Lecture 7- the genetic code

Lecture 7- the genetic code - DNA: Transcription,...

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DNA: Transcription, Translation and Mutation In the last lecture we sketched the outline of the central dogma. We showed how information is encoded in the sequence of bases in the DNA and how that information determines, via the tRNA, the sequence of amino acids in the protein encoded by a particular gene. But the information in the DNA is vast, 3 billion base pairs worth in humans, and there is no table of contents. In the first part of this lecture we discuss in more detail how the genetic information is extracted. First of all, as we alluded to, the RNA polymerase makes an RNA polymer that is complementary to one gene at a time. This process, where the DNA is transcribed into messenger RNA (mRNA), is called transcription . The resulting mRNA strand starts slightly before the stretch of DNA that actually encodes the protein and extends somewhat past the coding region then stops. So the RNA polymerase must know where genes start and stop. Furthermore, the two strands of DNA that constitute a gene have different sequences (the reverse complement of each other). So the RNA polymerase must also know which strand to transcribe to produce a functional protein. We have names to distinguish the strands of a coding region. The strand which is not transcribed but which has the same sequence as the mRNA is called the sense strand . The transcribed strand that has a sequence complementary to the mRNA is the template strand (or "antisense strand"). But how does the RNA polymerase know where a gene begins and which strand to transcribe? The DNA has special sequences called promoters that the RNA polymerase recognizes as the beginning of a gene. But the promoter sequence itself is not transcribed. Here is a case where the DNA transmits information directly to a protein. Not information that changes the sequence of the protein but information that tells the protein, "This is a gene, start transcribing here on this strand in this direction." Similarly, there is a termination sequence in the DNA telling the RNA polymerase when to stop.
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Thus, as you scan a stretch of DNA from one end to the other, you will find (as a hypothetical example) that the first gene is transcribed from the top strand, the next from the bottom, bottom, top, top, top bottom etc. in a basically random sequence (the DNA strand that is transcribed is often indicated by showing the direction of the transcript; because transcription is always 5' to 3', the direction indicates the strand). OK so you've made an mRNA and now you need to convert it into protein. This process
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Lecture 7- the genetic code - DNA: Transcription,...

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