Maturity functions

Most eukaryotic RNAs are synthesized as precursors that contain excess sequences which are removed prior to the generation of mature, functional RNA. 

The counterpart of DNA polymerases in replication is RNA polymerases in transcription. Just as there are several DNA polymerases in vertebrate cells, so there are several RNA polymerases. To be precise, there are three of them. The different RNA polymerases are associated with three of the classes of RNA molecules found in vertebrate cells. Specifically, RNA polymerase I is responsible for the synthesis of the precursors of most rRNAs. RNA polymerase II plays the same role for the precursors of mRNA. Finally, RNA polymerase III is responsible for the synthesis of the precursors to the tRNAs as well as a few other small RNA molecules. Note here that I have specifically referred to precursors of these classes of RNA molecules. The initial products of the action of the RNA polymerases undergo further metabolism to yield the mature, functional products. 

Those RNA molecules destined to function as mRNA are chemically modified following transcription. The immediate product of transcription is termed pre-mRNA. There are two classes of these modifications that lead to mature, functional mRNA alteration of the mRNA ends and spUcing. 

Epididymis histopathology Mating usually imaffected Fertility sperm maturation/function may be affected leading to a decrease in fertility/fecundity... 

Some newly synthesized RNA transcripts are the functional species, whereas others must be modified or processed into the mature functional species. Modifying enzymes add nucleotides to the 5 or 3 ends or alter bases within the RNA, such as by methylation of specific residues. Specific processing enzymes cleave RNA internally, splice together noncontiguous regions of a transcript, or remove nucleotides from the 5 or 3 ends. 

Aconitase is the trivial name for citrate dehydratase cw-aconitate hydratase (EC 4.2.1.3). It catalyzes the reversible isomerization reaction of citrate into isocitrate via the intermediate cA-aconitate (Figure 2). It is a water-soluble, monomeric protein. In eukaryotic cells aconitase is located in the mitochondrial matrix. In prokaryotes the enzyme occurs in the cytoplasma. The pig heart enzyme consists of 754 amino-acid residues, providing a molecular mass of 83 kDa [27], Aconitase from other sources has similar size. The porcine protein is synthesized with a mitochondrial targeting sequence. The mature, functional protein can be (over)expressed in Escherichia coli [28],... 

However, to use these cells in clinical protocols, it is necessary to understand the nature and properties of stem cells originating from different tissues, as well as the mechanisms that make them differentiate into mature, functional cells (Mayhall et al., 2004). 

The amino-terminal sequence information plays a critical role 1) when the entire primary structure is ready to be constructed from many sequences assigned for small peptide fragments 2) when protein processing is involved in which amino-terminal region of translated polypeptide receives site specific cleavage reaction to produce mature, functional protein 3) when post-synthetic modification is involved in which amino-terminal end group receives some sort of modification, such as acetylation. 

Prokaryotic mRNAs function in translation with no processing, whereas a very complex process is used by eukaryotes to produce a mature, functional mRNA. 

The mucosa of the small intestine has an enormous surface area because of the presence of villi. Villi are covered by absorptive columnar epithelial cells whose surface is further increased by microvilli (brush border), on which carbohydrate and peptide digestive enzymes and transport processes involved in absorption are situated. Pits between the villi contain undifferentiated cells which move up the villi, mature, function for a few days and are shed into the lumen of the gut. 

To achieve neuronal mature function, the neurons must communicate by forming cell-cell connections through synapses. In rats this process occurs over the first three weeks postnatally [85] and in humans through adolescence [86]. Neurons generally have thousands of synapses and retain their ability to form new synapses throughout life. 

Figure 5.7. Hematopoiesis differentiation of HSCs to specialized cells. The differentiation from HSCs (hematopoiesis stem cells) is a complex process which is shown here. The relationships of the intermediate cells to each of the final mature forms are shown from top to bottom in this figure. The development is completed with the generation and specific spatial placement of the various specialized mature functional cell forms. [This image was obtained from HEAL (Health Education Assets Library) whose goal to provide free digital resources for health education (http //www.healcentral.org).] See insert for color representation of this figure.

DC have the unique capacity to initiate primary and secondary immune responses. They take-up antigens in peripheral tissues and migrate to lymphoid organs where they present processed peptides to T-cells. During migration DC undergo maturation from a immature to a mature functional phenotype, characterized by the expression of co-stimulatory molecules, cytokine production and high ability to stimulate T-cell proliferation. 

It is probably unfair to expect that any single thymic polypeptide should have the capability of inducing the full maturation of precursor stem cells into mature functional T lymphocytes. It is perhaps more appropriate to hypothesize that the variety of well-defined thymic polypeptides act sequentially in the thymus to promote selected steps of T-cell differentiation similar to the sequential interactions seen in the complement and coagulation cas-case systems. Thus, the most informative studies concerning the mechanisms of thymic hormone actions are those that have employed prethymic precursor cells or thymocytes and specific assays to detect the influences of the preparation on various intracellular enzymes or cell surface markers that reflect the state of differentiation of the cell. A detailed schematic representation of the different steps in T cell differentiation (in man) was shown in Fig. 10 earlier in this chapter. Figure 10 has been generated from studies performed with both murine and human thymic precursor cells, and on the basis of such studies the hypothesized sites of actions of each of the well-characterized thymic factors are indicated. It would appear from the currently available information that some thymic factors (such as thymosin P3... 

During formation of a mature, functional mRNA, the introns are removed and exons are spliced together. For short transcription units, RNA splicing usually follows cleavage and polyadenylation of the 3 end of the primary transcript, as depicted In Figure 12-2. However, for long transcription units... 

Now that we ve seen how pre-mRNAs are processed into mature, functional mRNAs, we consider how regulation of this process can contribute to gene control. Recall from Chapter 10 that higher eukaryotes contain both simple and complex... 

Approximately 80 percent of the total RNA in rapidly growing mammalian cells (e.g., cultured HeLa cells) is rRNA, and 15 percent is tRNA protein-coding mRNA thus constitutes only a small portion of the total RNA. The primary transcripts produced from most rRNA genes and from tRNA genes, like pre-mRNAs, are extensively processed to yield the mature, functional forms of these RNAs. 

The transition from intruutcrinc life to an independent existence requires changes in many biochemical systems. Mature function may not be attained for weeks, months or even years after birth. 

In neonates over 1.5 kg glomerular filtration rate (GER) increases dramatically in the first 2 weeks following birth due to adaptive changes in renal blood flow and the recruitment of mature, functioning nephrons. In term neonates the GFR is approximately 2—4 mL/min per... 

Chen, ]., Zong, C.S. Wang, L.-H. (1994). Tissue and epithelial cell-specific expression of chicken proto-oncogene c-ros in several organs suggests that it may play roles in their development and mature functions. Oncogene, 9, 773-80. 

Coadministration of Echinacea purpurea root extract and melatonin to mice significantly reduced levels of mature, functional granulocyte progeny in the spleen and bone marrow as compared to either Echinacea or melatonin administered alone (Currier et al. 2001). 

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