Repeated sequences, problems sequencing

Other repeat sequences also appear to offer problems. Many begin in C and end in G. Thus, we have large numbers of repeats of CCG and CGG (that code for proline, and arginine) that are believed to give a disposition to certain nerve degeneration-related diseases. 

The Huntington scenario paints a sad picture for our ability to cure those with the disease. Are we to modify each chromosome 4 in the billions of cells in our brain, and if so, how It is not the sequence itself, but rather the length of the sequence, that is the problem. All of us have some of these repeat sequences and they are necessary for other essential activities. 

The chromosome end problem is solved in another way in eukaryotes. As discussed in Section B,l, telomeres (chromosome ends) contain repeated sequences of variable length. One DNA strand is always G-rich. For example, in human cells the sequence 5 -(TTAGGG) -3 , where n maybe 20, oc- curs at... 

There are at least two reasons. One is the problem of concerted evolution of repeat sequences which may complicate phylogenetic interpretation. A second potentially confusing phenomenon is the fact that many, perhaps most, repetitive sequences are (or were) mobile pieces of DNA that have the ability to move within genomes and likely between genomes. If such sequences can move across species boundaries, i.e. be horizontally transfered, they would be misleading in analyses of phylogenies. 

As Monod states in his treatise. On Symmetry and Function in Biological Systems, One may set aside the simple problem of fibrous proteins. Being used as scaffolding, shrouds or halyards, they fulfill these requirements by adopting relatively simple types of translational symmetries. Therefore, it was not anticipated that positive cooperativity, the effect Monod thought to be the second secret of life, second only to the structure of DNA, would be most beautifully demonstrated by designed variations of a repeating sequence of the mammalian elastic fiber based on translational symmetry. 

One of the most fantastic achievements of this scientific era has been deciphering the amino acid code. As Garen, Crick, and Nirenberg pointed out repeatedly, the problem amounted to translating a 4-letter alphabet (adenine, guanine, thymine, and cytosine) into a 20-letter alphabet—the 20 amino acids in the cell proteins—or rather finding how messenger RNA s made of only four different nucleotides dictate the sequence of 21 amino acids in the polypeptide chains [162-170]. 

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