QUESTION: 101 Exam Topic: 1, Biology

Chickens have three digits on each limb and the development of these digits is controlled by a region of cells called ZPA (zone of proliferating activity) cells. The digits are normally labeled 2', 3' and 4' to show that, over the course of evolution, chickens have lost digits 1' and 5' that mammals and most reptiles have retained. Below is a three-part experiment performed on chicken embryos, where some ZPA cells are transplanted from a donor embryo into the limb bud that is pictured. The limb bud is the limb of the developing chick before any differentiation of digits has taken place. Depending upon where the ZPA cells are transplanted, different results occur.

Although the developing embryo normally has only one active area of ZPA ("recipient's ZPA normal position"), when a second active area of ZPA is present as seen in experiment (a), what is the result?

Development proceeds as it normally does into three distinct digits
Two identical, mirror-image sets of digits develop from the limb bud
The ZPAs cancel each other out and no digits develop
Digits are oriented with the shortest digits closest to the ZPAs
The digits developing on the side of the original ZPA will be functional, while the digits developing from the transplanted ZPA will not.

Answer(s): B

Explanation:

As one can see from the diagram, two mirror-image sets of 2', 3', and 4' digits develop, both sets identical.

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QUESTION: 102 Exam Topic: 1, Biology

ZPA regions have no effect on limb development
implanting a second ZPA region will cause shorter digits, such as the 2' digit, to disappear
the chicken embryo has mechanisms in place to turn off duplicate ZPA regions
the ZPA regions work by secreting neurotransmitters to control limb development
ZPA regions have to capacity to induce development of digits on limb buds

Answer(s): E

Explanation:

ZPA regions are clearly inducers of digit development, although placement of the ZPA regions is crucial to normal development. When two ZPAs are in close proximity, interference can result and some digits may develop poorly or not at
all. In addition, the relative placement of digits can change as the placement of the second ZPA changes. There is no evidence presented for what type of chemical control the ZPA exerts over development, nor is there any evidence for mechanisms to turn off duplicate ZPAs.

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QUESTION: 103 Exam Topic: 1, Biology

Chickens have three digits on each limb and the development of these digits is controlled by a region of cells called ZPA (zone of proliferating activity) cells. The digits are normally labeled 2', 3' and 4' to show that, over the course of evolution, chickens have lost digits 1' and 5' that mammals and most reptiles have retained. Below is a three-part experiment performed on chicken embryos, where some ZPA cells are transplanted from a donor embryo into the limb bud that is pictured. The limb bud is the limb of the developing chick before any differentiation of digits has taken place. Depending

upon where the ZPA cells are transplanted, different results occur.

It is thought that the ZPA cells control the length of different digits by secreting hormone-like chemicals that build up in the limb bud tissues. What result in the experiment most clearly suggests that this is the case?

The longest digits develop in areas closest to the ZPAs, while digits furthest from the ZPAs are shortest.
Mirror image sets of digits developed in experiment (a) when ZPAs were placed in regions opposite one another.
The sets of three digits in experiment (c) were opposite in orientation to the sets that developed in experiment (a).
Digits 4' and 3' are consistently longer than digit 2'.
Digit 2' docs not develop in experiment (b).

Answer(s): A

Explanation:

In all three parts of the experiment, the longest digits (4' and 3') are consistently nearest to the ZPA regions. Digit 2', when it develops, is furthest. These results suggest that the more chemical messages that cells receive in developing digits (i.e. the closer to the ZPA cells), the longer those digits will become.

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QUESTION: 104 Exam Topic: 1, Biology

Chickens have three digits on each limb and the development of these digits is controlled by a region of cells called ZPA (zone of proliferating activity) cells. The digits are normally labeled 2', 3' and 4' to show that, over the course of evolution, chickens have lost digits 1' and 5' that mammals and most reptiles have retained. Below is a three-part experiment performed on chicken embryos, where some ZPA cells are transplanted from a donor embryo into the limb bud that is pictured. The limb bud is the limb of the developing chick before any differentiation of digits has taken place. Depending upon where the ZPA cells are transplanted, different results occur.

The bones of the limb are derived from which of the following embryonic germ layers'?

Ectoderm
Mesoderm
Endoderm
Notochord
Neural crest

Answer(s): B

Explanation:

Bones and muscle are derived from the mesoderm.

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QUESTION: 103 Exam Topic: 1, Biology

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QUESTION: 104 Exam Topic: 1, Biology

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