Highlights Nucleic Acids and DNA Synthesis
1. DNA can be described as relaxed or supercoiled. Relaxed DNA has 10.5 base pairs per turn of the helix. If one alters the number of turn of the helix for a DNA and keeps the base pairs the same, the 10.5 base pairs per turn value will change and cause strain for the DNA. This will result in the DNA responding by coiling itself more tightly - supercoiling.
2. Supercoiling may stress regions of the DNA to unwind. Since A-T base pairs have fewer hydrogen bonds (2) than G-C base pairs (3), regions of DNA rich in A-T may tend to come apart when supercoiling occurs.
3. Chromatin is a complex of DNA and proteins called histones.
4. Histones are proteins that associate with DNA and allow it to coil up. DNA-histone complexes are called nucleosomes. When you put many nucleosomes together, you get chromatin. Chromosomes contain chromatin.
5. DNA strands can easily be separated by heat, acid, or base.
6. Single strands of nucleic acid (DNA or RNA) absorb light at 260 nm more strongly than double strands of nucleic acid in the same quantity. Thus, one can monitor the denaturation of nucleic acid by monitoring its absorbance at 260 nm as the denaturation proceeds.
7. The transition temperature (Tm) for a nucleic acid denaturation is the mid-point of the Tm transition
8. Renaturation is the opposite of denaturation. Renaturation puts single strands together. Both denaturation and renaturation are important for anyone running the polymerase chain reaction (PCR).
9. RNA differs from DNA in having ribose (instead of deoxyribose) and uracil (instead of thymine).
10. There are three main types of RNA found in all cells. They are transfer RNA (tRNA), ribosomal RNA (rRNA), and messenger RNA (mRNA). We will talk about these later. For now, you should know that mRNAs carry the genetic code necessary for making proteins, tRNAs carry amino acids for translation and rRNAs are components of ribosomes that make proteins.
11. Genes (= segment of DNA that when copied into mRNA can be made into protein) are organized in clusters (called operons) in prokaryotes, but are not clustered in eukaryotes.
12. In eukaryotes (but not prokaryotes), genes are in segments. The 'pieces' must be put together to make a functional gene. This assembly of pieces occurs in the RNA. The segments that are removed during this process of assembling pieces (splicing) are called introns, whereas the pieces that remain after splicing are called exons.
13. tRNAs carry an amino acid at one end and a three base sequence called an anti-codon loop at the other. The anticodon loop is complementary to the three base codon found in mRNA. The amino acid is specific for each anti-codon and thus for each codon, as well. The genetic code requires matching the right amino acid with the right anti-codon on a tRNA. Please note that the genetic code is written as the pairing of the codon with the amino acid that the tRNA brings to it.
14. Ribosomal RNA is found in ribosomes and helps the ribosome by catalyzing the formation of peptide bonds during protein synthesis. We'll see at least one other function soon.
15. Information in cells flows from DNA to RNA (called transcription) and from RNA to Protein (called translation). This is known as the Central Dogma. Some retroviruses have an enzyme called reverse transcriptase that allows them to make DNA from RNA. This last process is called reverse transcription.