Question

Identify at least two events in the cell cycle that must be completed successfully for daughter cells to share an identical complement of chromosomes.

Short answer

The two essential events in the cell cycle for daughter cells to share an identical complement of chromosomes are: (1) DNA replication, which occurs during the S-phase of interphase, ensuring that the cell has two identical copies of its genetic material; and (2) sister chromatid separation during the anaphase of mitosis, in which spindle fibers evenly divide the chromatids between daughter cells. Both DNA replication and sister chromatid separation ensure that genetic material is equally and accurately distributed among the daughter cells.

Step-by-step solution

1. DNA Replication

During the cell cycle, the first crucial event is DNA replication. This event occurs during the S-phase (synthesis phase) of the interphase, in which the DNA molecule (double helix) unwinds, and each of the two strands serves as a template to synthesize a new complementary strand. At the end of DNA replication, the cell has two identical copies of its genetic material, called sister chromatids, which are later separated and distributed to two daughter cells during cell division.

2. Sister Chromatid Separation

Sister chromatid separation is the second critical event for ensuring identical sets of chromosomes in daughter cells, happening during mitosis (M-phase), specifically in the anaphase. During this phase, the spindle fibers, which are composed of microtubules, attach to the centromeres of each sister chromatid. The spindle fibers pull the sister chromatids towards opposite poles, equally dividing the genetic material. Once the sister chromatids are at separate poles of the cell, the cell moves into the next phase, telophase, starting the final stages of cell division. By understanding these two key events – DNA replication and sister chromatid separation – it is clear how daughter cells inherit identical sets of chromosomes from the parent cell after completing these stages of the cell cycle successfully.

Key concepts

DNA Replication

Our journey through the cell cycle begins with a foundational process known as DNA replication, which occurs during the S-phase of interphase. To ensure that each daughter cell receives an identical set of genetic material, the DNA within the parent cell must be duplicated. Think of DNA as a twisted ladder, or double helix. This helix unwinds so that each strand can serve as a template for a new complementary strand. Through a series of enzymatic reactions, each old strand pairs with new nucleotide bases forming two new, identical DNA molecules. These resulting structures are known as sister chromatids, which are crucial for the next steps in cell division.

Sister Chromatid Separation

Once DNA replication is complete, the focus shifts to another critical process: sister chromatid separation. This event occurs during the M-phase of the cell cycle, where mitosis takes place. Sister chromatids are essentially duplicated chromosomes that are joined together by a centromere. The goal is to divide these pairs so each new cell receives one chromatid from each chromosome.
During separation, spindle fibers attach to the centromeres which act as the connector between sister chromatids. These spindle fibers exert a pulling force that orchestrates the movement of sister chromatids toward opposite ends, ensuring an equitable sharing of genetic information.

Mitosis

A key player in the successful division of cells, mitosis, is divided into several stages, each with its own objectives. It's the process that ensures each daughter cell is a genetic replica of the parent cell. The stages of mitosis – prophase, metaphase, anaphase, and telophase – ensure that the sister chromatids formed during the DNA replication are properly divided.

• During Prophase, the chromosomes condense and become visible, and spindle fibers begin to form.
• In Metaphase, the chromosomes align in the middle of the cell, called the metaphase plate.
• Anaphase is where sister chromatids are pulled apart.
• Finally, in Telophase, the separated chromosomes reach opposite poles, and the nuclear envelope starts to reform.

Each stage is vital for ensuring that the genetic material is evenly distributed to the daughter cells.

Anaphase

Anaphase is often seen as the most dynamic stage of mitosis. It plays a pivotal role in ensuring that genetic information is evenly divided between two forming daughter cells. During anaphase, each chromatid pair that was aligned during metaphase is split apart.
The separation process is driven by the shortening of spindle fibers, which retract and pull the chromosomes apart towards opposite poles of the cell. This stage is crucial as it results in the equal partitioning of chromosomes, preparing the cell for the final stages of cell division. The success of anaphase determines the genetic consistency in daughter cells, highlighting its importance in the fidelity of cell division.