What is the regulation of lac operon by repressor referred to?
1. It consists of five genes coding for five enzymes catalysing the synthesis of tryptophan and thus constitute an anabolic pathway.
2. The structure of the operon is more or less similar to that of the lac operon and control of regulator gene R, promoter gene, operator gene O and structural gene. It manifests a functional variation.
3. Here the R gene product (equivalent to the gene of lac operon) produces protein, which by itself unable to operate. This is referred to as apo-repressor.
4. In presence of tryptophan (co-repressor), the functional repressor is formed that now binds to the operator, preventing the transcription of the operon and production of tryptophan.
â² Fig. 2.7. Tryptophan-Operon.
If the amount of tryptophan increases then it acts as a corepressor. The corepressor joins with the repressor synthesized by regulator gene. The complex of repressor and corepressor binds with operator and switch off the operon as further synthesis of enzyme coded by structural genes stops.
What is the basic purpose of translation
It consists of two main events :
(A) Transcription and (B) Translation.
Transcription. It is the copying of a complementary messenger RNA strand on DNA strand, the strand which acts as template is termed master strand or sense strand. The base pairing follow A = U and G=C. In eukaryotes transcription occurs in the nucleus. The mRNA synthesized come out into cytoplasm through nuclear pore.
Transcription requires a template (double stranded DNA), ribonucleoside triphosphates (ATP, GTP, CTP and UTP (RNA polymerase II and divalent metal ion.)
RNA chain starts at promoter region and ends at terminator region and synthesized in 5’→3’ direction. Chain termination is brought by Rho factor.
A Fig. 2.11—A. Sigma factor and coenzyme join to form RNA polymerase.
B. RNA polymerase attaches to initiation site 8 factor help in detecting the promoter or initiation site.
C. DNA unwinds, core enzyme catalyzes the synthesis of mRNA 8 factor separates out.
D. mRNA chain elongates.
E. Termination of chain brought by 8 factors.
F. mRNA strand transcribed by DNA.