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My book has a statement:
A single strand of mRNA is capable of forming a number of different polypeptide chains.
In my opinion this statement is wrong because a single strand of mRNA will have same sequence of codons. So, every time it gets translated, it produces same polypeptide chain. But, according to my book this statement is correct.
Am I wrong or is my book wrong?
I believe, your book refers to polycistronic mRNA. This is mRNA where multiple genes are encoded together in one mRNA and are often (but not necessarily) translated one after another. This is mainly found in prokaryotes where the proteins encoded on the same mRNA often form a metabolic pathway together.
You can come at mRNA from two directions. Remember that human genes are largely monocistronic, in other words they often code for a single protein. If we look at prokaryotes, however, polycistronic genes such as those in operons are very much possible. From a single lac mRNA you can get multiple proteins.
For humans with monocistronic genes, you can code for multiple isoforms, however. A good example is protein kinase C: You have PKC-α, PKC-δ, and so forth. So how can this be? In part, you can look at splicing. What gets initially transcribed from your human gene is a pre-mRNA, in that the entire gene contains introns and exons. The spliceosome knows where to cut out introns, and you technically get a mature mRNA that is made of only exons and codes for protein.
In special cases, however, RNA-binding proteins and—at times in concert with, at times to the detriment of—the spliceosome machinery alternatively splice some other combination of introns and exons (sometimes, introns are kept!). And so from the PKC gene, you still get PKC, but a different isoform of it. The different proteins that different types of cells or tissues express that can interact with the splicing machinery are capable of taking the same genetic information, the same pre-mRNA, and make different polypeptides out of it.
Apart from the polycistronic case there is another possibility of one mRNA yielding multiple proteins. By one mRNA, I mean that the RNA is not in any way altered because of RNA editing or other mechanisms.
Translation can be initiated at alternative sites leading to production of different protein products (Touriol et al., 2003). VEGF is one of the genes that exhibits this phenomenon. However, the precise factors that regulate the choice of the translation start site are not very well known. The basic mechanism, however, seems to be masking of the alternative start sites by different factors such as secondary structures and/or trans regulatory factors.
Similarly a stop codon can be skipped. This is frequently seen in the case of the UAG (Amber) stop codon. This phenomenon is called amber suppression (see this post). Again, the factors that regulate this phenomenon are not well known.
Having said all this, I am 100% sure that your textbook refers to polycistronic mRNAs only.
