rRNA Gene Location in Eukaryotes
Ribosomal RNA (rRNA) molecules are essential components of ribosomes, the cellular machinery responsible for protein synthesis. In eukaryotic cells, there are several types of rRNA, including the 18s, 5.8s, and 28s rRNA, which play crucial roles in forming the structure and function of ribosomes. The question asks about the specific location of the genes that code for these particular rRNA molecules within eukaryotic organisms.
Understanding Ribosomal RNA (rRNA) Types
Eukaryotic ribosomes are complex structures composed of both proteins and various rRNA molecules. The major rRNA components are:
- 18s rRNA: Forms part of the small ribosomal subunit (40s).
- 28s rRNA, 5.8s rRNA, and 5s rRNA: Form part of the large ribosomal subunit (60s). (Note: The 5s rRNA gene is typically located elsewhere, often amplified in tandem repeats on a different chromosome).
The genes encoding the 18s, 5.8s, and 28s rRNA are found together in specific regions of the eukaryotic genome.
Chromosomal Location of rRNA Genes
The genes responsible for transcribing the 18s, 5.8s, and 28s rRNA are located in specialized regions of certain chromosomes. These regions are known as Nucleolar Organizer Regions (NORs).
- Nucleolar Organizer Regions (NORs): These are specific chromosomal loci that contain tandem repeats of ribosomal DNA (rDNA).
- Function: NORs are the sites where transcription of the precursor rRNA molecule (which is processed into 18s, 5.8s, and 28s rRNA) occurs.
- Nucleolus Formation: These NORs congregate within the nucleus to form the nucleolus, a distinct structure where rRNA synthesis and ribosome assembly primarily take place.
Therefore, the genes coding for the 5.8s, 18s, and 28s rRNA in eukaryotes are situated within the nucleolar organizer regions of chromosomes.
Evaluating Other Options
Let's briefly consider why the other options are incorrect:
- Mitochondrial DNA: Mitochondria contain their own DNA, but it codes for mitochondrial proteins and its own distinct rRNAs (e.g., 12s and 16s rRNA in mammals), not the nuclear-encoded 18s, 5.8s, and 28s rRNA.
- Membrane of Endoplasmic Reticulum: The ER membrane is involved in protein synthesis (via attached ribosomes) and lipid synthesis, but it does not house the genes for rRNA.
- Membrane of Ribosome: Ribosomes themselves are composed of rRNA and proteins; they do not have a membrane, nor are their constituent genes located within a membrane structure.
Important Questions from Miscellaneous
A stone is thrown horizontally from the top of a 20 m high building with a speed of 12 m/s. It hits the ground at a distance R from the building. Taking g = 10 m/s2 and neglecting air resistance will give :
A mass is attached to a spring that hangs vertically. The extension produced in the spring is 6 cm on Earth. The acceleration due to gravity on the surface of the Moon is one-sixth of its value on the surface of the Earth. The extension of the spring on the Moon would be:
Directions: Each item in this section consists of a sentence with an underlined word followed by four words (a), (b), (c), and (d). Select the option that is opposite in meaning to the underlined word and mark your response in your Answer Sheet accordingly.
The deluge affected the population.The major source of vitamins and minerals for vegetarians is
Which of the following statements about the Deccan Riots Commission is/are correct?
1. The Commission did not hold enquiries in the districts which were not affected.
2. The Commission did record the statements of ryots, sahukars and eye-witnesses.
Select the correct answer using the code given below:
