In synchronous populations ofScenedesmus quadricauda the RNA amount in the cells increases in waves: periods of a high rate of RNA synthesis alternate with periods of a low rate in the course of the cell cycle. Each wave usually leads to the doubling of the RNA amount per cell. In cells growing under normal conditions the waves of RNA synthesis seem to be linked with consecutive rounds of DNA replication. The pattern of RNA synthesis in the course of the cell cycle, however, does not change, if DNA replication is prevented by application of 5-fluoro-deoxyuridine. In darkness the rate of RNA synthesis drops to zero and thereafter the RNA amount per cell decreases. In cells which have been induced to cellular division RNA synthesis may become restored in the dark in newly formed daughter cells. The lowering of RNA amount and its new increase during the dark period become more pronounced with increasing irradiance in the previous light period as well as with its increasing length. In the period of protoplast fissions RNA synthesis is arrested even if the cells divide in the light; whether a similar inhibition occurs during mitoses is not clear.
Not all cells divide through the process of mitosis. Organisms that also undergo a type of cell division called . Meiosis occurs in and is similar in process to mitosis. After a complete cell cycle in meiosis, however, four daughter cells are produced. Each cell contains one-half the number of chromosomes as the original parent cell. This means that sex cells are cells. When haploid male and female unite in a process called , they form one cell called a zygote.
Then, on the inside of the cell, ATP (Adenosine TriPhosphate) binds toanother site on the carrier and phosphorylates (adds one of its phospategroups, or -PO, to) one of theamino acids that is part of the carrier molecule.
Inside every cell, ribosomes read mRNA sequences and hook together protein building blocks called amino acids in the order specified by the code: Groups of three nucleotides in mRNA code for each of 20 amino acids.
An intermediate in this process, called mRNA (messenger ribonucleic acid), is made from the DNA template and serves as a link to molecular machines called ribosomes.
Depending whether a cell in interphase (see mitotic stages) is in the G1, S or G2 phase, it can thus carry unreplicated (single) chromosomes or -partially- replicated chromosomes (consisting of two chromatides).
The protein synthesis occurs by means of transcrition (in the nucleus: production of RNA with nitrogenous bases that are complementary to one of the ; thus DNA -> RNA) and translation (in polyribosomes and the both located in the cytoplasm: RNA codes -> specific chains of aminoacids, i.e.
Whether this cell division will eventually be a mitosis or a meiosis ( on differences), in either case during replication the entire DNA is copied in which the genetical code is encrypted as a sequence of nitrogenous bases.
The activity of those lat genes (yes or not in state of transcription) is often determined by a confluence of external factors and internal cellular signals.
Once a cell has completed the cell cycle, it goes back into the G 1 phase and repeats the cycle again. Cells in the body can also be placed in a non-dividing state called the Gap 0 phase (G 0) at any point in their life. Cells may remain in this stage for very long periods of time until they are signaled to progress through the cell cycle as initiated by the presence of certain growth factors or other signals. Cells that contain are permanently placed in the G 0 phase to ensure that they are not replicated. When the cell cycle goes wrong, normal cell growth is lost. may develop, which gain control of their own growth signals and continue to multiply unchecked.
In mitosis and , the contents of the dividing cell are equally distributed between two daughter cells. Mitosis has four phases: Prophase, Metaphase, Anaphase, and Telophase.
The sequence and numbers of aminoacids is determined by the genetic code, consisting of three consequent mRNA bases (called triplets or mRNA codons; see tabel here right).
During this segment of the cell cycle, a cell doubles its cytoplasm and synthesizes . It is estimated that a dividing cell spends about 90-95 percent of its time in this phase.
The time it takes for a to complete one cell cycle varies depending on the . Some cells, such as cells in , cells, and cells lining the stomach and intestines, divide rapidly and constantly. Other cells divide when needed to replaced damaged or dead cells. These cell types include cells of the , liver, and . Still other cell types, including , stop dividing once mature.