Science Weblecture for Unit 60
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I've used the Biology Hypertext at MIT for years for this lecture, but MIT has taken it down. Please read the MIT disclaimer at the MIT Biology Hypertext book site.
Luckily for us, Jay Ward inspired someone to create the WayBack machine, so along with Sherman, we will use the pages anyway.
The following sections cover "The Central Dogma" of biology: the structure of DNA and its Genetic code. Each link has multiple pages: be sure to follow the links at the site to read subsequent pages. WARNING: This is a LOT of reading. Don't try to memorize all the details: just get the main points of the story. If you can answer the homework questions, you have enough to go on for now. You'll get all this again in detail in a good survey course in biology.
Charles Darwin recognized that for his theory to work, he would need some mechanism to explain how organisms inherited the characteristics in whole or in part from their parents. He was unaware of Mendel's studies, but even those would have explained only the results of inheritance, not the processes involved. For several decades after the theory of evolution became generally accepted in scientific circles as a "likely story", biologists struggled to determine exactly how a cell passes its information to its daughter cells.
The following reading recaps the details of the experiments leading to the discovery of DNA. Pay attention to the terminology and the types of proof which convinced the experimenters and their peers of the results.
Once DNA had been identified as the "inheritance" material, the search was on to learn how it was structured, how such a simple compound could carry so much information. DNA instructions not only control the growth and functions within the cell, but they have to be copied when new cells are created. Every cell in your body has all the DNA necessary to describe your whole body system, but not every cell expresses all that information; so another question was how could DNA control the formation of specialized functions?
The answers to all of these lie in the structure of DNA. It is like a zipper. Phosphate ions and sugar molecules (with five carbon atoms instead of the usual six) called ribose alternate to form chain. Attached to each sugar molecule is one of four nucleic acids: cytosine, thymine, adenosine, guanine.
It takes two chains to actually make a strand of DNA. The chains are attached to each other by the nucleic acids. Because of the structure of the acids, cytosine can only combine with guanine, and adenosine can only combine with thymine. The two strands twist around each other, forming a double helix shape like a spiral staircase or twisted ladder.
Read the link below to find out more details about the structure of DNA.
DNA's double helix structure means that one side of the chain contains all the information to make both sides! If we pull the chains apart through the middle of the nucleic acid pairs (think of a zipper coming apart), we can use each half as a template to make a whole new strand, and wind up with two complete copies.
DNA contains the instructions to make up the thousands of enzymes that control all the functions of your body. These proteins are sequences of about 20 amino acids (the exact number and types vary from species to species). One of the great advances in genetics was the realization that any three-pair sequence of nucleic acids identifies one amino acid. Since each component of the pair defines the other half (if we have guanine, the other half has to be cytosine), we can determine the pair sequence by looking at one one half of the pair. A three-nucleic-acid sequence is called a codon.
Using combination mathematics, we can determine that for each of the three nucleic acids in a codon, we can have one of four possible acids. So there are 34 = 64 possible different codons. Each codon identifies one amino acid to be made by the cell, but we only need to be able to make 20 different amino acids. So we have more codons than amino acids and that means some of amino acids have more than one codon code. This gives the system redundancy and reduces the problems of mutations a bit: a mutation that changes the final nucleic acid of a codon often does not change the amino acid specified by the original codon. To see in detail how this works, read the following link.
It is possible for the replication mechanism that creates new DNA to fail, resulting in a change in the codon sequence. The results can vary from no problem (the codon still codes for the same amino acid), to serious (the codon now signifies a stop sequence, cutting short the pattern for an entire protein).
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