Question

Polyribosomes are aggregation of (a) ribosomes and rRNA (b) peroxisomes (c) several ribosomes held together by a string of \(\mathrm{mRNA}\) (d) RNA.

Short answer

Polyribosomes are aggregations of several ribosomes held together by a string of mRNA, so the answer is (c).

Step-by-step solution

Identify the Concept of Polyribosomes

Polyribosomes, or polysomes, are complexes of multiple ribosomes that simultaneously translate the same messenger RNA (mRNA) molecule. The key to identifying what polyribosomes aggregate is understanding their role in protein synthesis.

Eliminate Incorrect Options

Since polyribosomes involve translation, look for the option that includes both ribosomes and components directly involved in translation. Eliminate options that do not have a direct relationship with translation.

Choose the Correct Answer

Polyribosomes are actually several ribosomes held together by a string of mRNA, so the correct option is (c).

Key concepts

Protein Synthesis

Protein synthesis is a fundamental biological process where cells build proteins. This complex mechanism involves translating genetic information encoded in the DNA into functional proteins. The process begins in the cell nucleus, where DNA is transcribed into messenger RNA (mRNA). The mRNA then travels out of the nucleus and into the cytoplasm, where ribosomes latch onto it. Ribosomes are molecular machines that read the mRNA sequence and synthesize the corresponding protein, a process known as translation.

Proteins are constructed from amino acids, which are brought to the ribosomes by transfer RNA (tRNA). Each tRNA carries a specific amino acid and matches the mRNA codons—sequences of three nucleotides—through complementary base pairing. This ensures that the amino acids are added in the proper order, creating a polypeptide chain that folds into a functional protein. To enhance efficiency and the rate of protein production, multiple ribosomes can work on a single mRNA strand simultaneously, forming what is called a polyribosome, or polysome.

mRNA Translation

mRNA translation is the process where the genetic code carried by mRNA is decoded to produce a specific protein. After mRNA is transcribed from DNA, it undergoes a series of modifications before being exported to the cytoplasm. During translation, the ribosome reads the mRNA in sets of three nucleotides, called codons, each coding for a specific amino acid.

Translation occurs in three main stages: initiation, elongation, and termination. During initiation, the ribosome assembles around the start codon of the mRNA. In elongation, tRNAs bring amino acids to the ribosome, which adds them to the growing polypeptide chain. Each tRNA has an anticodon that pairs with a corresponding mRNA codon. Termination occurs when the ribosome reaches a stop codon, indicating the end of the protein-coding sequence. The completed polypeptide chain then detaches and undergoes folding and, sometimes, post-translational modifications to become functional.

Ribosome Aggregation

Ribosome aggregation refers to the clustering of ribosomes along a single strand of mRNA during protein synthesis. This assembly is called a polyribosome or polysome. The primary advantage of such an arrangement is efficiency; multiple ribosomes can simultaneously translate the same mRNA molecule. This increases the rate at which proteins can be synthesized within a cell.

When visualized under an electron microscope, these polyribosomes appear as a string of beads. Each 'bead' represents a single ribosome, and the string between them is the mRNA strand. The ribosomes move along the mRNA, reading its sequence and synthesizing proteins as they go. As one ribosome moves along, another can attach to the start of the mRNA, creating a continuous production line for protein synthesis. This allows for rapid response to cellular needs, and it is especially important in cells that require large amounts of specific proteins.