CHAPTER 17. FROM GENE TO PROTEIN. OVERVIEW. The information content of DNA is in the form of specific sequences of nucleotides along the DNA strands. The inherited instructions in DNA direct protein synthesis.
FROM GENE TO PROTEIN
1. Further research determined that the function of a gene was to direct production of a specific enzyme.
2. Cells synthesize and degrade organic compounds by metabolic pathways with each step being catalyzed by a specific enzyme.
3. George Beadle and Boris Ephrussi(1930’s) studied eye color in Drosophila and speculated that mutations affecting eye color block pigment synthesis by preventing enzyme production at certain steps in the pigment synthesis pathway.
4. Beadle and Edward Tatum demonstrated the relationship between genes and enzymes by studying Neurosporacrassa (red bread mold).
In prokaryotes, mRNA produced by transcription is immediately translated without more processing
DNA transcriptionRNA translation protein
1. Triplets of nucleotide bases are the smallest units that can code for all the amino acids.
2. Three consecutive bases, a codon, specify an amino acid.
3. Marshall Nirenberg determined the first match of the codon UUU for the amino acid phenylalanine.
4. 61 of 64 triplets code for amino acids,AUGcodes for the amino acid methionine and also indicates the start of translation.
5. Three codons signal the termination of translation.
6. During transcription, one of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript
7. During translation, the mRNA base codons, are read in the 5 to 3 direction
8. Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide
9. No codon specifies more than one amino acid
10. Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced
11. Genetic code is shared universally by nearly all organisms which indicates that this code was established very early in life’s history.
A. Molecular Components of Transcription
1. RNA Polymerase and Initiation of Transcription
Transcriptionfactors are proteins that help RNA polymerase recognize and bind to the promoter region so transcription can begin
1. Each end of a pre-mRNA molecule is modified in a particular way:
2. These modifications share several functions:
3. Three properties of RNA enable it to function as an enzyme:
A. Molecular Components of Translation
1. A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
2. Molecules of tRNA are not identical:
mRNA binding site
1. The three stages of translation:
2. All three stages require protein “factors” that aid in the translation process
1. During the elongation stage, amino acids are added one by one to the preceding amino acid
2. Each addition involves proteins called elongation factors and occurs in three steps:
1. A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome(or polysome)
2. Polyribosomes enable a cell to make many copies of a polypeptide very quickly
7. Polypeptides destined for the ER or for secretion are marked by a signal peptide
8. A signal-recognition particle (SRP) binds to the signal peptide
9. The SRP brings the signal peptide and its ribosome to the ER
A. Comparing Gene Expression in Bacteria, Archaea, and Eukarya
1. The idea of the gene itself is a unifying concept of life
2. We have considered a gene as:
3. In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule