Despite radical differences in what limbs do and what they look like,

Question

Despite radical differences in what limbs do and what they look like, the underlying blueprint of all limbs in land based animals, whether those limbs are wings in birds, flippers in penguins, or hands in humans, is the same - one bone, the humerus in the arm or the femur in the leg, articulates with two bones, which attach to a series of small blobs, which connect with the fingers or toes. Want to make a bat wing? Make the fingers really long. Want to make a horse? Elongate the middle fingers and toes and lose the outer ones. The differences between creatures lie in differences in the shapes and sizes of the bones and the numbers of blobs, fingers, and toes.

In the 1950s and 1960s a number of biologists, including Edgar Zwilling and John Saunders, did extraordinarily creative experiments on chicken eggs to understand how skeletal structure of limbs forms and uncovered some of the key mechanisms that build limbs that have the same architecture but are as different as bird wings, lizards webbings, and human hands . They discovered that two little patches of tissue essentially control the development of the pattern of bones inside limbs. These patches of tissue were named the zone of polarizing activity (ZPA). The cells in the ZPA made special molecules that then spread across the limb to instruct cells to make femur, articulates, and connecting toes. The concentration of these molecules was the important factor and decided the length of femur, articulates, connecting toes, and even the length of individual toe fingers. Later experiments with other animals such as the bat, frog, etc. proved that the mechanism to form limbs remained the same - formation of limbs in every creature was controlled by the ZPA – just the concentration of these special molecules varied corresponding to the desired structure.

The reason for this inherent commonality in architecture still remained a conundrum for decades. Why did nature not develop architectures better optimized to the functional needs of various organisms? The discovery of Tiktaalik, a transition between non-tetrapod vertebrates ("fish") to early tetrapods solved this mystery, providing evidence that all land based creatures that have limbs, hands, etc. share a common ancestor. The fact that our DNA and that of all land based animals can be traced back to Tiktaalik further provided evidence that all appendages, whether they are hands or limbs, are built by similar kinds of genes and that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in development of Tiktaalik’s limbs, in new ways to make wings in birds or various sized limbs with fingers and toes, explaining the diversity in shapes while maintaining the similarity in architecture.   1

1. According to the passage the mechanism that builds limbs as different as bird wings, penguin flippers, and human hands etc. is 
A. controlled through the ZPA, uses the same DNA and the same genes in different ways and only differs in the concentration of special molecules.
B. controlled through the ZPA and uses the same DNA and complementary genes suited to the shape and size of the limb.
C. defined by the DNA coming from fish and the corresponding complement of genes activated as per the needs of the shape, size and strength of the limb.
D. controlled through the ZPA in most tetrapods with some exceptions and uses DNA from corresponding ancestral heritage of tetrapods with suitable genes turned on.
E. not controlled through ZPA but differs from one vertebrate to another.

2

2. In the context of the passage, why was the discovery of the Tiktaalik important? 
A. It proved beyond doubt and without the need of experimentation the commonality of skeletal architectures of all land based organisms.
B. It proved the reason why the architecture of limbs in various organisms was optimized for the organism’s corresponding functional needs. 
C. It provided the reason for commonality of skeletal architecture described earlier in the passage.
D. It traced our ancestral heritage to the fish, providing the reason why we share majority of our DNA with aquatic animals.
E. It showed how ancient genes could be used in new ways for structures as diverse as wings in birds and fins in sharks.

3 
3. From the passage, what can be inferred about the architecture of limbs?
A. All land-based creatures that have limbs with similar architecture are also structurally similar.
B. It is different in land-based animals due to the difference in expression of the genes.
C. It evolved since the time of Tiktaalik to suit the functional needs of the land-based animals.
D. It may have been possible to develop an architecture better optimized for the functional needs of various organisms had nature not adhered to the common architectural scheme.
E. It is defined by the concentration of special molecules present in the cells of ZPA.

4

4. The primary purpose of the passage is 
A. To explain the reasons for the similarity in the architecture of the limbs of all land-based organisms.
B. To evaluate the implications of the experiments of Edgar Zwilling and John Saunders.
C. To trace the importance of the discovery of Tiktaalik.
D. To describe the role a common DNA plays in the unified architecture of the limbs of all land-based organisms.
E. To deduce how limbs of various sizes and shapes are built by the ZPA tissue.

OC2910 · Accepted Answer

Q1: A
according to passage question so refer to passage directly 
"The cells in the ZPA made special molecules that then spread across the limb to instruct cells to make femur, articulates, and connecting toes. The concentration of these molecules was the important factor and decided the length of femur, articulates, connecting toes, and even the length of individual toe fingers."

so only option A talks about special molecules and therefore the answer.

Q2:C
purpose question : why was tiktaalik mentioned 
 "Why did nature not develop architectures better optimized to the functional needs of various organisms? The discovery of Tiktaalik, a transition between non-tetrapod vertebrates ("fish") to early tetrapods solved this mystery, providing evidence that all land based creatures that have limbs, hands, etc. share a common ancestor"

so we see the thing was mentioned to answer why we had common architecture 
both B and C mentions the same but C has a broader scope : reading the question again (in context of passage is given so C best pick). B on other hand just mentions the scope of the question asked in the previous line.

Q3:D
A :All land-based creatures that have limbs with similar architecture are also structurally similar. (not all they have different structure so wrong )
B. It is different in land-based animals due to the difference in expression of the genes.(no mention of expression of genes is given).wrong
C. It evolved since the time of Tiktaalik to suit the functional needs of the land-based animals.(cant say it evolved to meet the functional needs there can be other reason)
D. It may have been possible to develop an architecture better optimized for the functional needs of various organisms had nature not adhered to the common architectural scheme.this sounds logical : Correct (possibilities are always good answers :GMAT tip)
E. It is defined by the concentration of special molecules present in the cells of ZPA.(molecules were not present in cells but made by cells and it was an important factor . saying it defined is too strong )

Q4:A
all other options ,mentions part of the passage only A gets the whole picture.

abhishekdadarwal2009 · Answer

3. From the passage, what can be inferred about the architecture of limbs?
A. All land-based creatures that have limbs with similar architecture are also structurally similar.
B. It is different in land-based animals due to the difference in expression of the genes.
C. It evolved since the time of Tiktaalik to suit the functional needs of the land-based animals.
D. It may have been possible to develop an architecture better optimized for the functional needs of various organisms had nature not adhered to the common architectural scheme.
E. It is defined by the concentration of special molecules present in the cells of ZPA.

In the above question the trick was to check the answer which can be INFERRED.i was tricked to pick E,but E was stated explicitly in the passage.

SO E was a trick option. correct answer is D

Nikhil · Answer

1. According to the passage the mechanism that builds limbs as different as bird wings, penguin flippers, and human hands etc. is

A. controlled through the ZPA, uses the same DNA and the same genes in different ways and only differs in the concentration of special molecules.

Correct  It is a rewording of the relevant information from the passage

B. controlled through the ZPA and uses the same DNA and complementary genes suited to the shape and size of the limb.

INCORRECT  There is no mention of any complementary genes in the passage

C. defined by the DNA coming from fish and the corresponding complement of genes activated as per the needs of the shape, size and strength of the limb.

INCORRECT  The passages doesn't mention anything about "DNA coming from the fish" and "genes activation"

D. controlled through the ZPA in most tetrapods with some exceptions and uses DNA from corresponding ancestral heritage of tetrapods with suitable genes turned on.

INCORRECT  The passage doesn't mention any exceptions.

E. not controlled through ZPA but differs from one vertebrate to another.

INCORRECT  This option is the opposite of the findings of the experiment. The mechanism is indeed controlled by the ZPA.

2. In the context of the passage, why was the discovery of the Tiktaalik important?

A. It proved beyond doubt and without the need of experimentation the commonality of skeletal architectures of all land based organisms.

INCORRECT  The discovery of Tiktaalik didn't prove the commonality. It provided the reason for the commonality.

B. It proved the reason why the architecture of limbs in various organisms was optimized for the organism’s corresponding functional needs.

INCORRECT  This choice state opposite information. The passage states that the architecture is not optimized. It is same as land based animals.
 
 C. It provided the reason for commonality of skeletal architecture described earlier in the passage.

CORRECT  The discovery of Tiktaalik provided the reason for the commonality of skeletal architecture

D. It traced our ancestral heritage to the fish, providing the reason why we share majority of our DNA with aquatic animals.

INCORRECT  Sharing of DNA with aquatic animals is not mentioned.

E. It showed how ancient genes could be used in new ways for structures as diverse as wings in birds and fins in sharks.

INCORRECT  Mechanism of how ancient genes were used in new ways is not discussed in the passage.

3. From the passage, what can be inferred about the architecture of limbs?

A. All land-based creatures that have limbs with similar architecture are also structurally similar.

INCORRECT  The passage doesn't say that structure of limbs is similar

B. It is different in land-based animals due to the difference in expression of the genes.

INCORRECT  This option states the opposite. The architecture of all land-based creatures is similar.

C. It evolved since the time of Tiktaalik to suit the functional needs of the land-based animals.

INCORRECT  As per passage, the architecture of limbs didn't evolve. The last line of the passage clearly states "maintaining the similarity in architecture". Hence the architecture didn't evolve.

D. It may have been possible to develop an architecture better optimized for the functional needs of various organisms had nature not adhered to the common architectural scheme.

Correct  This option can be inferred from following lines.

"The reason for this inherent commonality in architecture still remained a conundrum for decades. Why did nature not develop architectures better optimized to the functional needs of various organisms?"

E. It is defined by the concentration of special molecules present in the cells of ZPA.

INCORRECT  Architecture is not defined by the concentration of special molecules but the structure of limbs.

4. The primary purpose of the passage is

A. To explain the reasons for the similarity in the architecture of the limbs of all land-based organisms.

Correct  This is the pupose of the passage.

B. To evaluate the implications of the experiments of Edgar Zwilling and John Saunders.

INCORRECT  Partial scope. These experiments are discussed only in paragraph 2.

C. To trace the importance of the discovery of Tiktaalik.

INCORRECT  Partial scope. The importance of the discovery of Tiktaalik is discussed only in paragraph 3.

D. To describe the role a common DNA plays in the unified architecture of the limbs of all land-based organisms.

INCORRECT  Partial scope. This is discussed only in paragraph 3.

E. To deduce how limbs of various sizes and shapes are built by the ZPA tissue.

INCORRECT  Partial scope. This is discussed only in paragraph 2.

guptakashish02 · Answer

Hi ,
I marked E for Q3.
Can you please tell why E is wrong?
The passage clearly mentions that " The cells in the ZPA made special molecules that then  spread across the limb  to instruct cells to make femur, articulates, and connecting toes." . This basically tells us about the architecture of the limb.
Can you please help me with this?

Thanks

Ankit75 · Answer

The content of option E is explicitly stated in the passage. It needn't be inferred. But the question specifically asks us to infer some info about the architecture of limbs. Hope this clears it up.

TheNightKing · Answer

SajjadAhmad

This one is "e-GMAT" too.

VerbalBot · Answer

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Despite radical differences in what limbs do and what they look like,

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Despite radical differences in what limbs do and what they look like,

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