thumb|upright=1.35|Prophase is the first step of cell division in mitosis. As it occurs after G2 of interphase, DNA has been already replicated when prophase begins.
thumb|right|200px|[[Fluorescence microscope image of two mouse cell nuclei in prophase (scale bar is 5 μm).]]
Prophase () 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 reticulum and the disappearance of the nucleolus.
Various DNA stains are used to treat cells such that condensing chromosomes can be visualized as the move through prophase. During both meiotic and mitotic prophase, giemsa staining can be applied to cells to elicit G-banding in chromosomes. 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.
Mitotic prophase
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.
Condensation of chromosomes
DNA that was replicated in interphase is condensed from DNA strands with lengths reaching 0.7 μm down to
0.2-0.3 μm.
Movement of centrosomes
During prophase in animal cells, centrosomes move far enough apart to be resolved using a light microscope. Interdigitated interpolar microtubules from each centrosome interact with each other, helping to move the centrosomes to opposite poles.
Pachytene
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis. In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The adaptive significance of prophase I arrest is still not fully understood. However, it has been proposed that the arrest of oocytes at the four genome copy stage may provide the informational redundancy needed to repair damage in the DNA of the germline. Prophase arrested oocytes have a high capability for efficient repair of DNA damages.The system consists of multiple pathways (including the meiotic recombination checkpoint) that prevent the cell from entering metaphase I with errors due to recombination.
See also
- Prometaphase
- Metaphase
- Anaphase
- Telophase
- Meiosis
- Mitosis
- Cytoskeleton
- Homologous chromosome