跟读练习: The different types of mutations | Biomolecules | MCAT | Khan Academy - 通过YouTube学习英语口语
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Voiceover: So, today we're going to talk about the different types of genetic mutations that you would find in a cell.
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Voiceover: So, today we're going to talk about the different types of genetic mutations that you would find in a cell.
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But first, I want to review the central dogma of molecular biology and how the genetic information of a cell is stored in the form of DNA, which is then transcribed to form RNA and then translated to generate protein.
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Nucleotides from the DNA are transcribed to their complementary forms on RNA, which are then read as codons or groups of three, to code for specific amino acids in a larger protein.
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Now, if you mutate one of the nucleotides on DNA, like let's say turning this thymine-based into an adenine-based, then that will affect the RNA sequence and ultimately the protein that follows.
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So, we say that mutations are mistakes in a cell's DNA that ultimately lead to abnormal protein production.
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So, what are the different types of mutations?
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Well, the first type of mutations we're going to talk about are called point mutations.
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Now, here I've just written out a random sequence of DNA, which is just a repeating pattern of CTC, which would code for a repeating sequence of GAG in the RNA strand, and finally, a protein sequence of three glutamate amino acids.
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So, a point mutation is when one of our DNA bases is replaced with another.
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So, in this example, a thymine-based is being replaced with an adenine-based, which leads to a change in one RNA nucleotide and ultimately a change in one amino acid.
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Another type of mutation is called frame-shift, which works a little differently.
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So, first I'll write out the same DNA, RNA, and protein sequences from before, but now, instead of changing one base to another, I'm going to add one to the sequence, and here I've thrown in this extra cytosine base that I've written in blue.
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Now, naturally, this change would lead to an additional guanine base being in the resulting messenger RNA sequence, but what's interesting is that this mutation will change the reading frame of the RNA.
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Remember that RNA is read in groups of three or codons when being translated to form protein, but now, since we've added an extra G here, all of the codons coming after that extra G will look a little differently.
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Now, instead of having three GAG codons, we've swapped out two for GGA codons.
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This means that two of our amino acids in the final protein will be changed, and in this example, they'll be changed from glutamate to glycine.
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So, you can see that frame-shift mutations usually have more significant effects on the final protein than point mutations do.
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Now, it's important to recognize that both of these mutations are classified and named for how they affect the cell's DNA structure and aren't really named for how they affect the resulting protein.
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Now, our next type of mutations are non-sense mutations and missense mutations.
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Let's say we have a DNA sequence that normally generates RNA and codes for a cysteine amino acid.
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A non-sense mutation is any genetic mutation that leads to the RNA sequence becoming a stop codon instead.
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Now, missense mutations are a little different, and they're any genetic mutation that changes an amino acid from one to another.
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So, in this example, our mutation is changing the resulting amino acid from a cysteine to a tryptophan.
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Now, you can see that non-sense mutations probably affect the resulting protein a lot more than missense mutations do, since that new stop codon that we're creating could chop off a huge section of the protein, instead of just changing one amino acid to another.
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So, now we can divide the missense mutations even further into a bunch of smaller categories.
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Silent mutations are when the mutation doesn't actually affect the protein at all.
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Since many different RNA codons can code for the same amino acid, it's possible that the mutation might not affect the protein at all.
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So, in this example, CCA, CCG, CCT, and CCC in the section of DNA will all end up coding for glycine.
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So, if you change the third base, it wouldn't affect the final protein.
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Conservative mutations are where the new amino acid is of the same type as the original.
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So, here I have a glutamate and an aspartate, which are both acidic amino acids.
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So, a mutation that swapped out an aspartate for a glutamate would be a conservative mutation.
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Finally, a nonconservative mutation is one with a new amino acid is of a different type from the original.
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So, here we have a serine amino acid, which is a small polar amino acid, being replaced with phenylalanine, which is a large, nonpolar, aromatic amino acid, and this would be an example of a nonconservative mutation, since serine and phenylalanine are different types of amino acids.
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Now, I'll point out again that all of these mutations are classified and named for how they affect the resulting proteins and aren't really named for how they affect the cell's DNA.
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So, let's look at a quick example.
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Sickle cell disease is a disorder where hemoglobin or Hb, which is a protein found in human blood, is mutated into a less active form, which we're going to call HbS, and it results from a single glutamate residue being converted into a valine residue.
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Now, we can classify this mutation as a point mutation, since only one DNA base is affected, but we can also say that it's a nonconservative missense mutation, since glutamate is being swapped out for valine, and the two are different types of amino acids, since glutamate is an acidic amino acid, and valine is a nonpolar one.
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So, what did we learn?
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Well, first we learned that mutations originate at the DNA level, but show their effects on the protein level, and second, we learned that we can classify different types of mutations by either their effects on DNA or their effects on protein.
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In reference to DNA, we have point and frame-shift mutations, and in reference to protein, we have missense and non-sense mutations.
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为什么要通过这个视频练习说话？
练习英语口语是一项重要的技能，而观看诸如 "不同类型的突变" 的视频不仅可以帮助你了解科学内容，还能增强你的口语能力。通过模仿视频中的讲解者，你可以在一定程度上提高自己的发音、语速和表达能力。在这个视频中，讲解者使用清晰的句子结构和逻辑流畅的论述，使得学习者能够更容易地理解并跟读。同时，分析科学主题能让你在与他人讨论相关话题时显得更为自信。这种练习方式也正是“英语口语练习”的一个重要方法。
语法和表达在语境中的分析
在该视频中，有几个关键句式和表达方式值得学习：
- 被动语态 - 讲解者多次使用被动语态来强调突变的结果，例如“突变是细胞DNA中的错误”。这种结构在科学写作中非常常见。
- 条件句 - 例如“如果你改变DNA中的一个碱基……”。这种句式有助于表达假设情况，非常适合雅思口语练习。
- 名词性从句 - “我们说突变是细胞DNA中的错误”中的“突变是细胞DNA中的错误”充当主语，有助于学习如何用复杂的句子传达复杂的概念。
常见发音陷阱
在视频中，有一些可能导致发音困扰的词语和表达方式：
- Mutation - 发音时注意重音位置，初学者常会混淆其发音。“突变”的正确发音为/mjuːˈteɪʃən/。
- Transcription - 虽然这个词很常用，但发音可能令人困惑，很多学习者可能会发成/trænˈskrɪpʃən/，而正确的发音是/trænˈskrɪpʃən/。
- Codon - 在这个领域，"codon"（密码子）是一个重要词汇，应该练习其发音以确保清晰地表达概念。
通过观看和模仿视频，不仅可以提升你的英语口语能力，还能在实际交流中更加自信。通过这些指导，努力去“看YouTube学英语”，在发音和表达上不断进步！
什么是跟读法？
跟读法 (Shadowing) 是一种有科学依据的语言学习技巧，最初开发用于专业口译员的培训，并由多语言者Alexander Arguelles博士普及。这个方法简单而强大：您在听英语母语原声的同时立即大声重复——就像是一个延迟1-2秒紧跟说话者的影子。与被动听力或语法练习不同，跟读法强迫您的大脑和口腔肌肉同时处理并模仿真实的讲话模式。研究表明它能显着提高发音准确性，语调，节奏，连读，听力理解和口语流利度——使其成为雅思口语备考和真实英语交流最有效的方法之一。