In eukaryotes, ribosome assembly is a complex process involving more than assembly factors and spreads from the nucleolus to the cytoplasm. Cleavage of this emerging rRNA releases the earliest small ribosomal subunit precursor, the 90S. The remaining rRNA will continue to be synthetized and become the large ribosomal subunit precursor. Molecular biology: Prime-time progress.
Nature , All rights reserved. Figure Detail. One factor that helps ensure precise replication is the double-helical structure of DNA itself. In particular, the two strands of the DNA double helix are made up of combinations of molecules called nucleotides. DNA is constructed from just four different nucleotides — adenine A , thymine T , cytosine C , and guanine G — each of which is named for the nitrogenous base it contains. Moreover, the nucleotides that form one strand of the DNA double helix always bond with the nucleotides in the other strand according to a pattern known as complementary base-pairing — specifically, A always pairs with T, and C always pairs with G Figure 2.
Thus, during cell division, the paired strands unravel and each strand serves as the template for synthesis of a new complementary strand. Each nucleotide has an affinity for its partner: A pairs with T, and C pairs with G. In most multicellular organisms, every cell carries the same DNA, but this genetic information is used in varying ways by different types of cells.
In other words, what a cell "does" within an organism dictates which of its genes are expressed. Nerve cells, for example, synthesize an abundance of chemicals called neurotransmitters, which they use to send messages to other cells, whereas muscle cells load themselves with the protein-based filaments necessary for muscle contractions. Transcription is the first step in decoding a cell's genetic information. RNA molecules differ from DNA molecules in several important ways: They are single stranded rather than double stranded; their sugar component is a ribose rather than a deoxyribose; and they include uracil U nucleotides rather than thymine T nucleotides Figure 4.
Also, because they are single strands, RNA molecules don't form helices; rather, they fold into complex structures that are stabilized by internal complementary base-pairing.
Messenger RNA mRNA molecules carry the coding sequences for protein synthesis and are called transcripts; ribosomal RNA rRNA molecules form the core of a cell's ribosomes the structures in which protein synthesis takes place ; and transfer RNA tRNA molecules carry amino acids to the ribosomes during protein synthesis.
Other types of RNA also exist but are not as well understood, although they appear to play regulatory roles in gene expression and also be involved in protection against invading viruses. Some mRNA molecules are abundant, numbering in the hundreds or thousands, as is often true of transcripts encoding structural proteins.
Other mRNAs are quite rare, with perhaps only a single copy present, as is sometimes the case for transcripts that encode signaling proteins. In eukaryotes, transcripts for structural proteins may remain intact for over ten hours, whereas transcripts for signaling proteins may be degraded in less than ten minutes. Cells can be characterized by the spectrum of mRNA molecules present within them; this spectrum is called the transcriptome.
Whereas each cell in a multicellular organism carries the same DNA or genome, its transcriptome varies widely according to cell type and function. For instance, the insulin-producing cells of the pancreas contain transcripts for insulin, but bone cells do not.
Even though bone cells carry the gene for insulin, this gene is not transcribed. Therefore, the transcriptome functions as a kind of catalog of all of the genes that are being expressed in a cell at a particular point in time. Figure 5: An electron micrograph of a prokaryote Escherichia coli , showing DNA and ribosomes This Escherichia coli cell has been treated with chemicals and sectioned so its DNA and ribosomes are clearly visible. The DNA appears as swirls in the center of the cell, and the ribosomes appear as dark particles at the cell periphery.
Courtesy of Dr. Abraham Minsky Ribosomes are the sites in a cell in which protein synthesis takes place. Cells have many ribosomes, and the exact number depends on how active a particular cell is in synthesizing proteins. For example, rapidly growing cells usually have a large number of ribosomes Figure 5. Ribosomes are complexes of rRNA molecules and proteins, and they can be observed in electron micrographs of cells.
Sometimes, ribosomes are visible as clusters, called polyribosomes. In eukaryotes but not in prokaryotes , some of the ribosomes are attached to internal membranes, where they synthesize the proteins that will later reside in those membranes, or are destined for secretion Figure 6.
Although only a few rRNA molecules are present in each ribosome, these molecules make up about half of the ribosomal mass. The remaining mass consists of a number of proteins — nearly 60 in prokaryotic cells and over 80 in eukaryotic cells.
Within the ribosome, the rRNA molecules direct the catalytic steps of protein synthesis — the stitching together of amino acids to make a protein molecule. Eukaryotic and prokaryotic ribosomes are different from each other as a result of divergent evolution. These differences are exploited by antibiotics, which are designed to inhibit the prokaryotic ribosomes of infectious bacteria without affecting eukaryotic ribosomes, thereby not interfering with the cells of the sick host.
Figure 6: The endoplasmic reticulum of this eukaryotic cell is studded with ribosomes. Electron micrograph of a pancreatic exocrine cell section. When many ribosomes do this the structure is called a polysome.
Where there is rough endoplasmic reticulum the association between ribosome and endoplasmic reticulum ER facilitates the further processing and checking of newly made proteins by the ER. All factories need services such as gas, water, drainage and communications. For these to be provided there must a location or site. Protein production also needs service requirements.
A site requiring the provision of services is produced in a small ribosome sub-unit when a strand of mRNA enters through one selective cleft, and a strand of initiator tRNA through another. This action triggers the small sub-unit to lock-on to a ribosome large sub-unit to form a complete and active ribosome. The amazing process of protein production can now begin.
For translation and protein synthesis to take place many initiator and release chemicals are involved, and many reactions using enzymes take place. There are however general requirements and these have to be satisfied. The list below shows the main requirements and how they are provided:. The Protein Factory: What happens on the inside?
Now we have considered the requirements and provisions needed for the protein production machine to operate, we can look at the inner workings. As mentioned earlier many detailed biochemical reactions take place in the ribosome and only a brief outline is given here to illustrate the concept.
Sites A and P span both the ribosome sub-units with a larger part residing in the ribosome large sub-unit, and a smaller part in the smaller sub-unit. Site E , the exit site, resides in the large ribosome sub-unit. Table of binding sites, positions and functions in a ribosome please also see schematic of ribosome at end of section.
Peptide synthesis, consolidation, elongation and transfer of peptide chain to site A.
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