Exam 2 Microbiology Chapter 7

What is the difference between polypeptide and protein?

A polypeptide is simply a chain of amino acids, whereas a protein is a functional molecule made up of one or more polypeptides.

Bacterial Gene Regulation

A regulatory mechanism sometimes controls the transcription of only a limited number of genes, but in other cases, a wide array of genes is controlled coordinately.

Repressors

A repressor is a regulatory protein that blocks transcription (negative regulation). It does this by binding to an operator, a specific DNA sequence located immediately downstream of a promoter. When a repressor is bound to an operator, RNA polymerase canoe progress past that DNA sequence. Repressors are allosteric proteins, however, meaning that specific molecules can attach to them and change their shape. There are two general mechanisms by which different repressors can function: induction and repression

Operon

A set of regulated genes transcribed as a singly polycistronic message. One of the most well-charcterized examples is the lac operon, the set of genes required for transporting and hydrolyzing the disaccharide lactose. Separate operons controlled by a single regulatory mechanism constitute a regulon.

polycistronic

A transcript that carried multiple genes. The proteins encoded on a polycistronic message generally have related function, allowing a cell to express related genes as one unit.

monocistronic

A transcript that carries one gene. (a cistron is synonymous with a gene)

DNA replication's general tasks

All cells must accomplish two general tasks in order to multiply. First, the double-stranded DNA may be duplicated before cell division so that its encoded information can be passed on to the next generation.

Inducer Exclusion

Although a great deal of attention has been paid to the role of the activator in carbon catabolite repression, another mechanism of regulation called inducer exclusion might be more significant in E. coli. In this mechanism, when glucose is being moved into the cell, a glucose transport component binds to the lactose transporter (permease), locking it in a non-functional position. The locked perm ease cannot move lactose into the cell, so the lac operon will not be induced. Once the glucose supply diminishes, the glucose transporter becomes idle, so lactose can then be brought into the cell.

Activators

An activator is a regulatory protein that facilitates transcription (positive regulation). Genes controlled by an activator have an ineffective promoter preceded by an activator-binding site. The binding of the activator to the DNA enhances the ability of RNA polymerase to initiate transcription at that promoter. Like repressors, activators can be changes by the binding of other molecules. When a molecule called an inducer binds to an activator, the shape of the activator is altered so that it can now bing to the activator- binding site. Thus, the term "inducer" applies to a molecule that turns on transcription, either by stimulating the function of an activator or interfering with the function of a repressor.

Two-component regulatory system

An important mechanism that cells use to detect and react to changes in the external environment. Ex. E. coli. Two-component regulatory systems consist of two different proteins-a sensor and a response regulator. The sensor spans the cytoplasmic membrane. In response to specific environmental variations, the sensor chemically modifies a region on its internal portion, usually by phosphorylating a specific amino acid. The phosphate group is then transferred to a response regulator. When phosphorylated, the response regulator can turn genes either on or off, depending on the system.

Splicing

Another important modification is splicing, which removes specific segments of the transcript. Splicing is necessary because eukaryotic genes are often interrupted by non-coding sequences.

How did Beadle and Tatum generate mutant strains?

Beadle and Tatum treated the mold cultures with X rays and then grew the resulting cells on a nutrient-rich medium that supported the growth of both he original strain and any mutants. Next, they screened thousands of the progeny to find the nutrient-requiring mutants. To identify the metabolic defect of each one, they grew them separately in various types of media containing different nutrients.

5' end and 3'end

Because of the chemical structure of nucleotides and how they are joined to each other, a single stead of SNA will always have a 5'PO4 at one end and 3'OH at the other. These ends are often referred to as the 5'end (5 prime end) and the 3' end (3 prime end)

Regulating Gene Expression

Cells require mechanisms to regulate the expression of certain genes.

Constitutive

Constitutive enzymes are synthesized constantly; the genes that encode these enzymes are always active. Constitutive enzymes usually play indispensable roles in the central metabolic pathways. For example, the enzymes of glycolysis are constitutive.

Characteristics of DNA

DNA is a double stranded, helical structure. Each strand is composed of a chain of deoxyribonucleotide subunits (nucleotides). These subunits are joined together by a covalent bond between the 5'PO4 (5 prime phosphate) of one nucleotide and 3'OH (3 prime hydroxyl) of the next. The designation 5' and 3' refer to the numbered carbon atoms of the pentose sugar of the nucleotide. Joining the nucleotides in this manner creates a series of alternating sugar and phosphate units, called the sugar-phosphate backbone.

DNA Replication

DNA is replicated in order to create a duplicate molecule, so that the two cells generated during binary fission can each receive one complete copy. The replication process is generally bidirectional, meaning it proceeds in both directions from the starting point. This allows a chromosome to be replicated in half the time it would take if the process were unidirectional. The progression of bidirectional replication around a circular DNA molecule creates two advancing forks where DNA synthesis is occurring.

nucleotides

DNA itself is a simple structure, a string composed of only four different nucleotides.

diauxic growth

E. coli cells prioritize carbon/energy sources, a trait that cane demonstrated by growing them in a medium containing both glucose and lactose.Initally, the cells multiply using only glucose. Once the supply of that sugar is exhausted, growth stopes for a short period as the cells great up to begin metabolizing lactose. Then, they begin multiplying again, this time using lactose to fuel their growth. This characteristic two-phase growth pattern is called diauxic growth.

Termination of Translation

Elongation of the polypeptide terminates when the ribosome reaches a stop codon, a codon not recognized by a tRNA. At this point, enzymes free the polypeptide by breaking the covalent bond that joins it to the tRNA. The ribosome falls off the mRNA, dissociating into its two component subunits (30S and 50S). The subunits can then be reused to initiate translation at other sites.

DNA Polymerases

Enzymes called DNA polymerases synthesize DNA in the 5' to 3' direction, using one strand as a template to make the complement. DNA polymerase adds nucleotides onto the 3' end of a primer, powering the reaction with the energy released when a high-energy phosphate bond of the incoming nucleotide is hydrolyzed. Note that DNA polymerases add nucleotides only onto an existing DNA strand, so they cannot initiate synthesis. This explains why primers are required at the origin of replication-they provide the DNA polymerase with a molecule to which it can add nucleotides.

Primases

Enzymes called primases then synthesize short stretches of RNA complementary to the exposed templates.

pre-mRNA

Eukaryotic mRNA is synthesized in a precursor form, called pre-mRNA. The pre-mRNA must be processed, both during and after transcription to form mature mRNA.

What did Beadle and Tatum establish?

Eventually, Beadle and Tatum established that the metabolic defect was inherited as a single gene, which ultimately led to their conclusion that a single gene determines the production of one enzyme. That assumption has been modified somewhat, because we now know that some enzymes are made up of more than and polypeptide, and not all proteins are enzymes. In 1958, Beadle and Tatum were awarded a Nobel Prize, largely for these pioneering studies what ushered in the era of modern biology.

deoxyribonucleic acid

Every characteristic of each of these cells, from its shape to its function, is dictated by information within its deoxyribonucleic acid (DNA). DNA encodes the master plan, the blueprint, for all of an organism's features.

Gregor Mendel

In 1866, the Czech-Austrian monk Gregor Mendel determined that traits are inherited as physical units, not called genes.

RNA transcript

In making the RNA molecule, or transcript, the base-pairing rules apply except that uracil, rather than thymine, pairs with adenine. The interaction of DNA and RNA is only temporary, however, and the transcript quickly separates from the template.

mRNA in prokaryotes

In prokaryotes, mRNA molecules can carry the information for one or multiple genes.

Initiation of Translation

In prokaryotes, translation begins as the mRNA molecule is still being synthesized. Part of the ribosome binds to a sequence in mRNA called the ribosome-binding site; the first AUG after that site usually serves as the start codon. The complete ribosome assembles there, joined by an initiating tRNA that carries a chemically altered form of the amino acid methionine. The position of the first AUG is critical, as it determines the reading frame used for translation of the remainder of that protein. Note that AUG functions as a start codon only when preceded by a ribosome-binding site; at other sites, it simply encodes methionine.

Elongation of the RNA Transcript

In the elongation phase, RNA polymerase moves along DNA, using the (-) strand as a template to synthesize a single-stranded RNA molecule. As with DNA replication, nucleotides are added only to the 3' end; the reaction is fueled by hydrolyzing a high energy phosphate bond of the incoming nucleotide. When RNA polymerase advances, it denatures a new stretch of DNA and allows the previous portion to close. This exposes a new region of the template so elongation can continue. Once elongation has proceeded far enough for RNA polymerase to clear the promoter, another molecule of the enzyme can bind, initiating a new round of transcription. Thus, a single gene can be transcribed repeatedly very quickly.

Inducible

Inducible enzymes are not routinely produced at significant level; instead, their synthesis can be turned on when needed. An example is B-galactosidase, the enzyme that hydrolyzes lactose into its component monosaccharides-glucose and galactose. The genes for this enzyme are part of the lac operon, which is turned on only when lactose is present. This makes sense because a cell would waster precious resources if it expressed the operon when lactose is not available. Inducible enzymes are often involved in the transport and breakdown of specific energy sources.

Termination of Transcription

Just as an initiation of transcription occurs at a distinct site on the DNA, so does termination. When RNA polymerase encounters a sequence called a terminator, it falls off the DNA template and releases the newly synthesized RNA.

Molds

Neurospora species can grow up on media containing only sucrose, inorganic salts, and biotin.

CAP (catabolite activator protein)

One mechanism of carbon catabolite repression involves an activator called CAP (catabolite activator protein), which is required for transcription. To be functional, the a captivator must be bound by an inducer- an ATP derivative called cAMP (cyclic AMP). The inducer is made only when extracellular glucose levels are low, because the enzyme required for its synthesis is activated by the idle form of the glucose transporter component.

Post- Translational Modification

Polypeptides must often be modified after they are synthesized in order to become functional. Polypeptides destined for transport through the cytoplasmic membrane also must be modified.

The role of promoters in Transcription

Promoters identify the regions of a DNA molecule that will be transcribed into RNA. In doing so, they also orient the direction of the RNA polymerase on the DNA molecule, thereby dictating which strand will be used as a template. The direction of polymerase movement can be likened to the flow of a river. Because of this, the works "upstream" and "downstream" are used to describe relation positions of other sequences. As an example, promoters are upstream of the genes they control.

Characteristics of RNA

RNA is similar to DNA in ways, with a few important exceptions. One difference is that RNA is made up of ribonucleotides rather than deoxyribonucleotides, although in both cases these are usually referred to simply as nucleotides. Another distinction is that RNA contains the nuclease uracil in place of thymine found in DNA. Like DNA, RNA consists of a chain of nucleotides, but RNA is usually a single-stranded linear molecule much shorter than DNA. RNA is synthesized using a region of one of the two strands of DNA as a template.

Repressible

Repressible enzymes are produced routinely, but their synthesis can be turned off when they are not required. Repressible enzymes are generally involved in biosynthetic (anabolic) pathways, such as those that produce amino acids. Cells require a sufficient amount of a given amino acid to multiply; so, if an amino acid is not available in the environment, it needs to be produced by the cell. When the amino acid is available, however, synthesis of the enzymes used in its production would waste energy.

The Role of Ribosomes

Ribosomes serves as translation "machine," structures that string amino acids together. A ribosome does this by aligning two amino acids so that a ribosomal enzyme can easily create a peptide bond between them. Ribosomes also locate key punctuation sequences on the mRNA molecule, such as the point at which protein synthesis should begin. The ribosome then moves along the mRNA in the 5' to 3' direction, "presenting" each codon in a sequential order for deciphering, while maintaining the correct reading frame. Prokaryotic ribosomes are composed of a 30S subunit and a 50S subunit, each made up of protein and ribosomal RNA. The "S" strands for Svedberg unit, which is a measure of size.

Gene expression

Second, the information encoded by the DNA must be decoded so that the cell can synthesize the necessary gene products. This process, gene expression, involves two interrelated events-transcription and translation.

capped

Shortly after transcription begins, the 5' end of the pre-mRNA is capped by adding a methylated guanine derivative. This cap binds specific proteins that stabilize the transcript and enhance translation.

ribosomal RNA(rRNA) transfer RNA (tRNA)

Some genes are never translated into proteins; instead the RNAs themselves are the final products. These genes encode either ribosomal RNA (rRNA) or transfer RNA (tRNA), each of which plays a different but critical role in protein synthesis.

Quorum Sensing

Some organisms can "sense" the density of cells within their own population-a phenomenon called quorum sensing. This allows cells to activate genes that are only useful when expressed by a critical mass. As an example, the cooperative activities leading to biofilm formation are controlled by quorum sensing. Some pathogens use the mechanism to coordinate expression of genes involved with the infection process. Quorum sensing involves a process that allows bacteria to "talk" to each other by synthesizing one or more varieties of extra cellular signaling molecules. Some types of bacteria are able to detect and even interfere with the signaling molecules produced by other species. This allows them to "eavesdrop" and even obstruct "conversations" of other bacteria.

chaperones

Some polypeptides must be folded into a specific three-dimensional structure, a process that requires the assistance of proteins called chaperones.

What are used as templates for RNA synthesis?

Specific chromosomal regions are use as templates for RNA synthesis, generating numerous distinct transcripts. Either DNA strand may serve as the template, but only one of the two strands is transcribed in a given region.

Leading Strand

Synthesis of one new strand proceeds continuously as fresh template is exposed, because DNA polymerase simply add nucleotides to the 3' end. This strand is called the leading strand.

Lagging Strand

Synthesis on the lagging strand is more complicated. This is because DNA polymerases cannot add nucleotides to the 5' end, so as additional template is exposed, synthesis must be reinitiated. Each time synthesis is reinitiated, another RNA primer must be made first. The result is a series of small fragments, each of which has a short stretch of RNA at its 5' end. These fragments are called Okazaki fragments. As DNA polymerase adds nucleotides to the 3' end of one Okazaki fragment, it eventually reaches the 5' end of another. A different type of DNA polymerase then removes the RNA primer nucleotides and simultaneously replaces them with deoxynucleotides. The enzyme DNA ligase then seals the gaps between fragments by forming a covalent bond between the adjacent nucleotides.

polyadenylation

The 3' end of the molecule is also modified, even before transcription has been terminated. This process, polyadenylation, cleaves the transcript at a specific sequence and then adds about 200 adenine derivatives to the new 3' end. This creates a poly A tail, which is thought to stabilized the transcript as well as enhance translation.

terminator

The DNA sequence that can stop the process.

promoter

The DNA sequence to which RNA polymerase can bind and initiate transcription.

minus(-) strand

The DNA strand that serves as the template for transcription and its complement is called the plus(+) strand. Because the RNA is complementary to the (-) DNA strand, its nucleotide sequence is the same as the (+) DNA strand, except it contains uracil rather than thymine.

genome

The complete set of genetic information of a cell. Technically, this includes plasmids as well as the chromosome; however, the term "genome" is often used interchangeably with chromosome. The genome of all cells is composed of DNA, but some viruses have an RNA genome.

gene

The function unit of the genome. A gene encodes a product (called the gene product), most commonly a protein.

Lac operon and lactose

The lac operon uses a repressor that prevents transcription when lactose is not available; the repressor binds the operator, blocking RNA polymerase. When lactose is in the cell some of it is converted to allolactose, an inducer. This compound binds the repressor and, in doing so, changes the repressor's shape so that it can no longer graso the operator. With the operator unoccupied, RNA polymerase can begin transcribing the operon. However, this can happen only if glucose is not available in the growth medium.

The Role of mRNA

The mRNA is a temporary copy of genetic information; it carries encoded instruction for synthesis of a specific protein, or in the case of a polycistronic message, a specific group of proteins. That information is deciphered using the genetic code, which correlates a series of three nucleotides, a codon, with one amino acid. The genetic code is practically universal. Because a codon is a triplet of any combination of the four nucleotides, there are 64 different codons. Three are stop codons. The remaining 61 translate to the 20 different amino acids. This means that more than one codon can code for a specific amino acid. Because of this redundancy, the genetic code is said to be degenerate. The nucleotide sequence of mRNA indicates where the coding region begins and ends. The site at which it begins is particularly critical because the translation "machinery" reads the mRNA in groups of three nucleotides. As a consequence, any given sequence has three possible reading frames, or ways in which triplets can be grouped. If translation begins in the wrong reading frame, a very different, and generally non-functional, protein would by synthesized.

Mechanisms to control Transcription

The methods a cell used to prevent or facilitate transcription must be readily reversible, allowing cells to control the relative number of transcripts made. Two of the most common regulatory mechanisms are alternative sigma factors and DNA-binding proteins.

replisomes

The most critical of enzymes and proteins exist together in DNA-synthesizing "assembly lines" called replisomes.

Sigma Factor

The portion of RNA polymerase that recognizes the promoter is a loosely attached subunit called sigma factor. A cell can produce carious types of sigma factors, each recognizing different promoters. By controlling which sigma factors are made, cells can transcribe specialized sets of genes as needed. The RNA polymerases of eukaryotic cells and archaea use proteins call transcription factors to recognize promoters.

When was genes revealed?

The precise function of genes was not revealed until 1941, when George Beadle and Edward Tatum published a scientific paper reporting that genes direct the production of enzymes.

Induction

The repressor is synthesized as a form that binds to the operator, blocking transcription. When a molecule called an inducer attaches to the repressor, the shape of the repressor changes so that it can no longer grasp the operator. With the repressor unable to bind to DNA, RNA polymerase may transcribe the gene.

Repression

The repressor is synthesized as a form that cannot bind to the operator. However, when a molecule termed a corepressor attaches to the repressor, the corepressor-repressor complex can then bind to the operator blocking transcription.

Elongation of the Polypeptide Chain

The ribosome has two sites to which amino acid-carrying tRNAs can bind-the P-site and the A-site. At the start of translation, the initiating tRNA carrying the f-Met occupies the P-site. A tRNA that recognizes the next codon on the mRNA then fills the unoccupied A-stie. Once both sites are filled, an enzyme creates a peptide bond between the two amino acids carried by the tRNAs. This transfers the amino acid from the initiating tRNA to the amino acid carried by the incoming tRNA. After the initiating tRNA has donated its amino acid to the tRNA in the A-site, the ribosome advances a distance of one codon, moving along the mRNA in a 5' to 3' direction. The initiating tRNA is released through a region called the E-site. The remaining tRNA, which now carries both amino acids, occupies the P-site. The A-site is transiently empty. A tRNA that recognizes the codon in the A-site quickly attaches there, and the process repeats. Once translation of a gene has progressed far enough for the ribosome to clean the initiating sequences, another ribosome can bind. Thus, at any one time, multiple ribosomes can be translating a single mRNA molecule. This allows maximal protein synthesis from a single mRNA template. The assembly of multiple ribosomes attached to a single mRNA molecule is called a polyribosome, or a polysome.

genomics

The study and analysis of the nucleotide sequence of DNA.

The Role of Transfer RNA's

The tRNAs are segments of RNA that act as key to the genetic code. Each tRNA recognizes and base-pairs with certain codons and, in the process, delivers the appropriate amino acid to that site. This recognition is made possible because each tRNA has an anticodon-three nucleotides complementary to a codon in the mRNA. The anticodon of a tRNA molecule dictates which amino acid the molecule carries. Once a tRNA molecule has donated its amino acid during translation, it can be recycled. An enzyme in the cytoplasm recognizes the tRNA and then attaches the appropriate amino acid.

semiconservative

The two DNA molecules created through replication each contain one of the original strands paired with a newly synthesized strand. Because half of the original molecule is conserved in each molecule, replication is said to be semiconservative.

What happens when a circular bacterial chromosome is replicated?

The two replication forks eventually meet at a site opposite the origin of replication. Two complete DNA molecules have been produced at this point, and these can be passed on to the two daughter cells.

complementary

The two strands of DNA are complementary and are held together by hydrogen bonds between the nucleases. Wherever an adenine (A) is in one stand, a thymine (T) is in the other; these opposing A-T bases are held together by two hydrogen bonds. Similarly, wherever a guanine (G) is in one strand, a cytosine (C) is in the other.

Base pairing

These G-C bases are held together by two hydrogen bonds, a slightly stronger attraction than that of an A-T pair. The characteristic bonding of A to T and G to C is called base pairing and is a fundamental characteristic of DNA. Because of the rules of base-pairing, one strand can always be used as a template for the synthesis of the opposing strand.

What are the three different functional types of RNA?

Three different functional types of RNA are required for gene expression, and are transcribed from different sets of genes.

Origin of Replication

To initiate replication of a DNA molecule, specific proteins must recognize and bind to a distinct DNA sequence called an origin of replication. Prokaryotic chromosomes and plasmids typically contain only one of these initiating sites. A molecule that lacks this sequence will not be replicated. The proteins that bind to the origin of relocation cause localized melting of the double-stranded DNA, exposing single-stranded regions that can act as templates.

Initiation of RNA Synthesis

Transcription is initiated when RNA polymerase binds to a promoter. The binding melts a short stretch of DNA, creating a region of exposed nucleotides that serves as a template for RNA synthesis.

DNA-Binding Proteins

Transcription is often controlled by proteins that bind to specific DNA sequences. When a regulatory protein attaches to DNA, it can act either as a repressor, which blocks transcription, or an activator, which facilitates transcription.

Signal Transduction

Transmits information from outside a cell to the inside. This allows cells to monitor and react to environmental conditions.

How do the ribosomes of eukaryotes differ from those of prokaryotes?

Whereas the prokaryotic ribosome is 70S, made up of 30S and 50S subunits, the eukaryotic ribosome is 80S, made up of 40S and 60S subunits. The differences in ribosome structure are medically important because certain antibiotics bind to and inactivate bacterial 70S ribosomes, but not 80S ribosomes. This explains why those antibiotics kill bacteria without causing significant harm to mammalian cells.

signal sequence

a characteristic series of hydrophobic amino acids at their amino terminal end, which "tags" them for transport. The signal sequence must be removed by proteins in the membrane.

Alternative Sigma Factors

a loose component of RNA polymerase that functions in recognizing specific promoters. Standard sigma factors recognize promoters for genes that need to be expressed ruing routine growth conditions, but a cell can also produce Alternative sigma factors. These recognize different sets of promoters, thereby controlling the expression of specific groups of genes. In the endospore0former Bacillus subtilis, the sporulation process is controlled by a number of different alternative sigma facts. One controls the steps at the beginning of sporulation. Others then guise the states of development in the mother cell and spore. A cell can also express anti-sigma factors, which inhibit the function of specific sigma factors.

What does a set of three nucleotides encode?

a specific amino acid; in turn, a string of amino acids makes up a protein, the structure and function of which is dictated by the order of the amino acid subunits. Some proteins serve as structural components of a cell. Others,such as enzymes, direct cellular activities including biosynthesis and energy conversion. Together, all these proteins control the cell's structure and activities, dictating the overall characteristics of that cell.

Like DNA polymerase, RNA polymerase can...

add nucleotides only to the 3' end of a chain and therefore synthesizes RNA in the 5' and 3' direction. Unlike DNA polymerase, however, RNA polymerase can start synthesis without a primer.

Phase Variation

another mechanism of randomly altering gene expression is phase variation, which is the routine switching on and off of certain genes.

Natural selection

can also play a role in gene expression. The expression of some genes changes randomly in cells, enhancing surveil chances of at least a part of a population.

The RNA sequence made during transcription is...

complementary and antiparallel to the DNA template.

Transcription definition

copies the information encoded in DNA into a slightly different molecule, RNA. The RNA serves as a transitional, temporary form of the information and is the one actually deciphered.

nucleobase

each nucleotides contain a particular nucleobase. (also simply called a base): adenine (A), thymine (T), cytosine (C), and guanine (G).

In order for replication to progress....

enzymes called helicases must progressively "unzip" the DNA strands at each replication fork to reveal additional template sequences.

Francois Jacob and Jacques Monod

in the early 1960s, Francois Jacob and Jacques Monod described the lac operon of E.coli served as an important model for understanding the control of bacterial gene expression. This operon encodes proteins involved with the transport and degradation of lactose, and is only turned on when lactose is in the cell but glucose is not available. The fact that glucose prevents expression of the genes is significant biologically because it forces cells to use the most efficiently metabolized carbon source first.

Translation definition

interprets information carried by RNA to synthesize the encoded protein. The flow of information from DNA--> RNA--> protein is often referred to as the central dogma of molecule biology.

introns

intervening sequences that are transcribed along with the expressed regions, eons, and must be removed from pre-mRNA to create functional mRNA.

The duplex structure of double-stranded SNA

is generally quite storable because of the numerous hydrogen bonds that occur along its length. Short fragments of DNA have correspondingly fewer hydrogen bonds, so they are easily separated into single strands. Separating the two strands is called melting, or denaturing.

What is a fundamental aspect of gene regulation?

is the instability of mRNA. Within minutes of being produced, transcripts are degraded by cellular enzymes. Although this might seem wasteful, it actually provides cells with an important regulatory mechanism. If transcription of a gene is turned "on," transcripts will continue to be available for translation. If it is then turn "off," the number of transcripts will rapidly decline. By simply regulating the synthesis of mRNA molecules, a cell can quickly change the levels of protein production. Some cells have mechanisms to adjust the stability of RNA, providing an addition level of control.

Translation requires what three major structures?

mRNA, ribosomes, and tRNAs

messenger RNA (mRNA)

most gene encode proteins and are transcribed into messenger RNA (mRNA). The information encrypted in mRNA is deciphered according to the genetic code, which correlates each set of three nucleotides to a particular amino acid.

The mRNA in eukaryotic cells

must be transported out of the nucleus before it can be translated in the cytoplasm. Thus, unlike in prokaryotes, the same mRNA molecule cannot be synthesized and translated at the same time or even in the same cellular location. The mRNA of eukaryotes is generally monocistronic, and translation of the message typically begins at the first AUG in the molecule.

George Beadle and Edward Tatum

set out to discover how genes control metabolic reactions by studying common bread molds that have very simple nutritional requirements. Beadle and Tatum reasoned that if they created mutant strains which required additional nutrients, they could use them to gain insights into the relationship between genes and enzymes. For example, a mutant that requires a certain amino acid likely has a defect in an enzyme required to synthesize that amino acid.

primers

small fragments are critical in the next steps of replication.

RNA polymerase in Transcription

the enzyme RNA polymerase synthesizes single stranded RNA molecules from a DNA template. Nucleotide sequences in the DNA direct the polymerase where to start and where to end.

replication forks

the regions where the progression of bidirectional replication around a circular DNA molecule creates two advancing forks where DNA synthesis is occurring is called replication fords, ultimately meet at a terminating site when the process is complete.

Antigenic variation

the role of natural selection is readily apparent in bacteria that undergo antigenic variation, which is an alteration in the characteristics of certain surface proteins. Pathogens that do this can stay one step ahead of the body's defenses by altering the very molecules our immune systems must learn to recognize.

Antiparallel

the two strands of DNA in the double helix are also antiparallel. They are oriented in opposite directions. One strand is oriented in the 3' to 5' direction and its complement is oriented in the 3' to 5' direction.

Global Control

two-component regulatory systems often control regulons. The simultaneous regulation of numerous genes is called global control.

carbon Catabolite repression (CCR)

when glucose is available, the lac operon is not expressed because of a phenomenon called carbon catabolite repression (CCR). Carbon catabolite repression is a global control system that allows glucose to regulate expression of the lac operon as well as other sets of genes. Glucose does not act directly in the regulation, however. Instead, the cell's glucose transport system serves as a sensor of glucose availability. When the transport system is moving glucose molecules into the cell, catabolite repression prevents the lac operon from being expressed. When the transport system is idle, indicating that glucose is not available, then the lac operon can be turned on.