Question

6. How is transcription initiated and terminated? What is the importance of the promoter consensus sequence? 7. How are mRNAs modified in eukaryotes? What types of elements are sometimes present in RNA's but 8. What is the genetic code? What are the properties of the triplet codons? What does it mean that the code is How and why do consensus sequences differ from one another? usually not in DNA's? redundant and what useful purpose does such redundancy serve?

Answer 1

QUESTION 8

INITIATION OF TRANSCRIPTION: The RNA polymerase and the three eukaryotic enzymes ( RNA polymerase I, RNA polymerase II and RNA polymerase III in case of eukaryotes) attach, directly or via accessory proteins, to their promoter or core promoter sequences. Next this closed promoter complex is converted into an open promoter complex by breakage of a limited number of hydrogen bonds between base pairs around the transcription initiation site forming the transcription bubble. Finally, the RNA polymerase moves away from the promoter.

TERMINATION OF TRANSCRIPTION:

Bacteria have two termination mechanisms. In Rho-independent transcription termination, RNA transcription stops when the newly synthesized RNA forms a G-C-rich hairpin loop followed by a run of uracil bases. When the hairpin forms, the poly-U transcript is pulled out of the active site of the RNA polymerase, terminating transcription. In Rho-dependent type of termination, a protein factor called Rho destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex.

The termination sequence in eukaryotes is AATAAA followed by GT repeats. The termination is closely related to the post-transcriptional modification. Termination of mRNA synthesis is combined with polyadenylation.

PROMOTER CONSENSUS SEQUENCE: In bacteria, the target sequence for RNA polymerase attachment. In eukaryotes, the term ‘promoter’ is used to describe all the sequences that are important in initiation of transcription of a gene. Without the consensus sequence, the polymerases will mot bind to DNA and transcription will not occur. Even if it does occur, it will not be efficient.

QUESTION 9

MODIFICATION OF mRNA IN EUKARYOTES: The main modifications include intron splicing, capping at 5’ end and polyadenylation at the 3’ .

Uracil is a base which is present only in RNA and is complementary to adenine. RNA also has ribose sugar instead of deoxyribose sugar.

QUESTION 10

The genetic code is the way in which the nucleotide sequence in mRNA (or DNA) specifies the amino acid sequence in protein. Of the 64 codons, 61 specify one of the 20 amino acids. The other 3 -UAG, UGA and UAA codons are chain-terminating codons and do not specify any amino acid. AUG is used in the initiation of protein synthesis and codes for methionine. Amino acids that occur more frequently in proteins have the higher number of codons (e.g. Thr occurs more frequently than His).

Properties:

1. Universal: The genetic code is used by all species.
2. Directional: The sequence of triple-nucleotide codons is read in the direction of 5’ to 3’.
3. Commaless: The coding region in mRNA is read in a continuously without punctuation.
4. Degeneracy: Because 61 codons specify 20 amino acids multiple codons must decode the same amino acid.

Redundancy is the same as degeneracy. Multiple codons code for the same amino acid. For example Leu, Ser, and Arg each is specified by six different codons. If mutations occur, thanks to redundancy, it is likely that the mutation might result in a codon that codes for the same amino acid, thereby nullifying the effect of the mutation. It also helps in the study of mutations.