Cell growth
Cell growth (or interphase) is shorthand for the idea of "growth in cell populations" by means of cell reproduction.
It is the stage which cells are preparing for the next division, biochemical activities and reactions are taking place, however no obvious changes can be seen at this stage. Until cell division, a parent cell divides to form two or more new daughter cells.
Phase
- First growth phase (G1) - In this phase, new organelles like mitochondria or chloroplasts are made. These organelles help store and provide energy. It also has mechanism to ensure everything is ready for DNA synthesis.
- Synthesis phase (S) - DNA molecules are replicated.
- Second growth phase (G2) - The cell continues to grow until to its maximum size, therefore energy stores can be increased at the same time.
Cell populations
Cell populations go through a type of exponential growth called doubling. Thus, each generation of cells should be twice as numerous as the previous generation. But not all cells survive in each generation, as written by Richard Dawkins in 1997.
Cell reproduction
Cell reproduction is asexual.
The process of cell reproduction has three major parts. The first part of cell reproduction involves the replication of the parental cell's DNA. The second major part is the separation of the duplicated DNA into two equally sized groups of chromosomes. The third major aspect of cell reproduction is the physical division of entire cells, usually called cytokinesis.
Cell reproduction is more complex in eukaryotes than in other organisms. Prokaryotic cells such as bacterial cells reproduce by binary fission, a process that includes DNA replication, chromosome segregation, and cytokinesis. Eukaryotic cell reproduction either involves mitosis or a more complex process called meiosis. Mitosis and meiosis are sometimes called the two "nuclear division" processes.
Comparison of the three types of cell reproduction
The DNA content of a cell is duplicated at the start of the cell reproduction process. Prior to DNA replication, the DNA content of a cell can be represented as the amount Z (the cell has Z chromosomes). After the DNA replication process, the amount of DNA in the cell is 2Z (multiplication: 2 x Z = 2Z). During Binary fission and mitosis the duplicated DNA content of the reproducing parental cell is separated into two equal halves that are destined to end up in the two daughter cells. The final part of the cell reproduction process is cell division, when daughter cells physically split apart from a parental cell. During meiosis, there are two cell division steps that together produce the four daughter cells.
Immediately after DNA replication a human cell will have 46 "double chromosomes". In each double chromosome there are two copies of that chromosome's DNA molecule. During mitosis the double chromosomes are split to produce 92 "single chromosomes", half of which go into each daughter cell. During meiosis, there are two chromosome separation steps which assure that each of the four daughter cells gets one copy of each of the 23 types of chromosome.
Sexual reproduction
Cell reproduction that uses mitosis can reproduce eukaryotic cells. Eukaryotes bother with the more complicated process of meiosis because sexual reproduction such as meiosis confers a selective advantage.
Related pages
References
- Morgan DO. (2007) "The Cell Cycle: Principles of Control" London: New Science Press.
- Climbing Mount Improbable (1997) Richard Dawkins. ISBN 0-393-31682-3