If available, please reference B.A. Pierce, Genetics. A Conceptual Approach, 5th

Question

If available, please reference B.A. Pierce, Genetics. A Conceptual Approach, 5th ed., W.H. Freeman & Co., NY., 2014 (Ch. 22)

1. Describe the life cycle of the fruit fly. What, in particular, is a syncitial blastoderm?

2a. Prior to egg laying, where is bicoid mRNA concentrated in the egg? After fertilization and egg-laying,

what happens to this mRNA? Describe the gradient that results.

2b. Describe the similar mechanism involving nanos.

3. The word "bicoid" suggests "two tails." Since genes are often named for the results of mutation in the

gene, why might mutations in a mother's bicoid gene result in her offspring having "two tails?" [As you

probably know, fruit flies don't have tails.]

4. How do the products of bicoid and nanos affect expression (transcription and/or translation) of

hunchback?

5. What are the three categories of segmentation genes (in order of time of expression)?

6. How do the roles of homeotic genes differ from those of the genes that create the anterior-posterior

axis?

7. What structures are found in the four whorls of flowers?

8. Explain the effects of mutations in Class A, Class B, and Class C genes in Arabidopsis.

9. Contrast humoral and cellular immunity.

10. What is clonal selection and how does it relate to the primary immune response? What allows for a

secondary immune response?

11. How can so many different antibodies be produced by just a few genes? [There are several

contributors to this, aside from somatic recombination.]

12. What are the roles of the MHC genes?

Explanation / Answer

1. Embryonic development stages
-Zygote stage embryo undergoes rapid cleavages to generate morula stage embryo

-Morula stage cells cannot be distinguished morphologically from each other, embryo contains a few hundred cells that are morphologically equivalent to each other

-Blastula stage cells have differences

-Gastrulation precedes organogenesis, no embryonic axis.

-Organogenesis is longest stage of embryogenesis, main scaffold of embryo laid out, embryonic axes start to form
-ectoderm, endoderm, and mesoderm develop into the internal organs of the organism.

2. a) Bicoid-protein gradient is preceded by a gradient in bicoid mRNA in the egg cortex, with the bicoid mRNA being proposed to be transported along cortical microtubules.

After fertilization, translation of bicoid mRNA gives rise to the gradient in Bicoid protein, which provides positional information required for further patterning.

b) Before fertilization, bicoid mRNA is synthesized in the egg and other cells of the ovary and accumulated at the anterior end of the egg, where it is tethered to the cytoskeleton. Similarly, nanos mRNA accumulates at the posterior end. After fertilization, the asymmetrically distributed mRNAs are translated to form the corresponding proteins. Because these proteins are degraded rapidly, the proteins form a gradient from high at the end where they are created to low at the opposite end.

3. Master regulatory gene in the cytoplasm of the mother--needed for the establishment of the anterior and posterior portion of the drosophila fruit fly..maternal effects-maternal mRNA were the concentration of the Bicoid is the most will yield the anterior end and Less will yield the posterior end..if the concentration of Bicoid gene is uniformed..will yield two anterior end...if Bicoid is not expressed, it will result in two posterior end..Bicoid is a Master Regulatory gene that controls the gap genes.

4.Maternally transcribe bicoid and nanos mRNAs are localized to the anterior and posterior ends of the egg, respectively. After fertilization, these mRNAs are translated, and the protein diffuse to form opposing gradients: Bicoid protein concentrations are highest at the posterior end. Bicoid protein at the anterior acts as a transcription factor to activate transcription of hunchback, a gene required for the formation of head and thoracic structures. Nanos protein at the posterior end inhibits translation of hunchback mRNA, thereby preventing the formation of anterior structures in the posterior regions.

5. a) gap genes specify broad regions (multiple adjacent segments) along the anterior-posterior axis of the embryo. Interactions among the gap genes regulate transcription of the pair-rule genes.
b) Pair-rules genes compartmentalize the embryo into segments and regulate expression of the segment polarity genes. Each pair rule gene is expressed in alternating segments.
c) Segment polarity genes specify the anterior and posterior compartments within each segment.

6.Homeotic genes specify segment identity-expression of the homeotic genes informs cells of their location or address along the anterior-posterior axis.

Master regulatory gene that encode transcription factors for the expression of genes involved in body parts production-that is the "specify the defects among the 14 segments".

-body parts - eg. Bicoid is hometic gene

7. Sepals-
outermost flower whorl
functions to protect the young flower bud via anti herbivory compounds and from cold

Petals-
second flower whorl from the outside
colorful to attract pollinators
are usually reduced in plants that wind-disseminate pollen

Stamens-
third flower whorl from the inside
2 parts: filaments and anthers
functions to produce pollen "male structure"

Pistil-
innermost flower whorl
3 parts: stigma, style, ovary
produces the seeds and fruit also has carpels "female structure".

8.Expression of class B genes is inhibited in the second whorl but not the third whorl. Sepal-sepal-stamen-carpel. The first and second whorls, expressing only class A genes, would both produce sepals. The third whorl, with both class B and class C genes, would produce stamens, and the fourth whorl with only class C genes would produce carpels.

b.Expression of class C genes is inhibited in the third whorl but not the fourth whorl. Sepal-petal-petal-carpel. The first, second, and fourth whorls are unaffected and produce the normal floral parts. The third whorl, without class C gene products, would express class A genes as well as class B genes to specify petals.

c.Expression of class A genes is inhibited in the first whorl but not the second whorl. Carpel-petal-stamen-carpel. Without class A gene expression, the first whorl would express class C genes and produce carpels. The other whorls have normal gene expression and are unaffected.

d.Expression of class A genes is inhibited in the second whorl but not the first whorl. Sepal-stamen-stamen-carpel. Lack of class A gene expression in the second whorl would allow class C gene expression and result in stamens because of co-expression of class B and class C genes. The other whorls are unaffected and produce normal floral organs.

9.Humoral immunity is mediated by antibody molecules that are secreted by the B-lymphocyte cells that can neutralize specific pathogens outside the cells. Cell mediated immunity is mediated by T-cell antigen receptors made by thymus-derived T-cells.

10.Clonal selection-
A critical theme in the adaptive immune response, applying to both B cells & T cells
describes how the body can make a seemingly unlimited range of antibody specificities.

-lymphocytes migrate into secondary lymphoid organs from the peripheral bloodstream.

-antigens bind to lymphocytes that have a specific antigen receptor for an epitope on the antigen.

-the lymphocytes being activated need the antigen as a first signal, but also a second signal to confirm the danger and allow activation.

-when activation is confirmed, proliferation occurs.

-differentiation into effector cell form (plasma cell, helper T cell, cytotoxic T cell) occurs.

11.Recombination- Unique to lymphocytes. For many decades, biologists puzzled over how a mammalian immune system could produce billions of distinct antibody molecules when there's only about 25,000 genes in humans. The generation of diversity of antigen-specific receptors on both B and T lymphocytes uses a unique mechanism called V(D)J recombination. 99% of genes come in pieces (exons) and then need to be glued together at RNA level. In addition, the human body is able to produce billions of different B cell receptors (BCR) and T cell receptors (TCR) using a set of genes numbering only in the hundreds. This is accomplished by recombining gene "segments" in a variety of different ways to produce the enormous diversity of receptors present in the immune system. V(D)J recombination is mediated by two proteins: RAG-1 and RAG-2. RAG-1 and RAG-2 are expressed by developing B cells (bone marrow) and developing T cells (thymus). These proteins work together to pull together pieces of DNA, cut them, and allow them to be pasted together. In this way, the immune system is able to generate the large number of lymphocyte receptors necessary to cope with the universe of pathogens.

12. MHC protein functions include:
1. Positive selection of T cells in the thymus
2. Presentation of antigenic peptides to T cells
3. encodes some complement proteins, some cytokines and proteins involved in antigen processing
4. MHC class I and II proteins are also most important antigens recognized in the graft rejection process.

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