Amy S. answered • 07/24/19
MS in Molecular Genetics with 10 years of Teaching Experience
DNA and RNA polymerase recognize start sequences within the DNA and RNA respectively. The start codon in DNA is TAC and the start codon in mRNA is AUG. However the chromosome must be unwound first even to begin DNA replication in the nucleus. Chromosomes are tightly packed in the form of heterochromatin. Some areas of chromosomes have DNA that is methylated, which represses transcription because transcription initiation factors can't bind to the promoter to initiate transcription. So in this case, the gene will not be transcribed because RNA polymerase can't attach to the promoter because it is sterically hindered. However, if the gene is not metylated and has an open reading frame where the start codon is recognized by the polymerase then the chances of it being fully transcribed into a protein are higher. This is why methylation patterns in human development are so important. There are certain genes that we want turned on only during developmental processes, and then they should be silenced for the rest of our natural lives. One such example are the genes that encode for the webbing between our hands and feet. If these genes were never silenced we would be born with webbed hands and feet. Great question by the way! Thank you for asking!
Dwight H.
Is there a polymerase for each gene, or for each chromosome? Does the polymerase have to search through the entire genome looking for the right one? And what is the criteria for the right gene to transcribe?07/25/19
Amy S.
Dwight, the cell is a very complicated living thing just like we humans are. k regulation does not always start at the gene level. Often times it starts very far upstream. Cells are able to sense their environments through the cell membrane. For example, if we take a look the classic lac operon.... Even though humans (eukaryotes) don't have operons, the lac operon is a good example because it explains how gene regulation works at a more simplistic level. Genes are composed of promotors, operators, repressors, transcription factors as well as a host of other regulatory elements. The lac repressor functions to bind the operator portion of the gene when there is no lactose present. So in this instance RNA polymerase will not bind to the transcription start site because it is bound by the repressor. Only when lactose is present will allolactose bind to the lac repressor to remove it from the operator. Once the repressor is removed, THEN and only then can RNA polymerase bind and know what gene to transcribe. Cells are "smart" they don't waste resources by turning on unnecessary genes when they are not needed. Side note: I would take a look at Khan Academy, they are a great resource for explaining and showing science videos. I use them to help explain concepts to my students in class. In eukaryotes transcription is even more complicated. In general eukaryotic RNA polymerases rely on promoter sequences (generally TATA boxes at -25 to -35 base pairs upstream of the transcription start site) to drive the direction of transcription as well as what strand will be transcribed. Many transcription factors act to guide the RNA polymerase at this site to initiate transcription. If you are interested further I would look into transcription factors that encode certain genes. This may help you along in your learning. As far as eukaryotic RNA polymerases there are three and they have different functions each. RNAP2 is responsible for mRNA production. In prokaryotes they are different and serve even different functions. So to answer your question, "Is there a polymerase for each gene or chromosome", there is not. Just remember, the "criteria" for the right gene to be transcribed is sensed upstream, often far upstream, by the cell sensing its environment. This sets off an entire cellular cascade from the cellular membrane through the cytoplasm into the nucleus where transcription can either be upregulated or downregulated. I hope this helps Dwight! Great question and thank you for making me thin too! Ms S.07/25/19
Dwight H.
During any moment of normal processing a cell will require a particular protein to be made. How does the polymerase find that particular gene out of all the many chromosomes containing thousands of genes? I'm not asking about gene transcription in general. Out of all the thousands of genes that are not metylated, how does the polymerase know which gene to transcribe?07/25/19