Question

1. The rate of transcription in bacteria is approximately 40-80 nt per second for an actively transcribed gene. The rate of translation in bacteria is approximately 20 amino acids per second.

1. How long will it take to transcribe and translate a protein of 550 amino acids?

2. Considering that a codon consists of three bases, the rates of transcription and translation in bacteria are fairly similar on an amino acid basis. (In other words, of the total time needed to express a gene, about half is transcription and about half is translation.) What might be a consequence if one process occurred more rapidly than the other in bacteria?
3. In eukaryotes, the rates of transcription and translation are not as similar as they appear to be in bacteria, with translation being somewhat slower. What is the reason that the rates of transcription and translation in eukaryotes might not be similar?

Answer 1

Answer a.
Assuming two disconnected processes, i.e., no simultaneous occurrence of transcription and translation on the same elongating transcript, we see that 550 amino acids posses 550 * 3 = 1650 nucleotides coding transcript, because each 3 nucleotides make 1 codon encoding for 1 amino acid.
Processing 1650 nucleotides takes 1650 / {(40+80)/2} = 27.5 seconds.
Following which 550 amino acids, will take 550/20 = 27.5 seconds more.
In total, the time taken will be 27.5 + 27.5 = 55 seconds

Answer b.
If transcription occurred more rapidly than translation, then many mRNA transcripts will accumulate in the transcriptome, and by the time their proteins are translated, there would be a significant delay in feedback regulated processes, both for repressors and inducers.
If the translation occurs more rapidly than transcription then that is wasteful machinery with too many parts dedicated to the end product because the bottleneck step will always be the precursor transcription step, forming transcripts at a much slower rate than their protein products.

Answer c.
There are many reasons for translation being much slower in complex organisms such as eukaryotes, such as the presence of introns requiring post-processing to obtain a functional mRNA molecule before it can go for translation. Apart from that, we have quite a large variety of alternatively spliced mRNA transcripts, which further increases the pool of mRNA's as opposed to their protein products.
Also, eukaryotes have monocistronic genes which means each gene codes for one polypeptide unlike prokaryotes, that further adds a large pool of mRNA transcripts. This diversity implies that we need much faster transcription speeds than translation speeds.