Problem 163 - Objective Biology for NEET

Question

Select the incorrect statement: (a) Micro bodies contain various enzyme and are present in both plant and animal cells. (b) Few chromosomes have non-staining secondary constriction of constant location. This give the appearance of a small fragment called the satellite. (c) Nuclei are spherical structures present in nucleoplasm and it is a site for ribosomal RNA synthesis. (d) Every chromosome essentially has a secondary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present.

Step-by-Step Solution

Answer

The incorrect statement is (c). The statement confuses 'nuclei' and 'nucleoli'. Nucleoli, not nuclei, are the parts of the nucleus where ribosomal RNA synthesis takes place.

Step 1: Identifying statement (a)

This statement asserts that micro bodies contain various enzymes and are present in both plant and animal cells. Upon checking, it is found to be correct. Microbodies, also known as peroxisomes, are indeed present in both plant and animal cells and contain enzymes.

Step 2: Identifying statement (b)

This statement says that a few chromosomes have non-staining secondary constriction of constant location that appears as a small fragment called the satellite. This statement is correct as there are indeed chromosomes known as satellite chromosomes that present this feature.

Step 3: Identifying statement (c)

The statement holds that nuclei are spherical structures present in nucleoplasm and that they are the site of ribosomal RNA synthesis. This statement is incorrect because it confuses 'nuclei' and 'nucleoli'. In fact, nucleoli are the portions of the nucleus where ribosomal RNA is synthesized.

Step 4: Identifying statement (d)

This statement posits that every chromosome essentially has a secondary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present. This is indeed correct: every chromosome does indeed have a centromere, which serves as the point of attachment for the kinetochores.

Key Concepts

Microbodies

Microbodies, commonly known as peroxisomes, play a pivotal role in cellular metabolism. These small organelles are situated in the cytoplasm of plant and animal cells and are instrumental in various enzymatic reactions.
Their main function is to rid the cell of toxic substances by breaking them down with the aid of enzymes. Peroxisomes contain enzymes such as catalase and urate oxidase, which help in the breakdown of hydrogen peroxide, a harmful byproduct of cellular processes, into water and oxygen.
Apart from detoxification, they are involved in the metabolism of fatty acids and the biosynthesis of cellular lipids. Microbodies hence contribute to maintaining cellular health and balance in both plant and animal cells.

Chromosomes

Chromosomes are the thread-like structures composed of DNA and proteins. They are located within the nucleus of eukaryotic cells and serve as vehicles for genetic information.
Each chromosome possesses a primary constriction known as the centromere. This is a crucial region because it is where the kinetochores, which are protein structures, attach during cell division. The centromere ensures that chromosomes are correctly segregated into daughter cells during mitosis and meiosis.
Moreover, some chromosomes have secondary constrictions that are constant in location. These areas might not stain easily during microscopic examination and can result in small protrusions known as satellites. Such chromosomes are often referred to as "satellite chromosomes." This secondary constriction is different from the centromere and adds to the unique morphology of specific chromosomes.

Nucleolus

The nucleolus is a prominent, spherical structure within the nucleus known for its involvement in ribosomal RNA (rRNA) synthesis and ribosome assembly.
Though it lacks a surrounding membrane, the nucleolus is a highly organized region packed with DNA, RNA, and proteins. These components collaboratively create rRNA, which is crucial for assembling ribosomes—the protein synthesis machinery of the cell.
During the synthesis process, the nucleolus assembles the rRNA with ribosomal proteins, originating from the cytoplasm, into ribosome subunits. These subunits are then transported out of the nucleus into the cytoplasm, where they join together to form functional ribosomes, facilitating protein synthesis in the cell.

Ribosomal RNA synthesis

Ribosomal RNA (rRNA) synthesis is a key cellular process that takes place primarily in the nucleolus within the nucleus. rRNA, an essential component of the ribosome, is synthesized and processed here.
The synthesis of rRNA starts with the transcription of specific regions of DNA into precursor rRNA molecules by RNA polymerase I. These precursor molecules then undergo several modification and processing steps within the nucleolus, which trim and fold them into functional rRNA components.
Once fully processed, these rRNAs combine with ribosomal proteins, forming the small and large subunits of ribosomes. These subunits are then exported to the cytoplasm, where they join mRNA and tRNA to facilitate protein synthesis. This critical process ensures that cells have the necessary machinery for protein production, pivotal to cell growth and proliferation.