草叶,我把你西班牙线里的comments和图转过来,再转一篇我看到的文章by Geodakyan,是他这个系列的第一篇,后边的也很好玩儿。我前两天刚好看了一个关于左撇子的录像,又找到一本书,他这个系列里也有一篇关于左右手的。我们这里说对称,但是发现大家在研究的都是为什么不对称。
请教八十一子,这些理论我看着很make sense,只是hypothesis吗?
还有关于分子结构的非对称,除了极个别的,生物体内都是氨基酸左旋,糖右旋,虽然实验室合成的两个方向都可能。这个能给讲讲为什么吗?
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浮生, 当你说到重复性和对称性时, 我一下子想到动物世界,是如何从一个圆圆的细胞长成各个不同的种属型状, 但它们又多拥有共性, 如前后, 头尾,中轴。
复习了一下胚胎学, 所有的animal architecture都具有这样的共性: modularity and pattern repetition,symmetry and polarity,从低等动物如starfish 到人都encoded在胚胎的发育中。
人的那种对这些建筑in awe的情感, 是否来自与机体深埋的同样的机理?
现在我要上班, 呆会看能否上些图, pictures worth thousand words
手的x-ray, 是说明系列的reiterated 构建,不同的只是shape和number。
最后一个彩图是repetition in Fine scale: 这是放大的蝴蝶翅膀的花纹,每个小的unit是细胞level,每个细胞是一个single scale的颜色,collectively, 它们组成一个几何图案,和你及八爷谈的马赛克就是一回事。
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ZT: Evolutionary Theories of Asymmetrization of Organisms, Brain and Body (I)
Features of the symmetry are determined by the isotropy of the environment. Maximal extent of organism symmetry corresponds to a completely isotropic ecological niche.
First organisms on Earth floating in the depths of water unicellular and lower multicellular organisms had the maximum possible spherical symmetry. They appeared approximately 3.5 billon years ago. Anisotropic environment step by step was leading to asymmetry of the organisms.
Asymmetrization along the “top – bottom” axis occurred under the influence of gravity. This led to the appearance of the attached, low-mobile forms (plants and coelenterates) that had radial symmetry.
Asymmetrization along the “front – back” axis occurred due to the interaction with space, when rapid motion was required (to escape from the predator, or to chase a pray). As a result, the main receptors and the brain were moved to the front of the body. Organisms with bilateral symmetry were dominating last 650-800 million years. These are crustaceans, fish as well as the most progressive forms, i.e., mammals, birds, and insects.
V. Beklemishev (1964) distinguished three types of symmetry (spherical, radial, and bilateral) and arranged them in evolutionary array (Fig. 1). Forth type of triaxial asymmetry (aaa) he mistakenly assigned to a primitive organism (amoeba) and placed at the beginning of the array. Organisms of bilateral symmetry were considered the “crown” of evolution.
Amoeba → spherical → radial → bilateral symmetry
Fig. 1. The evolutionary array of organisms’ symmetry types.
c — symmetry, а — asymmetry of an organism on
each of the 3 axis of the three-dimensional space.
What determines asymmetry along the “left – right” axis was unknown.
Basic statements of the new theory:
1. The evolution of any structure goes from symmetry to asymmetry. The asymmetry of the organisms in the phylogenesis grows on all three axes (I — dorso-ventral (back – belly), II — anterior-posterior (nose – tail), III — and mediolateral (left – right).
2. Symmetry and asymmetry are features of a shape. Amoeba is shapeless and therefore should be removed from the evolutionary array. Extrapolation of evolutionary logic of the existing array: ccc ® аcc ® ааc have allowed to state a hypothesis about further asymmetrization of modern progressive forms and their natural transition from bilateral symmetry to the last type—triaxial asymmetry (TA) (аас ® ааа). Growing number of the facts of lateral asymmetry found in modern progressive forms (functional asymmetry of a brain, right handedness in humans, unilateral ovulation and unihemispheric sleep of dolphins) confirms this transition.
3. Which factor of the environment dictates the development of lateral asymmetry?
According to the idea of asynchronous evolution such factor can be time, so one side (organ) is more advanced ("vanguard" as, already in the future) and another one is a "rear-guard" (as yet in the past). Triaxial asymmetry type should be the most evolutionary advanced (Fig. 3).
Fig. 3. Adaptive asymmetrization of organisms in anisotropic environment
4. So, the information flows from the isotropic environment create asymmetry in organisms. How the field acts upon the organism and why it evolves? Let’s take one-dimensional (linear) organism — hydra in the field of gravity and earthworm (or snake) in space.
Phenotype appears from interrelation of genetic and environment information: P = G + E. The source of genetic information is zygote. Since the organism is developed from the zygote, in hydra it should be located at the bottom of the foot, and information flow should move to the top towards tentacles and at earthworm zygote at the end of the tail and genetic information flows to the nose, so tentacles are younger than the leg and nose — younger than the tail. Can development start from the nose? No, because ontogeny repeats phylogeny. Can it start in the middle of linear organism? Yes, if development starts at the border of two environments (ground – atmosphere as in the case of plants, then seed is growing up and down). Information from the environment always go towards (growth) and forms the gradient of potential. Then the new organ (character) should appear at the point of growth. If they are necessary their, like receptors or brain, they stay others drift along the gradient towards the tail. The Evolutionary Theory of Sex allows verifying this hypothesis. Hydra had all three types of reproduction: asexual (using kidneys), hermaphrodite, and dioecious, and earthworm had asexual and hermaphrodite. Theory of Sex predicts that ovaries at hydra should be between kidneys and testes, and at earthworm — closer to the tail than testes. That is so. Hydra has kidneys at the bottom, ovaries higher and testes even higher. Ovaries of earthworm are in the 13th segment and testes — only in the 10th.
5. Organisms of triaxial asymmetry keep two previous asymmetries. Lateral asymmetry appears on a background of previous ones (a ‘back–belly’ from a jellyfish, and ‘nose-tail’ from the opossum), therefore it should spread from front to back.
6. Increase in asymmetry can be seen in evolution of many systems:
The trend towards asymmetry can be followed in phylogeny of plant organs (flower, leaf, fruits, and seeds). It is known, that zygomorphic (bilateral symmetry) flowers [Gladiolus sp., Orchids, Eyebrights and Violets] are evolutionary more progressive, than actinomorphic (radial symmetry) flowers [Primula, Narcissus, Pyrola], but are less progressive, than triaxial asymmetric ones [Cannaceae and Valerianaceae].
The morphology of a leaf during evolution follows the same picture: spherical symmetry of chlorella, radial symmetry of pine needles, bilateral symmetry of Magnolia leafs, and triaxial asymmetry of Begonia or Elms leafs.
The same trend can be found in embryogenesis—spherical zygote, radial gastrula, bilateral embryo and triaxial asymmetric child. Vertebrate embryos (and indeed, many invertebrates) exhibit a strikingly conserved left-right asymmetry of the internal organs. Nearly all visceral organs of the thorax and abdomen are left-right asymmetrical in their anatomy, placement, and, in some cases, physiology. Directional left-right asymmetry is conserved throughout chordate evolution, although the details of anatomical asymmetry can vary among species.
One can conclude that asymmetrization is not especially human phenomenon, but general evolutionary phenomenon inherent to all live systems.
Narcissus, Orchid, Cannaceae
Chlorella, Pineneedles, Magnolia leaf, Begonia
- posted on 11/22/2009
国内八十年代有人提出一条“生物全息律”--即动、植物部分往往反应或对应着整体,书里有许多有趣的图片。这或许是对称性的另一种意义与表征。
在生物界的一个pattern还有可能就是对称与不对称在不同的层次上替换。比如氨基酸分子本身是不对称(只有某一类手性),而由氨基酸组成的蛋白质的二级或三级结构又有可能出现某些对称性(记得有种protein foldase 号称 lord of the rings,在四级结构上存在七重对称性)。
但许多药物分子(如著名的Plavix)往往是不对称的:即某种手性的分子相对而言它的镜象体有非常不同的生物性质(如活性、毒性等)。这称不对称当然也反映着人体内许多系统的不对称性。
我知道从前有一个生物学博士“不务正业”,用实验室的电子琴显微系统拍摄了许多漂亮的DNA等大生物分子照片,结果成了名人。作品四处展览、销售。
浮生、草叶不妨开拓一个新领域:分子美学吧。:) - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/22/2009
草叶周末还上班,辛苦了!
我等着读这线。 - posted on 11/22/2009
我要kitsch 一下,:) 啊!多么奇妙的科学世界, 太有趣了,若我儿子以后做scienctist, 我会比他做CEO还高兴。
浮生,我服了U,你总是不让我浅尝则止,好吧,我正在想开拓个第二职业,以后空闲时不要浪费生命和“才华” (前两天我一个雄心勃勃的医学院同学,打电话来骂我的,当然我觉得每日写grant, 也是一种浪费)。就从分子美学着手,浮生说真的,我想学一门医学的anthropology,专门浪费时间胡思乱想。
浮生我还看到一个allometry的图,是不是你说的那个数学公式?植物的size 和shape 能用数学公式来predict。
先说一下我的图,其实主要说明了modularity 和pattern repetition, 看了你引的文章,關於Symmetry, 我原来想得太浅显了,没有说明任何和进化的互动 。
手的x-ray, 是说明系列的reiterated 构建,不同的只是shape和number。
最后一个彩图是repetition in Fine scale: 这是放大的蝴蝶翅膀的花纹,每个小的unit是细胞level,每个细胞是一个single scale的颜色,collectively, 它们组成一个几何图案,和你及八爷谈的马赛克就是一回事。
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我觉得这些问题包涵了很多方面:从分子化学,细胞生物,还有基因,胚胎发育及生理解剖, 每个层面都有对称和不对称的现象。 我也不懂,只粗略想了一些不成熟的碎想,大家来讨论, 或举一具体例子更好理解。
1。 How it relate to other aspects of artificial life 和human behavior(如艺术)。我在想的是生物界的这些现象是不是暗合了很多人类的艺术创造,虽然艺术家和欣赏家并没有aware of 这种呼应,这种艺术反应在 :文字故事(maya提的原型, 前世的记忆); 建筑(symmetry and repetition),音乐?是否是intrinsical to human body then the mind respond to them?所谓的美学
2。Why?为什么会有这么一种现象in a grand scale, 哲学的角度,生物进化的角度? 就如你引的文章里谈到的从evolution 的角度,natural selection的角度来解释,但phyilosophyical的和具体技术细节角度很象还是有点儿lacking,
3。How?要知道自然界如何长成各种的形状,variation 和 morphogenesis , 自然make sense从胚胎发育成长的角度来看,modularity 和pattern repetition, 是进化所致,人类的四肢的gene tool kit和fly的tool kit 几乎相似, 有限的building blocks ,用 modular 和repetition 即能有級大的variation又可conserve, modularity and repetition as building pattern make lots sense。Symmetry and asymmetry 也是應該是同理的。
4。具体技术的层面,从cell biology, physiology necessity 来解释symmetry和asymmetry. 不仅形状是由于和enviroment interact 的产物,还有相邻cell之间的interaction。
5. 这个问题还能从很多角度看
functional vs anatomical.
如大脑,是functional asymmetry, anatomical symmetry. 而多数内脏如心,脾等等是anatomical 不对称,心脏的左右functional 功能也不一样。有些人是镜像反位(Situs inversus)
plasticity vs constraint,morphological phenotypic vs genotypic。
如双胞胎, 有一样的genotype , 但随着生活况境的不同,又会是独特的个体,包括外形和形为(phenotype)都有不同,但是他们的不同又是有限的,这就引到了plasticity 和constraint 上。symmetry 和asymmetry是phenotypic 还是genotypic? 还是both? - posted on 11/22/2009
我们这里说对称,但是发现大家在研究的都是为什么不对称。
请教八十一子,这些理论我看着很make sense,只是hypothesis吗?
还有关于分子结构的非对称,除了极个别的,生物体内都是氨基酸左旋,糖右旋,虽然实验室合成的两个方向都可能。这个能给讲讲为什么吗?
对称的物体常常由不对称的物体构成。研究对称就是研究非对称。
有机分子常常具有手征性。化学合成过程中,左右两种手性的产物都可能产生。但在生物体内却不一定如此。生物体的大分子是由各种小分子构成的。作为构成生物体的小分子化合物往往都有一定的手征性,例如糖类都是右旋分子,氨基酸都是左旋分子。这是为什么呢?
首先应该是生物大分子结构稳定性的需要。例如生物体的最重要的组成部分是DNA,负责携带遗传信息。遗传信息的载体的结构特点应该是既稳定又方便复制。DNA的双螺旋结构恰恰具备这两个特点。DNA的主要构件之一是核糖。双螺旋的成功构成要求参与结构的全部核糖分子都具有相同的手征性。同样地,蛋白质结构特点也要求其基本成分氨基酸的手性相同。
非对称性对于生物体的功能也至关重要。生物体的某一项功能的成功执行,例如生殖,是一系列信号在机体内各种组织和细胞之间成功传递的结果。非对称物体的另一个特征是其结构具备特异性,而特异性是信号本身及其传递过程的第一要素。
那么,为什么生物体选择了右旋糖而不是左旋糖,左旋氨基酸而不是右旋氨基酸呢?
我想,这是因为进化过程的两个特点:随机性和不可逆性。遗传信息的突变是随机的。进化过程做出的选择是不可逆的。生物体的最初形成过程目前尚属未知,但很可能经历了核酸、氨基酸的形成,核酸单体形成多聚体(DNA或RNA),氨基酸单体形成多聚体(蛋白质),DNA或RNA与蛋白质形成有自我复制能力的病毒,再进一步形成单细胞、多细胞生物。这个过程一旦起始便不可逆,除非过程中断,从头再来。好比在球赛开始前猜硬币的正反来决定起始进攻方向,一旦作出选择,球赛便开始。球赛开始后便不可逆转。
球赛开始前裁判掷出的硬币有正反两种情形,生命的基本载体也有左旋和右旋两种选择。我的猜测,我们所知的地球上现存的这种生命形式最初选择了右旋糖和左旋氨基酸。
宇宙里跟地球类似的行星估计有上亿颗,生命发生过程很有可能在别处再来一次。如果生物进化在别的行星上再来一次,最初的病毒在挑选核酸和蛋白质时,完全有可能选择跟我们的相反的手性化合物。但是,构成生物体的非对称小分子无论是是左旋还是右旋,构成的生物体的整体依然应该是对称的。 - posted on 11/22/2009
先举个人心脏胚胎发育的例子, 从对称到不对称。为什么人有一个心脏但有两个肺?
心脏和大血管的胚胎发育是很complex的,开始时只是管状的,原始的心脏和血管是在胚胎第三周开始出现的, 第22-23天心脏就开始跳动。血脉系统一开始是对称的形式,然后再经过一系列的obliterations, remodeling, and anastomoses. Obliterate 右边的动脉,左边的静脉,左右心就开始发育成不同的腔
發育第四周和第7周的心臟。
第八周的心脏, 开始不对称了, 我想driving force 是functionality
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/22/2009
球赛开始前裁判掷出的硬币有正反两种情形,生命的基本载体也有左旋和右旋两种选择。我的猜测,我们所知的地球上现存的这种生命形式最初选择了右旋糖和左旋氨基酸。
如果最初的氨基酸在地球不同的地方独立地产生,为什么它们有一致的手征性呢?
莫非所有的地球生物都起源于一个单一的源头?正象人类都起源于非洲? - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/22/2009
生命的发生看来是极为偶然的事件(否则我们早就应该遇到外星人了),有一个源头就算你运气了!:-)
gz wrote:
球赛开始前裁判掷出的硬币有正反两种情形,生命的基本载体也有左旋和右旋两种选择。我的猜测,我们所知的地球上现存的这种生命形式最初选择了右旋糖和左旋氨基酸。所有的地球生物都起源于一个单一的源头吗?如果最初的氨基酸在地球不同的地方独立地产生,为什么它们有一致的手征性呢?
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/22/2009
八十一子 wrote:
生命的发生看来是极为偶然的事件(否则我们早就应该遇到外星人了),有一个源头就算你运气了!:-)
也就是说所有的地球生命都起源于 *一个*偶然出现的氨基酸? 这是你的猜想还是 established 理论?
- posted on 11/22/2009
gz wrote:
如果最初的氨基酸在地球不同的地方独立地产生,为什么它们有一致的手征性呢?
在“原始热汤”里产生的核酸和氨基酸应该两种手征性都存在。对手征性作出选择的时间应该是多聚核酸形成能够依靠外界条件进行自我复制的原始病毒的时候。不难想像,有自我复制能力的化合物比随机反应的化合物具有的、多个数量级的优势,一旦出现便占据主导地位的可能性是存在的。
地球上的生命形式有共同的单一起源,这一点应该已是共识。可参看 SJ Gould "Wonderful Life". - posted on 11/22/2009
心脏只有一個,左右心腔有不同的功能, 而肺则是blilateral的, 两边的肺功能是一样的, 但解剖结构有些不同, 左肺的有三片肺叶, 右肺有两片肺叶, 两边的气管的大小和角度也不对称的。而不对称的driving force是因为受邻近器管的影响(心脏和食道), 而不是受功能的影响。
胃肠道最后发育期间, 要rotate 270度 的方向, 因为长长的肠子要全部进入小小的腹腔。 malrotation就会造成腹腔畸型,肠道梗阻。
- posted on 11/23/2009
八十一子 wrote:
gz wrote:在“原始热汤”里产生的核酸和氨基酸应该两种手征性都存在。对手征性作出选择的时间应该是多聚核酸形成能够依靠外界条件进行自我复制的原始病毒的时候。不难想像,有自我复制能力的化合物比随机反应的化合物具有的、多个数量级的优势,一旦出现便占据主导地位的可能性是存在的。
如果最初的氨基酸在地球不同的地方独立地产生,为什么它们有一致的手征性呢?
地球上的生命形式有共同的单一起源,这一点应该已是共识。
http://en.wikipedia.org/wiki/Chirality_(chemistry)
The origin of this homochirality in biology is the subject of much debate.[10] Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. However, there is some suggestion that early amino acids could have formed in comet dust. In this case, circularly polarised radiation (which makes up 17% of stellar radiation) could have caused the selective destruction of one chirality of amino acids, leading to a selection bias which ultimately resulted in all life on Earth being homochiral.[11]
http://www.simsoup.info/Origin_Issues_Homochirality.html
The Origin of Chiral Discrimination and Homochirality
Homochirality is essential to the correct functioning of many of the mechanisms of contemporary life-forms. But how did it arise in the first place? In order for homochiral macro-molecules to have evolved there must have been a mechanism which could discriminate between the molecular sub-units from their mirror images, and construct the macro-molecules from the sub-units of correct chirality. In contemporary life-forms this is done using complex mechanisms in which highly evolved (homochiral) molecules perform key functions. Clearly such highly evolved molecules would not have been available from the outset. We must therefore ask: What was the origin of chiral discrimination? This, as Graham Cairns-Smith pointed out in his book "Genetic Takeover" (Cairns-Smith, 1982) is the nub question. To date, there is no generally accepted answer to this question. Noam Lahav (in Lahav, 1999, page 257) indicates that the organic chemist William Bonner argued at a meeting in the mid 1990's that there was a big gap between the origin of homochirality and the origin of life, and that homochirality must have preceeded life. Bonner said "I happen to think that you have to understand the origin of homochirality before you can bridge that gap. Stepwise, one has to deal with the origin of homochirality first, and then how do you get to living organisms". He went on to say "I spent 25 years looking for terrestrial mechanisms for homochirality and trying to experimentally investigate them and didn't find any supporting evidence". Bonner has suggested an extraterrestrial source for homochiral molecules. Others, including Stanley Miller and Jeffrey Bada have argued that homochirality is an "artefact of life" rather than a precondition.
http://www.talkorigins.org/faqs/abioprob/originoflife.html
Origin of the homochirality of amino acids and sugars
The term chiral is used to describe an object which is non-superimposable on its mirror image. The mirror image forms are the enantiomers which were just mentioned. Life almost exclusively synthesizes L-amino acids and D-sugars. This homochirality is essential for the functioning of proteins as amino acid polymers, and for the structure of DNA and RNA, which requires incorporation of D-sugars.
How did this homochirality of amino acids and sugars arise? It is a question that has puzzled origin-of-life researchers for decades, yet a series of recent findings appears to address it astonishingly well.
Above experiments with stereospecific amino acid catalysis of the synthesis of sugars may provide a good start for an answer. The prebiotic stereospecific catalysis of D-sugar synthesis could have occurred by strongly enriched enantiomers of amino acids or small peptides. But how would these have been present in sufficient purity?
The answer to this question appears to be enrichment in two or three of the following steps:
1. An initial imbalance of enantiomeric forms of a catalytic amino acid
How could a greater presence of one enantiomeric form of an amino acid over the other (enantiomeric excess, abbreviated: ee), be it ever so slight, have arisen at all? After all, typical synthesis of an amino acid in the laboratory results in an exact 1:1 ratio of the L and D enantiomer – a so-called racemic mixture.
One possible source are meteorites. On the Murchison meteorite, a well-studied example, the L-form of some of the amino acids found is present in up to 9% ee (Cronin, Pizzarello 1997). These ee’s may have been induced by circularly polarized UV light (Bailey et al. 1998).
Yet while chemical evolution of early life could very well have built upon meteoritic material, sources of slight or even pronounced enantiomeric excesses of amino acids could have arisen in numerous places on the prebiotic earth.
Robert Hazen and colleagues found (Hazen et al. 2001) that crystals of the common rock-forming mineral calcite (CaCO3) can preferably adsorb D- or L-forms of aspartic acid (and in preliminary experiments, D- or L-forms of alanine as well) depending on the chirality of the crystal surface. Average ee was a few percentage points. The work is also described, with a vivid and personal look behind the scenes, in Hazen’s Gen-e-sis, chapter "Left and Right".
Even though such slight local enantiomeric excesses may be enough to trigger a series of amplifying events (see below), another study found much larger preferences of adsorption of amino acids to mineral surfaces (Wedyan, Preston 2005). While in their study quartz, kaolin and montmorillonite showed slight preferences for adsorption of enantiomers, ordinary sediments from estuaries exhibited strong selectivity. At the slightly acidic pH 4, typical D/L ratios were hugely different from 1, reaching at their most extreme up to 100 for serine. At pH 9 effects were still significant, even though less pronounced. Sediments were ashed in order to remove organic matter which could introduce chiral bias. The authors are cautious: "The possibility that the ashing process actually etches and activates the mineral surface as it burns off natural (chiral?) organic matter cannot be discounted." However, they also note that it is remarkable that such strong selectivity should occur at all.
An entirely different, attractive mechanism to achieve enantiomeric excess has been reported which, it seems, is robust (Kojo et al. 2004). It is based on the internal properties of amino acid mixtures. Most racemic amino acids (i.e. featuring a 1:1 ratio of the L and D enantiomer) form crystals that are also racemic. However, when racemic D, L-asparagine forms crystals (which may, for example, happen upon cooling of a hot solution, or slow evaporation of a solution), they are not racemic, but show varying degrees of excess of either the L-enantiomer or the D-enantiomer. When other amino acids are present, either their L- or their D-form preferentially co-crystallizes with the enantiomeric form (L or D) of asparagine that confers an excess during formation of the particular crystal at hand.
The internal consistency of the results of the study was quite amazing. If there was a bias towards L-asparagine in one crystal, practically all the other 12 co-crystallized amino acids showed a bias towards the L-form as well. The degree of bias towards L-asparagine varied between crystals; when the bias was more pronounced, the bias towards the L-form in the other amino acids was also more pronounced. The ee’s observed could be very high. If in yet another crystal there was a bias towards the D-form of asparagine, the other co-crystallized amino acids exhibited a bias towards their D-enantiomer as well.
Of course, the overall enantiomeric balance of the entire amino acid mixture (the sum of all crystals and the liquid phase above them) was still racemic. However, if, these kinds of crystals were formed from a solution on the prebiotic earth, and some of the crystals were then physically separated from others – which could have happened by many ordinary processes – and later redissolved, the amino acids in solution would automatically have exhibited an ee. Depending on the particular crystal where the solution derived from, this ee could have been high.
2. Enhancement of enantiomeric excess in solution by solid phase-liquid phase equilibria
Simultaneous independent studies by different groups have shown that, once an initial ee in a mixture of amino acids exists, even if it is just very slight, it can have an enormous effect. This effect can occur when solid and dissolved amino acids coexist in equilibrium, something that could well have happened in a prebiotic landscape – all that is needed is an aqueous environment where the concentration of a particular amino acid is so high that it partially falls out of solution and forms a solid phase. This could, for example, take place by change of the aqueous environment (temperature, salt, pH etc.) or simply by limited evaporation of an aqueous solution.
A detailed study was performed by the British group of Donna Blackmond (Klussmann et al. 2006). When an amino acid mixture shows an excess of one enantiomer, in most cases an equilibrium of solid and dissolved amino acids will consist of the following two or three components:
racemic crystals (crystals with a 1:1 ratio of the L- and D-enantiomers, no enantiomeric excess either way)
pure crystals of the enantiomer in excess (Whether just one of these two solid phases is present, or both coexist, will depend on the overall ee.)
an amino acid solution in equilibrium with the solid phase(s) which also exhibits a certain ee.
Yet as it turns out, for several amino acids this ee in solution is much higher than the overall ee of the total mixture (the solid material, dominated by racemic crystals, thus shows correspondingly less ee). At the most extreme, serine provides an almost enantiopure solution (> 99% ee) in water from a nearly racemic sample (only about 1% ee) under solid-liquid equilibrium conditions.
A smaller study, independently conducted around the same time, reports similar findings (Breslow, Levine 2006). Slow evaporation of an aqueous solution of phenylalanine at just 1% ee of the L-enantiomer led to a solution of this amino acid with 40% ee of the L-enantiomer above solid material. If, in turn, such a solution was allowed to evaporate, the resulting solution in equilibrium with the solid material had a 90% ee. Similar results were found with the D-enantiomer.
In both these studies the findings are discussed also in terms of amino acid catalysis of aldol reactions, which include sugar synthesis.
3. Amplification of enantiomeric excess in sugar synthesis catalyzed by amino acids
If all the above mechanisms were not enough, and an almost enantiopure amino acid solution, which could catalyze the stereospecific synthesis of sugars, was still not at hand in the local environment where primitive life first was shaped, the final boost towards an enantiopure sugar could have occurred the following way:
The group from Stockholm University showed that in the catalysis of sugar synthesis by amino acids a significant amplification of enantiomeric excess can occur (Cordova, Engquist et al. 2005). For example, a reaction mediated by the amino acid proline with an ee as low as 40% still yielded almost enantiopure hexose sugar. The reaction catalyzed by proline at 10% ee furnished the sugar with 33% ee. - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
You disappointed me, GZ. Thought you were the kind of person who uses his head, not his google. :-)
BTW, "decision on homochirality was made when the most primitive virus was formed" is my hypothesis. :-)
When we talk about the very root of all life forms on earth, we are talking about a certain multi-cell organism already. Too much is unknown about anything earlier.
- posted on 11/23/2009
八十一子 wrote:
You disappointed me, GZ. Thought you were the kind of person who uses his head, not his google. :-)
This topic is by no means anything close to what I know. I am not trying to argue one way or the other. I am just interested in this issue of origin of the homochirality, and google seems the shortest distance to a quick answer. :-)
When we talk about the very root of all life forms on earth, we are talking about a certain multi-cell organism already. Too much is unknown about anything earlier.
But I was talking about the origin of homochirality of amino acids, instead of multi-cell organisms. - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
gz wrote:
But I was talking about the origin of homochirality of amino acids, instead of multi-cell organisms.
虽然L-氨基酸是生物体内氨基酸的主要成分,D-氨基酸在生物体内并不是完全不存在。事实上,今年9月美国《科学》有过报道说,有一种细菌会制造D-氨基酸,并用来强化自己的细胞壁。 - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
So after all, homochirality does not exactly hold? - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
No. Not in absolute terms.
gz wrote:
So after all, homochirality does not exactly hold? - posted on 11/23/2009
八十一子说的是化学物理结构上的对称和不对称性, 我在想这个问题时是解剖意义上的对称和不对称。
假设对称和不对称同时以同等机率在baking life 的dish 上(这假设不知成立否?),当生命一步步adapt环境, 有些enviromental stress call for 不对称,比如前后不对称的就有more advantage,有些call for 对称,如两个眼睛,两个耳朵,两个鼻孔,因为这样能准确知道sensory 来源的立体方位。而有些开始是对称的,但受到周边器官的影响而不对称。所以我们现在看到的是各种因素叠加起来的结果,intrinsic 和extrinsic的因素都有。
所以我觉得对称和不对称都是adaptation所致,不能完全确定不对称是今后进化的方向。
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
所以我觉得对称和不对称都是adaptation所致,不能完全确定不对称是今后进化的方向。
几乎所有的鱼都是对称的,唯独比目鱼。这大概是进化的结果。但有趣的是,这有什么优势呢? - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
gz wrote:
几乎所有的鱼都是对称的,唯独比目鱼。这大概是进化的结果。但有趣的是,这有什么优势呢?
比目鱼是仰泳爱好者。 - posted on 11/23/2009
问问草医生:为什么胎生动物还有yolk sac? 是临时工棚吗?
草叶 wrote:
先举个人心脏胚胎发育的例子, 从对称到不对称。为什么人有一个心脏但有两个肺?
心脏和大血管的胚胎发育是很complex的,开始时只是管状的,原始的心脏和血管是在胚胎第三周开始出现的, 第22-23天心脏就开始跳动。血脉系统一开始是对称的形式,然后再经过一系列的obliterations, remodeling, and anastomoses. Obliterate 右边的动脉,左边的静脉,左右心就开始发育成不同的腔
發育第四周和第7周的心臟。
第八周的心脏, 开始不对称了, 我想driving force 是functionality
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
你是说侧泳爱好者? :-)
小时候《十万个为什么》里说,比目鱼一侧靠海底,一侧朝上,结果两只眼睛跑一边来了。听上去有些拉马克。
小蜜蜂 wrote:
gz wrote:比目鱼是仰泳爱好者。
几乎所有的鱼都是对称的,唯独比目鱼。这大概是进化的结果。但有趣的是,这有什么优势呢? - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
地球上的生命形式有共同的单一起源,这一点应该已是共识。
还是对这个话题有兴趣。
如此说来当年产生生命物质的一锅浓汤只在地球的某 *一个* 地方发生,虽然别处也有相同的条件,却没有产生生命物质?那么最初等的生命也有一个从一点逐渐走向全球的过程了?(就象人类走出非洲那样?)
能再多给点参考资料吗?这回连 google 都不会查了。:) - posted on 11/23/2009
You may want to start from a review article by Vaneechoutte and Fani entitled "From the primordial soup to the latest universal common ancestor", published in Research in Microbiology 160 (2009) 437-440
http://www.ncbi.nlm.nih.gov/pubmed/19751826?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=2
Another interesting paper is by Taylor in Nature 434, 705 (7 April 2005): Stirring the primordial soup
gz wrote:
还是对这个话题有兴趣。地球上的生命形式有共同的单一起源,这一点应该已是共识。
如此说来当年产生生命物质的一锅浓汤只在地球的某 *一个* 地方发生,虽然别处也有相同的条件,却没有产生生命物质?能给点参考资料吗?这回连 google 都不会查了。:) - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
八十一子 wrote:
你是说侧泳爱好者? :-)
小时候《十万个为什么》里说,比目鱼一侧靠海底,一侧朝上,结果两只眼睛跑一边来了。听上去有些拉马克。
八先生,我开玩笑呢。你们这贴谈的,我基本不懂。:)
用进废退挺有志者事竟成的。适合拿来做白日梦。
比目鱼为什么不对称?因为比目鱼不耐烦待见关中。LOL - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/23/2009
另外我看着也糊涂,动物和植物不需要分开来讲吗?动物是压倒性对称吧(这是听道金斯同志传的道)? - posted on 11/23/2009
跟着GZ继续ADD。继续生命起源话题。
在讨论生命起源时,有几个概念值得复习。
意大利人范尼(Fani)在最近一篇回顾文章(Research in Microbiology 160 (2009) 437-440
) 里指出,在讨论生命起源问题时,应该好好定义“生命”。他建议把“生命life”与“生活being alive”区别开来。“生活”指的是单细胞或多细胞生物代谢过程。病毒没有自己的代谢过程,因此谈不上生活。“生命”则指地球上现存的全体细胞的总和。当一个细菌分裂时,它不再活着,因为它被两个新的细菌代替了,但它的生命却由那两个新的细菌延续。从这个意义上讲,从生命发生之初到现在,“生命”这个东西已经在地球上连续存在了35亿年了。
俄国人欧巴林(Oparin)在1924年提出了“原始热汤(primordial soup)”的概念,意即在单细胞生物出现之前,生命发生在地球上灼热的原始海水里。那时的地球表面富含碳氮氧,高温的原始海洋里满载甲酸氨 (HCOONH4)和水。核酸、核糖核酸、多聚核糖核酸等有机物质极有可能出现,并造就最初的生命形式 - 能够自我复制的RNA病毒。
美国人沃日(Woese)指出,生物进化过程除了可遗传的DNA突变外,还有一个途径叫做横向基因传递(horizontal gene transfer; HGT),包括细胞融合,病毒、质粒、裸DNA等介导的、跨越原核-原核、原核-真核、真核-真核生物界限的基因传递。这个途径的重要性很可能并不小于经典遗传。我们今天看到的这个大千世界的出现很可能在很大程度上要归功于横向基因传递。 - posted on 11/24/2009
回八老师, 要知道, 很久以前考试时最秫头的就是胚胎学, 近来才重新有了兴趣是因为不需要考胚胎学啦。
爬行动物从水到陆地, 将蛋生在陆地而不是水中, 蛋壳,羊膜和羊水给Amniote胚胎一个相对于鱼卵更安全的环境。Amniote 动物都有york sac , 蛋要单独在母体外生存, 营养胚胎的养料是从york里来, york sac包着york , 养料透过sac 而提供给胚胎。
到了脯乳动物, 除了少数的蛋生动物(platypus)和marsupial类(袋鼠),子宫胎盘 提供胎儿更温暖更富于营养的母体内生长环境。
这种arragment 有利于父母一方的mobility, 不需坐着孵蛋了:) 但是孵育胎儿, placenta 对子宫的侵入, 和如寄生似的吸取母体的养份, 是用牺牲母体为代价 , 繁衍更好的后代。
言归正传,人体的york sac是早期时, 在胎儿的血循环系统和胎盘血供建立完善前, 提供胎儿血循环的结构,york sac 和胎儿的原始主动脉相接, 回流血到胎心。见上面第4周的心脏图。 以后就退化, 有的没全被吸收, 就变成人的肠道里一个多余的diverticulum。会有出血等症状, 有2%的孩童有此问题, 叫Meckel's diverticulum
不知这算是回答了你的疑问吗?
谢谢你提供的文章, 有空好好读, 请多讲讲, 很有兴趣学。
八十一子 wrote:
问问草医生:为什么胎生动物还有yolk sac? 是临时工棚吗?
- posted on 11/24/2009
小蜜蜂,
动植物可不能如此分得清的, 我们有很相同的祖先。见八老师写的这一段, 还有我帖的图, 生命的树, 我们本是同根生, 我们指动植物。
我见过一个数据, 但找不到出处了, 还欠浮生的哈:( 人和番茄share50-60% DNA 。
还有这里的对称指的是多方位的,突然想到人类都是圆滚滚的球会是如何一种社会? 你的政治学也要改写了吧:)
还有读到你和玛雅谈到的经血, 很有意思, 你读的那么多的女性文章里有没有谈到in anthropology point view, 这是怎么一回事, 这MB 只在primate 里有这现象的。
"生物进化过程除了可遗传的DNA突变外,还有一个途径叫做横向基因传递(horizontal gene transfer; HGT),包括细胞融合,病毒、质粒、裸DNA等介导的、跨越原核-原核、原核-真核、真核-真核生物界限的基因传递。这个途径的重要性很可能并不小于经典遗传。我们今天看到的这个大千世界的出现很可能在很大程度上要归功于横向基因传递。"
小蜜蜂 wrote:
另外我看着也糊涂,动物和植物不需要分开来讲吗?动物是压倒性对称吧(这是听道金斯同志传的道)?
- posted on 11/24/2009
草叶 wrote:
动植物可不能如此分得清的, 我们有很相同的祖先。见八老师写的这一段, 还有我帖的图, 生命的树, 我们本是同根生, 我们指动植物。
我见过一个数据, 但找不到出处了, 还欠浮生的哈:( 人和番茄share50-60% DNA 。
还有这里的对称指的是多方位的, 不只是动物的lateral 对称,突然想到人类都是圆滚滚的球会是如何一种社会? 你的政治学也要改写了吧:)
我知道生命同源。我说分开动植物,是指对称。
道金斯在他一本书里讲他写过一个EVOLUTION (reproduction + development)的SIMULATION小程序,纯粹为了从视觉上展示代际变异的效果。因为我平时处理数据时有时也要做SIMULATION,所以特别留意了一下。当然为了电脑展示的原因他大大提高了变异的RATE。他在输入基因时解释说他用的都是对称的,一是为了美学,二是因为他想得到“动物”形状的变异结果---而绝大多数“动物”,都是相当对称的。
因为训练的缘故,我对这样的措辞一向很敏感。这样的措辞只有当作者在不确定时才会使用。通常遇到不确定的情况,原则是要么特指,要么尽量模糊用语。道金斯在这里特指,意味着植物和动物不一样---于对称而言。
这是我当时读他书时的理解。你们这贴谈的我不怎么懂,一看头就大了。浮生贴的文章我也没细看,但我注意到那文章里第六条举高级不对称的例子都是植物。照片也都是植物。这是我的PUZZLE.
如果理解错了,一定要纠正我。
--------------
至于经血,以后有空说。
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/24/2009
抱谦没说清,还说错。 从浮生的那篇文章的意思, 人不是lateral 对称, 而是三轴不对称, 愈是高级的动物, 不对称的轴就愈多, 人在三个轴面上都不对称, 包栝lateral。虽然外表看上去lateral 是大致对称的, 但内部器官如心, 脑, 肠都不对称, 当然还有左右手之分。在植物也一样,从低级到高级植物,也是附和从球状对称到三轴不对称的现象, 所以推出进化的方向是不对称。
理解不对的, 浮生来纠正!
当然用什么定义对称,用什么参数? 绝对, 相对的, 也会有不同结论。如眼,耳,手 ,我覺得是對稱的。 - posted on 11/24/2009
生物体在形态学和解剖学上或许不对称,但在美学意义上是对称的。例如树木,侧面多为塔性,俯视多为圆形,都是对称的,虽然在严格意义上恐怕没有一棵树在任何一个维度对称。
草叶 wrote:
抱谦没说清,还说错。 从浮生的那篇文章的意思, 人不是lateral 对称, 而是三轴不对称, 愈是高级的动物, 不对称的轴就愈多, 人在三个轴面上都不对称, 包栝lateral。虽然外表看上去lateral 是大致对称的, 但内部器官如心, 脑, 肠都不对称, 当然还有左右手之分。在植物也一样,从低级到高级植物,也是附和从球状对称到三轴不对称的现象, 所以推出进化的方向是不对称。
理解不对的, 浮生来纠正!
当然用什么定义对称,用什么参数? 绝对, 相对的, 也会有不同结论。如眼,耳,手 ,我覺得是對稱的。 - posted on 11/24/2009
Great, 草叶,你的胡思乱想总是很有趣,一下子就给出这么多角度,完全看不过来了 :)Medical anthropology sounds cool, 具体研究什么?
谢谢八十一子,你随手一写就是很好的科普文章,多给讲讲哈。也很想听大家自己的hypothesis。
先说我对对称非对称的理解,刚读到一个搞心理学personality measurement的笑话:People are divided into two types: those who divide people into two types and those who don't. 对称有很多种,严格的定义可以去查。就说几何上的,上边转的文章里讲的都是轴对称(镜像对称),也是这线里说对称的assumption,对称非对称在这个意义上我理解为一个spectrum,就是有多少个对称轴,轴越多就可以说“对称性”越强,vice versa。比如球体有无数多个对称轴。从这个意义上进化的大致方向是对称轴数的减少。这个是在同一个scale上的。草叶你照片里给的那些重复的blocks也是对称之一种,通过translate,rotate, scale。在一个生物体上,我想不同的scale下也会有不同的对称性,比如人的内脏整体来说三个轴都不对称,而草叶说的眼耳鼻可能大致lateral对称(不过我两眼的度数可不一样哈)。两手功能上也是相当的不对称哈。
八十一子说的美学上的对称呢,是对严格数学意义上对称的近似,一方面表现的是生物本身的variations,另方面又正表明我们对对称性的内在的“向往”:)Simplification(这个字本身就包含着某种对称性),我想这也是我们认知上的倾向。
从功能上,对称的好处我的最粗浅的理解是有redundancy,保险,比如坏了一个肾还有另一个。对称减少的好处是variation,新功能,新的adaptability。这个对称也包括重复,有了重复才有多余的blocks去变化,这个又可以是在不同scale上的。比如我们的genome里有很多duplicated genes,植物基因中大规模的duplication比动物要多,其中一个解释就是植物不能动,只能适应环境不能逃离环境。这个又扯远了。
小蜜蜂,那些植物的图是我找的,到动物我不耐烦了就没接着找,不好意误导你了。
八十一子说的“进化过程的两个特点:随机性和不可逆性”的解释我觉很make sense。左旋还是右旋最初的随机性也是我的猜测(嘿嘿,又是对Occam's razor的下意识使用)。不过guanzhong给狗狗来的都蛮有道理的。我怎么这么susceptible啊?
再说我对单一起源的理解。即便是单一起源,也不是说只发生了一次,无论时间空间上。但是最终影响到我们的可能只有某一个,把生命”树“(现在已经不是树了)回溯到一个根,但这个根可能是一个更大的树/网的结点。用人的单一起源就好说了,现在是说现代人走出非洲两次,第一次的died out?还有Neanderthal和我们有共同祖先,但是走出非洲的我们的祖先和Neanderthal走出非洲时的祖先大概就不是一个。单一只能是looking back的结果,不能和looking forward混着看。
- posted on 11/24/2009
这段东西“原始热汤"是成熟的理论,还是假设? 有试验依据吗?
人类至今为此还没有成功地合成生命吧? 合成氨基酸蛋白质之类的还不是生命吧?
讨论生命的起源, 这是该线最关键的问题. 这个问题不谈明白, 其它的都是想当然, 只是STRETCH THE TRUTH, 不足为据.
八十一子 wrote:
跟着GZ继续ADD。继续生命起源话题。
在讨论生命起源时,有几个概念值得复习。
俄国人欧巴林(Oparin)在1924年提出了“原始热汤(primordial soup)”的概念,意即在单细胞生物出现之前,生命发生在地球上灼热的原始海水里。那时的地球表面富含碳氮氧,高温的原始海洋里满载甲酸氨 (HCOONH4)和水。核酸、核糖核酸、多聚核糖核酸等有机物质极有可能出现,并造就最初的生命形式 - 能够自我复制的RNA病毒。
- posted on 11/24/2009
Medical anthropology is a subfield of social and cultural anthropology that examines the ways in which culture and society are organized around or impacted by issues of health, health care and related issues.
浮生, 你总是把我想说的, 又没理清的说得那么好。 你最后一段就是我想说的, 这个单源说是从winner 的角度, 也就是survivor 是单源的, 多少losers 我们回看时, 并不一定能知, 当然winner也可能是撞了大运了。
浮生 wrote:
Great, 草叶,你的胡思乱想总是很有趣,一下子就给出这么多角度,完全看不过来了 :)Medical anthropology sounds cool, 具体研究什么?
再说我对单一起源的理解。即便是单一起源,也不是说只发生了一次,无论时间空间上。但是最终影响到我们的可能只有某一个,把生命”树“(现在已经不是树了)回溯到一个根,但这个根可能是一个更大的树/网的结点。用人的单一起源就好说了,现在是说现代人走出非洲两次,第一次的died out?还有Neanderthal和我们有共同祖先,但是走出非洲的我们的祖先和Neanderthal走出非洲时的祖先大概就不是一个。单一只能是looking back的结果,不能和looking forward混着看。
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/24/2009
这个说法听起来比较合理, 两种手性的竞争是个不稳定过程,最后只能导致一个 winner。
再查一次google:
Mirror-symmetry breaking and chiral amplification
http://en.wikipedia.org/wiki/Homochirality - posted on 11/24/2009
Primordial soup当然只是假说,但并不是臆想,也有一些试验依据。生物体内的全部有机分子基本都可以从含有两、三个碳原子的简单分子如乙醇和丙酮酸来开始合成。三十多亿年前的地球高温,空气富含氮气,这都是已知的。在人造“原始热汤”里甲酸氨等简单化合物也很容易产生。这些简单化合物在“前生物”世界存在是完全可能的。只是,最初的生命形式(可能是RNA病毒)的产生花了大约5亿年,从原始病毒的产生到单细胞生物的产生花了大约15到20亿年的时间,其间多少trial and error, 多少“艰辛”,不难想像。生物学家把这个过程“复盘”恐怕很需要些时间。
合成“生命”已不是难事,例如制造可以自我复制的核酸大分子,人类已经做到,只是还不能从头开始在原始热汤里做出来。
疑问 wrote:
这段东西“原始热汤"是成熟的理论,还是假设? 有试验依据吗?
人类至今为此还没有成功地合成生命吧? 合成氨基酸蛋白质之类的还不是生命吧?
讨论生命的起源, 这是该线最关键的问题. 这个问题不谈明白, 其它的都是想当然, 只是STRETCH THE TRUTH, 不足为据.
八十一子 wrote:
跟着GZ继续ADD。继续生命起源话题。
在讨论生命起源时,有几个概念值得复习。
俄国人欧巴林(Oparin)在1924年提出了“原始热汤(primordial soup)”的概念,意即在单细胞生物出现之前,生命发生在地球上灼热的原始海水里。那时的地球表面富含碳氮氧,高温的原始海洋里满载甲酸氨 (HCOONH4)和水。核酸、核糖核酸、多聚核糖核酸等有机物质极有可能出现,并造就最初的生命形式 - 能够自我复制的RNA病毒。
- posted on 11/24/2009
不能从原始热汤里做出生命,八教的解释仍然是空对空的假说(说是臆想也不为过),您的---{{合成“生命”已不是难事}}----(如果是指由无生命的有机物到生命的话),这一命题恐难成立。
而建立在一个没有实验基础的假说上的进一步演绎,其实就已经离科学很远了。当然当成有趣的文学作品来看那又是另外一回事。
具体到你这段谈的其他一些观点, 在谈到细节上, 俺认为也还是有问题。
比如,首先要定义生命的基本形式是什么,并根据这定义去讨论。--------“Primordial soup当然只是假说,但并不是臆想,也有一些试验依据。生物体内的全部有机分子基本都可以从含有两、三个碳原子的简单分子如乙醇和丙酮酸来开始合成。”----------这段就有些概念不清。道理很简单,“全部有机分子”并不能和生命划等号,仅仅有“含有两、三个碳原子的简单分子如乙醇和丙酮酸”仍然不是生命的开始。这些都不足以作为生命产生的试验依据,不难理解吧?
“最初的生命形式(可能是RNA病毒)的产生花了大约5亿年,”----这段还是臆想。因为生命的产生是偶然的,还是进化而来的(达尔文只谈了物种进化, 没有说生命是进化产生的,注意这区别),人类现在都没有搞明白, 谈“大约5亿年”又有什么意义?
至于“早期”生命形成(如果是自然发生的话,这本身是个疑问),理论上说,应该有一个稳态的环境,这样才能使生命活动的基本要素(假如早期生命也是单细胞,先有蛋,再有鸡)能集中在一起,同时有高浓度的胞浆,同时有mRNA和tRNA,以及复制和翻译所需要的酶,这些东西能稳定地聚集在一起不被海水冲走,等等等等,否则这自然产生的RNA也会马上死掉;而且这早期“自然产生”出现的RNA一经出现就非常完美地PROGRAMED能立刻适应环境。这种机率能有多大?八教相信JUNK YARD的一堆废铜烂铁经过“大约5亿年”能够自然产生一台丰田车吗?有合理的理论支持吗?
地球由自然界中“自然”地产生生命,不论这种观点冠以什么理论出现,到目前为止都还是缺乏有说服力的实验根据,也同样缺乏合理的理论去解释的。
所以{{生物学家把这个过程“复盘”恐怕很需要些时间。}}这段话仍然还是臆想,和声称{{科学家把永动机制造出来恐怕很需要些时间。}}区别大吗?
说明一点,即使地球生命“自然”发生假说被证伪,也不等于说神创说理所当然地由此被证明成立。
八十一子 wrote:
Primordial soup当然只是假说,但并不是臆想,也有一些试验依据。生物体内的全部有机分子基本都可以从含有两、三个碳原子的简单分子如乙醇和丙酮酸来开始合成。三十多亿年前的地球高温,空气富含氮气,这都是已知的。在人造“原始热汤”里甲酸氨等简单化合物也很容易产生。这些简单化合物在“前生物”世界存在是完全可能的。只是,最初的生命形式(可能是RNA病毒)的产生花了大约5亿年,从原始病毒的产生到单细胞生物的产生花了大约15到20亿年的时间,其间多少trial and error, 多少“艰辛”,不难想像。生物学家把这个过程“复盘”恐怕很需要些时间。
合成“生命”已不是难事,例如制造可以自我复制的核酸大分子,人类已经做到,只是还不能从头开始在原始热汤里做出来。
疑问 wrote:
这段东西“原始热汤"是成熟的理论,还是假设? 有试验依据吗?
人类至今为此还没有成功地合成生命吧? 合成氨基酸蛋白质之类的还不是生命吧?
讨论生命的起源, 这是该线最关键的问题. 这个问题不谈明白, 其它的都是想当然, 只是STRETCH THE TRUTH, 不足为据.
八十一子 wrote:
跟着GZ继续ADD。继续生命起源话题。
在讨论生命起源时,有几个概念值得复习。
俄国人欧巴林(Oparin)在1924年提出了“原始热汤(primordial soup)”的概念,意即在单细胞生物出现之前,生命发生在地球上灼热的原始海水里。那时的地球表面富含碳氮氧,高温的原始海洋里满载甲酸氨 (HCOONH4)和水。核酸、核糖核酸、多聚核糖核酸等有机物质极有可能出现,并造就最初的生命形式 - 能够自我复制的RNA病毒。
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/24/2009
氨基酸的两种手性是个有趣的问题。在谈“原始热汤”时这是个大问题。
右旋氨基酸如果加入CHAIN中,就和毒药一般,杀死早期的"生命”(如果有的话),理论上说(要形成生命)PROTEIN这关就可能过不了。不知道那些用电闪雷鸣试图“复制”原始热汤的科学家有没有想过这问题。
况且不谈核糖核酸和酶的事情。
gz wrote:
这个说法听起来比较合理, 两种手性的竞争是个不稳定过程,最后只能导致一个 winner。
- posted on 11/25/2009
GZ was right after all - Google is quite knowledgeable these days. I jsut googled "abiogenesis" and found a pretty good summary on - what else - "chemical evolution", the study of how life on Earth could have arisen from inanimate matter:
http://en.wikipedia.org/wiki/Abiogenesis#Origin_of_organic_molecules
Long live Google the Almighty!
Speaking of almighty, it just occurred to me that the search engine Bill Gates wants us all to use is called "Being". The implication is almost surreal. - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/25/2009
"圣经"说上帝按自己的形象造人.人是神的镜象.所以手性等正好相反.人类genome的序列完全解完解清后,反反手性,变变分子构型,神的大秘就将被解开.道教炼丹有云"顺成人,逆成仙,只在中间颠倒颠",亦合此理.
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/25/2009
:)
风子 wrote:
八十一子,that search engine is called "bing" -- Bill Gates is more subtle.
But it's true that my being is inevitably linked to search engines nowadays. The question I ask myself most frequently is "to google or not to google" :) Surely God had intended for it! - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/25/2009
浮生:谢谢你的总结。现在我清楚一点点了。
我的PUZZLE:你没有回答我这个动植物盲人呢:是不是动物比植物,至少在绝对数量上,更多对称(以你解释的对称)的现象?
植物学家XW也许可以谈谈他的看法。
你和八先生的科普文章都写得好。为什么你写这类东西可以挥洒自如,写游记时文字就有点“紧"? 这是我的最新PUZZLE. 你不用回答我。我有回答我自己。
- Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/25/2009
草叶 wrote:
先举个人心脏胚胎发育的例子, 从对称到不对称。为什么人有一个心脏但有两个肺?
不小心打开这条线,居然这么长,这些问题不早就讨论过了?为什么人只有一个鼻子但有两个耳朵? :))
内部不对称是为功能,但外部对称哪里是为美学?当然是为了平衡。你们这些生物学家白学了。 - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/25/2009
老瓦 wrote:
内部不对称是为功能,但外部对称哪里是为美学?当然是为了平衡。你们这些生物学家白学了。
哈。这是我前面困惑的一部分。动植物分一下,内外不该也分一下吗(指草叶贴的那些器官)。
生物学家不喜欢make a distinction?
- posted on 11/25/2009
老瓦,平衡就是功能,美学也不是为了美学,当年你找女朋友的时候,外表的attraction就在美学层面,这里一定有对称。但这个“美”encode的还是功能,一方面可能是生殖优势(比如body shape)另方面可能是less degree of malfunction :)
小蜜蜂,我不知道动植物具体在对称性上的区别,感觉每一个的variations都很大。如果你直觉动物的对称性更强,也许是因为我们更习惯于bilateral对称,而这个如上面ZT里说的跟动物往前移动有关,但量化上我完全不知道。但是,Dawkins对动物对称性的强调另有一个easier explanation: 他自己是zoologist。这就如同你问为什么草叶更注重器官,因为她是医生,所以肯定是对人体,develomental biology和pathology更有直接的expertise。对称肯定是分各个层次,整体局部宏观微观的。 - Re: 草叶请进:关于生物对称非对称的进化理论posted on 11/26/2009
浮生, what can I say, you are the most amazingly intelligent and empathic people , period。
It is not just natural selection also sexual selection。 Most of chromosome abnormality have dysmorphic features including asymmetry(we secretly call them Funny looking kids)
Those people mostly likely not going to survive or having children. We only see the winners.
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