G1 Phase: Videos & Practice Problems

The cell cycle begins with the G1 phase, a critical growth period that occurs after interphase and before DNA replication and cell division. During G1, the cell prepares for division by growing and synthesizing the necessary components for the upcoming mitotic process. This phase is essential for cell cycle regulation, as the transition from G1 to the S phase is a commitment point; once a cell enters S phase, it must either divide or face apoptosis (programmed cell death). If G1 does not adequately prepare the cell, it may lead to incomplete division or cell death, which is inefficient for cellular resources.

The transition between these phases is regulated by the start checkpoint, a crucial control mechanism that ensures the cell is ready to proceed. Mitogens, which are extracellular signals such as hormones, chemicals, or proteins, stimulate cell growth during interphase. When mitogens bind to cell receptors, they activate downstream pathways that promote the expression of cyclins and cyclin-dependent kinases (CDKs) necessary for G1 phase progression. These signals also inhibit S and M phase cyclins to prevent premature entry into subsequent phases, allowing the cell sufficient time to grow.

However, if the cell has sustained DNA damage, it must halt its progression in G1. The protein p53 plays a pivotal role in this process by acting as a transcription regulator that detects DNA damage and prevents the cell from entering S phase until the damage is repaired. The importance of p53 is underscored by its frequent mutation in various cancers, as its failure to function properly can lead to the division of damaged cells, contributing to tumorigenesis.

In cases where the cell is not ready for division due to size, DNA damage, or environmental stress, it may enter a non-dividing state known as G0. Cells in G0 can remain in this phase indefinitely, allowing them to repair themselves before re-entering G1. For instance, nerve cells typically enter G0 and remain there permanently, as they do not undergo division.

In summary, the G1 phase is a vital period for cellular growth and preparation for division, regulated by checkpoints and influenced by external signals. The p53 protein is crucial for maintaining genomic integrity by halting the cell cycle in response to DNA damage, thereby preventing the propagation of errors that could lead to cancer. Understanding these mechanisms is essential for insights into cell biology and cancer research.

In the cell cycle, once a cell has grown and confirmed that its DNA is intact, it prepares to divide by entering the S phase, or synthesis phase. This critical phase is primarily characterized by DNA replication, which is facilitated by specific proteins known as S cyclin-dependent kinases (SCDKs). These proteins play a pivotal role in triggering the transition into S phase.

The SCDKs activate essential proteins called DNA helicases, which are crucial for unwinding the DNA strands, allowing replication to occur. Additionally, SCDKs promote the formation of the pre-replicative complex, a key assembly that is recruited to specific sites on the DNA known as origins of replication. These origins are the designated starting points for DNA replication.

As SCDKs are recruited to these origins, they work alongside other proteins to ensure that the pre-replicative complex is properly established. This complex is vital for the initiation of DNA replication, marking the cell's readiness to proceed into the S phase. The successful formation of this complex signifies that the cell is prepared to duplicate its genetic material, a fundamental step before cell division can occur.