Highlights Lecture #19 Spring 2017

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. Three factors combine to make eukaryotic transcription more complex than prokaryotic transcription – 1) differentiation; 2) DNA wrapped in histones in eukaryotes; and 3) larger eukaryotic genomes.

3. Transcription factors are proteins that bind to specific sequences of DNA and affect transcription.

4. Enhancers are eukaryotic DNA sequence elements located as far away as several thousand base pairs from a gene. They can help activate transcription of the relevant gene when they are bound by transcription factor proteins. Enhancers 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).

5. 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

6. Splicing is a process that occurs in eukaryotic cells only. In it, 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.

End of material for exam 2 here

Start of material for exam 3 here

Highlights Translation

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 amino acids to build a protein. Information in mRNA is encoded in groups of three nucleotides (called a codon). 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 exceptions 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.

7. Amino acid activation can be thought of as the first step in the process of translation. Amino acids are activated by covalently linking them to the 3′ end of specific tRNAs. Enzymes called aminoacyl-tRNA-synthetases catalyze the covalent linkage of amino acids to tRNAs. There is one aminoacyl-tRNA synthetase for each amino acid. We decided in class to called aminoacyl-tRNA synthetases “Kevin”.

8. Amino acids are not linked randomly to tRNAs. Rather, aminacyl-tRNA-synthetases “look” at the anti-codon of a tRNA to decide which amino acid to put on the 3′ end of each tRNA. In this way, each tRNA with a particular anti-codon always has the same amino acid on the end of it. Therefore when a tRNA base pairs with a messenger RNA in translation, it is bringing the right amino acid to be incorporated into the growing protein chain.

9. The first amino acid put into proteins is methionine, since that is the amino acid specified in the genetic code for the first codon of a protein – AUG.

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