Appendix F: DNA and RN DNA and RNA DNA Structure DNA A polymer made up of monomers called nucleotides. Long strands Structure: form a double helix which runs antiparallel. Charge: DNA is negatively charged due to its phosphate backbone. Nucleotides: Each nucleotide has three parts: • 5-carbon sugar, (DNA uses deoxyribose) • Nitrogen-rich base • Phosphate Group • Note: A nucleoside lacks the phosphate group Example of a Nucleotide: Guanosine diphosphate (GDP) Nucleotide Pairs: • Adenine – Thymine 2 H-bonds • Guanine – Cytosine 3 H-bonds, stronger • Note: RNA has U instead of T Structural DNA backbone is held together via phosphodiester bonds that Bonds: form between the sugar and the phosphate groups. Hydrogen bonds hold the nucleotide bases together inside the double helix. Pyrimidines 1 ring: A pyrimidine ring Purines 2 rings: A pyrimidine ring fused to an imidazole ring Nitrogenous Bases Cytosine (DNA only) Thymine Uracil Adenine Guanine (RNA only) Pairing: purine + pyrimidine = uniform width purine + purine = too wide pyrimidine + pyrimidine = too narrow DNA Double DNA double helix has a diameter of 20 angstroms. Helix Width: RNA: Also a polymer of nucleotides, but differs from DNA in three major respects: 1. RNA is usually single stranded. 2. The sugar in RNA is ribose, which is more reactive than deoxyribose. 3. The nitrogenous base is Uracil (U), not thymine (T). mRNA: Messenger. Encodes AA sequence. tRNA: Transfer. Brings AA to ribosomes during translation. rRNA: Ribosomal. Form ribosomes. snRNA: Small nuclear. Form spliceosomes that remove introns. Levels of DNA Packaging • • • • Strands of DNA wrap around a histone protein forming nucleosomes Nucleosomes coil together forming chromatin Chromatin loops and coils together forming supercoils Supercoils bunch together forming chromosomes DNA vs. RNA Proofreading: DNA replication has proofreading while RNA transcription does not. This makes DNA replication more accurate than RNA transcription. Stability: RNA is less stable than DNA because it contains the sugar ribose compared to DNA’s deoxyribose. As a result, mRNA degrades rapidly in the cytoplasm. DNA Double Helix Histones at the core of Nucleosomes Chromatin Supercoiled DNA Chromosome