Cell growth
Oocytes can be unusually large cells in species for which embryonic development takes place away from the mother's body within an egg that is laid externally.
Biosynthesis of biomolecules is initiated by expression of genes which encode RNAs and/or proteins, including enzymes that catalyse synthesis of lipids and carbohydrates.
The protein TOR, part of the TORC1 complex, is an important upstream regulator of translation initiation as well as ribosome biogenesis.
[5] TOR is a serine/threonine kinase that can directly phosphorylate and inactivate a general inhibitor of eIF4E, named 4E-binding protein (4E-BP), to promote translation efficiency.
Nutrient availability influences production of growth factors of the Insulin/IGF-1 family, which circulate as hormones in animals to activate the PI3K/AKT/mTOR pathway in cells to promote TOR activity so that when animals are well fed they will grow rapidly and when they are not able to receive sufficient nutrients they will reduce their growth rate.
Recently it has been also demonstrated that cellular bicarbonate metabolism, which is responsible for cell growth, can be regulated by mTORC1 signaling.
[7] Though the proteins that control Cdk1 are well understood, their connection to mechanisms monitoring cell size remains elusive.
A postulated model for mammalian size control situates mass as the driving force of the cell cycle.
Chemical gradients are known to be partly responsible, and it is hypothesized that mechanical stress detection by cytoskeletal structures is involved.
This covalent modification of the molecular structure of Cdc2 inhibits the enzymatic activity of Cdc2 and prevents cell division.
When cells have reached sufficient size during G2, the phosphatase Cdc25 removes the inhibitory phosphorylation, and thus activates Cdc2 to allow mitotic entry.
This finding connects a physical location, a band of cortical nodes, with factors that have been shown to directly regulate mitotic entry, namely Cdr1, Cdr2, and Blt1.
A Cdr2 kinase mutant, which is able to localize properly despite a loss of function in phosphorylation, disrupts the recruitment of Wee1 to the medial cortex and delays entry into mitosis.
Thus, Wee1 localizes with its inhibitory network, which demonstrates that mitosis is controlled through Cdr2-dependent negative regulation of Wee1 at the medial cortical nodes.
Pom1 knockout cells were also shown to divide at a smaller size than wild-type, which indicates a premature entry into mitosis.
This finding shows how cell size plays a direct role in regulating the start of mitosis.
[14] The Pom1 polar gradient successfully relays information about cell size and geometry to the Cdk1 regulatory system.
Genetic studies of the fruit fly Drosophila have revealed several genes that are required for the formation of multinucleated muscle cells by fusion of myoblasts.
Most unicellular organisms are microscopic in size, but there are some giant bacteria and protozoa that are visible to the naked eye.
The second part of the cell cycle is the S phase, where DNA replication produces two identical sets of chromosomes.
The third part is the G2 phase in which a significant protein synthesis occurs, mainly involving the production of microtubules that are required during the process of division, called mitosis.
It produces four special daughter cells (gametes) which have half the normal cellular amount of DNA.
A male and a female gamete can then combine to produce a zygote, a cell which again has the normal amount of chromosomes.
The rest of this article is a comparison of the main features of the three types of cell reproduction that either involve binary fission, mitosis, or meiosis.
Recombination of genetic information between homologous chromosomes during meiosis is a process for repairing DNA damages.
However, in organisms with more than one set of chromosomes at the main life cycle stage, sex may also provide an advantage because, under random mating, it produces homozygotes and heterozygotes according to the Hardy–Weinberg ratio.
A series of growth disorders can occur at the cellular level and these consequently underpin much of the subsequent course in cancer, in which a group of cells display uncontrolled growth and division beyond the normal limits, invasion (intrusion on and destruction of adjacent tissues), and sometimes metastasis (spread to other locations in the body via lymph or blood).
Several key determinants of cell growth, like ploidy and the regulation of cellular metabolism, are commonly disrupted in tumors.
In epithelial tissues, misregulation of cellular size can induce packing defects and disperse aberrant cells.
All these assays may correlate well, or not, depending on cell growth conditions and desired aspects (activity, proliferation).
