1. There are three RNA polymerases in eukaryotes. RNA Polymerase I in eukaryotes catalyzes transcription of the large rRNAs. RNA Polymerase II in eukaryotes catalyzes transcription of the mRNAs. RNA Polymerase III in eukaryotes catalyzes transcription of tRNAs and other small RNAs.
2. Note that many proteins bind promoters in eukaryotes, whereas only a couple of proteins bind prokaryotic promoters. This is partly due to the fact that eukaryotes have more genes to control than prokaryotes and that eukaryotic genes are not organized in operons.
3. Enhancers are eukaryotic proteins that bind DNA sequence elements located as far away as several thousand base pairs from a gene. They they can help activate transcription of the relevant gene. Transcription factors are specific to specific tissues, thus allowing specific genes to be active in some tissues (which have the transcription factors) and not active in others (that lack the appropriate transcription factors).
4. Proteins that bind to DNA often have common structural features. (Common structural features shared between proteins are called domains) Many of them have a feature known as a 'zinc finger,' A zinc finger arises from zinc interacting with cysteine and histidine, causing a portion of the polypeptide chain to extend, much like a finger. Another common structural feature is a leucine zipper, which involves placement of leucines every seven residues in a protein. The leucines interact with similarly spaced leucines (hydrophobic interactions) in another protein. A third common structural feature of DNA binding proteins is called Helix-turn-Helix.
6. Splicing is a process that occurs in eukaryotic cells only in which RNAs have sections removed (introns) and the remaining sections joined (spliced) together. The remaining sections after splicing are called exons. Splicing allows eukaryotic cells to mix and match exons and create different proteins in different tissues from the same DNA.
7. tRNAs are modified chemically in both prokaryotes and eukaryotes. mRNAs are not modified in prokaryotes, but in eukaryotes, they are capped at the 5' end and have a poly-A "tail" at the 3' end.
End of material for exam 2 HERE
1. Translation is the process whereby proteins are synthesized from the information contained in mRNAs. We can think of protein synthesis (translation) as occurring in 4 steps - 1) activation of amino acids; 2) chain initiation (start of translation); 3) chain elongation; and 4) chain termination.
2. The information in mRNA is encoded as the Genetic Code. The genetic code specifies how nucleic acid information is converted to make a protein. Information in mRNA is encoded in groups of three nucleotides (called a codon or a triplet). There are 64 possible codons. 61 of them code for amino acids. Three of them (UAA, UGA, UAG) tell the ribosome to STOP translating. A start codon (AUG) is among the 61 above.
3. The genetic code is universal (all cells use the same code - only very rare exception have very slightly altered codes)
4. Codons you should know include the codon that always starts translation (AUG - specifies methionine) and three codons that STOP translation - UAA, UAG, UGA.
5. The third base of many (but not all) codons is unnecessary for identifying an amino acid. For example, glycine can be coded by GGG, GGA, GGC, and GGU. This could be written as GGX, where X stood for any base. The third base of a codon is therefore called the 'wobble' position of a codon, as it is not very specific. Note that in translation, the anticodon of the tRNA is base paired to the codon on the mRNA in an antiparallel fashion. Thus, the wobble base of the anticodon is the one at the 5' end of the anticodon.
6. tRNAs function to carry amino acids to the ribosomes for incorporation into proteins during the process of translation.