With two acetyl coenzyme As inside the mitochondrial matrix , we are finally able to start the steps of the citric acid cycle , or second stage of cellular respiration. The citric acid cycle is also known as the Krebs cycle . No matter what you call it, you'll notice the name fits the bill. This stage of cellular respiration is a cyclical process of 8 different chemical reactions. While all the reactions that occur are important, in this lesson, we're just going to focus on the reactions that are essential to create products that are important to the next phase of cellular respiration.
Cells use special proteins and checkpoint signaling systems to ensure that the cell cycle progresses properly. Checkpoints at the end of G1 and at the beginning of G2 are designed to assess DNA for damage before and after S phase. Likewise, a checkpoint during mitosis ensures that the cell’s spindle fibres are properly aligned in metaphase before the chromosomes are separated in anaphase. If DNA damage or abnormalities in spindle formation are detected at these checkpoints, the cell is forced to undergo programmed cell death, or apoptosis . However, the cell cycle and its checkpoint systems can be sabotaged by defective proteins or genes that cause malignant transformation of the cell, which can lead to cancer . For example, mutations in a protein called p53 , which normally detects abnormalities in DNA at the G1 checkpoint, can enable cancer-causing mutations to bypass this checkpoint and allow the cell to escape apoptosis.