Transcription in procaryotes

In the first phase of gene expression, one strand of DNA acts as a template for the production of a complementary strand of RNA. In the outline that follows, we shall describe how mRNA is synthesised, but remember that sometimes the product of transcription is rRNA or tRNA. An important point to note is that the coding strand is not the same for all genes; some are encoded on one strand, some on the other. Whereas in DNA replication the whole molecule is copied, an RNA transcript is made only of specific sections of DNA, typically single genes. The enzyme RNA polymerase, unlike DNA polymerase, is able to use completely single-stranded material, that is, no primer is required. It is able to synthesise an mRNA chain from scratch, according to the coded sequence on the template, and working in the 5' to 3' direction (Figure 11.7). In order to do this the RNA polymerase needs instructions for when to start and finish. First, it recognises a short sequence of DNA called a promoter, which occurs upstream of a gene. A protein cofactor called sigma (a) assists in attachment to this, and is released

Promoter

Terminator mmmmmmrnrnmrn *

RNA polymerase b)

5' rTTTTTTN

mRNA

5' rTTTTTTN

mRNA

5' 11111111 in 1111111111 3'

Figure 11.7 Transcription in procaryotes. (a) The gene to be transcribed is flanked by a promoter and a terminator sequence. (b) Following localised unwinding of the double-stranded DNA, one strand acts as a template for RNA polymerase to make a complementary copy of mRNA. (c) The mRNA is extended; only the most recently copied part remains associated with the DNA. (d) After reaching the termination sequence, both RNA polymerase and newly synthesised mRNA detach from the DNA, which reverts to its fully double-stranded state again shortly after transcription commences. The promoter tells the RNA polymerase where transcription should start, and also on which strand. The efficiency with which a promoter binds the RNA polymerase determines how frequently a particular gene will be transcribed. The promoter comprises two parts, one 10 bases upstream (known as the Pribnow box), and the other 35 bases upstream (Figure 11.8). RNA polymerase binds to the promoter, and the double helix of the DNA is caused to unwind a little at a time, exposing the coding sequence on one strand. Ribonucleotides are added one by one to form a growing RNA chain, according to the sequence on the template; this occurs at a rate of some 30-50 nucleotides per second. Remember from Chapter 2 that RNA has uracil rather than thymine, so that a 'U' is incorporated into the mRNA whenever an 'A' appears on the template. Transcription stops when a terminator sequence is recognised by the RNA polymerase; both the enzyme and the newly synthesised mRNA are released. Unlike the promoter sequence, the terminator is not transcribed. Some termination sequences are dependent on the presence of a protein called the rho factor (p). Groups of bacterial proteins having related functions may have their genes grouped together. Only the last one has a termination sequence, so a single, contiguous mRNA is produced, encoding several proteins (polycistronic mRNA).

Figure 11.8 The promoter sequence in E. coli. RNA polymerase attaches at a point 35 nucleotides upstream of the start of transcription; as the DNA unwinds, it binds to the Pribnow box, situated at -10 nucleotides. The actual sequences may differ from gene to gene; the sequences shown in the figure are consensus sequences. Note: only the non-template DNA strand is shown

Figure 11.8 The promoter sequence in E. coli. RNA polymerase attaches at a point 35 nucleotides upstream of the start of transcription; as the DNA unwinds, it binds to the Pribnow box, situated at -10 nucleotides. The actual sequences may differ from gene to gene; the sequences shown in the figure are consensus sequences. Note: only the non-template DNA strand is shown

Transcription in eucaryotes proceeds along similar lines, but with certain differences. The most important of these is that in eucaryotes, the product of transcription does not act directly as mRNA, but must be modified before it can undergo translation. This is because of the presence within eucaryotic genes of DNA sequences not involved in coding for amino acids. These are called in-trons (c.f. coding sequences = exons), and are removed to give the final mRNA by a process of RNA splicing (Figure 11.9).

Eucaryotic genes generally contain non-coding sequences (introns) in between the coding sequences (exons).

Was this article helpful?

0 0
How To Win Your War Against Bronchitis

How To Win Your War Against Bronchitis

Sick And Tired Of Your Constant Cough? Is Your Bad Immune System Leading You To The Path Of Fever And Sore Chest? You Sure Have A Reason To Panic BronchitisThere Is Always A Way Out And, This Is It Finally Discover Some Of The Most Effective Tips That Can Curb Bronchitis, And Its Repeated Bouts Learn How To Keep The Chronic Cough, And Sore Chest Away Breathe Free, And Feel The Whiff Of Fresh Air, With No Hassles

Get My Free Ebook


Post a comment