that the DNA double helix is unwound; this means that denaturation of the hydrogen bonds is needed to separate (melt) the strands

Melting temperature of DNA (T_m)

-can be determined experimentally by measuring the increase in absorbance of 26o nm light as the bases become exposed

-this effect is called hyperchromicity

-because CG pairs contain 3 hydrogen bonds, higher CG content among similarly sized strands generally coincides with a higher melting temperature

3 principal types of RNA found in cells

-Transfer RNA (tRNA)

-Ribosomal RNA (rRNA)

-messenger RNA (mRNA)

Transfer RNA (tRNA)

-functions as an adapter molecule in protein synthesis; transports individual amino acids to the ribosome

Ribosomal RNA (rRNA)

-contained within the ribosome along with protein; through to be the catalytic component of the ribosome

messenger RNA (mRNA)

-acts as an intermediary coding molecule between DNA and protein; directs the amino acid sequence in proteins

information transfer in cells

DNA→RNA→protein

-want replication with high fidelity/not a lot of mutations