跟读练习: PCR (Polymerase Chain Reaction) - 通过视频学习英语口语

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There are some pieces of technology you really develop an appreciation for.

Maybe it's your phone, your laptop, your drawing tablet, but for me, it is the copy machine.

Oh, the copy machine.

I know it's not possible, but I could promise you that whenever the copy machine was aware of my presence, that's when it decided to malfunction,

especially if it knew I was short on time.

And of course, when it jams, the fancy copy machines will even tell you what is wrong and how to fix it,

but my experience with trying to follow the instructions of fixing the copy machine makes it worse, and then by that point there's a line,

and you feel your heart pounding, and you're embarrassed because now you've jammed it for everyone.

Yeah, so while I've developed an appreciation and respect for the copy machine, I've definitely tried to limit my use of it over the years.

Saves paper anyway.

You may be wondering why is she spending so much time on this?

Well I want to call your attention to a technology, a biotechnology, that works almost like an amazing and fancy copy machine.

Except not for something on paper.

No, it's for DNA.

This biotechnology is PCR.

Polymerase Chain Reaction, or PCR for short.

It provides a way to make more copies of a portion of DNA.

Lots and lots and lots of DNA copies, and this technology does NOT need to happen in a cell either.

In fact, it can happen in a test tube.

And so you may wonder, number one, how does PCR work?

And number two, why?

Why make more copies of some specific portion of DNA?

So let's briefly answer those two questions.

To answer the first question, the how does this work question,

let's talk about what we need first before we get to the steps of how it works.

We need the DNA portion that we want to make copies of, we need some kind of buffer to put it in,

and then we need things that would be necessary to make more copies of the DNA.

So think about what that might mean from our DNA replication video.

We'll need primers.

Recall that primers help DNA polymerase, a building enzyme, know where to go to start its building.

We need DNA polymerase, the building enzyme.

Fun fact, the DNA polymerase used is often a heat -resistant type of DNA polymerase as PCR uses heat.

Typically, the polymerase chosen for the job is tach polymerase, a type of of heat -resistant DNA polymerase.

Tac polymerase is originally from a type of bacteria that can handle and live in really high temperatures in nature, in hot springs.

Pretty cool.

We also need DNA nucleotides for the DNA polymerase to build with.

So we can look at the PCR sequence in three major steps.

We'll illustrate with one double -stranded DNA molecule here, and let's assume this is what we want replicated.

Step 1.

Denaturation.

You've heard that word denature before, right, when we were talking about enzymes?

One way we had talked about denaturing enzymes was using heat.

And heat is what we will use in this step.

This step involves the addition of heat needed to separate the two strands of the DNA molecule.

Step 2.

Annealing.

This word means something a little different in biology.

Basically this is when the two DNA strands

that now have been separated by that heat are going to be cooled and be joined by the primers.

The temperature for this step should allow the primers to bind to the specific segment of DNA that you want to amplify,

which means make copies of.

Step 3.

DNA synthesis.

Remember, synthesis means to make something.

With DNA synthesis, we're going to make more copies of DNA, and to do so, DNA polymerase will begin to work on both of these strands

and it will use the DNA nucleotides as its building material to amplify the DNA.

We should note that the temperature at this step may be a little warmer than the previous step.

It needs to be a temperature that is ideal for the specific DNA polymerase used.

Now after finishing one cycle of this, you have two double -stranded DNA molecules, right?

Similar to how it would be in DNA replication within a cell.

But you can repeat this now, except this time you now have two double -stranded DNA molecules to start with.

So you repeat with the denaturation, annealing, and DNA synthesis steps, now you have four double -stranded DNA molecules.

You repeat the steps again, now you have eight.

And if the process is automated by a machine, which, why yes, they do have in science catalogs, you can actually do this fairly quickly.

Which brings you to, why?

Why do this?

Well, any technology that needs copies of a portion of DNA could find PCR useful, but But we'll just mention two examples in our limited time.

We mentioned DNA fingerprinting in our gel electrophoresis video, and we mentioned that DNA fingerprinting can be part of a crime scene investigation.

Well, in order to have enough copies of DNA samples to run in gel electrophoresis to analyze,

PCR can be performed to make copies of the of the fragments of DNA that are found at a crime scene.

Another example?

Diagnosis of a disease, such as one caused by a virus.

For an especially relevant example, I can mention one of the testing types done for the virus that causes COVID -19.

COVID -19 is a pandemic that we're experiencing right now in 2020.

And the virus that causes COVID -19 is called SARS -CoV -2.

You might have heard of one test type that uses a sample from a nose or throat swab in a PCR test,

but to be more specific, this test for this virus is a real -time reverse transcription PCR test.

The reason it has this fancier reverse transcription in there is

because this virus uses RNA as its genetic material instead of DNA,

and you have to use an enzyme called reverse transcriptase to convert the RNA into DNA.

So before we can do the regular PCR steps we've mentioned, we must convert isolated and purified RNA into DNA.

A specific primer is added that will bind to an area of viral RNA,

and then reverse transcriptase is used to convert viral RNA into cDNA .

Using specific primers and the TAC DNA polymerase,

the cDNA can be copied over and over each cycle in the familiar steps we've been mentioning of PCR.

See the goal is you need enough copies of the viral cDNA in order for it to be detectable.

The idea being if it's a positive result, you have the selective primers binding and you have the TAC DNA polymerase making more

and more copies of the viral cDNA each cycle.

In addition, specific fluorescent probes are also used for identification.

A certain level is needed for identification of a positive result.

And if the virus's genetic material is not present in the sample, then primers wouldn't bind, and there would be no cDNA copies produced.

If you'd like to learn more about this test as far as its limitations

and also its complexity as we're being pretty general here, we've included some further reading suggestions in the video details.

So those were just two examples of how PCR can be used,

but there are many more examples in our suggested further reading links in the video description.

Overall, PCR is such a useful and fascinating technology that will likely remain indispensable for future uses.

It's a rare day when I pull out that word, indispensable.

Well, that's it for the Amoeba Sisters, and we remind you to stay curious.

I'll see you next time.

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本课的词汇与口语要点

视频中反复出现的词有：Dna, Pcr, polymerase, step, copies。这段视频共有 83 个句子、1299 个单词可供跟读。讲话部分时长为 7:30。说话人语速自然，每分钟约 173 个词，接近日常对话。只有 81% 的单词属于英语最常用的 3,000 词，词汇难度较高。

视频中的重点词汇

视频中 15 个值得学习的单词，附发音和释义：

单词	发音	释义
strand 名词	/ˈstɹænd/	綹 /绺, 一綹頭髮 /一绺头发
virus 名词	/ˈvaɪ.ɹəs/	病毒
enzyme 名词	/ˈɛn.zaɪm/	酶, 酵素
molecule 名词	/ˈmɑ.lɪ.kjul/	分子
synthesis 名词	/ˈsɪnθəsɪs/	綜合 /综合
identification 名词	/aɪˌdɛn(t)ɪfɪˈkeɪʃn̩/	辨識 /辨识, 鑑定 /鉴定
jam 名词	/ˈd͡ʒæm/	果醬 /果酱
curious 形容词	/ˈkjʊɹ.i.əs/	好奇, 有好奇心
ideal 形容词	/aɪˈdi(ə)l/	理想的
recall 动词	/ɹɪˈkɔl/	罷免 /罢免, 召回
remind 动词	/ɹəˈmaɪnd/	提醒
tube 名词	/tuːb/	筒
sequence 名词	/ˈsiː.kwəns/	序列, 順序 /顺序
throat 名词	/ˈθɹoʊt/	嗓子, 喉嚨 /喉咙
segment 名词	/ˈsɛɡmɛnt/	片斷 /片断, 段

值得反复跟读的句子

视频中简短而完整的句子，可以直接用在日常对话里：

You’ve heard that word “denature” before, right?
When we were talking about enzymes?
Which brings you to…why?

视频中的语法

说话人最常用的结构，并附上视频中的原话：

结构	视频中的用法
被动语态 be + 过去分词 — 强调发生了什么，而不是谁做的	have been separated · is automated · can be performed
现在完成时 have/has + 过去分词 — 过去发生但与现在仍有关联的事	you've jammed · I've developed · You've heard

需要注意的发音

说话人用了 16 次缩略和弱读形式，例如 we'll, we're, we've。请按听到的简短形式来说。

“th” 音: synthesis /ˈsɪnθəsɪs/, throat /ˈθɹoʊt/
“sh” 和 “zh” 音: identification /aɪˌdɛn(t)ɪfɪˈkeɪʃn̩/, appreciation /əˌpɹiː.ʃiˈeɪ.ʃən/, transcription /tɹænˈskɹɪpʃən/, suggestion /səˈd͡ʒɛst.ʃən/, instruction /ɪnˈstɹʌkʃən/
长单词——注意重音位置: identification /aɪˌdɛn(t)ɪfɪˈkeɪʃn̩/, appreciation /əˌpɹiː.ʃiˈeɪ.ʃən/, biology /baɪˈɑ.lə.d͡ʒi/, complexity /kəmˈplɛk.sɪ.ti/, limitation /lɪmɪˈteɪʃən/

中文母语者容易读错的音：

/v/ — 不要读成 /w/，上齿轻触下唇: virus /ˈvaɪ.ɹəs/, reverse /ɹɪˈvɝs/, viral /ˈvaɪɹəl/, convert /kənˈvɝt/, involve /ɪnˈvɑlv/
词尾辅音 — 要读清楚，后面不要加元音: strand /ˈstɹænd/, virus /ˈvaɪ.ɹəs/, reverse /ɹɪˈvɝs/, synthesis /ˈsɪnθəsɪs/, bind /ˈbaɪ̯nd/
/r/ 和 /l/ 的区别: viral /ˈvaɪɹəl/, recall /ɹɪˈkɔl/, briefly /ˈbɹiːfli/, illustrate /ˈɪl.əˌstɹeɪt/, complementary /ˌkɑmplɪˈmɛnt(ə)ɹi/

如何用这段视频练习

先完整听一遍视频，不要开口，记下不认识的单词。
先用 0.75 倍速逐句跟读，熟练之后再回到正常速度。
录下自己的声音并与原声对比，特别注意 strand, virus, enzyme 这类单词。

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。

影子跟读法: 阅读完整分步指南 →

跟读练习: PCR (Polymerase Chain Reaction) - 通过视频学习英语口语

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