Question

1. Consider the cyanobacteria Anabaena. Individual cells grow attached to each other in long chains. Most perform photosynthesis, using photosystems I and II to generate ATP and NADPH that they use to power the fixation of carbon dioxide via the Calvin cycle. However, a small number form heterocysts, cell that instead of performing photosynthesis, fix nitrogen into ammonium. Heterocyts share their nitrogen compounds with their photosynthetic neighbors, who in turn share sugars with the heterocysts.

a)If you extracted all the RNA from a single heterocyst, and all the RNA from single photosynthetic cell, what information would this give you?

b)Would you find RNA that is the same between the two cell types? If yes suggest three examples.

c)Would you find RNA that is different between these two cell types? If yes, suggest three examples.

Answer 1

All living cells in the plant body contain the identical genomic sequence. However, each tissue exhibits its own tissue-specific RNA expression pattern. It is achieved by turning on specific transcription factors that are master regulators for each tissue type by epigenetic mechanisms. Some genes are expressed in all the tissues all the times. Such genes are known as constitutive genes. Ex: Genes that code for enzymes involved in glycolysis.

a. If we extract total RNA separately from a single heterocyst and a single photosynthetic cell, we would find hallmark RNA expression pattern specific for each tissue. The specific RNA expression profile would give insights about the function of each cell type.

b. Both the cells can express certain common genes which are involved in basal metabolism. Ex: Actin, hexokinase, cellulose synthase, and Glyceraldehyde-3-phosphate dehydrogenase.

c. Each tissue type expresses characteristic genes that are required for their specific cellular function.

For example, heterocysts express genes involved in nitrogen metabolism. Ex: Nitrogenase, nitrate reductase, and nitrite reductase.

Photosynthetic cells express genes involved in light absorption and carbon fixation. Ex: Rubisco, NADP⁺-reductase, and ATP synthase.