Ribosomal migration

When observed under the electron microscope, the yeast cytoplasm appears rich in ribosomes. These tiny granulations, made up of ribonucleic acids and proteins, are the center of protein synthesis. Joined to polysomes, several ribosomes migrate the length of the messenger RNA. They translate it simultaneously so that each one produces a complete polypeptide chain. 

All enveloped human vimses acquire their phospholipid coating by budding through cellular membranes. The maturation and release of enveloped influenza particles is illustrated in Fig. 3.8. The capsid protein subunits are transported flom the ribosomes to the nucleus, where they combine with new viral RNA molecules and are assembled into the helical capsids. The haemagglutinin and neuraminidase proteins that project fiom the envelope of the normal particles migrate to the cytoplasmic membrane where they displace the normal cell membrane proteins. The assembled nucleocapsids finally pass out from the nucleus, and as they impinge on the altered cytoplasmic membrane they cause it to bulge and bud off completed enveloped particles flxm the cell. Vims particles are released in this way over a period of hours before the cell eventually dies. 

Synthases differ with respect to their site of attachment to tRNA. Some synthases form the 2 ester, some form the 3 ester, and still others produce a mixture of the two. The specificity of the synthases was determined by analyzing their ability to act on tRNA derivatives lacking one or the other terminal hydroxyl group. Once esterified to the terminal ribose, the aminoacyl group can migrate between the vicinal 2 and 3 hydroxyl groups. Thus, in cells, amino-acyl-tRNAs are mixtures of 2 and 3 esters. Only the 3 derivative is a substrate for the subsequent transpeptidation reaction catalyzed by the ribosome. 

Genetic information is accessed by a process known as transcription, in which the double-stranded DNA splits and the genetic code is transcribed onto a single-strand messenger RNA(mRNA). The mRNA is comprised of the same bases as the DNA, arranged in the same sequence, but in a complementary fashion. The mRNA migrates out of the nucleus and into the cytoplasm, where it attaches to ribosomes. The ribosomes assemble amino acids to form protein molecules through a process known as translation. 

During transcription of information from DNA into mRNA, the two complimentary strands of the DNA partly uncoil. The sense strand separates from the antisense strand. The antisense strand of DNA is used as a template for transcribing enzymes that assemble mRNA (transcription), which, in the process produces a copy of the sense strand. Then, mRNA migrates into the cell, where other cellular structures called ribosomes read the encoded information, its mRNA s base sequence, and in so doing, string together amino acids to form a specific protein. This process is called translation. ... 

DNA stores the genetic information, while RNA molecules are responsible for transmitting this information to the ribosomes, where protein synthesis actually occurs. This complex process involves, first, the construction of a special RNA molecule called messenger RNA (mRNA). The mRNA is built in the cell nucleus on the appropriate section of DNA (the gene) the double helix is unzipped, and the complementarity of the bases is used in a process similar to that used in DNA replication. The mRNA then migrates into the cytoplasm of the cell where, with the assistance of the ribosomes, the protein is synthesized. 

Two-dimensional gel electrophoresis of RNA has been used for two purposes. One general problem has been to identify RNA species migrating in the form of nucleoprotein particles, e.g. ribosomes and viruses. In this case, the buffer used in the second dimension is one in which the RNA and protein components are dissociated. RNA species of known size can be run in parallel with the sample in the second dimension. 

In weak composite acrylamide-agarose gels Dahlberg et al. (1969) reported that not only ribosomes but polysomes could be made to migrate, and this provides indeed an elegant and convenient procedure for the analysis of polysome populations (Fig. 10.15). Addition of the antibiotic streptomycin which is known to bind to ribosomes, causes... 

The ribosomal subunits migrate through the nuclear pores into the cytoplasm where they complex with mRNA, forming 80S ribosomes. Because sedimentation coefficients reflect both shape and particle weight, they are not additive. 

The molecular mechanisms by which ATP is formed in mitochondria and chloroplasts are very similar, as explained in Chapter 8. Chloroplasts and mitochondria have other features in common both often migrate from place to place within cells, and they contain their own DNA, which encodes some of the key organellar proteins (Chapter 10). The proteins encoded by mitochondrial or chloroplast DNA are synthesized on ribosomes within the organelles. However, most of the proteins in each organelle are encoded in nuclear DNA and are synthesized in the cytosol these proteins are then incorporated into the organelles by processes described in Chapter 16. 

Messenger RNA (mRNA) contains the information (formerly residing in DNA) that is decoded in a way that enables the manufacture of a protein, and migrates from the nucleus to ribosomes in the cytoplasm (where proteins are assembled). A triplet of nucleotides within an RNA molecule (called a codon) specifies the amino acid to be incorporated into a specific site in the protein being assembled. A cell s population of mRNA molecules is very diverse, as these molecules are responsible for the synthesis of the many different proteins found in the cell. However, mRNA makes up only 5 percent of total cellular RNA. 

Fig. 14.15. rRNA and ribosome synthesis. The 5S rRNA is transcribed in the nucleoplasm and moves into the nucleolus. The other rRNAs are transcribed from DNA and mature in the nucleolus, forming the 40S and 60S ribosomal subunits, which migrate to the cytoplasm. 

In the production of cytoplasmic ribosomes in human cells, one portion of the 45S rRNA precursor becomes the 18S rRNA that, complexed with proteins, forms the small 40S ribosomal subunit (Fig. 14.15, circle 4). Another segment of the precursor folds back on itself and is cleaved, forming 28S rRNA, hydrogen-bonded to the 5.8S rRNA. The 5S rRNA, transcribed from nonnucleolar genes, and a number of proteins complex with the 28S and 5.8S rRNAs to form the 60S ribosomal subunit (Fig. 14.15, circle 5). The ribosomal subunits migrate through the nuclear pores. In the cytoplasm, the 40S and 60S ribosomal subunits interact with mRNA, forming the 80S ribosomes on which protein synthesis occurs. 

The main feature of normal animal cell is its compartmentalization [7]. The DNA of the animal cell is restricted to the nucleus at all cell cycle stages except during metaphase when no nucleus exists. The synthesis of RNA occurs in the nucleus and most of it remains there, but messenger RNA and transfer RNA migrate to the cytoplasm. Ribosomal RNA is synthesized in the nucleolus the two ribosomal subunits are partly assembled in the nucleolus and nucleus then migrate to the cytoplasm. All protein synthesis proceed in the cytoplasm. The mitochondria, which is located only in the cytoplasm, contains DNA-s, RNA- and protein-synthesizing systems of their own [7]. 

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