Key Points
Key Points
- Cells are the basic building blocks of all organisms. Cells that have a shared function and interconnect with each other form tissues. Tissues further combine to form organs and several organs comprise an organ system. There are four primary types of tissues in multicellular organisms: epithelial, connective, muscle and nervous.
- Many processes are undertaken for a cell to develop into a final cell type including cell proliferation, differentiation, movement and programmed cell death or apoptosis. Mitosis is a form of asexual replication and is the process by which somatic cells replicate and divide into two identical daughter cells. Meiosis is another form of replication that occurs in sex cells, in which haploid cells are formed, containing one set of chromosomes. Sexual reproduction requires the union of two haploid cells from two individual organisms which combine to create a diploid cell.
- Accurate duplication of the DNA contained within cells during replication is imperative to maintain cell function. Mistakes in the duplication or distribution of genetic material can result in mutations that may be passed on to every new cell that is created from the abnormal cell. To manage these processes, there are three main cell cycle checkpoints that can halt the cell cycle until conditions are favourable for replication. At G1 the cell commits to entering the cell cycle and commencing replication processes. The G2 checkpoint confirms that all DNA has been replicated and is not damaged before committing to the next stage of the cell cycle. The M checkpoint ensures that all sister chromatids are attached to spindle microtubules. In addition to ensuring the quality of genetic material prior to replication, several stimuli can influence the cell cycle, including a limit or excess of growth hormone, lack of available room to replicate due to increase cell number, and increased cell size resulting in reduced cell efficiency which stimulates replication.
- Several cell regulatory molecules are involved in the regulation of the cell cycle, and these can be positive or negative. Positive regulatory molecules include cyclins and cyclin-dependent kinases (Cdks). These proteins form complexes that work to progress the cell cycle to the next phase. Negative regulatory molecules block the progression of the cell cycle until the problematic condition that triggered the negative regulation is resolved. Tumour-suppressor proteins including retinoblastoma protein (Rb), p53 and p21 act primarily in the G1 checkpoint to ensure replication occurs in a controlled manner. When DNA is damaged, p53 halts the cell cycle and recruits’ enzymes to repair the DNA, or if this is not possible, triggers cell apoptosis. Increased levels of p53 trigger production of p21 which binds to Cdk/cyclin complexes to ensure the cell cycle does not progress. Rb monitors cell size, to ensure that replication does not occur until the cell reaches sufficient maturity. This is accomplished by binding to transcription factors that trigger transition into the cell cycle until the cell reaches sufficient size and these transcription factors are released.
