Well defined nucleus is absent in a. Blue-green algae b. Diatoms c. Algae d. Yeast
Hint: Prokaryotic organisms lack an ordered nucleus and other organelles attached to the membrane. Its DNA is located in a central part of the nucleoid called the cell. Sometimes, however, they have inclusions that compartmentalize their cytoplasm.
Complete answer: > Option A is correct. Cyanobacteria lack a membrane-bound nucleus, mitochondria, Golgi apparatus, chloroplasts, and endoplasmic reticulum like all other prokaryotes. Cyanobacteria or Blue-green algae are aquatic and photosynthetic, i.e. they live in the water, and can grow their own food. They are very tiny since they are bacteria, and can be unicellular or filamentous. Many have sheaths in colonies to bind other cells or filaments.
> Option B is incorrect. Diatoms are known as eukaryotic species whose membrane-bound nucleus of cells that differentiate them from archaea and bacteria of the prokaryote. Diatoms are the most common type of plankton, called phytoplankton. Common organelles found in most of the diatoms are the nucleus, mitochondria and prominent plastids (chromatophores). The diatoms are the very distinctive characteristic of their shells. The nucleus in diatoms is normally positioned centrally, moving to various cell sites as the diatom prepares for cell division.
> Option C is incorrect. Algae are eukaryotic, and contain three types of the double-membrane-bound organelles: nucleus, chloroplast, and mitochondrion. There is only one nucleus in most algal cells while some cells are multinucleated. Their nucleus in algae is surrounded, as in other eukaryotic species, by a double membrane nuclear shell, and contains DNA.
> Option D is incorrect. Yeasts are one-cell organisms that developed from multicellular ancestors, with certain species being able to establish multicellular characteristics by creating strings of linked budding cells known as pseudo-hyphae or false hyphae. While yeast is single-celled organisms, they have a similar cellular structure to that of higher organisms, including humans. Its genetic material is precisely found within a nucleus.
Hence, The correct answer is option (A).
Note: For a variety of reasons prokaryotic cells are important to all life on Earth. They play a vital role in nutrient recovery by decomposing dead species and facilitating re-use of their nutrients. They are important for a lot of metabolic processes as well.