The four types of nitrogen bases found in nucleotides are: adenine A , thymine T , guanine G and cytosine C. The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA. The complete DNA instruction book, or genome, for a human contains about 3 billion bases and about 20, genes on 23 pairs of chromosomes.
DNA contains the instructions needed for an organism to develop, survive and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies. Each DNA sequence that contains instructions to make a protein is known as a gene. The size of a gene may vary greatly, ranging from about 1, bases to 1 million bases in humans.
Genes only make up about 1 percent of the DNA sequence. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made. DNA's instructions are used to make proteins in a two-step process.
First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA. Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins. This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein.
This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins. But nearly a century passed from that discovery until researchers unraveled the structure of the DNA molecule and realized its central importance to biology.
For many years, scientists debated which molecule carried life's biological instructions. Most thought that DNA was too simple a molecule to play such a critical role. Instead, they argued that proteins were more likely to carry out this vital function because of their greater complexity and wider variety of forms. A ribosome receives the mRNA molecule and starts to build a chain of amino acids a protein that exactly matches the instructions within the mRNA.
Each codon tells the protein-making machinery which amino acid to add next. For instance, the codon AGA is an instruction to add the amino acid arginine to a growing protein — whether that protein is growing within bacterial cells or human cells.
In other words, every cell follows the same rules to make a new protein. See the article How to add foreign DNA to bacteria for more information. Within any cell, only a proportion of proteins are produced at any one time. Proteins that perform essential roles are produced constantly, while others are expressed only when they are needed.
Cells also need large amounts of some proteins such as the enzymes involved in continuous processes like transcription and translation and smaller amounts of others such as hormones.
But how does a cell decide which genes to express and how much to make? Promoters are the sequences of DNA that determine when a gene is expressed. The group found a transcription factor MYB10 that binds to the promoter of several genes that produce red pigment in apples, causing them to be expressed.
There is far more MYB10 in red-fleshed apples than in white-fleshed apples, so these pigment genes are expressed at a higher level and make more red pigment. Find out more in the article: Discovering what controls apple flesh colour.
Add to collection. Proteins: key examples on the Hub. Promoters and apple flesh colour. Go to full glossary Add 0 items to collection. In eukaryotes, mature mRNA molecules must leave the nucleus and travel to the cytoplasm , where the ribosomes are located.
On the other hand, in prokaryotic organisms, ribosomes can attach to mRNA while it is still being transcribed. In all types of cells, the ribosome is composed of two subunits: the large 50S subunit and the small 30S subunit S, for svedberg unit, is a measure of sedimentation velocity and, therefore, mass.
Each subunit exists separately in the cytoplasm, but the two join together on the mRNA molecule. The tRNA molecules are adaptor molecules—they have one end that can read the triplet code in the mRNA through complementary base-pairing, and another end that attaches to a specific amino acid Chapeville et al. The idea that tRNA was an adaptor molecule was first proposed by Francis Crick, co-discoverer of DNA structure, who did much of the key work in deciphering the genetic code Crick, The rRNA catalyzes the attachment of each new amino acid to the growing chain.
Interestingly, not all regions of an mRNA molecule correspond to particular amino acids. In particular, there is an area near the 5' end of the molecule that is known as the untranslated region UTR or leader sequence. This portion of mRNA is located between the first nucleotide that is transcribed and the start codon AUG of the coding region, and it does not affect the sequence of amino acids in a protein Figure 3. So, what is the purpose of the UTR? It turns out that the leader sequence is important because it contains a ribosome-binding site.
A similar site in vertebrates was characterized by Marilyn Kozak and is thus known as the Kozak box. If the leader is long, it may contain regulatory sequences, including binding sites for proteins, that can affect the stability of the mRNA or the efficiency of its translation. Figure 4: The translation initiation complex. When translation begins, the small subunit of the ribosome and an initiator tRNA molecule assemble on the mRNA transcript.
The small subunit of the ribosome has three binding sites: an amino acid site A , a polypeptide site P , and an exit site E. Here, the initiator tRNA molecule is shown binding after the small ribosomal subunit has assembled on the mRNA; the order in which this occurs is unique to prokaryotic cells.
In eukaryotes, the free initiator tRNA first binds the small ribosomal subunit to form a complex. Figure Detail Although methionine Met is the first amino acid incorporated into any new protein, it is not always the first amino acid in mature proteins—in many proteins, methionine is removed after translation.
In fact, if a large number of proteins are sequenced and compared with their known gene sequences, methionine or formylmethionine occurs at the N-terminus of all of them. However, not all amino acids are equally likely to occur second in the chain, and the second amino acid influences whether the initial methionine is enzymatically removed.
For example, many proteins begin with methionine followed by alanine. In both prokaryotes and eukaryotes, these proteins have the methionine removed, so that alanine becomes the N-terminal amino acid Table 1.
However, if the second amino acid is lysine, which is also frequently the case, methionine is not removed at least in the sample proteins that have been studied thus far. These proteins therefore begin with methionine followed by lysine Flinta et al. Table 1 shows the N-terminal sequences of proteins in prokaryotes and eukaryotes, based on a sample of prokaryotic and eukaryotic proteins Flinta et al.
In the table, M represents methionine, A represents alanine, K represents lysine, S represents serine, and T represents threonine. Once the initiation complex is formed on the mRNA, the large ribosomal subunit binds to this complex, which causes the release of IFs initiation factors. The large subunit of the ribosome has three sites at which tRNA molecules can bind. The A amino acid site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain.
The P polypeptide site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. Finally, the E exit site is the location at which the "empty" tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process.
The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond Figure 5. Figure 5: The large ribosomal subunit binds to the small ribosomal subunit to complete the initiation complex.
The initiator tRNA molecule, carrying the methionine amino acid that will serve as the first amino acid of the polypeptide chain, is bound to the P site on the ribosome.
The A site is aligned with the next codon, which will be bound by the anticodon of the next incoming tRNA. Next, peptide bonds between the now-adjacent first and second amino acids are formed through a peptidyl transferase activity.
For many years, it was thought that an enzyme catalyzed this step, but recent evidence indicates that the transferase activity is a catalytic function of rRNA Pierce, After the peptide bond is formed, the ribosome shifts, or translocates, again, thus causing the tRNA to occupy the E site.
The tRNA is then released to the cytoplasm to pick up another amino acid. In addition, the A site is now empty and ready to receive the tRNA for the next codon. This process is repeated until all the codons in the mRNA have been read by tRNA molecules, and the amino acids attached to the tRNAs have been linked together in the growing polypeptide chain in the appropriate order. At this point, translation must be terminated, and the nascent protein must be released from the mRNA and ribosome.
No tRNAs recognize these codons. Thus, in the place of these tRNAs, one of several proteins, called release factors, binds and facilitates release of the mRNA from the ribosome and subsequent dissociation of the ribosome. The translation process is very similar in prokaryotes and eukaryotes.
Although different elongation, initiation, and termination factors are used, the genetic code is generally identical. As previously noted, in bacteria, transcription and translation take place simultaneously, and mRNAs are relatively short-lived. In eukaryotes, however, mRNAs have highly variable half-lives, are subject to modifications, and must exit the nucleus to be translated; these multiple steps offer additional opportunities to regulate levels of protein production, and thereby fine-tune gene expression.
Chapeville, F. On the role of soluble ribonucleic acid in coding for amino acids. Proceedings of the National Academy of Sciences 48 , — Crick, F. On protein synthesis.
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