Scientist long believed that two nerve cluster in the human hypothalam

Hi MagooshExpert, SajjadAhmad, workout,

My query is with regard to Q1. In it I eliminated option B, C, and E, for obvious reasons then left with A and D. A and D almost felt like same hence went ahead with A instead D. Could you please assist to find where I made error.

First of all analyze the wording used in both options A and D

(A) challenge recent findings that appear to contradict earlier findings
(D) discuss evidence that has caused a longstanding belief to be revised

Below is the explanation by "mikemcgarry"

In Par 1, he talks about the role of the SCN in regulating circadian rhythms.
Par. 2, he cites new evidence that is not entirely compatible with the SCN running everything. The first paragraph really sets the stage of what the author wants to say. The second paragraph is really the author's main point, the juicy point he want to make--- the first paragraph just sets the stage so we have context for the second paragraph. The "SCN runs the whole thing" theory was called "long believed," so it is a longstanding theory that the new evidence calls into question, causing scientist to revise the longstanding theory. This is what (D) says.

Thanks SajjadAhmad for response.

I felt that A also could be answer as

"Par. 2, he cites new evidence that is not entirely compatible with the SCN running everything" Which can be also inferred as "recent findings that appear to contradict earlier findings"

Not sure if I making any mistake in understanding it.

For the main purpose question 1,I'm stuck between B and D. In p1, the author cites traditional belief and then he presents new belief in p2. Isn't it B? The option B deals with two sides of an ongoing scientific debate?

Can someone help me with option D in the 3rd question please?

3. The author of the passage would probably agree with which of the following statement about the SCNs?

(A) The SCNs are found in other organs and tissues of the body besides the hypothalamus.
(B) The SCNs play a critical but not exclusive role in regulating circadian rhythms.
(C) The SCNs control clock genes in a number of tissues and organs throughout the body.
(D) The SCNs are a less significant factor in regulating blood pressure than scientists once believed.
(E) The SCNs are less strongly affected by changes in light levels than are by other external cues.

While I agree that the 2nd paragraph says - "While scientists do not dispute the role of the SCNs in controlling core functions such as the regulation of body temperature and blood pressure", it also says that there are other organs and tissues that are performing this role as well. So doesn't it imply that SCNs are a less significant factor in regulating blood pressure than they used to be?

You are very close. As you are clear that SCNs are not playing key role but there are other factors also , as observed in other species .

It is important to add : scientists once believed.
because this passage is based on hypothesis. There is no change in functionalities of SCNs from beginning. Only at some point , scientists discovered that SCN doesn't play key role.
If you don't mention about belief of scientists in D, it appears that there has been some change in process in body. But that's not what passage implies.

I hope it is clear.

Hi vatsal323, my two cents on this, the passage states in the end that in addition to SCN, other factors also play the role, but one key thing to note here is author didnt made any comparison between two, take for instance, we can have two metrics working to produce an output, not it could be the case that both are equally significant.

I hope this helps.

I am not able to bring my head around the question 3 " The author of the passage would probably agree with which of the following statement about the SCNs? " . As per the passage the author is explaining what the scientists have observed and its not mentioned as to whether the Author agrees or disagrees with it. So on what line of thinking is the option A correct ?

Please help explain

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KarishmaB GMATNinja Vinit800HBS

For Q3 we have choice (A) The SCNs are found in other organs and tissues of the body besides the hypothalamus.
The passage states the following:

"Four critical genes governing circadian cycles have been found to be active in every tissue, however, not just the SCNs, of flies, mice, and humans."

If we dissect the sentence structure we get: "Four critical genes governing circadian cycles have been found to be active in every tissue, however, not just the SCNs, of flies, mice, and humans."

Doesn't it seem as if the sentence is saying "not just SCNs but four critical genes have been found to be active in every tissue of flies, mice, and humans?"

So based on the sentence structure and based in the meaning we get (if I am correct) how do we reject (B)

I am sorry. I meant choice (A) The SCNs are found in other organs and tissues of the body besides the hypothalamus. I have edited my doubt accordingly.

I agree that (B) is the answer but based on the SC mentioned above, how to we eliminate (A)?

Circadian rhythms are the biological cycles that recur approximately every 24 hours. Every 24 hrs, there are light patterns, temperature changes etc that recur. As per those, our biological cycles are synchronised (we will get sleepy in the night - no light, lower temperature etc)

Long standing belief:
Scientist long believed that two nerve cluster in the human hypothalamus, called suprachiasmatic nuclei (SCNs), were what controlled our circadian rhythms.

SCNs control our circadian rhythms.

Revision:
scientists now believe that circadian clocks in other organs and tissues may respond to external cues other than light—including temperature changes—that recur regularly 24 hours.

Initially, scientists believed that SCNs controlled our circadian rhythms. Now they believe that circadian genes in other organs and tissues also have a role to play in our circadian rhythms. These genes in other organs respond to temperature changes etc that recur regularly every 24 hrs (day is hot, night is cold) etc.
So SCNs still respond to light cycles but other factors such as temp changes over 24 hours could be taken into account by other tissues/organs.

ag153

Great question! I have the same doubt. Please experts, do respond to this one.

ag153
sanjayparihar16

Long standing belief was that SCNs exclusively controlled circadian rhythms.
Now they have revised it to 'SCNs play an important role but other organs also play a role in controlling circadian rhythms'.

Both are talking about biological changes in us in 24 hr intervals (which is called our circadian rhythm).

The passage tells us that SCNs are synced to light changes every 24 hrs but other genes in other organs are synced to temperature changes in 24 hrs, to say food cycle every 24 hrs, to variation in magnetic field every 24 hrs etc.
With this understanding, read the passage again.

A. As per the passage and as per your previous message, isn’t the revised belief the part after “scientists now believe......” , which is the fact that genes in other organs are affected by external cues AND NOT REALLY that rythms are affected by other organs as well? Given the structure of the pssage and how it’s written, it seems the last sentence is the revised belief and not the first sentence of the second para. Pls clarify

In your previous message, you have mentioned 2 different revised beliefs: light's affect in SCN vs genes of other organs as one revision and SCNs only vs SCNs being one of the factors affecting light as the other revision. Looking at the structure I am not clear which one is supposed to be revision

And how does the rat thing serve as an evidence for the revision if the revision is that "genes in other organs also affect rythms". I mean this was already concluded before the evidence was given so I am not sure how the evidence had any role to play.

B. On a separate note: when it says 4 genes are present in every tissue, other than SCNs - I’m confused in the interpretation because of parallelism- is it implying :
1) “4 genes are present in every tissue other than IN SCNs” thereby implying same genes are present in SCNs and in other organs too but they are affected by external cues in the other organs?
Or
2) does it imply “besides SCNs , there are also 4 genes, which are present in every tissue and that effect the rythms”?

C) Also are both of these the evidence or only the rat sentence? Evidence: 1) 4 genes have also been found and 2) The peak activity in rats.....

KarishmaB

Thank you for your helpful explanation!

My first question:
What confused me about the passage was the temperature part where it notes "the SCNs control daily fluctuations in blood pressure, BODY TEMPERATURE, activity level, and alertness..."

and then later mentions "scientists now believe that circadian clocks in other organs and tissues may respond to external cues other than light- including TEMPERATURE changes -that recur every 24 hours."

To confirm that my understanding is correct, SCNs control fluctuations in temperature but do not respond to temperature changes in the external environment itself? In other words, SCNs only control temperature when they are exposed to light?

My second question:
For number 463, the use of "critical" in choice B. seemed strong to me at first given that you see the word "critical" in the passage when referring to the "four critical genes".

Are the SCNs critical because they control "core functions" whereas the passage does not describe ways how the other four genes control "core functions"? Is that where the inference is made... between "critical" and "core"?

So in all, to clarify, is B mainly correct because the passage does NOT say that these other four genes control "core functions", rather, that they just play a ROLE?

Thank you in advance

Edited accordingly.

Thank you!