跟讀練習: PCR (Polymerase Chain Reaction) - 通過影片學習英語口語

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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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本課的詞彙與口語要點

影片中反覆出現的詞有：Dna, Pcr, polymerase, step, copies。這段影片共有 83 個句子、1299 個單詞可供跟讀。講話部分時長為 7:30。說話人語速自然，每分鐘約 173 個詞，接近日常對話。只有 81% 的單詞屬於英語最常用的 3,000 詞，詞彙難度較高。

影片中的重點詞彙

影片中 15 個值得學習的單詞，附發音和釋義：

單詞	發音	釋義
strand 名詞	/ˈstɹænd/	綹 /綹, 一綹頭髮 /一綹頭髮
virus 名詞	/ˈvaɪ.ɹəs/	病毒
enzyme 名詞	/ˈɛn.zaɪm/	酶, 酵素
molecule 名詞	/ˈmɑ.lɪ.kjul/	分子
synthesis 名詞	/ˈsɪnθəsɪs/	綜合 /綜合
identification 名詞	/aɪˌdɛn(t)ɪfɪˈkeɪʃn̩/	辨識 /辨識, 鑑定 /鑑定
jam 名詞	/ˈd͡ʒæm/	果醬 /果醬
curious 形容詞	/ˈkjʊɹ.i.əs/	好奇, 有好奇心
ideal 形容詞	/aɪˈdi(ə)l/	理想的
recall 動詞	/ɹɪˈkɔl/	罷免 /罷免, 召回
remind 動詞	/ɹəˈmaɪnd/	提醒
tube 名詞	/tuːb/	筒
sequence 名詞	/ˈsiː.kwəns/	序列, 順序 /順序
throat 名詞	/ˈθɹoʊt/	嗓子, 喉嚨 /喉嚨
segment 名詞	/ˈsɛɡmɛnt/	片斷 /片斷, 段

值得反覆跟讀的句子

影片中簡短而完整的句子，可以直接用在日常對話裡：

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

影片中的語法

說話人最常用的結構，並附上影片中的原話：

結構	影片中的用法
被動語態 be + 過去分詞 — 強調發生了什麼，而不是誰做的	have been separated · is automated · can be performed
現在完成時 have/has + 過去分詞 — 過去發生但與現在仍有關聯的事	you've jammed · I've developed · You've heard

需要注意的發音

說話人用了 16 次縮略和弱讀形式，例如 we'll, we're, we've。請按聽到的簡短形式來說。

“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/
長單詞——注意重音位置: identification /aɪˌdɛn(t)ɪfɪˈkeɪʃn̩/, appreciation /əˌpɹiː.ʃiˈeɪ.ʃən/, biology /baɪˈɑ.lə.d͡ʒi/, complexity /kəmˈplɛk.sɪ.ti/, limitation /lɪmɪˈteɪʃən/

中文母語者容易讀錯的音：

/v/ — 不要讀成 /w/，上齒輕觸下唇: virus /ˈvaɪ.ɹəs/, reverse /ɹɪˈvɝs/, viral /ˈvaɪɹəl/, convert /kənˈvɝt/, involve /ɪnˈvɑlv/
詞尾子音 — 要讀清楚，後面不要加母音: strand /ˈstɹænd/, virus /ˈvaɪ.ɹəs/, reverse /ɹɪˈvɝs/, synthesis /ˈsɪnθəsɪs/, bind /ˈbaɪ̯nd/
/r/ 和 /l/ 的區別: viral /ˈvaɪɹəl/, recall /ɹɪˈkɔl/, briefly /ˈbɹiːfli/, illustrate /ˈɪl.əˌstɹeɪt/, complementary /ˌkɑmplɪˈmɛnt(ə)ɹi/

如何用這段影片練習

先完整聽一遍影片，不要開口，記下不認識的單詞。
先用 0.75 倍速逐句跟讀，熟練之後再回到正常速度。
錄下自己的聲音並與原聲對比，特別注意 strand, virus, enzyme 這類單詞。

什麼是跟讀法？

跟讀法 (Shadowing) 是一種有科學依據的語言學習技巧，最初開發用於專業口譯員的培訓，並由多語言者Alexander Arguelles博士普及。這個方法簡單而強大：您在聽英語母語原聲的同時立即大聲重複——就像是一個延遲1-2秒緊跟說話者的影子。與被動聽力或語法練習不同，跟讀法強迫您的大腦和口腔肌肉同時處理並模模擬實的講話模式。研究表明它能顯著提高發音準確性，語調，節奏，連讀，聽力理解和口語流利度——使其成為雅思口語備考和真實英語交流最有效的方法之一。

影子跟讀法: 閱讀完整分步指南 →

跟讀練習: PCR (Polymerase Chain Reaction) - 通過影片學習英語口語

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