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3 definitions found
 for Ras
From The Collaborative International Dictionary of English v.0.48 :

  Ras \Ras\ (r[aum]s), n.
     See 2d Reis.
     [1913 Webster]

From The Collaborative International Dictionary of English v.0.48 :

  ras \ras\ (r[aum]s), n. [from rat sarcoma.]
     The name and genetic symbol for a mutant gene that has been
     identified as one of those associated with certains types of
     cancer; -- it is a form of oncogene. It was first observed in
     rats, but analogues have been found in humans and other
     animals.
     [PJC]
  
           During the 1960s and 1970s, a great deal of research
           was done on a class of viruses that affects rodents and
           birds and causes tumors in those species. The
           motivation for a lot of this research was the idea that
           similar viruses might cause tumors in humans, but in
           fact it's turned out that there are very few viruses
           that cause tumors in humans. Nevertheless, the study of
           these rodent viruses has been enormously fruitful in
           helping us to understand human cancer, and that's the
           basis of this story.
           One of the viruses that was studied in those years had
           two peculiarities. One was that it had lost most of the
           genes that it needed to reproduce itself. It could only
           reproduce if a helper virus was present in the same
           cell to supply the missing functions. The second
           peculiarity was that in place of the genes that were
           required for reproduction of the virus was another gene
           that had actually been picked up at some point in the
           history of this virus when it went through rats, and it
           picked up a rat gene and incorporated it into its own
           genome.
           At the same time that a lot of work was going on on
           these viruses, other scientists were studying other
           aspects of tumor formation, in particular, the action
           of carcinogenic agents, chemicals and X-rays and
           ultraviolet light. As you all know, human cells can
           turn into tumor cells under the influence of such
           agents. The tumor-like properties of those cells are
           inherited by all the daughter cells through many
           generations and, moreover, almost all chemicals that
           turn out to be carcinogens are also able to cause
           mutations.
           Another observation was that in tumor cells, many of
           the chromosomes seemed to have altered structures. So,
           all of these observations and others certainly
           suggested that changes in DNA might be involved in the
           development of tumor cells. By about 1980, it became
           possible to test that hypothesis directly.
           If you have human tumor cells produced in laboratory
           dishes or isolated from the tumor itself, then perhaps
           they have a gene or genes in them which is responsible
           for the fact that they're tumor cells. If you isolate
           the DNA from the cells and cut it up into more or less
           gene-sized pieces and then put it on top of mouse cells
           growing in a dish, the mouse cells can take up pieces
           of this DNA, and any mouse cell that picks up a piece
           of DNA that carries on it a gene that can cause a tumor
           will begin to grow like tumor cells, and its progeny
           will grow rapidly and form a tight little cluster on
           the cell.
           Now it's possible to pick such cells off and isolate
           the DNA from them and also separate the human DNA
           sequence that might have caused the tumor-like property
           from the bulk of the mouse sequences and to clone that
           DNA. And when you do that and put that DNA, which is
           now pure sequence, back in mouse cells, many of the
           cells become tumor-like rather than just a rare few.
           And such a gene, such a DNA sequence, bears the name of
           an oncogene.
           When such DNA segments are cloned, the DNA can also be
           used to probe, to find out whether matching DNA
           sequences occur only in tumor cells or whether there
           are similar DNA sequences in normal cells. And the
           answer has been for a whole group of oncogenes, that
           very similar DNA sequences are present in normal cells.
           To find out just how similar, the sequences of the
           normal genes were compared with those from the genes
           that were isolated from these tumor cells.
           The first such oncogene isolated was from a human
           bladder tumor, and everyone was surprised by the
           results. First of all, the gene isolated from the
           bladder tumor was almost identical to the normal human
           gene and almost identical to the gene that was present
           in the tumor virus that infected rodents that I told
           you about before. This gene has become known as "ras",
           because it was originally isolated from rats with
           sarcoma, and it caused sarcomas and it's called that,
           and it's protein is called that. And the only really
           significant difference between the normal human gene,
           the bladder tumor gene, and the rodent virus gene was a
           change in one codon, Codon XII, and therefore a change
           in amino acids.
           So the normal human gene has a sequence GGC, encodes
           the amino acid glycine, and does not cause tumors. But
           the bladder tumor gene has GTC; it encodes valine. The
           rodent virus has AGA; it encodes arginine, and both of
           these cause tumors. In fact, any change that leads to a
           loss of the glycine at Codon XII can change this normal
           gene, ras, into a gene that would cause tumors. So
           there were two different ways in which the ras gene
           turned up. First, as a rat gene in a tumor virus and
           second of all as the gene that could account for the
           tumor-like properties of the bladder tumor.
           Well by now, many of the questions that occurred to the
           scientists working on this have occurred to you. What
           is the ras protein normally (if anything), and what
           does the altered ras protein do that differently, and
           how can a change in one amino acid in a protein change
           cells from normal to tumor cells?
           It turns out that the ras gene and the ras protein are
           important for a lot of things, but more particularly
           for regulating the growth of cells. Normal cells need
           to have a good ras gene in order to grow, in order to
           make new DNA, to time it all right so they don't grow
           out of control. Moreover, the ras gene occurs in
           virtually all living things. For example, yeast cells
           also have two ras genes. If either one of them is
           knocked out, the yeast cells can still grow very well
           and multiply. But if both ras genes are knocked out,
           the yeast cells cannot multiply, and they die.
           Astonishingly, if a human ras gene is applied to these
           yeast cells, it completely takes the place of the
           yeast's own ras genes. So we know from this that the
           ras gene is very important to all living cells and that
           it's probably been around for a couple billion years,
           ever since the very first cells were formed on the
           planet.
           So ras does something important and the question is,
           what does it do? David Golde told you before about
           receptors that span cell membranes that bind molecules
           outside the cell and provide a signal inside the cell,
           and it turns out that what the ras protein does is to
           help convey that signal from the receptor at the
           surface down into the cell and into the gene where it
           results in a change in gene expression. The ras protein
           itself actually sits right under the cell membrane,
           very well positioned to do this.
           Well, how can it do that? To tell you about that I need
           to tell you a couple of things about the ras protein
           and what it does. First of all, ras combines two small
           molecules called GDP and GTP, and they differ only in
           the presence of one more phosphate, three in GTP and
           two in GDP. This G is related; it's in fact the same
           kind of molecule as the G that occurs in DNA. Moreover,
           ras protein can catalyze the removal of one phosphate,
           so you go from ras GTP to ras GDP and a phosphate is
           lost. Furthermore, the ras GDP can lose the GDP and
           pick up the GTP, and there are extra proteins in the
           cell that foster either this exchange, back to GTP or
           this loss of the phosphate to GDP. And the whole trick
           is the ratio of the GTP to the GDP. So if you have ras
           GTP, it's active and it stimulates growth, but if you
           have ras GDP, then it's inactive and you don't
           stimulate growth.
           In fact, the change in Codon XII from a glycine results
           in a change in the amount of ras GTP, so that there's
           more ras GTP collecting in the cell than the ras GDP,
           and therefore the cell is constantly under pressure to
           make DNA and grow and divide. And this is the critical
           reason for this change, this oncogenic change in those
           versions of ras that cause tumors or are related to
           tumor formation as opposed to the natural protein.
           How can that happen, a small change like that? You've
           heard a little bit about the importance of shapes of
           proteins. If one looks closely at the atoms in the
           proteins then you see that the whole shape of the
           protein changes as you go from GTP to GDP.
           Now one ras gene and protein all by itself would be
           interesting, but it turns out that there's a whole
           family of ras genes and ras proteins. Two of them are
           specially similar to the type that I've been
           describing, and mutations in those genes are associated
           with a whole variety of human tumors including some
           that are believed to be the result of the reaction to
           environmental agents.
           A mutant in one of those two related genes, which was
           also first discovered in a tumor virus, is very
           frequently associated with human tumors of the colon
           and rectum. And again, it's Codon XII in that similar
           gene that is altered in the oncogenic form of this kind
           of ras. Tumors of the colon and rectum are the third
           most common human malignancy worldwide, and surgical
           removal of the tumors can actually cure the disease in
           many cases, but only if the tumor is detected very,
           very early. Recent work has shown that you can, in
           fact, detect the change in the gene even by looking at
           the DNA in the stool of people who are suspected of
           having the colon tumor.
           Even though the mutant DNA only occurs in a very small
           percentage of the cells in the stool, namely the cells
           that come from the tumor, not from all the normal cells
           or all the bacterial cells that are there, it is
           possible to amplify the amount of a possible abnormal
           ras gene and test directly for it. So, for example in
           this test, DNA from the stool of patient #1 matched a
           probe for the normal ras gene, but DNA isolated from
           the stool of patient #2 matched a probe not only from a
           normal ras gene but also from a ras gene with a
           mutation at Codon XII, thereby permitting a very early
           diagnosis of a colon tumor and thereby providing real
           hope that such tumors can be detected early, when the
           tumor is small enough to be removed surgically with a
           successful cure.                         --Maxine
                                                    Singer
                                                    (http://www.accessexcellence.org/AB/BA/Ras_Gene_and_Cancer.html)
     [PJC]

From The Collaborative International Dictionary of English v.0.48 :

  Reis \Reis\ (r[imac]s), n. [Ar. ra["i]s head, chief, prince.]
     A common title in the East for a person in authority,
     especially the captain of a ship. [Written also rais and
     ras.]
     [1913 Webster]

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