Prophase

Prophase (from the Greek πρό, 'before' and φάσις, 'stage') is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin and the disappearance of the nucleolus. Prophase (from the Greek πρό, 'before' and φάσις, 'stage') is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin and the disappearance of the nucleolus. Microscopy can be used to visualize condensed chromosomes as they move through meiosis and mitosis. Various DNA stains are used to treat cells such that condensing chromosomes can be visualized as the move through prophase. The giemsa G-banding technique is commonly used to identify mammalian chromosomes, utilizing the technology on plant cells was difficult due to the high degree of chromosome compaction in plant cells. G-banding was fully realized for plant chromosomes in 1990. During both meiotic and mitotic prophase, giemsa staining can be applied to cells to elicit G-banding in chromosomes. Silver staining, a more modern technology, in conjunction with giesma staining can be used to image the synaptonemal complex throughout the various stages of meiotic prophase. To perform G-banding, chromosomes must be fixed, and thus it is not possible to perform on living cells. Fluorescent stains such as DAPI can be used in both live plant and animal cells. These stains do not band chromosomes, but instead allow for DNA probing of specific regions and genes. Use of fluorescent microscopy has vastly improved spatial resolution. Prophase is the first stage of mitosis in animal cells, and the second stage of mitosis in plant cells. At the start of prophase there are two identical copies of each chromosome in the cell due to replication in interphase. These copies are referred to as sister chromatids and are attached by DNA element called the centromere. The main events of prophase are: the condensation of chromosomes, the movement of the centrosomes, the formation of the mitotic spindle, and the beginning of nucleoli break down. DNA that was replicated in interphase is condensed from molecules with lengths reaching 4 cm to chromosomes that are measured in micrograms. This process employs the condensin complex. Condensed chromosomes consist of two sister chromatids joined at the centromere. During prophase in animal cells, centrosomes move far enough apart to be resolved using a light microscope. Microtubule activity in each centrosome is increased due to recruitment of γ-tubulin. Replicated centrosomes from interphase move apart towards opposite poles of the cell, powered by centrosome associated motor proteins. Interdigitated interpolar microtubules from each centrosome interact with each other, helping to move the centrosomes to opposite poles. Microtubules involved in the interphase scaffolding break down as the replicated centrosomes separate. The movement of centrosomes to opposite poles is accompanied in animal cells by the organization of individual radial microtubule arrays (asters) by each centromere. Interpolar microtubules from both centrosomes interact, joining the sets of microtubules and forming the basic structure of the mitotic spindle. In cells without centrioles chromosomes can nucleate microtubule assembly into the mitotic apparatus. In plant cells, microtubules gather at opposite poles and begin to form the spindle apparatus at locations called foci. The mitotic spindle is of great importance in the process of mitosis and will eventually segregate the sister chromatids in metaphase.