dna 10
The Flow of Genetic Information from DNA to RNA to Protein (10.6-10.16)
Genotype controls phenotype through the production of proteins.
The Genetics of Viruses and Bacteria (10.17-10.23)
Viruses and bacteria are useful model systems for the study of nucleic acids.
The Structure of the Genetic Material
Experiments Showed That DNA Is the Genetic Material- Scientists routinely manipulate DNA to change heritable traits of cells.
Early in the 20th century, the molecular basis for inheritance was a mystery.
Genes were known to be located on chromosomes.
The two chemical components of chromosomes―DNA and protein―were the leading candidates to be the genetic material.
Until the 1940s, proteins seemed more structurally complex.- Proteins: Made from 20 different amino acid building blocks.
DNA: Made from just four kinds of nucleotides.
Biologists established the role of DNA in heredity through experiments with bacteria and the viruses that infect them.
Discovery of the Genetic Role of DNA:- 1928: Frederick Griffith tried to develop a vaccine against pneumonia.
Studied two strains of bacterium:- Harmless strain.
Pathogenic (disease-causing) strain.
Griffith found that when he killed the pathogenic bacteria and then mixed the bacterial remains with living harmless bacteria, some living bacterial cells became pathogenic.
Descendants of the transformed bacteria inherited the newly acquired ability to cause disease.
Some chemical component of the dead bacteria caused a heritable change in live bacteria.
1952: Alfred Hershey and Martha Chase showed DNA to be the genetic material of T2, a virus that infects the bacterium Escherichia coli (E. coli).- Viruses that exclusively infect bacteria are called bacteriophages ("bacteria-eaters"), or phages.
Hershey and Chase knew that T2 could reprogram its host cell to produce new phages, but they did not know what component of the virus conferred this capability.
The structure of phage T2 consists solely of two types of molecules DNA and protein.
Experiment:- Grew T2 with E. coli in a solution containing radioactive sulfur.- Protein contains sulfur but DNA does not, so the radioactive sulfur atoms were incorporated only into the proteins of the bacteriophage.
Grew a second batch of phages in a solution containing radioactive phosphorus.- Nearly all the phage's phosphorus is in DNA, this labeled only the phage DNA.
Allowed the two batches of T2 to infect separate samples of nonradioactive bacteria.
Agitated the cultures in an ordinary kitchen blender to shake loose any parts of the phages that remained outside the bacterial cells.
Collected the mixtures in tubes and spun the tubes in a centrifuge.- The cells were deposited as a solid pellet at the bottom of the centrifuge tubes, but phages and parts of phages—because they were lighter—remained suspended in the liquid.
Measured the radioactivity in the pellet and in the liquid.
Results:- When the bacteria had been infected with T2 phages containing labeled protein, the radioactivity ended up mainly in the solution within the centrifuge tube, which contained phages but not bacteria.- The phage protein did not enter the cells.
When the bacteria had been infected with phages whose DNA was tagged, most of the radioactivity was in the pellet of bacterial cells at the bottom of the centrifuge tube.- When these bacteria were returned to a liquid growth medium, they soon lysed, or broke open, releasing new phages that contained some radioactive phosphorus in their DNA.
Replication cycle of phage T2:- After the virus attaches to the host bacterial cell, it injects its DNA into the host.- Virtually all of the viral protein is left outside of the bacterium.
Once injected into the bacteria, the viral DNA causes the bacterial cells to produce new phage proteins and DNA molecules—indeed, complete new phages—which soon cause the cell to lyse, releasing the newly produced phages.
These phages may then attach to other host bacterial cells.
Once DNA was shown to be the molecule of heredity, understanding its structure became the most important quest in biology.
DNA and RNA Are Polymers of Nucleotides
DNA and its close chemical cousin RNA are nucleic acids, consisting of long chains (polymers) of chemical units (monomers) called nucleotides.
Each type of DNA nucleotide has a different nitrogen-containing base: adenine (A), cytosine (C), guanine (G), or thymine (T).
Nucleotides can occur in a polynucleotide in any sequence and because polynucleotides can be very long, the number of possible polynucleotides is enormous.
Each nucleotide consists of three components: a nitrogenous base (in DNA: A, C, T, or G), a sugar, and a phosphate group.
The nucleotides are joined to one another by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a sugar-phosphate backbone with a repeating pattern of sugar-phosphate-sugar-phosphate.
The nitrogenous bases are arranged like ribs that project from the backbone.
The phosphate group has a phosphorus atom at the center with four surrounding oxygen atoms.
The sugar has five carbon atoms, called out in red in the figure for emphasis-four in its ring and one extending above the ring. The ring also includes an oxygen atom.
The sugar is called deoxyribose because, compared with the sugar ribose, it is missing an oxygen atom.
The C atom in the lower right corner of the ring is bonded to an H atom instead of to an -OH group, as it is in ribose.
Therefore: DNA is "deoxy"-which means "without an oxygen"-compared to RNA.
The full name for DNA is deoxyribonucleic acid: deoxyribo refers to its form of the sugar, nucleic because DNA is located in the nuclei of eukaryotic cells, and acid because the phosphate group.
Every nucleotide contains a nitrogenous base (thymine, in our example at the right in Figure 10.2A).
Nitrogenous bases are basic.
Each base has a single or double ring consisting of nitrogen and carbon atoms with various functional groups attached.
The main role of the functional groups is to determine which other kind of bases each base can form hydrogen bonds with.
For example, the group hanging off cytosine is capable of forming a hydrogen bond to the group hanging off guanine but not with the group protruding from adenine.
The chemical groups of the bases are therefore responsible for the specific base pairing found in DNA.
The four nucleotides found in DNA differ only in the structure of their nitrogenous bases.- Thymine (T) and cytosine (C) are single-ring structures called pyrimidines.
Adenine (A) and guanine (G) are larger, double-ring structures called purines.
As its name—ribonucleic acid—implies, its sugar is ribose rather than deoxyribose
Instead of thymine, RNA has a nitrogenous base called uracil (U).
Except for the presence of ribose and uracil, an RNA polynucleotide chain is identical to a DNA polynucleotide chain.
DNA Is a Double-Stranded Helix
1952 Hershey-Chase experiment convinced most biologists that DNA was the material that stored genetic information.
American James D. Watson and Englishman Francis Crick determined the structure of DNA.
Watson saw an X-ray image of DNA produced by Wilkins's colleague, Rosalind Franklin.
Watson deduced the basic shape of DNA to be a helix (spiral) with a uniform diameter and the nitrogenous bases located above one another like a stack of dinner plates.
The thickness of the helix suggested that it was made up of two polynucleotide strands, forming a double helix.
Franklin had concluded that the sugar-phosphate backbones must be on the outside of the double helix, forcing the nitrogenous bases to swivel to the interior of the molecule.
A double-ringed base (purine) on one strand must always be paired with a single-ringed base (pyrimidine) on the opposite strand to produce a molecule of uniform thickness.
Adenine can best form hydrogen bonds with thymine and only thymine, and guanine with cytosine and only cytosine.
In the biologist's shorthand, A pairs with T, and G pairs with C
A is also said to be "complementary" to T and G to C.
Chargaff had discovered that the amount of adenine in the DNA of any one species was equal to the amount of thymine and that the amount of guanine was equal to that of cytosine.
Chargaff's rules, as they are called, are explained by the fact that A on one of DNA's polynucleotide chains always pairs with T on the other polynucleotide chain, and G on one chain pairs only with C on the other chain.
The side ropes represent the sugar-phosphate backbones, and the rungs represent pairs of nitrogenous bases joined by hydrogen bonds.
A C-G base pair has functional groups that form three hydrogen bonds, whereas an A-T base pair has functional groups that form two hydrogen bonds.
The two sugar-phosphate backbones of the double helix are oriented in opposite directions.
The sequence of bases can vary in countless ways, and each gene has a unique order of nucleotides, or base sequence.
In 1962, Watson, Crick, and Wilkins received the Nobel Prize for their work.
The Watson-Crick model gave new meaning to the words genes and chromosomes—and to the chromosome theory of inheritance.
The genetic information in a chromosome must be encoded in the nucleotide sequence of the molecule.
DNA Replication
DNA Replication Depends on Specific Base Pairing- The primary function of DNA is to encode and store genetic information.
Genes are passed along from one cell to the next during cell division and from one generation to the next during reproduction.
Watson and Crick proposed that the specific pairing of complementary bases accounts for the ability of DNA to be copied.
The two strands of parental DNA separate.
Each strand becomes a template for the assembly of a complementary strand from a supply of free nucleotides available within the nucleus.
Enzymes link the nucleotides to form the new DNA strands.
The completed new molecules, identical to the parental molecule, are known as daughter DNA.
Watson and Crick's model predicts that when a double helix replicates, each of the two daughter molecules will have one old strand from the parental molecule and one newly created strand.
This model for DNA replication is known as the semiconservative model because half of the parental molecule is maintained (conserved) in each daughter molecule.
DNA Replication Proceeds in Two Directions at Many Sites Simultaneously- Replication of a chromosomal DNA molecule begins at particular sites called origins of replication, short stretches of DNA having a specific sequence of nucleotides.
Proteins that initiate DNA replication attach to the DNA at an origin of replication, separating the two strands of the double helix.
Replication then proceeds in both directions, creating replication "bubbles."
The DNA molecule of a eukaryotic chromosome has many origins where replication can start simultaneously
The sugar-phosphate backbones run in opposite directions.
Each strand has a 3' ("three-prime") end and a 5' ("five-prime") end.
At one end of each DNA strand, the sugar's 3' carbon atom is attached to an -OH group; at the other end, the Sugar's 5' carbon is attached to a phosphate group.
The opposite orientation of the strands is important in DNA replication.
The enzymes that link DNA nucleotides to a growing daughter strand are called DNA polymerases.
These enzymes add nucleotides only to the 3' end of the strand, never to the 5' end.
A daughter DNA strand can only grow in the 5' → 3' direction.
One of the daughter strands can be synthesized in one continuous piece by a DNA polymerase working toward the forking point of the parental DNA.
To make the other daughter strand, polymerase molecules must work outward from the forking point.- The new strand is synthesized in short pieces as the fork opens up.
These pieces are called Okazaki fragments.
DNA ligase, then links, or ligates, the pieces together into a single DNA strand.
DNA polymerases carry out a proofreading step that quickly removes nucleotides that have base-paired incorrectly during replication.
DNA polymerases and DNA ligase are also involved in repairing DNA damaged by harmful radiation, such as ultraviolet light and X-rays, or toxic chemicals in the environment, such as those found in tobacco smoke.
The Flow of Genetic Information from DNA to RNA to Protein
Genes Control Phenotypic Traits Through the Expression of Proteins- An organism's genotype, its genetic makeup, is the heritable information contained in the sequence of nucleotide bases in DNA.
The phenotype is the organism's physical traits.
The DNA inherited by an organism specifies traits by dictating the synthesis of proteins (or, in some cases, just RNAs).
Proteins are the links between genotype and phenotype.
A gene dispatches instructions in the form of RNA, which in turn programs protein synthesis.
The molecular "chain of command" is from DNA in the nucleus of the cell to RNA to protein synthesis in the cytoplasm.
The two main stages are transcription, the synthesis of RNA under the direction of DNA, and translation, the synthesis of protein under the direction of RNA.
Genes dictate phenotypes through enzymes, proteins that catalyze specific chemical reactions.
An inherited disease reflects a person's inability to make a particular enzyme.
Cells make and break down biologically important molecules via metabolic pathways.- Each step in a metabolic pathway is catalyzed by a specific enzyme.
Individuals lacking one of the enzymes for a pathway are unable to complete that pathway.
Beadle and Tatum studied strains of the mold that were unable to grow on a simple growth medium.- Each of these nutritional mutants turned out to lack an enzyme in a metabolic pathway that synthesized some molecule the mold needed, such as an amino acid.
Each mutant was defective in a single gene.
The function of an individual gene is to dictate the production of a specific enzyme.
The RNA transcribed from some genes is not translated but nonetheless performs important functions itself.
A gene: a region of DNA that can be expressed to produce a functional product that is either a polypeptide or an RNA molecule.
Genetic Information Written in Codons Is Translated into Amino Acid Sequences- Genes provide the instructions for making specific proteins.
DNA is transcribed into RNA, which is then translated into protein.
Information within the cell flows as DNA → RNA → protein.
To understand how genetic information passes from genotype to phenotype, we need to see how the chemical language of DNA is translated into the different chemical language of proteins.
Both DNA and RNA are polymers made of nucleotide monomers strung together in specific sequences that convey information.
In DNA, there are four types of nucleotides, which differ in their nitrogenous bases (A, T, C, and G).
The same is true for RNA, although it has the base U instead of T.
Transcription: the nucleic acid language of DNA has been rewritten (transcribed) as a sequence of bases on RNA.
Translation: the conversion of the nucleic acid language to the polypeptide language.
Like nucleic acids, polypeptides are polymers, but the monomers that compose them are the 20 different kinds of amino acids.
The sequence of nucleotides of the RNA molecule dictates the sequence of amino acids of the polypeptide.
The RNA acts as a messenger carrying genetic information from DNA.
Triplets of bases are the smallest "words" of uniform length that can specify all the amino acids.
The genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping three-base "words" called codons.
Three-base codons in the DNA are transcribed into complementary three-base codons in the RNA, and then the RNA codons are translated into amino acids that form a polypeptide.
The Genetic Code Dictates How Codons Are Translated into Amino Acids- Molecular biologists used a series of elegant experiments to crack the genetic code, the amino acid translations of each of the nucleotide triplets.
61 of the 64 triplets code for amino acids.
The triplet AUG has a dual function: It codes for the amino acid methionine (Met) and also can provide a signal for the start of a polypeptide chain.
Three codons (UAA, UGA, and UAG) do not designate amino acids but serve as stop codons that mark the end of translation.
There is redundancy in the code but no ambiguity.
The genetic code is nearly universal, shared by organisms from the simplest bacteria to the most complex plants and animals.
Transcription Produces Genetic Messages in the Form of RNA- Transcription is the transfer of genetic information from DNA to RNA.
Prokaryotic cells, which is a simpler process than in eukaryotic cells.
One strand serves as a template for a new RNA molecule; the other DNA strand is unused.
The transcription enzyme RNA polymerase moves along the gene, forming a new RNA strand by following the base-pairing rules-but remember that in RNA, U replaces T.
A specific nucleotide sequence called a promoter acts as a binding site for RNA polymerase and determines where transcription starts.
RNA polymerase adds RNA nucleotides until it reaches a sequence of DNA bases called the terminator, which signals the end of the gene.
Eukaryotic RNA Is Processed Before Leaving the Nucleus as mRNA- The kind of RNA that encodes amino acid sequences is called messenger RNA (mRNA) because it conveys genetic messages from DNA to the translation machinery of the cell.
In prokaryotic cells, which lack nuclei, transcription and translation occur in the same place: the cytoplasm.
In eukaryotic cells mRNA molecules must exit the nucleus via the nuclear pores and enter the cytoplasm, where the machinery for polypeptide synthesis is located.
Eukaryotic transcripts are modified, or processed- The addition of extra nucleotides to the ends of the RNA transcript.- A small cap (a modified form of a G nucleotide) at the 5' end.
- A long tail (a chain of 50 to 250 A nucleotides) at the 3' end.<!-- -->The cap and tail facilitate the export of the mRNA from the nucleus, protect the mRNA from degradation, and help ribosomes bind to the mRNA.
The cap and tail themselves are not translated into protein.
Introns ("intervening sequences") are noncoding stretches of nucleotides that interrupt the nucleotides that actually code for amino acids.
The coding regions—the parts of a gene that are expressed—are called exons.
Introns are removed, and the exons are joined to produce an mRNA molecule with a continuous coding sequence.
This cutting-and-pasting process is called RNA splicing.
In most cases, RNA splicing is catalyzed by a complex of proteins and small RNA molecules.
Translation is a conversion between different languages-from the nucleic acid language to the protein language--and it involves more elaborate machinery than transcription."
The first important ingredient required for translation is the processed mRNA.
Translation requires enzymes and sources of chemical energy, such as ATP.
Translation requires two heavy-duty components: ribosomes and a kind of RNA called transfer RNA.
Transfer RNA Molecules Serve as Interpreters During Translation- Translation of any language into another language requires an interpreter, someone or something that can recognize the words of one language and convert them to another.
A cell uses a molecular interpreter, a special type of RNA called transfer RNA (tRNA).
The function of a tRNA is to transfer amino acids from the cytoplasmic pool to a growing polypeptide in a ribosome.
tRNA molecules must carry out two functions:- Picking up the appropriate amino acids.
Recognizing the appropriate codons in the mRNA.
A tRNA molecule is made from a single strand of RNA-one polynucleotide chain-consisting of about 80 nucleotides.
A cloverleaf structure consisting of four arms.- Each arm consists of nucleotides held together by hydrogen bonds.
tRNAs contain special bases that have been chemically modified to be different than the standard versions.
A single-stranded loop at one end of the folded molecule contains a special triplet of bases called an anticodon.
The anticodon is complementary to a codon triplet on mRNA.
During translation, the anticodon on the tRNA recognizes a particular codon on the mRNA by using base-pairing rules.
At the other end of the tRNA molecule is a site where one specific kind of amino acid attaches.
There is a slightly different variety of tRNA for each amino acid.
Each amino acid is joined to the correct tRNA by a specific enzyme.
These enzymes,one enzyme for each amino acid, specifically bind one type of amino acid to all tRNA molecules that code for that amino acid, using a molecule of ATP as energy to drive the reaction.
The resulting amino acid-tRNA complex can then contribute its amino acid to a growing polypeptide chain.
Ribosomes Build Polypeptides- The final components in translation are the ribosomes, structures in the cytoplasm that coordinate the functioning of mRNA and tRNA and catalyze the synthesis of polypeptides.
A ribosome consists of two subunits-a large subunit and a small subunit-each made up of proteins and a kind of RNA called ribosomal RNA (rRNA).
A fully assembled ribosome has a binding site for mRNA on the small subunit and binding sites (referred to as the P site and the A site) for tRNA on the large subunit.
The subunits of the ribosome act like a vise, holding the tRNA and mRNA molecules close together, allowing the amino acids carried by the tRNA molecules to be connected into a polypeptide chain.
An Initiation Codon Marks the Start of an mRNA Message- Translation can be divided into three phases: initiation, elongation, and termination.
The initiation process brings together the mRNA, a tRNA bearing the first amino acid, and the two subunits of a ribosome.
The initiation process establishes exactly where translation will begin, ensuring that the mRNA codons are translated into the correct sequence of amino acids.
An mRNA molecule binds to a small ribosomal subunit.
A special initiator tRNA base-pairs with the specific codon, called the start codon, where translation is to begin on the mRNA molecule.- The initiator tRNA carries the amino acid methionine (Met); its anticodon, UAC, base-pairs with the start codon, AUG.
A large ribosomal subunit binds to the small subunit, creating a functional ribosome.
The initiator tRNA fits into a tRNA binding site on the ribosome.- This site, called the P site, will hold the growing polypeptide.
The other tRNA binding site, called the A site, is vacant and ready for the next amino-acid-bearing tRNA.
Elongation Adds Amino Acids to the Polypeptide Chain Until a Stop Codon Terminates Translation- Amino acids are added one by one to the growing chain of amino acids.
The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.
The polypeptide separates from the tRNA in the P site and attaches by a new peptide bond to the amino acid carried by the tRNA in the A site.
The ribosome catalyzes the formation of the peptide bond, adding one more amino acid to the growing polypeptide chain, which snakes out of the ribosome via a tunnel through the molecule.
The P site tRNA (which is now lacking an amino acid) leaves the ribosome, and the ribosome translocates (moves) the remaining tRNA (which holds the growing polypeptide) from the A site to the P site.- The codon and anticodon remain hydrogen-bonded, and the mRNA and tRNA move as a unit.
This movement brings into the A site the next mRNA codon to be translated, and the process can start again.
Elongation continues until a stop codon reaches the ribosome's A site.
Stop codons-UAA, UAG, and UGA-do not code for amino acids but instead act as signals to stop translation.
The completed Polypeptide is freed from the last tRNA, and the ribosome splits back into its separate subunits.
Review: The Flow of Genetic Information in the Cell Is DNA → RNA → Protein- In transcription (DNA → RNA), the mRNA is synthesized from a DNA template.- In eukaryotic cells, transcription occurs in the nucleus, and the messenger RNA is processed before it travels to the cytoplasm (although the processing step is not shown here).
- In prokaryotes, transcription occurs in the cytoplasm.Translation (RNA → protein) can be divided into four steps, all of which occur in the cytoplasm.
When the polypeptide is complete, the two ribosomal subunits come apart, and the tRNA and mRNA are released.
Translation is rapid; a single ribosome can make an average-sized polypeptide in less than a minute.
An mRNA molecule is translated simultaneously by a number of ribosomes.
As it is made, a polypeptide coils and folds, assuming a three-dimensional shape, its tertiary structure.
Several polypeptides may come together, forming a protein with quaternary structure.
Transcription and translation are the main processes whereby genes control the structures and activities of cells.
The chain of command originates with the information in a gene, a specific linear sequence of nucleotides in DNA.
The gene serves as a template, dictating transcription of a complementary sequence of nucleotides in mRNA.
mRNA dictates the linear sequence of amino acids in a polypeptide.
The proteins determine the appearance and the capabilities of the cell and organism.
Mutations Can Affect Genes- Any change to the genetic information of a cell or virus is called a mutation.
A nucleotide substitution is the replacement of one nucleotide and its base-pairing partner with another pair of nucleotides.- Some substitution mutations have no effect at all. (silent mutation)
A missense mutation changes one amino acid to another.
Nonsense mutations change an amino acid codon into a stop codon.
Frameshift mutation: adding or subtracting nucleotides may alter the reading frame (triplet grouping) of the genetic message.
Spontaneous mutations result from errors during DNA replication or recombination.
Other mutations are caused by physical or chemical agents called mutagens.- High-energy radiation, such as X-rays or ultraviolet light, is a physical mutagen.
Some chemical mutagens are molecules that are similar to normal DNA bases but disrupt DNA replication.
Occasionally, a mutation leads to a protein that enhances the success of the mutant organism and its descendants.
Much more often, mutations are harmful to an organism.
Mutations are an important source of the rich diversity of genes in the living world, a diversity that makes evolution by natural selection possible.
The Genetics of Viruses and Bacteria
Viral DNA May Become Part of the Host Chromosome- A virus is an infectious particle consisting of little more than "genes in a box": a bit of nucleic acid wrapped in a protein coat called a capsid and, in some cases, a membrane envelope.
A viral genome may consist of DNA or RNA, and may be single- or double-stranded. Viral genomes usually consist of a single molecule of nucleic acid, which may be linear or circular.
Viruses are parasites that can reproduce only inside cells.
The host cell provides most of the components used to produce new viruses.
Lytic cycle: results in the lysis (breaking open) of the host cell and the release of the newly produced viruses.
Lysogenic cycle: viral DNA replication occurs without destroying the host cell.- Viral DNA is inserted into the bacterial chromosome.
Once inserted, the phage DNA is referred to as a prophage, and most of its genes are inactive.
Every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own chromosome and passes the copies on to daughter cells.
The lysogenic cycle enables viruses to spread without killing the host cells on which they depend.
Occasionally, an environmental signal triggers a switchover from the lysogenic cycle to the lytic cycle.- This causes the viral DNA to be excised from the bacterial chromosome, eventually leading to death of the host cell.
Sometimes, the few prophage genes active in a lysogenic bacterium can cause medical problems.- The bacteria that cause diphtheria, botulism, and scarlet fever would be harmless to people if it were not for the prophage genes they carry.
Certain of these genes direct the bacteria to produce the toxins responsible for making people ill.
Many Viruses Cause Disease in Animals and Plants- A typical animal virus has a membranous outer envelope and projecting spikes of glycoprotein (protein molecules with attached sugars).
The envelope helps the virus enter and leave the host cell.
Many animal viruses have RNA rather than DNA as their genetic material.- Examples of RNA viruses include those that cause the common cold, measles, mumps, polio, and AIDS.
Examples of diseases caused by DNA viruses include hepatitis, chicken pox, and herpes infections.
The replication cycle of a typical enveloped RNA virus:- Glycoprotein spikes attach to receptor proteins on the cell's plasma membrane.
The viral envelope fuses with the cell's membrane, allowing the protein-coated RNA to enter the cytoplasm.
Enzymes then digest the protein coat.
An enzyme that entered the cell as part of the virus uses the virus's RNA genome as a template for making complementary strands of RNA.- The new strands have two functions:- They serve as mRNA for the synthesis of new viral proteins.
- They serve as templates for synthesizing new viral genome RNAThe new coat proteins assemble around the new viral RNA.
The viruses leave the cell by cloaking themselves in the host cell's plasma membrane.- Thus, the virus obtains its envelope from the host cell, leaving the cell without necessarily lysing it.
Not all animal viruses replicate in the cytoplasm.- Herpesviruses-which cause chicken pox, shingles, cold sores, and genital herpes-are enveloped DNA viruses that replicate in the host cell's nucleus.- While inside the nuclei of certain nerve cells, herpesvirus DNA may remain permanently dormant, without destroying these cells.
- Physical stress may stimulate the herpesvirus DNA to begin production of the virus, which then infects cells at the body's surface and causes symptoms.The amount of damage a virus causes our body depends partly on how quickly our immune system responds to fight the infection and partly on the ability of the infected tissue to repair itself.
Plants, like animals, are susceptible to viral infections.- Viruses that infect plants can stunt plant growth and diminish crop yields.
Most known plant viruses are RNA viruses.
To infect a plant, a virus must first get past the plant's outer protective layer of cells (the epidermis).
Once a virus enters a plant cell and begins replicating, it can spread throughout the entire plant through plasmodesmata, the cytoplasmic connections that penetrate the walls between adjacent plant cells.
Plant viruses may spread to other plants by insects, herbivores, humans, or farming tools.
There are no cures for most viral diseases of plants.
Emerging Viruses Threaten Human Health- Emerging viruses are ones that seem to burst on to the scene, becoming apparent to the medical community quite suddenly.
Some diseases like the Ebola virus cause hemorrhagic fever, an often fatal syndrome characterized by fever, vomiting, massive bleeding, and circulatory system collapse.
A number of other dangerous newly recognized viruses cause encephalitis, inflammation of the brain(West Nile virus) .
The Zika virus is spread by mosquitoes and causes Zika fever.
Three processes contribute to the emergence of viral diseases:- Mutation
Contact among species
Spread from isolated populations
The AIDS Virus Makes DNA on an RNA Template- HIV, the virus that causes AIDS, is an RNA virus with some special properties.
HIV contains two identical copies of its RNA instead of one.
HIV also has a different mode of replication.- It is a retrovirus, an RNA virus that reproduces by means of a DNA molecule.
Retroviruses carry molecules of an enzyme called reverse transcriptase, which catalyzes reverse transcription: the synthesis of DNA on an RNA template.
Reverse transcriptase uses the RNA as a template to make a DNA strand and then adds a second, complementary DNA strand.
The resulting double-stranded viral DNA enters the cell's nucleus and inserts itself into the chromosomal DNA.
The host's RNA polymerase transcribes the incorporated DNA into RNA, which can then be translated into viral proteins.
HIV infects and kills white blood cells of the immune system.
The loss of such cells causes the body to become susceptible to other infections that it would normally be able to fight off.
Such secondary infections cause the syndrome (a collection of symptoms) that can kill an AIDS patient.
Prions Are Infectious Proteins- Prions are infectious proteins that cause a number of brain diseases in animals.
A prion consists solely of a misfolded form of a normal brain protein.
When the prion gets into a cell containing the normal form of the protein, the prion somehow converts normal protein molecules to misfolded versions.
The misfolded proteins then clump together, disrupting brain functions.
Bacteria Can Transfer DNA in Three Ways- Most of a bacterium's DNA is found in a single chromosome, a closed loop of DNA with associated proteins.
Bacterial cells reproduce by replication of the bacterial chromosome followed by binary fission.
Because binary fission is an asexual process involving only a single parent, the bacteria in a colony are genetically identical to the parental cell.
In the bacterial world, there are three mechanisms by which genes can move from one cell to another:- Transformation: the uptake of foreign DNA from the surrounding environment.
Transduction: transfer of bacterial genes by a phage.
Conjugation: physical union of two bacterial cells and the DNA transfer between them.
The donor cell has hollow appendages called sex pili.
The donor then transfers DNA to the recipient.
The donor cell replicates its DNA as it transfers it, so the cell doesn't end up lacking any genes.
Once new DNA gets into a bacterial cell,