Interphase, a pivotal stage in the life of a cell, encompasses a significant majority of the cell cycle. This phase involves the cell undergoing crucial processes of growth, DNA replication, and preparation for division. It's segregated into three distinct phases: G1, S, and G2.
G1 Phase: Growth and Preparation for DNA Synthesis
The G1 phase, denoting 'Gap 1', represents the initial phase of the interphase where the cell manifests its growth and conducts its regular metabolic processes. Its length varies considerably and is relatively short in cells, like embryonic cells, which divide at an expedited pace.
- Cellular Activity: In G1, the cell is metabolically active, synthesising proteins and growing in size. It duplicates its organelles and generates the molecular building blocks, such as nucleotides and amino acids, needed for the upcoming phases.
- G1 Checkpoint: Known as the restriction point in mammalian cells and Start in yeast, this is a critical control mechanism that ensures appropriate conditions before committing to DNA replication. If the cell fails to meet the necessary criteria, including cell size, availability of nutrients, growth factors, and absence of DNA damage, it might transition into a quiescent stage known as the G0 phase.
S Phase: DNA Replication
The S phase, standing for 'Synthesis', succeeds the G1 phase. Here, the cell replicates its DNA, ensuring identical genetic material for the subsequent daughter cells.
- DNA Replication: Each DNA molecule is unzipped, forming a replication fork where DNA polymerases can begin their work. Using the parent DNA strand as a template, DNA polymerases add nucleotides to the new DNA strand in a 5' to 3' direction, replicating the DNA. By the end of the S phase, each chromosome comprises two identical sister chromatids, conjoined at a region known as the centromere.
- Replication Checkpoint: This checkpoint safeguards the integrity of the genetic material, as the cell meticulously checks for DNA replication errors and potential DNA damage.
G2 Phase: Final Preparations for Cell Division
After the S phase, the cell transitions into the G2 phase, representing 'Gap 2'. Here, the cell continues its growth and prepares for cell division.
- Cellular Activity: During G2, the cell continues to synthesise proteins and grow. Particularly crucial are the proteins necessary for mitosis and cytokinesis, which are produced during this phase. The cell also prepares the energy reserves required for the impending mitotic phase.
- G2 Checkpoint: This crucial checkpoint examines the cell for DNA damage and ensures that all chromosomes have been completely and accurately replicated. If these conditions are satisfied, the cell proceeds to the M phase, where mitosis and cytokinesis occur.
DNA Replication: A Closer Look
In the S phase of the cell cycle, the DNA replication process is of immense importance, ensuring that future daughter cells receive identical copies of genetic material. DNA replication is a semi-conservative process, resulting in two DNA molecules, each comprising one parent strand and one newly synthesised strand.
- Initiation of DNA Replication: The process begins with the unzipping of the DNA double helix, catalysed by the enzyme helicase. This unwinding forms a replication fork, a Y-shaped region where new DNA strands are synthesised.
- RNA Primer Synthesis: An enzyme named primase synthesises a short RNA primer complementary to the DNA strand. This primer acts as a starting point for DNA polymerase.
- Elongation of DNA Strands: DNA polymerase recognises the RNA primer and adds nucleotides in a 5' to 3' direction, synthesising the new DNA strand.
- Finalisation of DNA Replication: The enzyme ligase seals any gaps left by the RNA primer removal, thereby finalising the replication process, resulting in two complete DNA molecules.
Preparation for Mitosis
Upon completion of interphase, the cell prepares for mitosis, the process where the cell's nucleus divides. Preparation includes ensuring DNA replication has been successfully carried out and that the cell has attained the necessary size.
Additionally, proteins vital for chromosome condensation and spindle formation, such as condensin and tubulin, are synthesised during the G2 phase. These proteins aid in the appropriate segregation of chromosomes during mitosis, ensuring that each daughter cell receives the exact genetic content.
FAQ
Despite DNA replication being a highly accurate process, errors known as mutations can occasionally occur. These mutations can lead to genetic variation if they occur in cells that will become gametes. Additionally, during replication, certain segments of DNA can undergo recombination, creating new combinations of genes, further adding to genetic diversity.
In cancer, cells divide uncontrollably, and this is often due to disruptions in the regulation of the cell cycle, particularly the S phase. Certain genes called oncogenes can stimulate cells to proceed through the cell cycle and replicate their DNA even when they shouldn't, potentially leading to cancer.
All cells that divide undergo interphase. However, mature cells like neurons and muscle cells rarely divide and typically stay in the G0 phase, a quiescent stage where cells continue to function but do not prepare for cell division.
If a cell does not pass the G1 or G2 checkpoints, it will not progress to the next stage of the cell cycle. The checkpoints ensure the cell is ready for DNA replication (G1) or mitosis (G2). If DNA is damaged, the cell attempts to repair it. If repair is impossible, the cell may enter a resting state, called G0, or undergo programmed cell death (apoptosis) to prevent the propagation of erroneous DNA.
During interphase, DNA is not tightly condensed as it is during mitosis. Instead, it exists as chromatin, a less condensed structure that allows for DNA replication and transcription. The DNA wraps around proteins called histones to form a "beads on a string" structure known as nucleosomes, facilitating efficient packaging of DNA within the nucleus.
Practice Questions
The G1 phase, the first phase of interphase, involves cell growth and preparation for DNA synthesis. The cell increases in size, duplicates organelles, and synthesises proteins and other molecular building blocks. It also passes the G1 checkpoint to ensure conditions are suitable for DNA replication. Conversely, the G2 phase follows DNA replication in the S phase and involves further cell growth and preparation for mitosis. The cell synthesises proteins crucial for mitosis and cytokinesis and passes the G2 checkpoint to ensure DNA has been accurately replicated and the cell is ready for division.
During the S phase, DNA replication occurs, which is a semi-conservative process. DNA helicase unwinds the DNA double helix, forming a replication fork. An enzyme named primase synthesises a short RNA primer that acts as a starting point for DNA polymerase. DNA polymerase then recognises the RNA primer and adds nucleotides in a 5' to 3' direction, synthesising the new DNA strand. Finally, the enzyme ligase seals any gaps left by the RNA primer removal, completing the DNA replication process. The end result is two complete DNA molecules, each consisting of one parent and one newly synthesised strand.
