Question

S-phase is not characterized by (a) DNA duplication (b) No increase in chromosome number (c) DNA replication (d) Duplication of centriole in nucleus of eukaryotic animal cell

Short answer

The correct answer is (d) Duplication of centriole in nucleus of eukaryotic animal cell. This process takes place during the G2 phase, not the S phase of the cell cycle. Therefore, this option does not characterize the S-phase.

Step-by-step solution

Understanding the S-phase

The S-phase is the part of the cell cycle where DNA is replicated. This phase is characterized by DNA replication and duplication, and although the quantity of DNA is doubled during this phase, the number of chromosomes does not increase because each chromosome now consists of two identical sister chromatids.

Analyzing the options

Option (a) states DNA duplication, and option (c) states DNA replication. Both these options characterize the S-phase as DNA is indeed replicated and thus duplicated in this phase. Option (b) states No increase in chromosome number, which is also characteristic of S-phase. Even though the DNA amounts are doubled, chromosomes as discrete structures are not increased. They only contain two identical copies of DNA (sister chromatids) after DNA replication.

Finding the correct answer

Option (d) is Duplication of centriole in nucleus of eukaryotic animal cell. The centriole duplication takes place during the G2 phase, not the S phase of the cell cycle. Therefore, this process does not characterize the S-phase. Hence, option (d) is the correct answer.

Key concepts

DNA replication in S-phase

The S-phase stands for 'synthesis' phase and it's a period in the cell cycle devoted to copying the cell's genetic material.

During this phase, an essential process called DNA replication occurs. DNA replication is a critical process in which the DNA double helix unwinds, and enzymes such as DNA polymerase build a new complementary strand alongside each of the original strands. This results in the creation of two identical DNA molecules from one, effectively doubling the total DNA content within the cell.

Every part of DNA must be accurately and fully replicated to ensure integrity in the genetic code. Mistakes during this process can lead to mutations, which sometimes result in serious consequences such as cancer. However, because of built-in proofreading mechanisms, DNA replication is remarkably accurate, ensuring faithful transmission of genetic information from one generation of cells to the next.

Chromosome number during S-phase

A common misconception is that the chromosome number changes during the S-phase; however, this is not the case.

Although the amount of DNA in the cell doubles as each chromosome is copied, the actual number of chromosomes remains the same. This is because the structure of each chromosome changes from having a single chromatid to having two sister chromatids that are still considered to be one chromosome.

These sister chromatids are connected by a centromere, and it's only during mitosis, particularly during the anaphase stage, that they will separate and be distributed to two new daughter cells. This distinction is crucial for understanding the difference between the quantity of genetic material and chromosome count in cells.

Cell cycle phases

The cell cycle is a series of events that take place in a cell leading to its division and replication. It consists of four distinct phases: G1, S, G2, and M phase.

• G1 Phase (First Gap): This is a period of cell growth where the cell increases in size and prepares the enzymes and energy required for DNA replication.
• S Phase (Synthesis): This phase is dedicated to the replication of the cell's genetic material, DNA.
• G2 Phase (Second Gap): During G2, the cell continues to grow and begins preparing for the division process. This includes the duplication of organelles and the synthesis of proteins needed for cell division.
• M Phase (Mitosis): In this phase, the cell divides, a process made up of several stages including prophase, metaphase, anaphase, and telophase, which culminate in the formation of two daughter cells.

The conventional understanding dictates that centriole duplication, as mentioned in the question, occurs during the G2 phase and thus does not characterize the S-phase. It's important for students to grasp the chronology and specific events of each cycle phase to fully understand cell division and replication mechanisms.