Ch. 19 Viruses

Lytic cycle steps

1. attachment 2. penetration- entry 3. replication- synthesis of viral genomes and proteins 4. assembly 5. lysis- ruptures host cell

HIV is a retrovirus

2 copies of single-stranded RNA and two molecules of reverse transcriptase Packaged into a capsid, surrounded by an envelope Retroviruses utilize reverse transcription to produce viral DNA that can be integrated into the host cell genome

Structural

All viruses have a capsid (protein coat) but it varies in shape and complexity Some have viral envelope derived from host cell plasma membrane smallest is 20nm

Genome

DNA vs. RNA, Single stranded (ss) vs. Double stranded (ds), linear vs. circular

Digestive system

Hepatitis B virus- viral hepatitis Rotavirus- viral gastroenteritis Norwalk virus- viral gastroenteritis

Skin

Herpes simplex I- cold sores Variola virus- smallpox

Reproductive system

Herpes simplex II- genital herpes Papillomavirus- warts, cervical cancer

Host range

There is limited range of host cells species that each type of virus can infect.

Lysongenic Cycle steps

integration, replication, and excision

Temperate phages

phages that use both the lytic and lysogenic cycles for replication Environmental conditions influence integration and length of latency called lambda

Release

Phages must lyse (rupture) their host cell to escape Enveloped viruses bud from the host cell *Remember that a "phage" (also known as a bacteriophage) only infects bacteria, but viruses infect all life forms.

A new pathogenic form of influenza A can emerge when __________. a mutation occurs in a virus as it passes from one host to another a virus with a novel genetic makeup recombines with viruses that circulate widely among humans there is a concurrent infection of a pig or bird by two or more different strains of influenza A the genomes of two or more influenza A strains mix and match during viral assembly within a host All of the listed responses are correct.

All of the listed responses are correct. a mutation occurs in a virus as it passes from one host to another a virus with a novel genetic makeup recombines with viruses that circulate widely among humans there is a concurrent infection of a pig or bird by two or more different strains of influenza A the genomes of two or more influenza A strains mix and match during viral assembly within a host *Mutations in viral genomes and recombination between different viral strains within a pig or bird host both contribute to the emergence of viral strains that are unfamiliar to the human immune system.

Which of the following, if any, may be a component of a virus? double-stranded (ds) DNA phospholipid bilayer single-stranded (ss) RNA protein All of the listed responses are correct.

All of the listed responses are correct. double-stranded (ds) DNA phospholipid bilayer single-stranded (ss) RNA protein *Viral genomes may consist of double-stranded or single-stranded DNA, double-stranded or single-stranded RNA, and many contain an envelope made up of a phospholipid bilayer.

Entry

Bacteriophages (or phages) injects only DNA into bacteria HIV fuses with host membrane and the entire virus enters One or several viral genes are expressed immediately Virus may proceed to synthesis of viral components OR integrate into host chromosome

Brain and CNS

Flavivirus- yellow fever Rhabdovirus- rabies

Creutzfeldt-Jakob disease

Rapidlly progressive chronic dementia, fatal disease of the central nervous system caused by a prion. 175 deaths in UK since 1996

HIV reproductive cycle

Reverse transcriptase lacks a proofreading function Makes more errors and tends to create mutant strains of HIV

Latency

Some viruses can integrate their genomes into a host chromosome Prophage or provirus is inactive or latent Most viral genes silenced during latency

Transduction

When the new viruses are assembled, coat proteins occasionally surround a piece of bacterial DNA instead of phage genetic material Inserts the bacterial DNA fragment into the recipient cell, which then incorporates this fragment into its own chromosome Mechanism of transduction is actually an error in a phage lytic cycle

Arrange the following steps in a viral reproductive cycle in order from first to last. Release Synthesis Assembly Attachment Entry Integration

__1__ Attachment __2__ Entry __3__ Integration __4__ Synthesis __6__ Release

Viruses that infect bacteria are called __________. retroviruses proviruses bacteriophages capsomeres bacterioviruses

bacteriophages Viruses that infect bacteria are called bacteriophages or simply phages.

To cause a human pandemic, the H5N1 avian flu virus would have to become much more pathogenic. develop into a virus with a different host range. spread to primates such as chimpanzees. arise independently in chickens in North and South America. become capable of human-to-human transmission.

become capable of human-to-human transmission.

Plant viruses spread throughout the plant by way of __________. (Concept 19.3) SHOW HINT the lymphatic system plasmodesmata chloroplasts central vacuoles vertical transmission Plasmodesmata connect the cytoplasm of different cells and allow nutrients and fluids to move throughout the plant. Viruses can spread through these connections.

plasmodesmata *Plasmodesmata connect the cytoplasm of different cells and allow nutrients and fluids to move throughout the plant. Viruses can spread through these connections.

Capsid

protein coat enclosing the viral genome a virus different shapes, rod-shaped, polyhedral, or more complex

Question 24: Birds act as a natural __________ for the influenza __________ virus. buffer ... B pandemic ... B defense ... A reservoir ... C reservoir ... A

reservoir ... A *Birds are a major reservoir for influenza A, and their ability to migrate all over the world is a major factor in the spread of avian flu.

Prophage

the viral DNA that is embedded in the host cell's, when integrated into the bacterial chromosome

Immune system

Rubella virus- measles Human immunodeficiency virus- AIDS Epstein-Barr virus- mononucleosis

Viral infection steps

1) Attachment 2)Entry 3)Integration 4)Synthesis of viral component 5)Assembly 6)Release

In the lysogenic cycle _____. host DNA is destroyed and viral DNA is replicated a bacterium replicates without passing viral DNA to its daughter cells viral DNA is destroyed and host DNA is replicated a bacterium divides once before the lytic cycle is initiated viral DNA is replicated along with host DNA

viral DNA is replicated along with host DNA

A prophage is a(n) __________. viral genome that has been incorporated into a bacterial cell's chromosome emerging virus prion that has been integrated into a bacterial cell's chromosome virus that infects other viruses type of retrovirus

viral genome that has been incorporated into a bacterial cell's chromosome

Restriction enzymes

restricts a phage's ability to replicate in a bacterium, by identifying the foreign DNA and cutting it up by cellular enzymes

A bacterium is infected with an experimentally constructed bacteriophage composed of the T2 phage protein coat and T4 phage DNA. The new phages produced would have T2 protein and T4 DNA. a mixture of the DNA and proteins of both phages. T2 protein and T2 DNA. T4 protein and T2 DNA. T4 protein and T4 DNA.

T4 protein and T4 DNA

A microbiologist analyzes chemicals obtained from an enveloped RNA virus that infects monkeys. He finds that the viral envelope contains a protein characteristic of monkey cells. Which of the following is the most likely explanation? The virus fools its host by mimicking its proteins. Its presence is a result of the monkey's immunological response. The virus is a prophage. The virus forced the monkey cell to make proteins for its envelope. The viral envelope forms as the virus leaves the host cell.

The viral envelope forms as the virus leaves the host cell. An enveloped virus is wrapped in host plasma membrane or nuclear membrane (which contains host proteins).

Restriction enzymes help defend bacteria against viral infections by __________. preventing the binding of the virus to the cell surface cutting viral DNA once it has entered the cell preventing entry of the viral DNA into the cell preventing integration of the viral genome into the host chromosome preventing the synthesis of viral capsomeres in the cell

cutting viral DNA once it has entered the cell

Scrapie (Prion disease)

disease in sheep and pigs - intense itching, animals scrape against trees and other objects, followed by neurodegenration

Assembly

Some viruses self-assemble Other are too complicated to self-assemble Proteins modify capsid proteins or serve as scaffolding

Vaccine

A harmless variant or derivative of a pathogen that stimulates a host's immune system to mount defenses against the pathogen

Viral envelope

A membrane, derived from membranes of the host cell, that cloaks the capsid, which in turn encloses a viral genome.

Lysogenic cycle

A phage replication cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host.

Lytic cycle

A type of viral (phage) replication cycle which copies a virus are made within a host cell, resulting in the release of new phages by lysis (and death) of the host cell, by bursting open, releasing new viruses.

Bacteriophages (phages)

A virus that infects bacteria most complex capsids are found among these viruses

Which one of the following statements about human immunodeficiency virus is incorrect? It is widely accepted to be the causative agent of AIDS. It is an example of an emerging virus. It can be transmitted from mother to unborn child. Infection ultimately leads to susceptibility to opportunistic infections. All of the above statements about HIV are correct.

All of the above statements about HIV are correct. It is widely accepted to be the causative agent of AIDS. It is an example of an emerging virus. It can be transmitted from mother to unborn child. Infection ultimately leads to susceptibility to opportunistic infections.

Emerging viruses can originate from which of the following sources? (Concept 19.3) SHOW HINT animal viruses the mutation of existing human viruses viruses previously confined to small, isolated populations that can now spread due to technological or social changes such as the development of affordable international travel All of the listed responses are correct. None of the listed responses is correct.

All of the listed responses are correct. animal viruses the mutation of existing human viruses viruses previously confined to small, isolated populations that can now spread due to technological or social changes such as the development of affordable international travel

In the lysogenic cycle of phages __________. the viral nucleic acid inserts itself into the host chromosome only a small number of the viral genes are expressed the nucleic acid core of the phage is all that enters the host cell the viral nucleic acid is replicated along with the host DNA All of the listed responses are correct.

All of the listed responses are correct. the viral nucleic acid inserts itself into the host chromosome only a small number of the viral genes are expressed the nucleic acid core of the phage is all that enters the host cell the viral nucleic acid is replicated along with the host DNA

The symptoms of a viral infection in a person can be caused by __________. toxic viral components, such as envelope proteins the death of infected cells the production of toxins by infected cells the reaction of the individual's immune system to the infection All of the listed responses are correct.

All of the listed responses are correct. toxic viral components, such as envelope proteins the death of infected cells the production of toxins by infected cells the reaction of the individual's immune system to the infection

Which of the following events occurs during the process of transcription? An mRNA attaches to a ribosome. A polypeptide is synthesized. DNA is replicated. An RNA is synthesized.

An RNA is synthesized. *Transcription is the synthesis of RNA using the information in DNA. Translation is the synthesis of a polypeptide using the information in mRNA. The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains.

Prions

Infectious proteins that cause several brain diseases Composed entirely of protein Proteinaceous infectious agent Prion converts normal proteins to abnormal conformation Cause several types of neurodegenerative brain diseases in humans and livestock Group of diseases called transmissible spongiform encephalopathies (TSE)

Which of the following is an example of vertical transmission of a virus in plants? Viral particles are carried from one plant to another by a pair of pruning shears. Viral particles are carried by the wind from one plant to another. Two neighboring plants touch each other, allowing viruses present in one plant to infect the other plant. An infected plant produces seeds that contain the virus, giving rise to infected progeny. All of the listed responses are correct.

An infected plant produces seeds that contain the virus, giving rise to infected progeny. *Vertical transmission refers to the spread of a virus from parent to offspring.

Which statement below is a correct comparison of a "regular" RNA virus and an RNA retrovirus? Only the RNA retrovirus performs translation. Only RNA retroviruses produce DNA using DNA replicase. Only the regular RNA virus performs transcription. Both produce protein coats via translation of mRNA. Only the regular RNA virus produces DNA from an RNA template.

Both produce protein coats via translation of mRNA.

Viroids infect Plants

Composed solely of a single-stranded circular RNA molecule a few hundred nucleotides long Infect plant cells Some replicate in host cell nucleus, others in chloroplast RNA genome does not code for proteins Cause abnormal development, stunted growth

Blood

Ebola virus- hemorrhagic fever Hantavirus- hemorrhagic fever with renal syndrome

Which one of the following statements about prions is incorrect? Prions induce wild-type proteins to fold incorrectly. Exposure to nucleic acid-damaging radiation renders a prion uninfectious. Prions are widely accepted to be the causative agent of spongiform encephalopathies. Misfolded prion proteins can spread through the body after being excreted out of a cell and into the bloodstream. All of the above statements about prions are correct.

Exposure to nucleic acid-damaging radiation renders a prion uninfectious.

What is the function of hemagglutinin in the influenza virus? Hemagglutinin is the name of the reverse transcriptase enzyme in the influenza virus. Hemagglutinin is the protein that helps the influenza virus attach to host cells. Hemagglutinin helps release new viruses from infected cells. Hemagglutinin is involved in assembling the membrane envelope that the virus uses as a cloak when it leaves an infected cell. Hemagglutinin is part of the protein capsid of the influenza virus

Hemagglutinin is the protein that helps the influenza virus attach to host cells. *There are 16 types of hemagglutinin in various strains of influenza, but they all function in recognition and attachment.

Why can flare-ups of herpesvirus infection recur throughout a person's life? Herpesvirus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores. The herpesvirus becomes a prophage during the lysogenic cycle of the infection, so the viral genome is recombined with the host DNA. Herpesvirus is very common in the environment, so it is easy for a person to become re-infected. Herpesvirus may cloak itself in a cell's nuclear envelope, making it very difficult for the immune system to recognize it. The herpesvirus always kills its host cell when it leaves. Cycles of cell death cause the recurring flare-ups.

Herpesvirus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores. *Herpesvirus remains in a person's cells virtually for their whole lifetime, so flare-ups are always a possibility.

Synthesis

Host cell enzymes such as DNA polymerase make many copies of the phage DNA and transcribe the genes within these copies into mRNA

AIDS and HIV

Human immunodeficiency virus (HIV) is the causative agent of acquired immune deficiency syndrome (AIDS) It is an enveloped animal virus with a single-stranded RNA genome AIDS is primarily spread by sexual contact between infected and uninfected individuals Can also be spread by the transfusion of HIV-infected blood, by the sharing of needles among drug users, and from mother to unborn child AIDS results from viral destruction of helper T cells in the immune system Individual becomes highly susceptible to opportunistic infections nWould not normally occur in a healthy person

Respiratory tract

Influenza virus- flu Rhinovirus- common cold

What is the function of the HIV viral enzyme, reverse transcriptase? It catalyzes the formation of a polypeptide from an RNA template. It catalyzes the formation of RNA from an RNA template. It catalyzes the formation of RNA from a DNA template. It catalyzes the formation of DNA from a polypeptide template. It catalyzes the formation of DNA from an RNA template.

It catalyzes the formation of DNA from an RNA template.

What is the origin of the phospholipid membrane that envelops many animal viruses? It is produced by viral enzymes and contains only viral proteins and phospholipids. It is assembled from free phospholipids floating in the cell's cytoplasm. It is "stolen" from the host cell and contains only host cell proteins and phospholipids. It is assembled in the ER based on signals sent out by the viral genome. It is "stolen" from the host cell, but it contains some proteins encoded by the viral genome.

It is "stolen" from the host cell, but it contains some proteins encoded by the viral genome. *Newly formed viruses "cloak" themselves in phospholipid membrane derived from the host, but certain components encoded by the viral genome are also included in the envelope.

Which of the following can a virus do without a host cell? produce nucleotides for use in replication and transcription produce ATP for energy synthesize proteins transcribe DNA

None of the listed responses is correct. Viruses are obligate intracellular parasites. They require a host cell to carry out all of the above cellular processes (and more).

Which one of the following statements about plasmids is correct? Some plasmids approach the size of an E. coli chromosome. Replication of plasmid DNA is initiated from the origin of replication on the bacterial chromosome. Plasmids are typically not essential for survival in wild-type conditions. Plasmids confer no selective advantage on the cells carrying them. A plasmid that can integrate into the bacterial chromosome is most accurately referred to as a col-plasmid.

Plasmids are typically not essential for survival in wild-type conditions.

Reverse transcription, carried out by retroviruses, is the process by which __________. DNA is duplicated information is copied from a protein molecule into RNA DNA information is copied into RNA RNA information is copied into DNA RNA information is "read" to form a protein molecule right answer feedback: Correct. Retroviruses are equipped with an enzyme called reverse transcriptase, which makes a DNA copy of an RNA template, providing information flow from RNA to DNA.

RNA information is "read" to form a protein molecule *Retroviruses are equipped with an enzyme called reverse transcriptase, which makes a DNA copy of an RNA template, providing information flow from RNA to DNA.

When comparing DNA and RNA viruses, which mutate more quickly, and why? DNA viruses, because they usually have larger genomes and thus more loci for mutations to occur RNA viruses, because no proofreading is done on RNA molecules RNA viruses, because RNA is single-stranded and thus more prone to mutations DNA viruses, because the host cell's proofreading enzymes do not work on viral DNA They all mutate at the same rate.

RNA viruses, because no proofreading is done on RNA molecules

How do retroviruses, such as HIV, differ from other viruses? They can transcribe a DNA copy from a RNA template. They can reproduce only inside living cells. They have much simpler reproductive cycles than other RNA viruses. They contain nucleic acids that code for proteins. They contain DNA that is used as a template to make RNA.

They can transcribe a DNA copy from a RNA template. * Retroviruses contain the enzyme reverse transcriptase, which transcribes DNA from an RNA template, which is opposite of the usual direction of information flow.

Question 2: What determines whether two proteins will bind to each other? They will bind to each other if they are identical or similar proteins. They will bind to each other if they are located in the same region of the cell. They will bind to each other if they have complementary shapes and chemical properties. They will bind to each other if one protein has an overall positive charge and one protein has an overall negative charge.

They will bind to each other if they have complementary shapes and chemical properties. *A protein's specific structure determines how it works, and in almost every case, the function of a protein depends on its ability to recognize and bind to some other molecule. This fit is very specific, something like a lock and key, and depends on both the shape and the overall chemical properties of the regions where the proteins might bind to each other.

First virus discovered

Tobacco Mosaic Virus (TMV)

Prophage- HIV is an RNA virus

Uses viral reverse transcriptase to make complementary DNA strand that will be template for double stranded viral DNA Integrates as a provirus into the host cell's genome

Attachment

Usually specific for one kind of cell due to binding to specific molecules on cell surface

Integration

Viral gene for integrase enzyme Integrase cuts host chromosomal DNA and inserts viral genome Phage integrated into the bacterial DNA is called a prophage -May excise later and proceed to synthesis

The genetic material of a virus may consist of any of the following except: single-stranded DNA double-stranded DNA single-stranded RNA double-stranded RNA a combination of DNA and RNA (single- or double-stranded)

a combination of DNA and RNA (single- or double-stranded)

Viruses

a nonliving infectious particle consisting of genes (genetic material), nucleic acid surrounded by a protein coat. requires a host cell

Prions are __________ that are thought to cause disease by __________. abnormally shaped proteins ... inducing similar but normally shaped proteins in the brain to adopt the abnormal form an abnormal type of capsid ... dramatically enhancing the rate of viral infection DNA molecules ... jumping around the genome and mutating genes RNA molecules ... encoding toxic proteins mutant DNA molecules ... encoding toxic proteins

abnormally shaped proteins ... inducing similar but normally shaped proteins in the brain to adopt the abnormal form *The abnormally shaped proteins are thought to clump together, causing microscopic holes in the brain that are ultimately fatal.

Emerging viruses arise by the spread of existing viruses to new host species. all of the above mutation of existing viruses. none of the above the spread of existing viruses more widely within their host species.

all of the above the spread of existing viruses to new host species. mutation of existing viruses. none of the above the spread of existing viruses more widely within their host species.

Mad cow disease (Prion disease)

found in cow (and cow meat) changes in posture and temperament, loss of coordination and neurodegeneration

Which of the following characteristics, structures, or processes is common to both bacteria and viruses? independent existence ribosomes cell division genetic material composed of nucleic acid metabolism

genetic material composed of nucleic acid

Vaccines for viral diseases are __________ and help prevent infection by __________. antibiotic formulations ... killing bacteria that assist viruses in infecting animal cells harmless derivatives of pathogenic viruses ... stimulating the immune system to mount a defense against the actual pathogen nucleoside inhibitors ... inhibiting the replication of the viral genome antibiotic formulations ... specifically killing infected cells protease inhibitors ... preventing synthesis of envelope proteins

harmless derivatives of pathogenic viruses ... stimulating the immune system to mount a defense against the actual pathogen

RNA viruses require their own supply of certain enzymes because these enzymes penetrate host cell membranes. host cells rapidly destroy the viruses. host cells lack enzymes that can replicate the viral genome. these enzymes cannot be made in host cells. these enzymes translate viral mRNA into proteins.

host cells lack enzymes that can replicate the viral genome.

Kuru (Prion disease)

human disease once common in native tribes of New Guinea, from eat dead brain with infection (ritual cannibalism) - loss of coordination, dementia, end in 1958

Provirus

integrated viral DNA that never leaves the host's genome, remaining as a resident

The avian flu virus H5N1 is considered a greater long-term threat than the swine flu virus H1N1 because __________. it is more readily transmitted between human hosts it has a significantly higher mortality rate more people eat chicken meat than pork H5N1 is a DNA virus and the similarity of genome chemistry can promote greater virulence and therefore the likelihood of death in humans. H5N1 has a greater tendency to become a prophage

it has a significantly higher mortality rate *Although transmission between humans appears to be rare, the mortality rate among those who contract the virus from birds is close to 50%, whereas the rate for H1N1 is about 1%. A mutation and/or recombination event could transform H5N1 into a virus that could be transmitted between humans.

A phage that inserts itself into the host DNA is called __________. lysogenic semipermeable a bacteriophage a bud a capsomere

lysogenic

The phage reproductive cycle that kills the bacterial host cell is a __________ cycle, and a phage that always reproduces this way is a __________ phage. lysogenic ... temperate lytic ... virulent virulent ... lytic lysogenic ... virulent lytic ... lysogenic

lytic ... virulent

When a virus infects an E. coli cell, what part of the virus enters the bacterial cytoplasm? the tail fibers the protein capsid and enclosed nucleic acid the entire virus only the nucleic acid the protein capsid only

only the nucleic acid *After attaching to receptors on the surface of the bacterial cell, the virus injects its DNA into the cell.

A viral particle which has been integrated into a bacterial genome is most accurately referred to as a: phage. viroid. prophage. provirus. plasmid.

prophage.

Lytic cycle process

synthesis, assembly, and release

Synthesis- HIV

the DNA provirus is not excised from the host chromosome. Instead, it is transcribed in the nucleus to produce many copies of viral RNA Translated to make viral proteins Serve as genome for new viral particle

In the lytic life cycle of phages __________. DNA replication is not part of the life cycle the entire phage is taken into the bacterium the viral capsid is assembled according to the genetic information of the bacterium the cell typically dies, releasing many copies of the virus phage DNA is incorporated into the host cell's genome

the cell typically dies, releasing many copies of the virus

Viruses can vary with respect to all of the following characteristics EXCEPT __________. DNA or RNA as the genetic material the type of host cell it can infect the presence or absence of a membranous envelope the presence or absence of metabolic machinery single- or double-stranded nucleic acids

the presence or absence of metabolic machinery Viruses are not classified by the presence or absence of metabolic machinery, because viruses do not contain metabolic machinery.

HIV, the virus that causes AIDS, only infects certain cells within the immune system. This is because __________. infection requires the presence of a specific DNA sequence that is only present in the genome of certain immune system cells the virus binds to specific receptors that are only present on certain immune cells other cells produce toxins that destroy the virus before infection can take place the virus is not very mobile within the body and only comes into contact with a limited number of immune cells the virus gets into all cells, but the viral RNA is immediately destroyed in all but a small number of immune system cells

the virus binds to specific receptors that are only present on certain immune cells *The virus binds to receptors such as CD4 that are only present on certain cells such as helper T cells.

The arrow is pointing to the _____. (head of virus) bacterium's plasma membrane viral protein coat bacterium's nucleic acid genome viral nucleic acid genome viral envelope

viral protein coat

Circular RNA molecules that function like a virus in plants are termed __________. viroid prion rhabdovirus retrovirus None of the listed responses is correct.

viroid

RNA particles that infect plant cells are most accurately referred to as: phage. retroviruses. viroids. prions. lysogens

viroids.

Virulent phages

virus reproduces through the lytic cycle only, always kill the host cells they enter a lytic cycle immediately

Which of the following clues would tell you if a cell is prokaryotic or eukaryotic? whether or not the cell carries out cellular metabolism whether or not the cell is partitioned by internal membranes the presence or absence of ribosomes the presence or absence of a rigid cell wall

whether or not the cell is partitioned by internal membranes *All cells are bounded by a selective barrier called the plasma membrane, and all cells have ribosomes, tiny complexes that make proteins according to the instructions from DNA. Eukaryotic cells, however, are generally much larger than prokaryotic cells. In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, whereas in prokaryotic cells, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid.