Chapter1 Diversity and Universality of Organisms

Chapter2 Replication of Genetic Information

Chapter3 Expression of Genes

Chapter 4 Regulation of Gene Expression

Chapter 5 Cell Membrane Structure and Organelles

Chapter 6 Cytoskeleton

Chapter 7 Metabolism

Chapter8 Energy

Chapte9 Signal Transduction and Cell Growth

Chapte10 Development and Differentiation

Chapte11 Intercellular Communication and Tissue Architecture

Chapte12 Reproduction and Meiosis

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4Problems

The expression of the β-galactosidase gene in E. coli is regulated both positively and negatively. Briefly explain these two mechanisms.

In eukaryotic multicellular organisms, each differentiated cell type has a unique gene expression pattern - a characteristic that is preserved even after cell division. In hepatocytes, for example, genes involved in differentiation unique to cells are expressed, and the expression pattern is different from that in osteocytes. Although it may appear that these cells have different genes and pass them on to their progeny cells, the genes are in fact the same, but in many cases may have undergone changes.

1)

Provide the name of the gene-level change behind these phenomena.

2)

Describe the mechanism that causes such changes.

 

Organisms are considered to adapt to their habitat by effectively regulating gene expression while responding to environmental conditions and external substances. Using an example, describe genes that are regulated in response to environmental factors.

 

In eukaryotes, a mRNA molecules is normally transcribed from single gene (i.e., monocistronic mRNA), whereas in prokaryotes, it is often transcribed from multiple related genes in series (i.e., polycistronic mRNA). Describe the advantages and disadvantages of these two mechanisms in the regulation of gene expression.

The expression of the β-galactosidase gene in E. coli is regulated both positively and negatively. Briefly explain these two mechanisms.

The operator is located inside the promoter upstream of the β-galactosidase gene. When a repressor binds to the operator, the action of RNA polymerase is suppressed (negative regulation). Conversely, the binding of CRP bound with cAMP (a cAMP-CRP complex) to the promoter allows RNA polymerase to bind to that promoter, thus causing transcription (positive regulation, see Figs. 4-1 and 4-2).
The factors that negatively regulate β-galactosidase in E. coli are repressor proteins constitutively expressed from the i gene. Lactose is converted to allolactose in a cell, and this allolactose binds to the repressor, thus preventing it from binding to the operator.

In eukaryotic multicellular organisms, each differentiated cell type has a unique gene expression pattern - a characteristic that is preserved even after cell division. In hepatocytes, for example, genes involved in differentiation unique to cells are expressed, and the expression pattern is different from that in osteocytes. Although it may appear that these cells have different genes and pass them on to their progeny cells, the genes are in fact the same, but in many cases may have undergone changes.

1)

Provide the name of the gene-level change behind these phenomena.

Epigenetic change

2)

Describe the mechanism that causes such changes.

When the C in the 5’-GC-3’ sequence located in genes and expression regulation regions is methylated, proteins that recognize the methylated C bind to it and form condensed chromatin (heterochromatin), thereby strongly inhibiting transcription from the region (see Fig. 4-8).
[Commentary]
As an example, the hemoglobin gene is strongly methylated in all cells except red cells, and is never expressed. In DNA replication, if the C in the 5’-GC-3’ sequence in the parent strand is methylated, the C in the corresponding 5’-GC-3’ sequence in the daughter strand also becomes immediately methylated; the methylated state is therefore passed on from parent to progeny cells. It may appear that this particular characteristic is passed on to progeny cells, but in fact there is no change in the DNA sequence. This phenomenon is called epigenetic change.

 

Organisms are considered to adapt to their habitat by effectively regulating gene expression while responding to environmental conditions and external substances. Using an example, describe genes that are regulated in response to environmental factors.

Many genes are more or less regulated in accordance with environmental changes. In particular, a group of proteins called heat shock proteins are induced by heat or other types of stress, thus allowing the cell under stress to survive. Many hormone genes also act to maintain biological homeostasis in response to environmental changes, generally in a way that mitigates environmental effects.

 

In eukaryotes, a mRNA molecules is normally transcribed from single gene (i.e., monocistronic mRNA), whereas in prokaryotes, it is often transcribed from multiple related genes in series (i.e., polycistronic mRNA). Describe the advantages and disadvantages of these two mechanisms in the regulation of gene expression.

Advantages: With regard to polycistronic mRNA, the expression of multiple genes can be collectively regulated by a single promoter. As for monocistronic mRNA, in cases where each gene needs to be expressed differently in accordance with various environmental conditions, this mechanism allows detailed gene expression regulation for single genes.
Disadvantages: With polycistronic mRNA, although the expression of multiple genes can be collectively regulated, detailed expression regulation for single genes is difficult. With monocistronic mRNA, on the other hand, although detailed gene expression regulation is possible in cases where single genes need to be expressed differently in accordance with various environmental conditions, the mechanism is complex. (See the Column in 4.2)

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