DavidCadogan.ca Forums » DavidCadogan.ca

the First 10 Billion Years Biologi Molekuler

(1 post)
  • Started 12 years ago by 87h9so54s7

  1. Anonymous
    Unregistered

    the First 10 Billion Years Biologi Molekuler,http://houdeblog.com/cheapjerseys/<br><br>Cosmologists believe that the universe began some 14 billion years ago with the gigantic 'primordial fireball called the Big Bang. Mathematical models suggest that after about 4 billion years galaxies began to fragment from the clouds of gas emitted by the Big Bang, and that within our own galaxy the solar nebula condensed to form the Sun and its planets about 4.6 billion years ago ( Figure 15.1 ). The early Earth was covered with water and it was in this vast planetary ocean that the first biochemical systems appeared,leedsconcerts.co.uk, cellular life being well established by the time land masses began to appear, some 3.5 billion years ago. But cellular life was a relatively late stage in biochemical evolution, being preceded by selfreplicating polynucleotides that were the progenitors of the first genomes. We must begin our study of genome evolution with these precellular systems.<br><br>15.1.1. The origins of genomes<br><br>The first oceans are thought to have had a similar salt composition to those of today but the Earth atmosphere,winesforspice.co.uk, and hence the dissolved gases in the oceans, was very different. The oxygen content of the atmosphere remained very low until photosynthesis evolved, and to begin with the most abundant gases were probably methane and ammonia. Experiments attempting to recreate the conditions in the ancient atmosphere have shown that electrical discharges in a methaneammonia mixture result in chemical synthesis of a range of amino acids,aldisplays.co.uk, including alanine,http://winesforspice.co.uk, glycine, valine and several of the others found in proteins (Miller, 1953). Hydrogen cyanide and formaldehyde are also formed, these participating in additional reactions to give other amino acids, as well as purines, pyrimidines and,wholesale jerseys, in less abundance,christian louboutin on sale, sugars. At least some of the building blocks of biomolecules could therefore have accumulated in the ancient chemosphere.<br><br>The first biochemical systems were centered on RNA<br><br>Polymerization of the building blocks into biomolecules might have occurred in the oceans or could have been promoted by the repeated condensation and drying of droplets of water in clouds (Woese, 1979). Alternatively, polymerization might have taken place on solid surfaces, perhaps making use of monomers immobilized on clay particles (Wchtershuser, 1988),christian louboutin oulet, or in hydrothermal vents (Wchtershuser, 1992). The precise mechanism need not concern us; what is important is that it is possible to envisage purely geochemical processes that could lead to synthesis of polymeric biomolecules similar to the ones found in living systems. It is the next steps that we must worry about. We have to go from a random collection of biomolecules to an ordered assemblage that displays at least some of the biochemical properties that we associate with life. These steps have never been reproduced experimentally and our ideas are therefore based mainly on speculation tempered by a certain amount of computer simulation. One problem is that the speculations are unconstrained because the global ocean could have contained as many as 1010 biomolecules per liter and we can allow a billion years for the necessary events to take place. This means that even the most improbable scenarios cannot be dismissed out of hand and a way through the resulting maze has been difficult to find.<br><br>Progress was initially stalled by the apparent requirement that polynucleotides and polypeptides must work in harness in order to produce a selfreproducing biochemical system. This is because proteins are required to catalyze biochemical reactions but cannot carry out their own selfreplication. Polynucleotides can specify the synthesis of proteins and selfreplicate, but it was thought that they could do neither without the aid of proteins. It appeared that the biochemical system would have to spring fully formed from the random collection of biomolecules because any intermediate stage could not be perpetuated. The major breakthrough came in the mid1980s when it was discovered that RNA can have catalytic activity. Those ribozymes that are known today carry out three types of biochemical reaction:<br><br>Selfcleavage, as displayed by the selfsplicing Group I, II and III introns and by some virus genomes ( Table 10.4 and Section 10.2.3);<br><br>Cleavage of other RNAs (as carried out by, for example, RNase P; Table 10.4 and Section 10.2.2);<br><br>Synthesis of peptide bonds, by the rRNA component of the ribosome (Section 11.2.3 and Research Briefing 11.1).<br><br>In the test tube,christian louboutin sale, synthetic RNA molecules have been shown to carry out other biologically relevant reactions such as synthesis of ribonucleotides (Unrau and Bartel, 1998), synthesis and copying of RNA molecules (Ekland and Bartel, 1996; Johnston et al., 2001) and transfer of an RNAbound amino acid to a second amino acid forming a dipeptide, in a manner analogous to the role of tRNA in protein synthesis (Section 11.1; Lohse and Szostak, 1996). The discovery of these catalytic properties solved the polynucleotidepolypeptide dilemma by showing that the first biochemical systems could have been centered entirely on RNA (Bartel and Unrau,cheap uggs, 1999).<br><br>Ideas about the RNA world have taken shape in recent years (Robertson and Ellington, 1998). We now envisage that RNA molecules initially replicated in a slow and haphazard fashion simply by acting as templates for binding of complementary nucleotides which polymerized spontaneously ( Figure 15.2 ). This process would have been very inaccurate so a variety of RNA sequences would have been generated,ugg uk sale, eventually leading to one or more with nascent ribozyme properties that were able to direct their own, more accurate selfreplication. It is possible that a form of natural selection operated so that the most efficient replicating systems began to predominate, as has been shown to occur in experimental systems. A greater accuracy in replication would have enabled RNAs to increase in length without losing their sequence specificity, providing the potential for more sophisticated catalytic properties, possibly culminating in structures as complex as presentday Group I introns (see Figure 10.26 ) and ribosomal RNAs (see Figure 11.11 ).<br><br>To call these RNAs 'genomes is a little fanciful, but the term protogenome has attractions as a descriptor for molecules that are selfreplicating and able to direct simple biochemical reactions. These reactions might have included energy metabolism, based,cheap nfl jerseys, as today,christian louboutin oulet uk, on the release of free energy by hydrolysis of the phosphatephosphate bonds in the ribonucleotides ATP and GTP, and the reactions might have become compartmentalized within lipid membranes, forming the first celllike structures. There are difficulties in envisaging how longchain unbranched lipids could form by chemical or ribozymecatalyzed reactions, but once present in sufficient quantities they would have assembled spontaneously into membranes, possibly encapsulating one or more protogenomes and providing the RNAs with an enclosed environment in which more controlled biochemical reactions could be carried out.<br><br>How did the RNA world develop into the DNA world? The first major change was probably the development of protein enzymes, which supplemented, and eventually replaced, most of the catalytic activities of ribozymes (Freeland et al.,louboutin on sale, 1999). There are several unanswered questions relating to this stage of biochemical evolution, including the reason why the transition from RNA to protein occurred in the first place. Originally,uggs on sale, it was assumed that the 20 amino acids in polypeptides provided proteins with greater chemical variability than the four ribonucleotides in RNA, enabling protein enzymes to catalyze a broader range of biochemical reactions, but this explanation has become less attractive as more and more ribozymecatalyzed reactions have been demonstrated in the test tube. A more recent suggestion is that protein catalysis is more efficient because of the inherent flexibility of folded polypeptides compared with the greater rigidity of basepaired RNAs (Csermely, 1997). Alternatively, enclosure of RNA protogenomes within membrane vesicles could have prompted the evolution of the first proteins,louboutin oulet uk, because RNA molecules are hydrophilic and must be given a hydrophobic coat, for instance by attachment to peptide molecules, before being able to pass through or become integrated into a membrane (Walter et al., 2000).<br><br>The transition to protein catalysis demanded a radical shift in the function of the RNA protogenomes. Rather than being directly responsible for the biochemical reactions occurring in the early celllike structures, the protogenomes became coding molecules whose main function was to specify the construction of the catalytic proteins. Whether the ribozymes themselves became coding molecules, or coding molecules were synthesized by the ribozymes is not known, although the most persuasive theories about the origins of translation and the genetic code suggest that the latter alternative is more likely to be correct ( Figure 15.3 ; Szathmry, 1993). Whatever the mechanism, the result was the paradoxical situation whereby the RNA protogenomes had abandoned their roles as enzymes, which they were good at, and taken on a coding function for which they were less well suited because of the relative instability of the RNA phosphodiester bond,http://centrical.co.uk, resulting from the indirect effect of the 2OH group (Section 1.1.2). A transfer of the coding function to the more stable DNA seems almost inevitable and would not have been difficult to achieve, reduction of ribonucleotides giving deoxyribonucleotides which could then be polymerized into copies of the RNA protogenomes by a reversetranscriptasecatalyzed reaction ( Figure 15.4 ). The replacement of uracil with its methylated derivative thymine probably conferred even more stability on the DNA polynucleotide, and the adoption of doublestranded DNA as the coding molecule was almost certainly prompted by the possibility of repairing DNA damage by copying the partner strand (Sections 14.2.2 and 14.2.3).<br><br>According to this scenario, the first DNA genomes comprised many separate molecules, each specifying a single protein and each therefore equivalent to a single gene. The linking together of these genes into the first chromosomes, which could have occurred either before or after the transition to DNA, would have improved the efficiency of gene distribution during cell division, as it is easier to organize the equal distribution of a few large chromosomes than many separate genes. As with most stages in early genome evolution, several different mechanisms by which genes might have become linked have been proposed (Szathmry and Maynard Smith, 1993).<br><br>If the experimental simulations and computer models are correct then it is likely that the initial stages in biochemical evolution occurred many times in parallel in the oceans or atmosphere of the early Earth. It is therefore quite possible that 'life arose on more than one occasion, even though all presentday organisms appear to derive from a single origin. This single origin is indicated by the remarkable similarity between the basic molecular biological and biochemical mechanisms in bacterial, archaeal and eukaryotic cells. To take just one example, there is no obvious biological or chemical reason why any particular triplet of nucleotides should code for any particular amino acid, but the genetic code, although not universal, is virtually the same in all organisms that have been studied. If these organisms derived from more than one origin then we would anticipate two or more very different codes.<br><br>If multiple origins are possible, but modern life is derived from just one, then at what stage did this particular biochemical system begin to predominate? The question cannot be answered precisely, but the most likely scenario is that the predominant system was the first to develop the means to synthesize protein enzymes and therefore probably also the first to adopt a DNA genome. The greater catalytic potential and more accurate replication conferred by protein enzymes and DNA genomes would have given these cells a significant advantage compared with those still containing RNA protogenomes. The DNARNAprotein cells would have multiplied more rapidly, enabling them to outcompete the RNA cells for nutrients which,blossomsandaccents.com/jerseys.html, before long, would have included the RNA cells themselves.<br><br>Are life forms based on informational molecules other than DNA and RNA possible? Orgel (2000) has reviewed the possibility that RNA was preceded by some other informational molecule at the very earliest period of biochemical evolution and concluded that a pyranosyl version of RNA, in which the sugar takes on a slightly different structure, might be a better choice than normal RNA for an early protogenome because the basepaired molecules that it forms are more stable (Beier et al., 1999; Eschenmoser, 1999). The same is true of peptide nucleic acid (PNA),http://www.airsoftontario.com, a polynucleotide analog in which the sugarphosphate backbone is replaced by amide bonds ( Figure 15.5 ). PNAs have been synthesized in the test tube and have been shown to form base pairs with normal polynucleotides. However,louboutin sale, there are no indications that either pyranosyl RNA or PNA were more likely than RNA to have evolved in the prebiotic soup.：

    Posted 12 years ago #

RSS feed for this topic

Reply

You must log in to post.