chp 13- genetic code and transcription
Why was it believed the genetic code was made up of triplet sequences and was nonoverlapping?
- Genetic code is a set of rules by which information encoded in genetic material is translated into proteins. -The code consists of 3 bases called codons made up of the 4 bases. - There are 64 codons that code for 20 amino acids. - The triplet nature of code was suggested because, given 20 amino acids, minimal use of the 4 DNA bases would require 3 bases per amino acid, thus providing 64 possible code words. -4 bases of DNA will form a triplet codon i..e. 4^3 = 64 possible combinations in triplets can be made out of these 4 bases. - non overlapping nature of the code was suggested because of the limitations that an overlapping code would place on the peptide sequences. - Each amino acid is coded by a triplet codon. -The codes are read in successive manner without overlapping. - In addition, with an overlapping code, single nucleotide substitutions would alter 2 adjacent amino acids which was not observed in mutant proteins that had been examined at the time. - The triplet, non overlapping of the code along with other characteristics were demonstrated primarily by studies on frameshift mutations in the k12 of E.coli.
what evidence do we have that the expression of the information encoded in DNA involves an RNA intermediate?
DNA doesnt directly produce proteins but instead mRNA transcript is utilized first. Researchers found the RNA in viruses was used instead of DNA, to form a protein product.
how were the experimentally derived triplet codon assignments verified in studies using bacteriophage MS2?
by matching the sequence of the RNA phage MS2 with protein products of MS2
How do we know that the initial transcript of a eukaryotic gene contains noncoding sequences that must be removed before accurate translation into proteins can occur?
researchers observed the splicing activities on RNA eukaryotic cells. They also found that RNA contains characteristics giving them enzymatic properties to splice themselves. This led to the discovery the RNA required to be spliced before leaving the nucleus.
