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. When protein production is not being carried out, the two subunits of a ribosome are separated.
In , the complete three-dimensional structure of the large and small subunits of a ribosome was established. Evidence based on this structure suggests, as had long been assumed, that it is the rRNA that provides the ribosome with its basic formation and functionality, not proteins.
Apparently the proteins in a ribosome help fill in structural gaps and enhance protein synthesis, although the process can take place in their absence, albeit at a much slower rate. The units of a ribosome are often described by their Svedberg s values, which are based upon their rate of sedimentation in a centrifuge.
The ribosomes in a eukaryotic cell generally have a Svedberg value of 80S and are comprised of 40s and 60s subunits. Prokaryotic cells, on the other hand, contain 70S ribosomes, each of which consists of a 30s and a 50s subunit. As demonstrated by these values, Svedberg units are not additive, so the values of the two subunits of a ribosome do not add up to the Svedberg value of the entire organelle.
This is because the rate of sedimentation of a molecule depends upon its size and shape, rather than simply its molecular weight. There are three adjacent tRNA binding sites on a ribosome: the aminoacyl binding site for a tRNA molecule attached to the next amino acid in the protein as illustrated in Figure 1 , the peptidyl binding site for the central tRNA molecule containing the growing peptide chain, and an exit binding site to discharge used tRNA molecules from the ribosome.
Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the cytoplasm in prokaryotes or to the Golgi apparatus in eukaryotes.
There, the proteins are completed and released inside or outside the cell. Ribosomes are very efficient organelles. Small proteins can therefore be made fairly quickly but two to three hours are needed for larger proteins such as the massive 30, amino acid muscle protein titin. Ribosomes in prokaryotes use a slightly different process to produce proteins than do ribosomes in eukaryotes.
Fortunately this difference presents a window of molecular opportunity for attack by antibiotic drugs such as streptomycin. Unfortunately some bacterial toxins and the polio virus also use it to enable them to attack the translation mechanism.
For an overview diagram of protein production click here. The diagram will open in a separate window. This is an electron microscope image showing part of the rough endoplasmic reticulum in a plant root cell from maize. The dark spots are ribosomes. Ribosomes are macro-molecular production units. They are composed of ribosomal proteins riboproteins and ribonucleic acids ribonucleoproteins.
Ribosomes can be bound by a membrane s but they are not membranous. Each complete ribosome is constructed from two sub-units. A eukaryotic ribosome is composed of nucleic acids and about 80 proteins and has a molecular mass of about 4,, Da.
Ribosomes are found in prokaryotic and eukaryotic cells; in mitochondria, chloroplasts and bacteria. You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? This article has been posted to your Facebook page via Scitable LearnCast. Change LearnCast Settings.
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