Unit 7 The Double Helix
Introduction
I. Listen to the text “Red Bread Mold Provided Insight into the Role of Genes”. Are these sentences true or false? Correct the false sentences.
1 Neurospora is very independent because it can synthesize the organic compounds it needs. T /F
2 Neurospora cannot grow on a minimal medium. T / F
3 Neurospora has just one copy of each gene that is why it is very useful in experiments. T / F
4 George Beadle and Edward Tatum wanted to see the DNA of Neurospora with the help of X-ray diffraction. T / F
5 The mutations caused by X-rays caused Neurospora to need more nutrients. T / F
6 The experiment of Beadle and Tatum proved that each enzyme is encoded by one gene. T / F
7 The experimental mutant mold could not grow unless citrulline was added to the medium. T / F
8 The experimental mutant mold lacked many enzymes. T / F
II. Are the following definitions correct? Change the wrong ones.
a A gene is a functional segment of DNA located at a particular place of a chromosome.
b Nucleic acid consists of four very similar subunits called nucleotides.
c Each nucleotide of DNA consists of three parts: a phosphate group, deoxyribose and a nitrogen-containing base that has a single-ringed structure.
d Chromosomes are long strands of DNA which are in eukaryotes complexed with lipids.
e Replication is a process of copying DNA to produce two identical DNA double helices.
f In transcription, the information contained in the whole of the DNA is copied into messenger RNA (mRNA).
g In translation, transfer RNA (tRNA) and ribosomal RNA (rRNA) convert the information of the base sequence in messenger RNA into a specific amino acid sequence and so help synthesize the protein.
Reading
Read the text. The exercise after the text will help you with the new words.
The Discovery of the Double Helix
IIn the early 1950s, many biologists realized that the key to understanding inheritance lay in the structure of DNA. They also knew that whoever deduced the correct structure of DNA would receive recognition from fellow biologists, fame in the popular press, and very possibly the Nobel Prize. Less obvious were the best methods to employ and who would be the person to do it.
IIThe betting favorite in the race to discover the structure of DNA had to be Linus Pauling of Caltech. Pauling probably knew more about the chemistry of large organic molecules than did any person alive, and he had realized that accurate models could aid in deducing molecular structure. Like Rosalind Franklin and Maurice Wilkins, Pauling was an expert in X-ray diffraction techniques. Finally, he was almost frighteningly brilliant. In 1950s, he demonstrated these traits by showing that many proteins were coiled into single-stranded helices. Pauling, however, had two main handicaps. First, for years he had concentrated on protein research, and therefore he had little data about DNA. Second, he was active in the peace movement. During the early 1950s, some government officials, including Senator Joseph MrCarthy, considered such activity to be potentially subversive and possibly dangerous to national security.
IIIThe second most likely competitors were Wilkins and Franklin, the English scientists who had set out to determine the structure of DNA by the most direct procedure, namely the careful study of the X-ray diffraction patterns of DNA. They were the only scientists who had very good data about the general shape of the DNA molecule. Unfortunately for them, their methodical approach was also slow.
IVThe door was open for the eventual discoverers of the double helix, James Watson and Francis Crick, two young scientists (American and English, respectively) with neither Pauling’s tremendous understanding of chemical bonds nor Franklin and Wilkins’s expertise in X-ray analysis. They did have three crucial advantages: (1) the knowledge that models could be enormously helpful in studying molecular structure, a lesson learned from Pauling’s work on proteins; (2) access to the X-ray data; and (3) a driving ambition to be first.
VWatson and Crick did no experiments in the ordinary sense of the word; rather, they spent their time thinking about DNA, trying to construct a molecular model that made sense and fit the data. Because they were based in England and because Wilkins was very open about his and Franklin’s data, Watson and Crick were familiar with all the X-ray information relating to DNA. This information was just what Pauling lacked. Because of Pauling presumed subversive tendencies, the U.S. State Department refused to issue him a passport to leave the United States, so he could neither attend meetings at which Wilkins presented the X-ray data nor visit England to talk with Franklin and Wilkins directly.
VIWatson and Crick knew that Pauling was working on DNA structure and were terrified that he would beat them to it. In his book ‘The Double Helix’, Watson recounts his belief that, if Pauling could have seen the X-ray pictures, “in a week at most, Linus would have the structure.”
VIIYou might be thinking now, “But wait just a minute! That’s not fair. If the goal of science is to advance knowledge, then everyone should have access to all the data. If Pauling was the best, he should have discovered the double helix first.” Perhaps so. But science is an activity of scientists who, after all, are people too. Although virtually all scientists want to see the advancement and benefit of humanity, each individual also wants to be the one responsible for that advancement and to receive the credit and the glory. The ambition to be first helps inspire the intense concentration, the sleepless nights, and the long days in the laboratory that ultimately produce results.
VIIILinus Pauling remained in the dark about the correct X-ray pictures of DNA and was beaten to the correct structure. When Watson and Crick discovered the base-pairing rule that was the key to DNA structure, Watson wrote a letter about it to Max Delbruck, a friend and advisor at Caltech. He asked Delbruck not to reveal the contents of the letter to Pauling until their structure was formally published. Delbruck, perhaps more of a model scientist, firmly believed that scientific discoveries belong to the public domain and promptly told Pauling all about it. With the class of a great scientist and a great person, Pauling graciously congratulated Watson and Crick on their brilliant solution of the DNA structure. The race was over.
Words, Words, Words
I. In paragraphs I-VIII of the text find words meaning the following:
praise, respect or admiration (I) ………
the person or animal who is expected to win the competition (II) ………
correct or true in every detail (II) ………
very intelligent (II) ………
a disadvantage that prevents you from doing something well (II) ………
intended to destroy the power of the government (II) ………
to start working in order to achieve an aim (III) ………
in the order in which they were mentioned (IV) ………
special skill or knowledge that you get from experience, training or studying (IV) ………
to do something before someone else (VI) ………
to say what happened (VI) ………
praise for something you have done or achieved (VII) ………
after a process or activity has ended (VII) ………
to make known (something secret or hidden) (VIII) ………
II. As you learned from “The Discovery of the Double Helix”, scientists in different laboratories often compete with one another to make new discoveries. Do you think this competition helps to promote scientific discoveries? Sometimes, researchers in different laboratories collaborate with one another. What advantages does collaboration offer over competition? What factors might provide barriers to collaboration and lead to competition?
Advantages of Collaboration:
1.
2.
3. |
Advantages of Competition:
1.
2.
3.
|
Factors providing barriers to collaboration and leading to competition: 1.
2.
|
|
Language focus 1
Modal Auxiliary Verbs: Overview
Modal auxiliary (“helper”) verbs are: can, could, may, might, will, would, shall, should, must and ought to. Sometimes verbs be, dare, have and need act as modal verbs, too.
Modal verbs
a) do not add –s in the third person singular;
b) form questions, negatives, question tags and short answers without do;
c) need an infinitive without to after them (except for be to, need to, ought to);
d
)
do not have infinitives or participles (to may, maying, musted);
e) can be followed by perfect or continuous infinitives (must have been, could be doing, etc.) to express ideas of past or progressive;
f) have contracted forms (can’t, won’t, ‘d (for would), ‘ll (for will), etc.).
Modal auxiliary verbs have many meanings. Among them are: 1) ability 2) obligation to do something, 3) permission, 4) degrees of certainty, 5) conditional meaning.
I. Determine the meaning of the modal verb in each of the sentences.
Can swim ten lengths of the swimming pool without stopping.
You should be more attentive during the test: you can make 5 mistakes in one word!
You are a princess and you must never say such words!
May I come in?
He might have missed the last train and is now stranded at the station.
How could I be such a fool?
That must have been a mistake!
You can borrow my car, if you want.
If I hadn’t been such a snob I might have stayed with my parents.
The text can’t be so small! It is just an introduction.
II. Work with a partner. Use the phrases below in situations of your own:
1. He might have seen you wrapping it!
2. How long must I stand here like a statue?
3. You should do it as quickly as you can.
4. Can you show me the way?
Language focus 2
Can and Could: Ability
To express the notion of ability, modal verbs can/could are used in English. The meaning of ability is the primary meaning of these verbs. For example:
I can read Italian, but I can’t speak it.
Can gases freeze?
When I was 5 years old, I could swim two lengths of the swimming pool. My friends couldn’t swim so far.
However, we can not use these two verbs in every tense. That is why the expression to be able to is used in other tenses:
Present: I can climb trees. – I am able to climb trees. (No difference in meaning, can sounds better.)
Past: I could draw people when I was 5. = I was able to draw people when I was 5. (General ability.) BUT: I was able to draw that man (was able to = managed to, succeeded in)
Future: You’ll be able to walk soon. (Only one form is used for future abilities.)
Infinitive: I like to be able to run fast. (Can is not used in the infinitive.)
Perfect: I’ve always been able to play games well. (Can is not used in the perfect.)
Gerund: Being able to practice parachute-jumping is very important for me. (Can is not used in the –ing form.)
Participle: Being able to collect a lot of money by singing, he decided to do so.
I. Interview your group-mates and find the person who:
(name)
was able to read when he was three ………….…………………………….
(Ask everybody: Were you able to read when you were 3?)
can drive a car …………….………………………..……….
has always been able to fight well ……….………………………………..…….
will be able to visit Europe in summer …………………….………………….
could watch horror movies when he was five .………………………..……….
II. Write in your notebook five words or expressions on the following in random order.
Something you used to be able to do but can’t do now.
Something you would like to be able to do in the future.
Something that you like being able to do.
Something you’ve been able to do this week.
Something you’ll be able to do when you graduate from the University.
Now swap your notes with your partner and ask questions looking at his/her notes. (For example: Did you use to be able to play the piano well?)
III. Read the following dialogue. Who is speaking? Finish the dialogue. Act similar dialogues in class, changing the names and other information.
Hello, Mr. Parker. I am John Stevens.
Hello, Mr. Stevens. So, you are applying for the position of …
Of a photographer. I can write small articles to the photographs as well.
Good. We’re always in a hurry here. Are you able to act quickly and efficiently?
Yes, I am. In my previous job I was able to act very quickly.
What did you do?
I… er… was a scuba diving photographer. When you dive, you know, you must be very attentive and make decisions quickly. I was able to get away from that shark… and even save my camera.
Oh!..
I can also climb mountains, sail a boat, and fly a small airplane…
That will do, thank you. Can you develop films?
Oh yes, sir.
Our magazine is a comic one. So we need comic photographs. Can you make comic photographs? …
Can or Will Be Able to
We use can to talk about future actions which we will be able to do because of present ability, present circumstances, present decision, etc. Will be able to is used specially to indicate future ability to do something.
IV. Fill in the gaps with can or will be able to.
1. I …………. give you a hand with this work, if you like.
2. I …………. solve this problem in an hour or two.
3. I …………. explain it to you once again.
4. I …………. lend you my notebook for a few days, to prepare for the test.
5. I …………. spend more time with my husband when I leave the university.
Language focus 3
Ability: Could Have Done
1 Could have done is a special structure to talk about unrealized past ability or opportunities – to say that somebody was able to do something, but did not try to do it; or that something was possible, but did not happen.
I could have married anybody I wanted to. (Unrealized past ability.)
That was a bad place to go skiing – you could have broken your leg. (It was possible, but it didn’t happen.)
I couldn’t have won, so I didn’t go in for the race. (In negatives – inability in the past.)
NB: I couldn’t win the race. (It is uncertain whether I took part in it.)
I couldn’t have won the race even if I had tried. (I did not take part in the race at all.)
2 Sometimes could have done is used to criticize actions:
You could have helped me – why did you just sit and watch?! (Criticism of a past action.)
3 The same forms are used in past and present conditionals:
I could have given up all research, if I hadn’t met that professor. (Unrealized condition in the past.)
I could give up all research, if I was not interested in it. (Unrealized condition in the present.)
These forms exist in both active and passive voice:
|
Active |
Passive |
Present |
could do |
could be done |
Past |
сould have done |
could have been done |
I. Put the verb in brackets into the correct form.
1. You could (kill) the bird! You were not careful when you took it!
2. You could (listen) to what I am saying.
3. The book could (read) more attentively if everything had been explained beforehand.
4. The work could (do) with more precision if the techniques we used had been more accurate.
5. You could (eat) first and (play) after.
6. I could (be) in prison now if not for you.
7. That could (be) an ideal solution if we didn’t know of the existence of that factor.
8. The experiment could (conduct) with due care if the second laboratory assistant was present.
9. The event could (place) into the list of the most wonderful discoveries in world history if it had been known to anybody but the scientist who died the next day.
10. I could (produce) a better impression! But I began to worry long before I saw the employer and couldn’t (do) better.
II. Use the following word combinations in phrases of your own.
1. … could possibly revise …
2. … can’t determine …
3. I can replace …
4. … couldn’t have decided …
5. … was able to calculate …
6. …could have counted …
7. … could have been achieved if …
8. I could try ……..
Language focus 4
Uncertainty, Doubt, Improbability
1 Can it be true? Неужели это правда?
Could (more uncertain)
Can/ Could she be telling lies? (The continuous infinitive is used)
Can/Could she have said it? (To show that something happened in the past use the perfect infinitive)
Can/Could she really have been waiting for us so long? (The action began in the past and continues into the present)
2 It can’t be true Не может быть, чтобы это было правдой!
couldn’t
He can’t be telling lies!
couldn’t
He can’t have said this! (Something happened in the past)
couldn’t
She can’t/couldn’t have been waiting for us so long! (The action began in the past and continues into the present.)
I. Substitute an English verb in the correct form for the Russian one in the brackets.
1. He can’t (сказал) that to your mother!
2. Can it (происходит) every day in our faculty?
3. Could he (работал и до сих пор работает) in this strange organization?
4. We couldn’t (видели) this so many times!
5. You can’t (смотришь) at these things and not (делаешь) anything with that!
6. Could you (ждал) so long?
7. They couldn’t (были) there.
II. Use two or more of the phrases from I in situations of your own.
III. Translate into English.
1. Не может быть, чтобы он это написал!
2. Ты мог бы переписать дискету, пока был у меня дома.
3. Вы не могли бы подержать мою сумку?
4. Он умел плавать в три года!
5. Она смогла только подняться с постели и сделать три шага.
6. Можно мне взять твою расческу?
7. Разве такое может случиться в нашей семье?
8. Неужели вы столько лет исследуете одну только плесень?
9. Он мог отдать вам все материалы и больше никогда не вспомнить об этом, но он этого не сделал.
10. Ты мог бы сейчас загорать где-нибудь в Испании…
11. Из нашего окна можно увидеть только каменную стену и маленький кусочек голубого неба.
12. Разве он до сих пор проводит подобные эксперименты?
13. Не может быть, чтобы новое средство не подействовало! Мы его опробовали, и оно дало хороший результат.
14. Разве вы не искали меня?
15. Ты не мог бы быть внимательнее, когда решаешь такие сложные задачи?
16. Эту книгу можно купить в любом магазине.
Render in English.
Образование и-РНК по матрице ДНК
Биосинтез белков идет в каждой живой клетке. Наиболее активен он в молодых растущих клетках, где синтезируются все виды белков. Основная роль в определении структуры белков принадлежит ДНК. Участок молекулы ДНК, содержащий информацию о структуре одного белка, называют геном. Поэтому информацию, которая содержит ДНК, называют генетической.
Информация, содержащаяся в ДНК, переписывается на и-РНК. Этот процесс называют транскрипцией. Специальный фермент – РНК-полимераза, двигаясь по ДНК, подбирает по принципу комплементарности нуклеотиды и соединяет их в единую цепочку. Если в нити ДНК стоит тимин, то полимераза включает в цепь и-РНК аденин, если гуанин – включает цитозин, если аденин – то урацил (в состав РНК не входит тимин).
По длине каждая из молекул и-РНК в сотни раз короче ДНК. Информационная РНК – копия не всей молекулы ДНК, а только части ее, одного гена или группы рядом лежащих генов, несущих информацию о структуре белков, необходимых для выполнения одной функции.
В начале каждой группы генов находится своего рода посадочная площадка для полимеразы. Только присоединившись к ней, полимераза способна начать синтез и-РНК. В конце группы генов фермент встречает сигнал (в виде определенной последовательности нуклеотидов), означающий конец переписывания. Готовая и-РНК отходит от ДНК, покидает ядро и направляется к месту синтеза белков – рибосоме, расположенной в цитоплазме клетки.
В клетке генетическая информация передается только от ДНК к белку.
Unit 8