Exam 2

Exam 2

1. Central dogma

   • 1. Replication
     • 2. Transcription
       • 3. Translation

2. Nucleotide

   • Nucleoside: has sugar & base only
   • have bases: planar, aromatic, & heterocyclic
     • Purines: double ringed
       • aromaticity & electron rich environment of purines & pyrimidines enable them to undergo keto enol tautomerism
         • keto tautomers of uracil, thymine, and guanine predominate at pH 7
           • enol form of cytosine predominates at pH 7
     • Pyrimidine: single ringed
   • Aromaticity also accounts for strong absorption of UV light
   • Protonation states of the nitrogens determines whether they can serve as H-bond donors or acceptors
   • Properties of Pyrimidines and Purines Can Be Traced to Their Electron-Rich Nature
   • The phosphate group may be attached to the sugar at either of two positions – if at C3’, it is called a 3’ nucleotide – if at C5’, it is called a 5’ nucleotide
   • DNA and RNA are polymers that are linked by phosphodiester bonds that bridge the 3’ and 5’ positions on the sugars of adjacent nucleotides
     • Sequence is always read 5' to 3' in proteins its N-terminus to C-terminus
   • DNA and RNA are polyanions
     • Phosphates in these polynucleotides are acidic (lose proton easily)
     • At physiological pH, nucleic acids are polyanions

3. Differences in DNA vs RNA Structure

   • RNA is important for its numerous functions… But if RNA is damaged, it can be degraded and regenerated.

4. Why does DNA contain thymine?

   • Deamination of cytosine forms uracil
     • In DNA if C is deaminated to form U, repair systems recognize U as foreign and needing to be replaced by C. U is foreign in DNA because DNA uses T in place of U