The number of chromosomes in a somatic cell of onion is 16. How many DNA molecules are there in a root tip cell of onion during metaphase stage of cell division ?

Question

Accepted Answer

The correct option is 3: 32

To determine the number of DNA molecules in a cell during metaphase, we must first understand the state of the cell cycle and the structure of a chromosome at that stage. The problem states that the somatic cell (diploid, $2n$) of the onion has 16 chromosomes. A root tip cell undergoes mitosis, a process of cell division that ensures the daughter cells are genetically identical to the parent cell. Before the cell enters the stages of mitosis (prophase, metaphase, anaphase, telophase), it must first pass through the S-phase (synthesis phase) of Interphase. During the S-phase, the cell replicates its entire genetic material, meaning the amount of DNA is doubled. Therefore, by the time the cell reaches metaphase, every one of the 16 chromosomes has been duplicated. Although the chromosome number is defined by the number of centromeres, which remains $2n=16$ in metaphase, each chromosome is now composed of two identical structures called sister chromatids. Since each chromatid represents one complete, double-stranded DNA molecule, the total number of DNA molecules is simply twice the number of chromosomes. The calculation is 16 chromosomes $\times$ 2 DNA molecules per chromosome, which gives a total of **32 DNA molecules**. This ensures that when the sister chromatids separate during anaphase, each resulting daughter cell will receive 16 individual DNA molecules, restoring the $2n=16$ condition of the parent cell's G1 phase.

The incorrect option is 1: 8

The number 8 represents the haploid chromosome number ($n$) for the onion, as the somatic cell number ($2n$) is 16. A haploid cell would contain 8 chromosomes, each consisting of one DNA molecule (in G1 phase). However, a root tip cell is a somatic cell and is diploid ($2n$). Furthermore, the cell is specified to be in metaphase, a stage that occurs *after* DNA replication, meaning the DNA content has already doubled from its G1 state. Therefore, 8 is the number of chromosomes in the gamete (haploid) or the number of DNA molecules *only* in a haploid cell during the G1 phase, neither of which is the correct condition.

The incorrect option is 2: 16

The number 16 correctly identifies the diploid chromosome number ($2n$) of the somatic onion cell. This value would represent the number of DNA molecules only if the cell was in the G1 phase of the cell cycle, which occurs *before* DNA synthesis. In the G1 phase, each of the 16 chromosomes is unreplicated, consisting of a single DNA molecule. Since the question specifies the metaphase stage, which follows the S-phase (DNA replication), the DNA content must be twice the G1 amount. Therefore, 16 is an intermediate step, representing the chromosome number at metaphase, but not the number of DNA molecules.

The incorrect option is 4: 64

The number 64 is double the correct count of 32 DNA molecules. This scenario would imply that the cell had either gone through an additional, unstated round of DNA replication without division, leading to an $8C$ DNA content, or that the initial somatic cell chromosome number was $2n=32$. Since the problem explicitly states $2n=16$, and we are considering a normal mitotic metaphase (where the DNA content is $4C$), the value 64 is biologically incorrect for this specific division process in the onion root tip cell.