章节大纲

  • Types of Life
    ::生命种类

    The first instance of life was likely very simple and single-celled. Today, life is fantastically varied and complex. We separate the life we see today into three   domains based on shared cellular structure and genetic material. The three domains of life,  archaea, bacteria, and eukaryotes, are a diversification of the Last Universal Common Ancestor.   
    ::生命的首个例子很可能非常简单,并且是单一的。 今天,生命是千变万化而复杂的。 我们把今天所看到的生活分成三个领域,以共享细胞结构和遗传材料为基础。 生命的三个领域,即古代、细菌和eukaryotes,是最后的普世共同祖先的多样化。

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    Tree of Life shows genetic relationship between all life on Earth. The stem represents the concept of the Last Universal Common Ancestor.
    ::生命树显示了地球上所有生命之间的基因关系。干叶代表了最后一个世界共同祖先的概念。

    • Bacteria   make up  the largest domain with both the greatest number of  individual species and  a biomass that exceeds the combined biomass of all plants and animals. Bacteria  were one of the earliest forms of life on our planet and they are found in most habitats. B acteria can rapidly recombine their genes with other bacteria to allow for genetic innovations, such as resistance to antibiotics.  
      ::细菌构成最大的领域,其个别物种数量最多,生物量超过所有动植物的生物量总和。 细菌是地球上最早的生命形式之一,在大多数生境中都有。 细菌可以与其他细菌一起迅速恢复基因,以便进行基因创新,比如抗抗生素抗药性。
    • Archaea are genetically distinct from bacteria  with their own, separate domain. Archaea are known to generate energy in a variety of ways and have been found in some of the most extreme environments on Earth.
      ::Archaea在遗传上与细菌不同,细菌有自己的、单独的领域,Archaea以各种方式产生能源,在地球上一些最极端的环境中发现。
    • Eukaryotes are distinct from prokaryotes in containing a central nucleus enclosed in a membrane and also contain other membrane-bound organelles. Examples of eukaryote organelles include chloroplast, the site of photosynthesis in plants and some algae, and mitochondria, where energy is generated in a cell. Eukaryotes can be uni- or multi-cellular, allowing for larger and more complex organisms. 
      ::Eukaryotes与prokaryotes不同,它含有嵌入膜内的一个核心核,并含有其他含有膜的有机物。 eukaryote 有机物的例子包括氯板、植物和某些藻类中的光合作用场所、以及细胞产生能量的mitochondria。 Eukaryotes可以是单细胞的,也可以是多细胞的,允许更大和更复杂的有机物。

     

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    A comparison of eukaryotic cells to prokaryotic cells. Note how the nucleoid, or DNA, of the prokaryotic cell is free floating while the DNA of the eukaryotic cell is contained with a nuclear membrane. While both have certain organelles, such as ribosomes, only eukaryotes possess membrane-bound organelles like mitochondria.
    ::将乳腺细胞与蛋白质细胞作比较。 请注意,蛋白质细胞的核素或DNA是如何自由漂浮的,而蛋白质细胞的DNA却用核膜封住。 虽然两者都有某种器官,如脊椎细胞,但只有eukaryotes才拥有有膜的有机物,如米托孔德里亚。

     

    Both bacteria and  archaea   are   prokaryotes:  single- celled microbes that do  not contain membrane-bound organelles.  However, t he  membranes in archaea incorporate a different type of lipid than either bacteria or eukaryotes. Genetic analysis reveals that archaea are closer to eukaryotes in an evolutionary sense than they are to bacteria. A rchaea and eukaryotes use  many of the same enzymes  for DNA translation.
    ::细菌和考古都是近似体:单细胞微生物不含有膜带有机细胞。然而,考古中的膜含有一种不同于细菌或eukaryotes的脂质。遗传分析表明,在进化意义上,考古比细菌更接近于eukaryotes。考古和水星使用许多相同的酶进行DNA翻译。

    Endosymbiosis
    ::内生共生

    Eukaryotic cells have gained membrane-bound organelles and increased complexity through the process of endosymbiosis.   Endosymbiosis is  a process whereby  primitive organisms benefited by living inside other organisms. Chloroplasts and mitochondria are examples of highly complicated organelles in eukaryotes that have their own membrane. They retained the DNA, messenger RNA, transfer RNA, and ribosomes of their bacterial ancestors  before they   were symbiotically incorporated into larger eukaryotic cells. The larger cell presumably provided protection and easy access to organic molecules while the chloroplast and mitochondria provided energy to the larger cell. This beneficial relationship led to the creation of larger, more efficient cells.
    ::内生生物是一种过程,使原始生物从其他生物中受惠。氯和mitochondria是拥有自己薄膜的eukaryotes中高度复杂的有机体的例子。它们保留了其细菌祖先的DNA、送信者RNA、转移RNA和肋骨,然后才被共生地纳入更大的电子水生细胞。较大的细胞可能提供保护和容易获得有机分子,而叶片和mitochondria则为更大的细胞提供能量。这种有益的关系导致创建更大、更高效的细胞。

       Mother's Mitochondria
    ::母亲的密托昆德里亚

    With sexual reproduction, the offspring ends up with a combination of the mother's and father's DNA. However, the DNA found in mitochondria is exclusively the mother's DNA. When cells are replicated, the mitochondria split themselves as needed and so maintain a self-consistent set of DNA. With mammals, the egg destroys most of the mitochondria in sperm when they merge. In addition, most of the sperm's mitochondria is positioned in the tail to provide energy and does not make it into the egg.
    ::生殖后,后代最终会结合母亲和父亲的DNA。然而,在米托乔因德里亚发现的DNA完全属于母亲的DNA。当细胞被复制时,米托乔因德里亚会按需要分裂自己,从而保持一组自成一体的DNA。随着哺乳动物,卵在精子合并时会摧毁大部分的精子。此外,大部分精子的米托乔因德里亚(mitochondria)被放在尾巴里以提供能量,而不会进入卵中。

    This preservation of maternal mitochondrial DNA is often used to trance ancestries. Because it is contained only in the mitochondria and is  infrequently used, mitochondrial DNA also suffers fewer mutations. It is therefore also helpful in determining the ancestry of different species and how they fit onto the tree of life.
    ::这种保存母线粒体DNA的做法常常被用于摇晃动脉,因为它只包含在米托乔因德里亚,而且很少使用,因此,线粒体DNA的突变也较少,因此也有助于确定不同物种的祖先以及它们如何与生命树相适应。

    Cyanobacteria
    ::阴阳杆菌

    The rise of oxygen likely began with organisms known as cyanobacteria, an early type of photosynthetic bacteria thought to be the first organism to produce oxygen as a byproduct. Photosynthesis is the process by which organisms can harness the energy of the sun to generate energy for their own use. Cyanobacteria are the only know prokaryotes to produce oxygen, and this adaptation  brought about the destruction of  many other organisms. 
    ::氧气的上升可能始于被称为氰化物的生物,一种早期的光合作细菌,被认为是第一个作为副产品生产氧的有机体。 光合作用是生物能够利用太阳的能量为自身使用而产生能量的过程。 氰化物是唯一知道产生氧气的原生虫,这种适应导致了许多其他生物的毁灭。

     

       Oxidation Reactions and the Free Radicals
    ::氧化物反应和自由激进党

    Oxidation  describes a chemical process in which a molecule, atom, or ion loses an electron. Oxygen is  a particularly good oxidizing agent because its nucleus strongly attracts electrons to fill its valence shell. Oxidation often produces free radicals, which are very reactive.  They  can attack and break apart bonds in other molecules, inciting a chain reaction that is  damaging  for  bio chemical reactions.
    ::氧化性是指分子、原子或离子失去电子的化学过程。 氧是一种特别好的氧化剂,因为其核极能吸引电子以填充其价值外壳。氧化性经常产生自由基,这些自由基反应性很强。它们可以攻击其他分子并分解其联结,引发对生化反应有害的连锁反应。

    Antioxidants  are compounds that inhibit oxidation and thereby prevent the formation of free radicals. Plants and animals have many natural antioxidant systems in place to guard against this or use antioxidant vitamins such as vitamin A, vitamin C, and vitamin E.  A certain amount of antioxidants is required in a well-balanced diet. However, clinical studies have been unable to prove benefits of artificially increasing antioxidant intake or antioxidant supplements.
    ::植物和动物拥有许多天然抗氧化剂系统,可以防范或使用抗氧化剂维生素A、维生素C和维生素E等抗氧化剂维生素。 饮食平衡需要一定数量的抗氧化剂,但临床研究无法证明人工增加抗氧化剂摄入量或抗氧化剂补充剂的好处。

    The rise in atmospheric oxygen was far from smooth and steady (  below). Cyanobacteria slowly became more abundant , but there was still a significant delay in the build up of oxygen. There are several processes that  would have hindered the rise in oxygen. Oxygen  would have reacted with various chemicals, mainly iron, in the oceans, and these reactions would trap oxygen, prevent it from  building up the atmosphere.
    ::大气氧的上升远非平稳和稳定(下面)。 氰化细菌慢慢地变得更加丰富,但氧的积累仍然严重拖延。 有几个过程会阻碍氧气的上升。 氧本会对各种化学物质(主要是海洋中的铁 ) 作出反应,而这些反应会困住氧气,防止氧气形成大气层。

     

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    The amount of oxygen in the atmosphere as a function of billions of years ago (Gyr). While the start of photosynthesis corresponds to an increase an oxygen, there is a notable plateau before oxygen jumps up to current day levels.
    ::数十亿年前(Gyr),大气中的氧含量相当于数十亿年前(Gyr ) 。 光合作用开始与氧的增加相对应,但在氧气跳升到当前水平之前,有显著的高度。

    Oxygen may also have been  taken up  by microbes in metabolic pathways that generate energy. O rganisms that used the oxygen to oxidize ammonia  appear to have been plentiful at the time and  could have been effective in the reduction of  free-floating oxygen.
    ::氧也可能被产生能源的代谢路径中的微生物吸收。 使用氧氧化氨的生物当时似乎非常丰富,可以有效地减少自由漂浮的氧气。

    Other organisms produced methane as a byproduct  that could have acted as a sink for atmospheric oxygen.  However, organisms that are known to produce methane require nickel to carry out the necessary reactions, and concentrations of nickel were dropping .  The decrease of nickel  would mean less excreted methane,  and would  allow oxygen to begin  accumulating .
    ::其他生物作为副产品产生的甲烷本可作为大气氧的吸收槽,然而,已知生产甲烷的生物需要镍来进行必要的反应,而镍的浓度正在下降。 镍的减少意味着排泄的甲烷减少,并允许氧气开始积累。

    The increase of oxygen in the atmosphere was poisonous for anaerobic organisms. M ost life before the Great Oxidation Event was anaerobic, so the rise of atmospheric oxygen surely resulted in one of the most significant extinction events in Earth's history. The fossil record shows a mass extinction of anaerobic life around 2.4-1.6 billion years ago, coincident with the rise of aerobic life.
    ::大气中氧气的增加对厌氧生物来说是有毒的。 大氧化事件之前的多数生命都是厌氧的,因此大气氧气的上升必然导致地球历史上最重要的灭绝事件之一。 化石记录显示,厌氧生物在24亿到16亿年前大量灭绝,与有氧生命的上升同时。

    Anaerobic vs Aerobic
    ::有氧对有氧

    The rise of atmospheric oxygen  coincided with the appearance of far more complicated life forms. There is every reason to believe that the rise in oxygen would have been  responsible since  aerobic metabolisms are more efficient . 
    ::大气氧气的上升与更为复杂的生命形式的出现同时发生。 有充分理由相信,氧气的上升本来是应该负责的,因为有氧新陈代谢效率更高。

    As  David Bowie sang : Ch-Ch-Ch-Ch-Changes. This is a key attribute of life! 
    ::正如大卫·鲍伊唱的:Ch -Ch -Ch -Ch -Ch

     

     

    Respiration, in biological terms, describes the process by which organisms convert nutrients into usable energy by forming ATP bonds. Respiration begins  with glycolysis, wherein glucose, a sugar, is broken down to form two molecules of a compound called pyruvate along with two molecules of ATP. Without oxygen, organisms have no choice but to undergo anaerobic respiration, or fermentation. Anaerobic respiration of yeast is what makes bread rise and beer bubbly.  In this scenario, pyruvate is shuttled down a pathway which produces just two molecules of ATP.
    ::呼吸用生物术语描述生物通过形成 ATP 键将营养素转换成可用能量的过程。 呼吸从胶解过程开始, 甘糖( glucose, 糖) 分解成一种化合物的两种分子, 一种叫做Uvruvate, 一种叫做ATP 的两种分子。 没有氧, 生物别无选择, 只能进行厌氧呼吸, 或发酵。 酵母的厌氧呼吸是面包上升和啤酒泡泡的产物。 在这种情形下, 春酸会经过一条路径, 只产生两种ATP 的分子 。

       Sore Winners
    ::赢家 赢家 赢家 赢家 赢家 赢家 赢家 赢家 赢家 赢家

    A byproduct of anaerobic respiration is a molecule called  lactic acid . Lactic acid should be very familiar to any athlete or any student who has been very late to class and had to make a run for it. When our bodies overexert themselves, we begin to use up more oxygen than we  can take in . In order to produce the energy needed to keep running, the body switches to anaerobic respiration, which leads to a build up of lactic acid. Lactic acid can damage  muscle cells and hinder recovery. The process of producing this lactate is also the cause of next-day muscle soreness.

    In the presence of oxygen, aerobic respiration becomes possible. With the help of oxygen, pyruvate can be broken down and enters a more complicated pathway known as the Krebs Cycle or the Citric Acid Cycle   (Note that organisms exhibit an enormous variety of metabolic pathways; the citric acid cycle merely represents one of the more common and well understood pathways). Through the Citric Acid Cycle, organisms can produce from 30 to 36 ATP from just one molecule of glucose. Though more complicated to assemble, and therefore likely taking longer to evolve, this process can be up to  18-fold more efficient than anaerobic respiration. With more energy, it is possible to carry out  more biochemical processes. This would have allowed cells to become increasingly complex and trend towards the more varied, multicellular life we see today. With great metabolism comes great complexity.
    ::在氧气存在的情况下,有氧呼吸是可能的。在氧气的帮助下,回旋剂可以分解,进入被称为Krebs循环或Citric酸循环的更复杂的路径(请注意,有机体呈现出多种多样的代谢途径;柠檬酸循环只是更常见和更为人熟知的途径之一 ) 。通过碳酸循环,有机体可以从仅仅一个葡萄糖分子中产生30到36ATP。虽然集聚起来比较复杂,因此可能需要更长的时间才能进化,但这一过程可能比厌氧呼吸效率高18倍。有了更多的能量,就有可能进行更多的生物化学过程。这将使细胞变得日益复杂,朝着我们今天看到的更多样化的多细胞生活发展趋势。随着巨大的新陈代谢过程变得非常复杂。

     

    Viruses?
    ::病毒吗?

    V iruses are even more abundant than bacteria.  A virus  consists of three functional parts: (1) genetic material, (2) protein coat, and typically (3) an envelope of lipids outside the protein coat. For genetic material, viruses have been discovered to use both DNA and RNA. The protein coat, also known as the   capsid, encases and protects this genetic material. The envelope of lipids adds an additional layer of protection.
    ::病毒比细菌更丰富,病毒由三个功能部分组成伤心1) 基因材料,(2) 蛋白质大衣,以及(3) 典型的(3) 蛋白质大衣外的脂肪包。对于基因材料而言,已经发现病毒同时使用DNA和RNA。蛋白质大衣,又称为卡皮、内壳和保护这种基因材料。脂肪包包又增加了一层保护层。

    However, the question  of   whether viruses can be considered life is hotly debated. Viruses have their own genetic material and are even capable of evolving through natural selection. Viruses survive and replicate by infecting a host cell. After attaching to a cell, the virus injects its DNA into the host cell. Now, the virus is capable of taking over the host cell's replication mechanisms. In doing so, the virus   can now create  copies of itself until it has exhausted the cell's resources. Millions of viruses can be made in this period before the cell dies and viruses escape to infect new host cells. Because viruses require the  metabolism   of a host cell to produce energy and reproduce, they can not be called self-sustaining. In this way, viruses fail the NASA definition of life.
    ::然而,病毒能否被视为生命的问题却引起了激烈的辩论。病毒有其自身的遗传物质,甚至能够通过自然选择而演变。病毒通过感染宿主细胞而生存和复制。病毒附着在细胞之后,病毒将其DNA注入宿主细胞。现在,病毒能够接管宿主细胞的复制机制。在这样做的时候,病毒现在可以创造自己的副本,直到它耗尽了细胞的资源。在细胞死亡和病毒逃脱感染新宿主细胞之前的这段时间里,可以制造出数百万个病毒。由于病毒需要宿主细胞的代谢性来生产能量和繁殖,它们不能被称为自我维持。因此,病毒无法满足美国航天局的生命定义。

    Despite their differences, evidence of viruses appear wherever life does, suggesting that viruses and life evolved together.  The history of viruses is traced through their DNA or RNA and has given rise to three theories on how viruses came to be.
    ::尽管病毒存在差异,但病毒的证据在生命存在的地方都有,这表明病毒和生命是一起演变的。 病毒的历史通过它们的DNA或RNA来追溯,并引出了三个关于病毒是如何形成的理论。

    Regressive evolution theory   proposes that viruses were once components of small, parasitic cells. Similar to the idea of endosymbiosis, viruses may have begun as small structures within larger cells that over time became separated. In fact, there exist today some bacteria that, like viruses, can only reproduce in a host cell. As they evolved, these small parasitic structures regressed further from cell-like characteristics, becoming the viruses we see today. However, there is no evidence of types of cells today that could serve as an intermediary between early and present day viruses. Even the smallest cellular parasites fail to really resemble viruses at all.
    ::倒退进化理论认为病毒曾经是小的寄生细胞的组成部分。与内生生物概念类似,病毒可能开始作为大细胞中的小结构,随着时间推移而分离。事实上,今天存在一些细菌,像病毒一样,只能在宿主细胞中繁殖。随着它们的进化,这些小寄生体结构从类似细胞的特性中进一步倒退,成为我们今天所看到的病毒。然而,今天没有证据表明各种细胞可以作为早期病毒和现今病毒的中间体。即使是最小的细胞寄生虫也完全不能像病毒一样。

    Another theory is the   escaped gene theory , which proposes that viruses got their start as DNA or RNA that escaped from the genome of a larger organism. Surprisingly mobile DNA has recently come to play a large role in biology.   Plasmids   (  below) are circular units of DNA and separate from the genome of an organism. They are most commonly found in bacteria and have been known to move between cells.  Scientists  have also recently discovered  transposons, or "jumping genes," which are large segments of DNA that can move around within a cell's genome. While either of these mechanisms could have provided the genetic material for viruses, it remains unclear where the complicated capsids enclosing these genes arose from.
    ::另一个理论是逃生基因理论,该理论建议病毒从较大的生物基因组中脱颖而出,以DNA或RNA为起点。 令人惊讶的是,移动式DNA最近在生物学中扮演了重要角色。 Plasmids(下面)是DNA的圆形单位,与生物基因组分离。它们通常在细菌中发现,并已知可以在细胞间移动。科学家最近还发现了转基因子,或“跳动基因 ” , 它们是基因组中可以移动的很大一部分DNA。 尽管这两种机制都可以为病毒提供遗传材料,但目前还不清楚这些基因的复杂特性来自何处。

     

     

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    Plasmids, pictured as small, blue loops. Being much smaller than chromosomal DNA, plasmids can be transcribed more easily and move from one organism to another.
    ::象蓝色小环状的等离子体。 比染色体DNA要小得多, 质子可以更容易地被转录, 从一个生物体转移到另一个生物体。

    The   pre-cellular origin theory   proposes that viruses formed from complex proteins that spontaneously arose even before RNA. Viruses could have evolved alongside the first cells or even before. However, this fails to account for the dependency viruses have on host cells. If viruses need the mechanism of a host cell to survive, how did they manage before cells even evolved?
    ::预细胞起源理论认为病毒是由复杂的蛋白质形成的,这些蛋白质在RNA之前就自动生成了。 病毒本可以和第一批细胞一起演变,甚至在此之前。 但是,这不能说明病毒对宿主细胞的依赖性。 如果病毒需要宿主细胞的机制来生存,那么它们是如何在细胞进化之前管理的呢?

    The exact origin of viruses, much like the origin of life, remains an open question. This vein of research is being pushed forward through analysis of viral and host DNA sequences. What we can say is that genetic comparisons show that the origins of viruses  may have   predated life splitting into the three different domains. Perhaps the precursors to viruses evolved from the self-replicating molecules that dominated the RNA world. Similar to RNA, viruses are capable of self-assembling in host cells. 
    ::病毒的确切来源,与生命起源一样,仍然是一个未决问题。这种研究的脉络正在通过病毒和主机DNA序列的分析推向前进。我们可以说,基因比较表明病毒的起源可能早于生命,已经分裂到三个不同的领域。也许病毒的先质是从主宰RNA世界的自我复制分子中演变而来的。和RNA一样,病毒能够在宿主细胞中自我组装。

    Viruses are now well enough understood to be used as tools in biomedical research. In a process called gene therapy,   researchers use viruses to  inserting genes into specific cells, offering possible  treatments  for diseases like cystic fibrosis.  Some   viruses   will  seek out and destroy  cancer cells, while leaving healthy cells alone, allowing for a highly targeted and effective   treatments .
    ::如今,人们已经足够理解病毒可以用作生物医学研究的工具。 在所谓的基因治疗过程中,研究人员利用病毒将基因插入特定的细胞,为细胞纤维化等疾病提供可能的治疗方法。 一些病毒将寻找并摧毁癌症细胞,而将健康细胞单独留给健康细胞,从而能够进行高度定向和有效的治疗。