Here's a conundrum: identical twins originate from the same DNA,
这里有一个谜题：同一双胞胎源于相同的DNA，
so how can they turn out so different even in traits that have a significant genetic component?
那么为什么他们会出现巨大的差异，即使他们拥有一样的基因组成特征？
For instance, why might one twin get heart disease at 55, while her sister runs marathons in perfect health?
举个例子，为什么双胞胎中的一个可能在55岁时患上心脏病，然而她的姐姐却以十分健康的状态跑完马拉松？
Nature versus nurture has a lot to do with it,
先天和后天有很多关系，
but a deeper related answer can be found within something called epigenetics.
但是更深层的答案可以在表观遗传学中找到。
That's the study of how DNA interacts with the multitude of smaller molecules found within cells,
这是一门关于DNA如何同大量在细胞中的微小分子相互作用的科学，
which can activate and deactivate genes.
这个过程可以激活或隐藏基因。
If you think of DNA as a recipe book, those molecules are largely what determine what gets cooked when.
如果你把DNA想象成为一本食谱，这些分子就是决定什么时候煮什么的主要因素。
They aren't making any conscious choices themselves,
它们自身并没有做出任何有意识的决定，
rather their presence and concentration within cells makes the difference.
但是它们在细胞内的存在和集中让一切都不同了。
So how does that work?
所以一切是怎么发生的呢？
Genes in DNA are expressed when they're read and transcribed into RNA,
当基因从DNA中读取和转录到RNA时会发生挤压，
which is translated into proteins by structures called ribosomes.
通过结构向蛋白质转化，成为核糖体。
And proteins are much of what determines a cell's characteristics and function.
并且蛋白质很大程度上决定了一个细胞的特性和功能。
Epigenetic changes can boost or interfere with the transcription of specific genes.
表观遗传变异会促进或妨碍特定基因的转移。
The most common way interference happens is that DNA, or the proteins it's wrapped around, gets labeled with small chemical tags.
最普遍的干扰方式发生在当DNA，或者蛋白质在被缠绕，得到化学标记时。
The set of all of the chemical tags that are attached to the genome of a given cell is called the epigenome.
所有的化学标记都依附于染色体组，属于一个特定的细胞，被称作是表观基因组。
Some of these, like a methyl group, inhibit gene expression
它们其中的一些，像甲基，抑制基因的表达
by derailing the cellular transcription machinery or causing the DNA to coil more tightly, making it inaccessible.
通过阻碍细胞的转录或导致DNA的盘绕更紧密，使得它们难以看到。
The gene is still there, but it's silent.
基因依旧存在，但它是静止的。
Boosting transcription is essentially the opposite.
促进转录有重要的反作用。
Some chemical tags will unwind the DNA, making it easier to transcribe,
一些化学标记将展开DNA，让它们更加容易被转录，
which ramps up production of the associated protein.
这会加强相关蛋白质的生产。
Epigenetic changes can survive cell division, which means they could affect an organism for its entire life.
表观遗传变异能够使细胞分裂继续，这意味着它们能够影响一个有机体的全部生命周期。
Sometimes that's a good thing.
有的时候这是一件好事。
Epigenetic changes are part of normal development.
表观遗传变异是正常发育的一部分。

The cells in an embryo start with one master genome.
胚胎上的细胞始于一个最先的基因组。
As the cells divide, some genes are activated and others inhibited.
当细胞开始分裂，一些基因被激活，其他的被抑制。
Over time, through this epigenetic reprogramming,
一段时间后，通过表观遗传学的重编程序，
some cells develop into heart cells, and others into liver cells.
一些细胞成长为心脏细胞，其他的成为肝脏细胞。
Each of the approximately 200 cell types in your body has essentially the same genome but its own distinct epigenome.
身体大约有200种的细胞，基本上，每种都有相同的基因组，但各自拥有独特的表观基因组。
The epigenome also mediates a lifelong dialogue between genes and the environment.
表观基因组也会促成基因与环境间终生的对话。
The chemical tags that turn genes on and off can be influenced by factors including diet, chemical exposure, and medication.
开关基因的化学标记会受一些因素影响，包括饮食、化学物质的暴露和药物。
The resulting epigenetic changes can eventually lead to disease,
所造成的表观遗传变化可能最后导致疾病，
if, for example, they turn off a gene that makes a tumor-suppressing protein.
比如它们会关闭制造抑制肿瘤蛋白质的基因。
Environmentally-induced epigenetic changes
环境导致的表观遗传变化，
are part of the reason why genetically identical twins can grow up to have very different lives.
部分解释了为什么同卵双胞胎长大后会有极大不同的生活。
As twins get older, their epigenomes diverge, affecting the way they age and their susceptibility to disease.
当双胞年纪渐长，他们的表观基因组出现差异，影响他们老化的方式和对疾病的易感性。
Even social experiences can cause epigenetic changes.
甚至社交经验也能造成表观遗传变化。
In one famous experiment, when mother rats weren't attentive enough to their pups,
在一个著名的实验中，当母鼠对其幼鼠关心不足，
genes in the babies that helped them manage stress were methylated and turned off.
幼鼠体内协助处理压力的基因会被甲基化且关闭。
And it might not stop with that generation.
而且可能不会在那一代就停止。
Most epigenetic marks are erased when egg and sperm cells are formed.
当卵子和精子细胞形成时，大多数表观遗传的标记会被去除。
But now researchers think that some of those imprints survive, passing those epigenetic traits on to the next generation.
但现在研究者认为，有些印记会留存下来，将那些表观遗传的特征传到下一代。
Your mother's or your father's experiences as a child, or choices as adults, could actually shape your own epigenome.
你的父母小时候的经验或成人时的抉择，可能塑造你自己的表观基因组。
But even though epigenetic changes are sticky, they're not necessarily permanent.
虽然表观遗传变化很黏人，但它们未必是永久的。
A balanced lifestyle that includes a healthy diet, exercise, and avoiding exposure to contaminants
一个均衡的生活形态，包括健康的饮食、运动和避免暴露到污染物，
may in the long run create a healthy epigenome.
从长远看，可以创造健康的表观基因组。
It's an exciting time to be studying this.
现在是研究这门科学的热络时刻。
Scientists are just beginning to understand how epigenetics could explain mechanisms of human development and aging,
科学家正开始去了解表观遗传学如何解释人类发育和老化的机制，
as well as the origins of cancer, heart disease, mental illness, addiction, and many other conditions.
以及癌症、心脏疾病、精神病、成瘾和许多其他疾病的源起。
Meanwhile, new genome editing techniques are making it much easier to identify which epigenetic changes really matter for health and disease.
同时，新的基因组编辑技术使得我们更容易辨识哪些表观遗传变化对健康和疾病是真正重要的。
Once we understand how our epigenome influences us, we might be able to influence it, too.
一旦我们了解表观基因组是如何影响的我们，我们可能也可以影响它。