MadSci Network: Molecular Biology |
During transcription can mRNA transcribe several genes at the same time? The answer depends on the interpretation of the question. To ensure that you get the desired information, and (perhaps more importantly) to help clarify the situation, I will begin with a general description and definition of terms before listing some precise questions and their answers. The central dogma of molecular genetics is very important and worth memorizing. It states that the order of information flow is from DNA to RNA to protein. But it is a little more subtle than that because the DNA does not make the RNA and the RNA does not make the protein. It is more mechanistic to say: DNA is transcribed to make RNA that is translated to make protein. But how to remember the order of these phonetically simmilar processes? DNA is very important and we don't want it galavanting around the cell. Instead, we make a copy: a *transcript*. This transcript (mRNA) is smaller than, and slightly modified from, the original (DNA) but still contains the same information in basically the same way. To remember the structural similarity of the mRNA and DNA, notice that they are both nucleic acids (the NA ind D*NA* and in R*NA*.) Protein is very different from DNA and RNA. Protein is not a nucleic acid and uses a completely different language to represent the same information. We must *translate* the language of the nucleic acids into the language of the protein. Now we make a final addition to our representation of the central dogma: DNA directs the process of transcription that is effected by RNA polymerase (II) to create mRNA that directs the process of translation that is effeted by ribosomes to create protein. In the preceeding paragraph, the term *directed* indicates that the sequence of the one directs the sequence of the other, and the term *effected* indicates the machinery that completes this task. Also note that there are three forms of RNA polymerase. It is RNApol(II) that creates mRNA (in the precursor form ... but don't worry too much about that, it's more of a disclaimer.) *** Now for some questions: Can mRNA translate anything? No. mRNA is a transcript that can only *direct* translation. Can a ribosome translate anything? Yes. A ribosome is the machinery that can translate mRNA (but not DNA.) Can multiple genes be transcribed at once? Yes. There are many copies of RNA polymerase. Each copy can effect transcription, so many independant transcription events can (and do) occur simultaneously. Can multiple genes be transcribed together to create a single mRNA? Depends. This is usually associated with prokaryotes (bacteria) and the resulting mRNA is called a polycistronic mRNA. Most of the mRNA created by prokaryotes is polycictronic. Eukaryotic polycistronic mRNA is very rare. Eukaryotic mRNAs generally represent a single gene (monocistronic mRNA.) Can translation occur along a mRNA that is currently being transcribed? Depends. This can only occur in prokaryotes because they lack a nuclear membrane. In eukaryotes, the nuclear membrane excludes ribosomes. If there are no ribosomes in the nucleus, then there are no machines to effect translation. In eukaryotes, translation only occurs after transcription is completed and the mRNA has moved out of the nucleus. Can a single mRNA transcript act as the template for multiple ribosomes simultaneously? Yes. But remember that each ribosome can only create one protein during a single translation event. The complex of a single mRNA together with many actively translating ribosomes is called a polysome or polyribosome. In eukaryotes polysomes are only found in association with membranes. In prokaryotes, polysomes are found free in the cytoplasm. Can a single cell contain many different monocistronic mRNAs all at once? Absolutely! This is essential to the survival of a cell. Many genes are in the process of transcription or translation or both all at the same time within the same cell. Is transcription regulated? Yes. This regulation is very important; consider that your skin cells produce different proteins than your liver cells, yet they both contain the same DNA sequence. This differential gene expression is acheived through regulation at many parts of the processes involved in protein production. *** It appears to me that your question refers to the differences between prokaryotic and eukaryotic processes with respect to polycistronicity. I will therefore expand a bit on the reanslation of polycistronic mRNA. *PROKARYOTIC* polycistronic translation: The bacterial site of translational initiation is an AUG sequence. But this is a message to the ribosome that is already on the transcript; the ribosome must first bind to the transcript. Ribosome binding occurs at an area called a Shine-Dalgarno sequence, which is a purine (adenine or guanine) rich sequence of 3-9 nucleotides which can base pair to sequences within the ribosomal RNA. This lines up the ribosome to begin translation at the AUG start codon that is usually located 5-10 bases downstream. In order to have translation in prokaryotes, we simply need a Shine- Dalgerno sequence upstream of the AUG codon that is to signal translational initiation. This ribosome binding occurs rather infrequently without any additional help. The mRNA must also contain additional helper sequences (enhancers, promoters, operators ...) which can be learned by studying the topic: operons. *EUKARYOTIC* polycistronic translation: Polycistronic translation in eukaryotes can occur by a few mechanisms. One way to acheive multiple proteins from a single polycistronic mRNA is for the first start codon (AUG) to be in a poor context for translational initiation. This means that due to structural configurations of the mRNA (determined by mRNA sequence), some ribosomes will miss the first start codon and slide down the message to the second start codon, at which they will intitiate translation. Remember that the ribosome binds somewhere upstream of the translation initiation point and slides down the transcript until it encounters an AUG codon. The upstream binding of ribosomes is a little different in eukaryotes than it is in prokaryotes. Eukaryotic mRNA is usually capped at the 5' end somewhere after transcription but before translation (post-transcriptional mRNA processing) and this 5' cap binds to factors that increase the chance that a ribosome will attatch to the transcript in this area. The 5' cap is refered to as an inverted guanosine cap and is actually a 7-methyl- guanosine triphosphate attatched to the 5' end of the unprocessed mRNA. If this terminology is not yet familiar to you, ask you teacher about the process of translation with respect to codons or look here: htt p://www.madsci.org/posts/archives/oct2000/972943713.Bc.r.html htt p://www.madsci.org/posts/archives/nov2000/973721404.Cb.r.html htt p://www.madsci.org/posts/archives/aug2000/965227551.Cb.r.html And don't worry about the chemical nature of the 5' cap. It is enough to know that it exists. A second mechanism to acheive multiple proteins from a polycistronic mRNA is to have an internal ribosome entry site (IRES) after the first gene transcript but before the second gene transcript. The ribosome will enter the mRNA here and slide down the transcript until it reaches an AUG codon. A third (and not well understood) mechanism involves the enhancement of ribosomes binding to the mRNA downstream from a point of translational termination. This is mechanistically different from the second mechanism because here it is the same ribosome that transcribes both gene transcripts, briefly hopping off the first and then back on before the second. In the previous mechanism, any ribosome may enter at the IRES without having previously translated that particular mRNA. *** FURTHER THOUGHTS: Since bacterial mRNAs are mostly polycistronic, it would make sense if their information was arranged in this manner. How do you think genes would be best be arranged to take advantage of polycistronicity? It would be advantageous for you to read about bacterial operons. The lac (lactose, a sugar) operon is very well studied and is a good place to start. Here are some links. http://www.kean.edu /~rkliman/BIO3705/lacoper.htm http://biochem. senecac.on.ca/humphreys/lacoperon.htm The concept of an operon was deceloped by Jacob and Monod. If you are interested in the development of scientific ideas, this is a wonderful place to start. After you have learned about the lac operon, read this again and try to understand how the lac operon fits into the bacterial model of polycistronicity and how it differs from the eukaryotic models. I am sure you will have more question at that time, but don't despair .. that is a good sign. If you have some specific questions, it means that you understand some parts of the model. Self-directed learning requires the formulation of questions and the ability to recognise what things you understand and what things you do not. It is a skill that will set you free on an adventure whose journey will encompase your life. Good sailing. *** REFERENCES: You can find information on this an most cell biology or molecular cell biology textbooks. The one I am using is "Molecular Cell Biology" by Lodish. You can find this book in any university library and likely in your high school. It is a bestseller. Chris Neale neale@innocent.com
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