7. RNA and the Genetic Code

Elongation is a three-step cycle that is repeated for each amino acid added to the protein after the initiator methionine. During elongation, the ribosome moves in the 5′ to 3′ direction along the mRNA, synthesizing the protein from its amino (N) to carboxyl (C) terminus. The ribosome contains three very important binding sites:

-The A site holds the incoming aminoacyl-tRNA complex. This is the next amino acid that is being added to the growing chain, and is determined by the mRNA codon within the A site. -The P site holds the tRNA that carries the growing polypeptide chain. It is also where the first amino acid (methionine) binds because it is starting the polypeptide chain. A peptide bond is formed as the polypeptide is passed from the tRNA in the P site to the tRNA in the A site. This requires peptidyl transferase, an enzyme that is part of the large subunit. GTP is used for energy during the formation of this bond. -The E site is where the now inactivated (uncharged) tRNA pauses transiently before exiting the ribosome. As the now-uncharged tRNA enters the E site, it quickly unbinds with the mRNA and is ready to be recharged.

3′ Poly-A Tail

A polyadenosyl (poly-A) tail is added to the 3′ end of the mRNA transcript and protects the message against rapid degradation. It is composed of adenine bases. Think of the poly-A tail as a fuse for a "time bomb" for the mRNA transcript: as soon as the mRNA leaves the nucleus, it will start to get degraded from its 3′ end. The longer the poly-A tail, the more time the mRNA will be able to survive before being digested in the cytoplasm. The poly-A tail also assists with export of the mature mRNA from the nucleus.

Transcription factors that bind to the DNA can recruit other coactivators such as histone acetylases. These proteins are involved in chromatin remodeling, because they acetylate lysine residues found in the amino terminal tail regions of histone proteins.

Acetylation of histone proteins decreases the positive charge on lysine residues and weakens the interaction of the histone with DNA, resulting in an open chromatin conformation that allows for easier access of the transcriptional machinery to the DNA. Specific patterns of histone acetylation can lead to increased gene expression levels. On the other hand, gene silencing can occur just as easily with chromatin remodeling. Histone deacetylases are proteins that function to remove acetyl groups from histones, which results in a closed chromatin conformation and overall decrease in gene expression levels in the cell.

5′ Cap

At the 5′ end of the hnRNA molecule, a 7-methylguanylate triphosphate cap is added. The cap is actually added during the process of transcription and is recognized by the ribosome as the binding site. It also protects the mRNA from degradation in the cytoplasm.

DNA methylation is also involved in chromatin remodeling and regulation of gene expression levels in the cell. DNA methylases add methyl groups to cytosine and adenine nucleotides; methylation of genes is often linked with the silencing of gene expression.

During development, methylation plays an important role in silencing genes that no longer need to be activated. Heterochromatin regions of the DNA are much more heavily methylated, hindering access of the transcriptional machinery to the DNA.

In eukaryotic cells, DNA is packaged in the nucleus as chromatin, which requires chromatin remodeling to allow transcription factors and the transcriptional machinery easier access to the DNA. Heterochromatin is tightly coiled DNA that appears dark under the microscope; its tight coiling makes it inaccessible to the transcription machinery, so these genes are inactive.

Euchromatin, on the other hand, is looser and appears light under the microscope; the transcription machinery can access the genes of interest, so these genes are active. Remodeling of the chromatin structures regulates gene expression levels in the cell.

During translation, the codon of the mRNA is recognized by a complementary anticodon on a transfer RNA (tRNA). The anticodon sequence allows the tRNA to pair with the codon in the mRNA. Because base-pairing is involved, the orientation of this interaction will be antiparallel. For example, the aminoacyl tRNA Ile-tRNA has an anticodon sequence 5′—GAU—3′, allowing it to pair with the isoleucine codon 5′—AUC—3′,

Every preprocessed eukaryotic protein starts with the exact same amino acid: methionine. Because every protein begins with methionine, the codon for methionine (AUG) is considered the start codon for translation of the mRNA into protein. There are also 3 codons that encode for termination of protein translation; there are no charged tRNA molecules that recognize these codons, which leads to the release of the protein from the ribosome. The three stop codons are UGA, UAA, and UAG.

At this point, when only the exons remain and the cap and tail have been added, the cell has created the mature mRNA that can now be transported into the cytoplasm for protein translation. Untranslated regions of the mRNA (UTRs) will still exist at the 5′ and 3′ edges of the transcript because the ribosome initiates translation at the start codon (AUG) and will end at a stop codon (UAA, UGA, UAG).

For some genes in eukaryotic cells, however, the primary transcript of hnRNA may be spliced together in different ways to produce multiple variants of proteins encoded by the same original gene. This process is known as alternative splicing, and it is illustrated in Figure 7.10. By utilizing alternative splicing, an organism can make many more different proteins from a limited number of genes.

Response elements outside the normal promoter regions can be recognized by specific transcription factors to enhance transcription levels. Several response elements may be grouped together to form an enhancer, which allows for the control of one gene's expression by multiple signals. Signal molecules, like cyclic AMP (cAMP), cortisol, and estrogen, bind to specific receptors.

For the examples given, these receptors are cyclic AMP response element-binding protein (CREB), the glucocorticoid (cortisol) receptor, and the estrogen receptor, respectively; all are transcription factors that bind to their respective response elements within the enhancer. Note that the large distance between the enhancer and promoter regions for a given gene means that DNA often must bend into a hairpin loop to bring these elements together spatially.

Cells can also increase the expression of a gene product by duplicating the relevant gene. Genes can be duplicated in series on the same chromosome, yielding many copies in a row of the same genetic information.

Genes can also be duplicated in parallel by opening the gene with helicases and permitting DNA replication only of that gene; cells can continue replicating the gene until hundreds of copies of the gene exist in parallel on the same chromosome.

Eukaryotic ribosomes contain four strands of rRNA, designated the 28S, 18S, 5.8S, and the 5S rRNAs; the "S" values indicate the size of the strand. The genes for some of the rRNAs (28S, 18S, and 5.8S rRNAs) used to construct the ribosome are found in the nucleolus. RNA polymerase I transcribes the 28S, 18S, and 5.8S rRNAs as a single unit within the nucleolus, which results in a 45S ribosomal precursor RNA. This 45S pre-rRNA is processed to become the 18S rRNA of the 40S (small) ribosomal subunit and to the 28S and 5.8S rRNAs of the 60S (large) ribosomal subunit. RNA polymerase III transcribes the 5S rRNA, which is also found in the 60S ribosomal subunit; this process takes place outside of the nucleolus. The ribosomal subunits created are the 60S and 40S subunits; these subunits join during protein synthesis to form the whole 80S ribosome.

In comparison with eukaryotes, prokaryotes have 50S and 30S large and small subunits, which assemble to create the complete 70S ribosome. Note that the "S" value is determined experimentally by studying the behavior of particles in a ultracentrifuge; thus, the numbers of each subunit and each rRNA are not additive because they are based on size and shape, not size alone.

The three nucleotides of a codon are referred to as the reading frame. Point mutations occur when one nucleotide is changed, but a frameshift mutation occurs when some number of nucleotides are added to or deleted from the mRNA sequence.

Insertion or deletion of nucleotides will shift the reading frame, usually resulting in changes in the amino acid sequence or premature truncation of the protein. The effects of frameshift mutations are typically more serious than point mutations, although it is heavily dependent on where within the DNA sequence the mutation actually occurred.

The nascent polypeptide chain is subject to posttranslational modifications before it will become a functioning protein, similar to how hnRNA is modified prior to being released from the nucleus. One essential step for the final synthesis of the protein is proper folding. There is a specialized class of proteins called chaperones, the main function of which is to assist in the protein-folding process.

Many proteins are also modified by cleavage events. A common example of this is insulin, which needs to be cleaved from a larger, inactive peptide to achieve its active form. In peptides with signal sequences, the signal sequence must be cleaved if the protein is to enter the organelle and accomplish its function. In peptides with quaternary structure, subunits come together to form the functional protein. A classic example is hemoglobin, which is composed of two alpha chains and two beta chains.

If a mutation occurs and it affects one of the nucleotides in a codon, it is known as a point mutation. Although we've already discussed the silent point mutation in the wobble position, other point mutations can have a severe detrimental effect depending on where the mutation occurs in the genome. Because these point mutations can affect the primary amino acid sequence of the protein, they are called expressed mutations. Expressed point mutations fall into two categories: missense and nonsense.

Missense mutation: a mutation where one amino acid substitutes for another Nonsense mutation: a mutation where the codon now encodes for a premature stop codon (also known as a truncation mutation)

If a gene sequence is a "sentence" describing a protein, then its basic unit is a three-letter "word" known as the codon, which is translated into an amino acid. Genetic code tables, serve as an easy way to determine the amino acid that is translated from each mRNA codon. Each codon consists of three bases; thus, there are 64 codons. Note how all codons are written in the 5′ → 3′ direction, and the code is unambiguous, in that each codon is specific for one and only one amino acid.

Note that 61 of the codons code for one of the 20 amino acids, while three codons encode for the termination of translation. This code is universal across species

KEY CONCEPT 2

Note that 61 of the codons code for one of the 20 amino acids, while three codons encode for the termination of translation. This code is universal across species

KEY CONCEPT 7

Operons include both inducible and repressible systems, and offer a simple on-off switch for gene control in prokaryotes.

Other biomolecules may be added to the peptide via the following processes:

Phosphorylation: addition of phosphates by protein kinases to activate or deactivate proteins Carboxylation: addition of carboxylic acid groups, usually to serve as calcium-binding sites Glycosylation: addition of oligosaccharides as proteins pass through the ER and Golgi apparatus to determine cellular destination Prenylation: addition of lipid groups to certain membrane-bound enzymes

KEY CONCEPT 8

Positive control is accomplished by inducible systems, in which a repressor is removed from the operon by the inducer to promote transcription of a gene. Negative control is accomplished by repressible systems, in which a repressor-corepressor complex binds to the operon to prevent transcription.

In eukaryotes, there are three types of RNA polymerases, but only one is involved in the transcription of mRNA:

RNA polymerase I is located in the nucleolus and synthesizes rRNA RNA polymerase II is located in the nucleus and synthesizes hnRNA (pre-processed mRNA) and some small nuclear RNA (snRNA) RNA polymerase III is located in the nucleus and synthesizes tRNA and some rRNA

Repressible systems allow constant production of a protein product. In contrast to the inducible system, the repressor made by the regulator gene is inactive until it binds to a corepressor. This complex then binds the operator site to prevent further transcription.

Repressible systems tend to serve as negative feedback; often, the final structural product can serve as a corepressor. Thus, as its levels increase, it can bind the repressor, and the complex will attach to the operator region to prevent further transcription of the same gene. Repressible systems are sometimes referred to as negative control mechanisms. The trp operon, described above, operates in this way. When tryptophan is high in the local environment, it acts as a corepressor. The binding of two molecules of tryptophan to the repressor causes the repressor to bind the operator site. Thus, the cell turns off its machinery to synthesize its own tryptophan, which is an energetically expensive process because of its easy availability in the environment.

Elongation factors (EFs) assist by locating and recruiting aminoacyl-tRNA along with GTP, while helping to remove GDP once the energy has been used.

Some eukaryotic proteins contain signal sequences, which designate a particular destination for the protein. For peptides that will be secreted, such as hormones and digestive enzymes, a signal sequence directs the ribosome to move to the endoplasmic reticulum (ER), so that the protein can be translated directly into the lumen of the rough ER. From there, the protein can be sent to the Golgi apparatus and be secreted from a vesicle via exocytosis. Other signal sequences direct proteins to the nucleus, lysosomes, or cell membrane.

KEY CONCEPT 6

Terminology and 5′ → 3′ DNA → DNA = replication: new DNA synthesized in 5′ → 3′ direction DNA → RNA = transcription: new RNA synthesized in 5′ → 3′ direction (template is read 3′ → 5′) RNA → protein = translation: mRNA read in 5′ → 3′ direction

KEY CONCEPT 9

The DNA regulatory base sequences (such as promoters, enhancers, and response elements) are known as cis regulators because they are in the same vicinity as the gene they control. Transcription factors, however, have to be produced and translocated back to the nucleus; thus they are called trans regulators because they travel through the cell to their point of action.

Transcription factors are transcription-activating proteins that search the DNA looking for specific DNA-binding motifs. Transcription factors tend to have two recognizable domains: a DNA-binding domain and an activation domain.

The DNA-binding domain binds to a specific nucleotide sequence in the promoter region or to a DNA response element (a sequence of DNA that binds only to specific transcription factors) to help in the recruitment of transcriptional machinery. The activation domain allows for the binding of several transcription factors and other important regulatory proteins, such as RNA polymerase and histone acetylases, which function in the remodeling of the chromatin structure.

By sharing a single common promoter region on the DNA sequence, these genes are transcribed as a group. This type of structure is called an operon—a cluster of genes transcribed as a single mRNA; this particular cluster in E. coli is known as the trp operon. Operons are incredibly common in the prokaryotic cell.

The Jacob-Monod Model is used to describe the structure and function of operons. In this model, operons contain structural genes, an operator site, a promoter site, and a regulator gene. The structural gene codes for the protein of interest. Upstream of the structural gene is the operator site, a nontranscribable region of DNA that is capable of binding a repressor protein. Further upstream is the promoter site, which is similar in function to promoters in eukaryotes: it provides a place for RNA polymerase to bind. Furthest upstream is the regulator gene, which codes for a protein known as the repressor. There are two types of operons: inducible systems and repressible systems.

KEY CONCEPT 5

The MCAT commonly tests post-transcriptional processing: Intron/exon splicing 5′ cap 3′ poly-A tail

When any of the three stop codons moves into the A site, a protein called release factor (RF) binds to the termination codon, causing a water molecule to be added to the polypeptide chain.

The addition of this water molecule allows peptidyl transferase and termination factors to hydrolyze the completed polypeptide chain from the final tRNA. The polypeptide chain will then be released from the tRNA in the P site, and the two ribosomal subunits will dissociate.

Each type of amino acid is activated by a different aminoacyl-tRNA synthetase that requires two high-energy bonds from ATP, implying that the attachment of the amino acid is an energy-rich bond.

The aminoacyl-tRNA synthetase transfers the activated amino acid to the 3′ end of the correct tRNA. Each tRNA has a CCA nucleotide sequence where the amino acid binds. The high-energy aminoacyltRNA bond will be used to supply the energy needed to create a peptide bond during translation.

RNA polymerase travels along the template strand in the 3′ → 5′ direction, which allows for the construction of transcribed mRNA in the 5′ → 3′ direction. Unlike DNA polymerase, RNA polymerase does not proofread its work, so the synthesized transcript will not be edited.

The coding strand (or sense strand) of DNA is not used as a template during transcription. Because the coding strand is also complementary to the template strand, it is identical to the mRNA transcript except that all the thymine nucleotides in DNA have been replaced with uracil in the mRNA molecule.

Although DNA contains the actual coding sequence for a protein, the machinery to generate that protein is located in the cytoplasm. DNA cannot leave the nucleus, as it will be quickly degraded, so it must use RNA to transmit genetic information.

The creation of mRNA from a DNA template is known as transcription, and while mRNA is the only type of RNA that carries information from DNA directly, there are many other types of RNA that exist, two of which will play important roles during protein translation: transfer RNA (tRNA) and ribosomal RNA (rRNA).

KEY CONCEPT 3

The degeneracy of the genetic code allows for mutations in DNA that do not always result in altered protein structure or function. Usually, a mutation within an intron will also not change the protein sequence because introns are cleaved out of the mRNA transcript prior to translation.

Maturation of the hnRNA includes splicing of the transcript to remove noncoding sequences (introns) and ligate coding sequences (exons) together. Splicing is accomplished by the spliceosome. In the spliceosome, small nuclear RNA (snRNA) molecules couple with proteins known as small nuclear ribonucleoproteins (also known as snRNPs, or "snurps"). The snRNP/snRNA complex recognizes both the 5′ and 3′ splice sites of the introns. These noncoding sequences are excised in the form of a lariat (lasso-shaped structure) and then degraded.

The evolutionary function of introns in eukaryotic cells is not currently well-understood; however, scientists hypothesize that introns play an important role in the regulation of cellular gene expression levels and in maintaining the size of our genome. The existence of introns has also been hypothesized to allow for rapid protein evolution. Many eukaryotic proteins share peptide sequences in common, suggesting that the genes encoding for these particular peptides may employ a modular function; that is, they contain standard sequences that can be swapped in and out, depending on the needs of the cell.

Bacteria can digest lactose, but it is more energetically expensive than digesting glucose. Therefore, bacteria only want to use this option if lactose is high and glucose is low. The lac operon is induced by the presence of lactose; thus, these genes are only transcribed when it is useful to the cell.

The lac operon is assisted by binding of the catabolite activator protein (CAP). CAP is a transcriptional activator used by E. coli when glucose levels are low to signal that alternative carbon sources should be used. Falling levels of glucose cause an increase in the signaling molecule cyclic AMP (cAMP), which binds to CAP. This induces a conformational change in CAP that allows it to bind the promoter region of the operon, further increasing transcription of the lactase gene.

The small ribosomal subunit binds to the mRNA. In prokaryotes, the small subunit binds to the Shine- Dalgarno sequence in the 5′ untranslated region of the mRNA. In eukaryotes, the small subunit binds to the 5′ cap structure. The charged initiator tRNA binds to the AUG start codon through basepairing with its anticodon within the P site of the ribosome. The initial amino acid in prokaryotes is N-formylmethionine (fMet); in eukaryotes, it's methionine.

The large subunit then binds to the small subunit, forming the completed initiation complex. This is assisted by initiation factors (IFs) that are not permanently associated with the ribosome.

Translation occurs in the cytoplasm in prokaryotes and eukaryotes. In prokaryotes, the ribosomes start translating before the mRNA is complete; in eukaryotes, however, transcription and translation occur at separate times and in separate locations within the cell.

The process of translation occurs in three stages: initiation, elongation, and termination. Specialized factors for initiation (initiation factors, IF), elongation (elongation factors, EF), and termination (release factors, RF), as well as GTP are required for each step.

While nucleotides play a crucial role in maintaining our genetic identity from generation to generation, it is the proteins they encode that help organisms develop and perform the necessary functions of life. The major steps involved in the transfer of genetic information are illustrated in the central dogma of molecular biology. Classically, a gene is a unit of DNA that encodes a specific protein or RNA molecule, and through transcription and translation, that gene can be expressed. Although this sequence is now complicated by our increased knowledge of the ways in which genes and nucleic acids may be expressed, it is still useful as a general working definition of the processes of DNA replication, transcription, and translation.

The relationship between the sequence found in double-stranded DNA, single-stranded RNA. Messenger RNA is synthesized in the 5′ → 3′ direction and is complementary and antiparallel to the DNA template strand. The ribosome translates the mRNA in the 5′ → 3′ direction, as it synthesizes the protein from the amino terminus (Nterminus) to the carboxy terminus (C-terminus).

The ribosome is composed of proteins and rRNA. In both prokaryotes and eukaryotes, there are large and small subunits; the subunits only bind together during protein synthesis. The structure of the ribosome dictates its main function, which is to bring the mRNA message together with the charged aminoacyl-tRNA complex to generate the protein.

There are three binding sites in the ribosome for tRNA: the A site (aminoacyl), P site (peptidyl), and E site (exit). These are described further in the section on translation below.

Once the transcription complex is formed, basal (or low-level) transcription can begin and maintain moderate, but adequate, levels of the protein encoded by this gene in the cell.

There are times, however, when the expression must be increased, or amplified, in response to specific signals such as hormones, growth factors, and other intracellular conditions. Eukaryotic cells accomplish this through enhancers and gene duplication.

Enhancer regions in the DNA can be up to 1000 base pairs away from the gene they regulate and can even be located within an intron, or noncoding region, of the gene.

They differ from upstream promoter elements in their locations because upstream promoter elements must be within 25 bases of the start of a gene. By utilizing enhancer regions, genes have an increased likelihood to be amplified because of the variety of signals that can increase transcription levels.

Transcription produces a copy of only one of the two strands of DNA. During initiation of transcription, several enzymes, including helicase and topoisomerase, are involved in unwinding the double-stranded DNA and preventing formation of supercoils.

This step is important in allowing the transcriptional machinery access to the DNA and the particular gene of interest. Transcription results in a single strand of mRNA, synthesized from one of the two nucleotide strands of DNA called the template strand (or the antisense strand). The newly synthesized mRNA strand is both antiparallel and complementary to the DNA template strand.

RNA is synthesized by a DNA-dependent RNA polymerase; RNA polymerase locates genes by searching for specialized DNA regions known as promoters. In eukaryotes, RNA polymerase II is the main player in transcribing mRNA, and its binding site in the promoter region is known as the TATA box, named for its high concentration of thymine and adenine bases.

Transcription factors help the RNA polymerase locate and bind to this promoter region of the DNA, helping to establish where transcription will start. Unlike DNA polymerase III, which we reviewed during DNA replication, RNA polymerase does not require an RNA primer to start generating a transcript.

KEY CONCEPT 4

Transcription is subject to the 5′ → 3′ rule, just like DNA synthesis. Synthesis of nucleic acids always occurs in the 5′ → 3′ direction

In the vicinity of a gene, a numbering system is used to identify the location of important bases in the DNA strand. The first base transcribed from DNA to RNA is defined as the +1 base of that gene region. Bases to the left of this start point (upstream, or toward the 5′ end) are given negative numbers: -1, -2, -3, and so on. Bases to the right (downstream, or toward the 3′ end) are denoted with positive numbers: +2, +3, +4, and so on. Thus, no nucleotide in the gene is numbered 0. The TATA box, where RNA polymerase II binds, usually falls around -25.

Transcription will continue along the DNA coding region until the RNA polymerase reaches a termination sequence or stop signal, which results in the termination of transcription. The DNA double helix then reforms, and the primary transcript formed is termed heterogeneous nuclear RNA (hnRNA). mRNA is derived from hnRNA via posttranscriptional modifications.

Once the mRNA transcript is created and processed, it can exit the nucleus through nuclear pores. Once in the cytoplasm, mRNA finds a ribosome to begin the process of translation—converting the mRNA transcript into a functional protein.

Translation is a complex process that requires mRNA, tRNA, ribosomes, amino acids, and energy in the form of GTP.

The genetic code is degenerate because more than one codon can specify a single amino acid. In fact, all amino acids, except for methionine and tryptophan, are encoded by multiple codons. we can see that for the amino acids with multiple codons, the first two bases are usually the same, and the third base in the codon is variable. We refer to this variable third base in the codon as the wobble position.

Wobble is an evolutionary development designed to protect against mutations in the coding regions of our DNA. Mutations in the wobble position tend to be called silent or degenerate, which means there is no effect on the expression of the amino acid and therefore no adverse effects on the polypeptide sequence. The amino acid glycine, for example, requires that only the first two nucleotides of the codon be GG. The third nucleotide could be A, C, G, or U, and the amino acid composition of the protein would remain the same.

Before the hnRNA can leave the nucleus and be translated to protein, it must undergo three specific processes to allow it to interact with the ribosome and survive the conditions of the cytoplasm.

You can think of the nucleus as the happy home of the cell; the DNA strands are the parents, and the hnRNA is their child. The child must mature if he or she is to survive.

Messenger RNA (mRNA)

carries the information specifying the amino acid sequence of the protein to the ribosome. mRNA is transcribed from template DNA strands by RNA polymerase enzymes in the nucleus of cells. Then, mRNA may undergo a host of posttranscriptional modifications prior to its release from the nucleus. mRNA is the only type of RNA that contains information that is translated into protein; to do so, it is read in three-nucleotide segments termed codons. In eukaryotes, mRNA is monocistronic, meaning that each mRNA molecule translates into only one protein product. Thus, in eukaryotes, the cell has a different mRNA molecule for each of the thousands of different proteins made by that cell. In prokaryotes, mRNA may be polycistronic, and starting the process of translation at different locations in the mRNA can result in different proteins.

Transfer RNA (tRNA)

is responsible for converting the language of nucleic acids to the language of amino acids and peptides. Each tRNA molecule contains a folded strand of RNA that includes a three-nucleotide anticodon. This anticodon recognizes and pairs with the appropriate codon on an mRNA molecule while in the ribosome. There are 20 amino acids, each of which is represented by at least one codon. To become part of a nascent polypeptide in the ribosome, amino acids are connected to a specific tRNA molecule; such tRNA molecules are said to be charged or activated with an amino acid. tRNA is found in the cytoplasm, and is the second most abundant type of RNA in the cell, after mRNA.

Ribosomal RNA (rRNA)

is synthesized in the nucleolus and functions as an integral part of the ribosomal machinery used during protein assembly in the cytoplasm. Many rRNA molecules function as ribozymes; that is, enzymes made of RNA molecules instead of peptides. rRNA helps catalyze the formation of peptide bonds and is also important in splicing out its own introns within the nucleus.

KEY CONCEPT 1

mRNA is the messenger of genetic information. DNA codes for proteins but cannot perform any of the important enzymatic reactions that proteins are responsible for in cells. mRNA takes the information from the DNA to the ribosomes, where creation of the primary protein structure occurs.

inducible systems

the repressor is bound tightly to the operator system and thereby acts as a roadblock. RNA polymerase is unable to get from the promoter to the structural gene because the repressor is in the way. To remove that block, an inducer must bind the repressor protein so that RNA polymerase can move down the gene. Inducible systems operate on a principle analogous to competitive inhibition for enzyme activity: as the concentration of the inducer increases, it will pull more copies of the repressor off of the operator region, freeing up those genes for transcription. This system is useful because it allows gene products to be produced only when they are needed. Inducible systems are sometimes referred to as positive control mechanisms.