As diagrammed above, first RNA polymerase binds during initiation to the unwound DNA structure, forming the beginning of a new RNA strand. Then, the new strand is elongated by addition of nucleotides complimentary to the DNA strand being transcribed. Finally, termination occurs, and the RNA drops off the DNA and is free to be transported to the cytoplasm for translation.
Promoters are sequences in the DNA just upstream of transcripts that define the sites of initiation. The role of the promoter is to attract the necessary components which will then attract the RNA polymerase to the correct start site so transcription of one gene can be initiated.
Figure 8. This is a rough diagram of a promoter sequence on a DNA molecule.
Promoters determine when a gene is transcribed and turned on. Different factors go into recruiting the RNA polymerase, and this logic is specific to every gene. Promoters are typically located about 30-50 nucleotides long and are located directly upstream (to the left of) the gene. However, there might be a larger region (about 1-2000 nucleotides long) upstream of the promoter which interacts with the different factors that in the end allow the RNA polymerase to bind to the promoter itself.
III. Genetics in the 20th century
Genetics in the 20th century studied and shed light mainly on two general questions of m o l e c u l a r b i o l o g y : h o w i n f o r m a t i o n i s s t o r e d a n d t r a n s l a t e d t o a w o r k i n g o r g a n i s m , a n d h o w e v o l u t i o n o c c u r s . T h e f i r s t q u e s t i o n w a s c o m p l e t e l y a n s w e r e d i n t h e 2 0 t h c e n t u r y , a s a l r e a d y d i s c u s s e d i n t h e p r e c e d i n g s e c t i o n s . T h e s e c o n d q u e s t i o n i s o l d e r , b u t i t s m o l e c u l a r b a s i s w a s a l s o u n d e r s t o o d d u r i n g t h e 2 0 t h century. The general answer is that evolution occurs as a result of random changes in DNA, some of which are rejected because they are harmful to the individual and some are accepted and over time get incorporated into the genetic makeup of the populations which can be observed today.
Speciation, or separation of one species into two separate ones, occurs as a result of various possible scenarios. For example, it might occur when an island, which is home to a particular species, gets separated in such a way that the representatives from different halves do not interact with each other. Slowly, two different species might evolve, because the random beneficial mutations that occur in one half will never get incorporated into the other half of the population.
One very important thing to understand about the 20th century is that genetics was mainly a descriptive science. We learned how things worked, but not precise quantities. For example, we know what a gene is, but don’t know all the genes that exist. We know how a gene gets turned on, but not how many genes are turned on at different times in the cell. So as a result, geneticists can tell very interesting stories about the central dogma and evolution, but they don’t know enough to reverse-engineer and change the genetic make-up, for example.