Cellular information transfer from

Figure 24. Illustration of hase-pair analogues effector conception (26, 97, 98, 99, 100). Survey on simplified schemes of carho- and heteroaromatic systems (a) (preceding page) cytokinines (b) (this page, top) steroids (c) (this page, bottom) and structural analogues and differently shaped, effectors (d) (following page) of cellular information transfer from nucleic acids to protein.

Fig. 5. Molecular information transfer from DNA to cellular protein during bio-cellular network development...

The site of synthesis of numerous proteins is remote from their site of function. During transfer from one site to the other, proteins must, therefore, cross cellular membranes [43] [44], Proteins are usually synthesized as precursors containing an amino terminal extension, called the signal (leader) peptide, the sequence of which contains the necessary information to guide the protein to and across a specific membrane. After transmembrane transport (called translocation), the signal peptide is cleaved off by specific signal peptidases, which are found in the rough endoplasmic reticulum, and the... 

It is a commonly held belief that RNA preceded DNA in the early evolution of living systems. If this is the case then the first DNA polymerases must have been capable of transferring sequence information from RNA to DNA. Enzymes of this sort are called reverse transcriptases because they do the reverse of common transcriptases (see chapter 28). Reverse transcriptases no longer play the central role in genetic information transfer, but they are still found in all species and function in a number of capacities in both cellular and viral metabolism. 

Without some means to transfer information between control system elanents, there can be no precise control. Intact eyes without an optic nerve to convey visual information to the brain are no better than no eyes at all. Cellular insulin receptors without access to blood flow cannot function as intended. Likewise, communication among individuals in an ecological community is jnst as necessary as scouting reports to an army together, the group can respond to the information obtained from the outside. 

The description of the enzymic equipment of the nucleus is not solely of academic interest. If each nucleus has a specific enzyme mosaic, this pattern may play a considerable role in determining cellular differentiation. In that case, nuclei from different tissue would have an identical stock of chromosomes surrounded by different enzymic patterns, which may affect the screening of the genetic information transferred to the cytoplasm. 

The central dogma of molecular biology is that information is transferred from DNA to RNA to proteins. The proteins (which include the enzymes and structural components of cells) are directly responsible for most cellular activities and functions. The information needed for all functions of all organisms is stored in the genomic DNA sequence, which contains discrete units defined as genes. Each gene encodes a protein whose function and activity are determined by its primary sequence. The discovery of colinearity of the DNA nucleotide sequence and the amino acid sequence of the encoded polypeptide in prokaryotes and their viruses led to the discovery of the genetic code... 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

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