Chapter 33 Flashcards
What are the functions of nucleotides and nucleic acids?
Nucleic acids are polymers of nucleotides used for:
- Storage of genetic info (DNA).
- Transmission of genetic info (mRNA).
- Processing of genetic information (ribozymes).
- Protein synthesis (tRNA and rRNA).
Nucleotides are also used in the monomer form for cellular functions:
- Energy for metabolism (ATP).
- Enzyme cofactors (NAD+).
- Signal transduction (cAMP).
What are the 3 components of nucleotides?
Nitrogenous base.
Sugar (pentose).
Phosphate (at least one).
What are nitrogenous bases?
Aromatic heterocycles containing C and N atoms.
Planar or almost planar structures.
Absorb UV light around 260 nm.
Derivatives of pyrimidine or purine.
What are the purines?
Adenine and Guanine.
What are the pyrimidines?
Cytosine, Uracil, and Thymine.
What is the phosphate group?
Negatively charges at neutral pH because of phosphate.
Typically attached to 5’ position.
- Nucleic acids are built using the 5’-triphosphates version of the nucleotide: ATP, GTP, TTP, CTP.
- Two of the three phosphates used for building nucleic acids form a leaving group, and completed nucleic acids contain one phosphate moiety per nucleotide.
May be attached to other positions for specialized function.
Name coenzymes (other functions of nucleotides).
Coenzyme A, Nicotinamide adenine dinucleotide (NAD+), Flavin adenine dinucleotide (FAD).
Explain polynucleotides.
Phosphodiester bonds covalently link successive nucleotide units.
DNA chain has a 5’ end, which is usually attached to a phosphoryl group, and a 3’ end, which is usually a free hydroxyl group.
Explain the complementarity of DNA strands.
Two chains differ in sequence (read from 5’ to 3’).
Two chains are complementary,
Two chains run antiparallel.
Explain how the double helix is stabilized by hydrogen bonds and the hydrophobic effect.
Adenine always forms hydrogen bonds with thymine, whereas guanine forms hydrogen bonds with cytosine.
Helix is stabilized by hydrogen bonds between base pairs as well as by hydrophobic interactions and van der Waals forces, called stacking forces, between adjacent bases.
Explain DNA denaturation.
Covalent bonds remain intact.
- Genetic code remains intact.
Hydrogen bonds are broken.
- Two strands separate.
Base stacking is lost.
- UV absorbance increases.
Denaturation can be induced by high temperature, or change in pH.
May be reversible: annealing.
What is thermal DNA denaturation (melting)?
DNA exists as double helix at normal temperatures.
Two DNA strands dissociate at elevated temperatures.
Two strands re-anneal when the temperature is lowered.
Reversible thermal denaturation and annealing form the basis for the polymerase chain reaction.
DNA denaturation is commonly monitored by UV spectrophotometry at 260 nm.
What are factors that affect DNA denaturation?
Midpoint of melting (Tm) depends on base composition.
- High CG increases Tm.
Tm depends on DNA length.
- Longer DNA has higher Tm.
- Important for short DNA.
Tm depends on pH and ionic strength.
- High salt increases Tm.
Why is denaturation of large DNA molecules not uniform?
AT-rich regions melt at a lower temperature than GC-rich regions.
