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9.1: Viral Morphology - Biology

9.1: Viral Morphology - Biology


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Learning Objectives

Discuss the basics of virus structure

Viruses are acellular, meaning they are biological entities that do not have a cellular structure. They therefore lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane. Viruses are sometimes called virions: a virion is a ‘complete’ virus free in the environment (not in a host). A virion consists of at least a nucleic acid core and an outer protein coating or capsid; sometimes a virus will have an outer envelope made of protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins, such as enzymes. The most obvious difference between members of viral families is their morphology, which is quite diverse. An interesting feature of viral complexity is that the complexity of the host does not correlate with the complexity of the virion. Some of the most complex virion structures are observed in bacteriophages, viruses that infect the simplest living organisms, bacteria.

Types of Nucleic Acid

Unlike nearly all living organisms that use DNA as their genetic material, viruses may use either DNA or RNA as theirs. The virus core contains the genome or total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot get from the host cell. This genetic material may be single- or double-stranded. It may also be linear or circular.

DNA viruses cause human diseases, such as chickenpox, hepatitis B, and some venereal diseases, like herpes and genital warts. Human diseases caused by RNA viruses include hepatitis C, measles, and rabies.

Morphology

Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family. All virions have a nucleic acid genome covered by a protective layer of proteins, called a capsid. The capsid is made up of protein subunits called capsomeres. Some viral capsids are simple polyhedral “spheres,” whereas others are quite complex in structure.

Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors (Figure 1).

Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.

Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification (Figure 2).

Practice Question

Which of the following statements about virus structure is true?

  1. All viruses are encased in a viral membrane.
  2. The capsomere is made up of small protein subunits called capsids.
  3. DNA is the genetic material in all viruses.
  4. Glycoproteins help the virus attach to the host cell.

[reveal-answer q=”243497″]Show Answer[/reveal-answer]
[hidden-answer a=”243497″]Statement d is true.[/hidden-answer]


9.1: Viral Morphology - Biology

Viruses are diverse entities. They vary in their structure, their replication methods, and in their target hosts. Nearly all forms of life—from bacteria and archaea to eukaryotes such as plants, animals, and fungi—have viruses that infect them. While most biological diversity can be understood through evolutionary history, such as how species have adapted to conditions and environments, much about virus origins and evolution remains unknown.

Learning Objectives

  • Describe how viruses were first discovered and how they are detected
  • Discuss three hypotheses about how viruses evolved
  • Recognize the basic shapes of viruses
  • Understand past and emerging classification systems for viruses

Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA - never both - and they replicate using the replication protein of a host cell.

According to hypothetic suggestion viruses generates from free-living cells, originated from DNA and RNA molecules that depart from host cell. Viral morphology included that viruses are acellular, having no cellular structure, consists of nucleic acid core along with an external coating of protein called capsid, these capsid are further divided into four categories: head-and-tail, helical, icosahedral, enveloped.

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How They Have Been Used during Recent Disease Outbreaks

Scientists began creating viral vectors in the 1970s. Besides being used in vaccines, viral vectors have also been studied for gene therapy, to treat cancer, and for molecular biology research. For decades, hundreds of scientific studies of viral vector vaccines have been done and published around the world. Some vaccines recently used for Ebola outbreaks have used viral vector technology, and a number of studies have focused on viral vector vaccines against other infectious diseases such as Zika, flu, and HIV.


Genome Structure

Coronaviruses belong to the Nidovirales family, which is characterized by an enveloped, positive-sense RNA virus [2]. Compared to other viruses, coronaviruses have a relatively long genome of 30 kilobases [2]. Twenty of these kilobases encode non-structural proteins, and the remaining ten of these kilobases are responsible for structure and accessory functions. The genome for coronaviruses also includes a 5’ cap and 3’ poly-A tail, which allow the virus to act as an mRNA molecule to translate replicase polyproteins [2]. The function of accessory proteins is not entirely known, but one possible hypothesis is that they are used for viral pathogenesis [2].

Further research shows some coronaviruses lack exoribonuclease activity, which can limit their ability to infect a host cell [8]. Microorganisms without exoribonuclease activity are able to increase the rate of mutations compared to microorganisms with exoribonuclease activity [8]. Furthermore, coronaviruses lacking exoribonuclease activity are less able to invade host organisms [8]. Exoribonuclease expression in the genome is critical in order for the virus to replicate therefore, the genes for expression of exoribonuclease may be a viable target for inhibition in future vaccinations for coronavirus [8].


DISCUSSION

In this paper we have developed the use of positively charged Nanogold to follow the endocytic pathway in yeast at the ultrastructural level. Positively charged Nanogold fulfills all of the criteria for an endocytic marker. Its internalization is time-, temperature-, and energy-dependent, and it is only found on the cell surface or within membrane-bound internal compartments. Its internalization is greater than 20-fold reduced in the end3mutant, which has previously been shown to be required for the internalization of a number of endocytic markers, including a fluid phase marker, lucifer yellow CH (Riezman, 1985), a membrane phase marker, FM4–64 (Vida and Emr, 1995 Wendland et al., 1996), as well as several specific markers, including α-factor, pheromone receptors, and permeases (Davis et al., 1993 Raths et al., 1993 Volland et al., 1994 Lai et al., 1995). Positively charged Nanogold is a nonspecific probe that has several advantages as a marker for absorptive endocytosis. It binds strongly and evenly to the surface of spheroplasts, and it can be used to generate an electron-dense aggregate that can be visualized in the electron microscope even though it is, itself, relatively small, and cannot be degraded, allowing its visualization throughout the endocytic pathway, including after delivery to the vacuole. Cationized ferritin has been used to follow endocytosis in yeast spheroplasts (Wendlandet al., 1996). This marker is also electron dense however, it is much larger than positively charged Nanogold and is subject to degradation, introducing another variable into the experimental conditions. Therefore, the use of positively charged Nanogold should be of general use in the analysis of endocytic compartments and mutants.

The use of positively charged Nanogold and mutant cells has allowed us to visualize and confirm the existence of distinct organelles along the endocytic pathway, including primary endocytic vesicles, a vesicular/tubular structure that we term an early endosome, late endosomes, and vacuoles. As in animal cells, there are two biochemically and morphologically distinct organelles in the endocytic pathway in addition to the primary endocytic vesicles. Overall, it is remarkable that the endocytic organelles from yeast appear quite similar to those described for endocytic organelles in animal cells. The late endosome, which has been visualized previously using antibodies to the α-factor receptor (Hicke et al., 1997), is a multivesicular compartment similar to that found for animal cells (McDowall et al., 1989). The organelle is rather large, being one of the more conspicuous organelles in yeast after the nucleus and vacuole. After fixation, embedding, and visualization, the lumen of the organelle contains internal membranes but is not very electron dense, consistent with its distribution in low-density fractions in equilibrium density gradients (Singer-Krüger et al., 1993). The late endosome is found most frequently, but not exclusively, close to the vacuole, and profiles can sometimes be seen where the late endosome may be fusing with the vacuole, but further analysis will be necessary to address this point. It is interesting to note that most of the labeling was found on the internal structures of the late endosome, suggesting that the gold bound to plasma membrane molecules and early endosomes finds it way into the interior of this organelle. We cannot be certain, however, that these membranes are really internal. We cannot rule out that they are the result of multiple infoldings of the outer membrane of the structure, even though we did not detect any such infoldings in the multiple structures we examined.

At early time points of incubation with positively charged Nanogold at 15°C we could detect a labeled vesicular/tubular organelle. The low-temperature incubations allowed us to demonstrate that appearance of positively charged Nanogold in this structure preceded its appearance in the late endosome. This structure is not generated due to the low temperature incubations because it was also easily seen after internalization experiments at 30°C (our unpublished observations) and 32°C (Figure 9). This structure may be one of the morphological counterparts of biochemically defined early endosomes (Singer-Krüger et al., 1993) because the structure is labeled early during time course incubations and is found mainly in the cell periphery in accordance with immunofluorescence studies (Hickeet al., 1997). This structure resembles early endosomes from animal cells. In animal cells early endosomes have been postulated to be an interconnected structure with tubular and vesicular components (Hopkins et al., 1990 Stoorvogel et al., 1996). The vesicular components in animal cells are sometimes coated with clathrin (Stoorvogel et al., 1996). Our early endosomal profiles could be very similar because we found both tubular and vesicular structures that were consistently associated with each other. Even if these components are not actually interconnected, there must be some underlying structure because the profiles seen are fairly distinct and well ordered. The gold labeling of these structures was found in both the tubular and vesicular parts but was more often associated with the tubular components. Some of the vesicular profiles were clearly unlabeled. This would be expected if some of the vesicular profiles resulted from incoming vesicles from the secretory pathway, e.g., from the trans-Golgi. Endocytic traffic from early to late endosomes requires a continuous input from the secretory pathway (Hicke et al., 1997) therefore, one would expect such incoming traffic.

Putative Primary Endocytic Vesicles

Putative primary endocytic vesicles were visualized with the aid of the sec18 mutant. In sec18 cells, only small uniform vesicles of approximately 30–50 nm diameter were seen labeled with positively charged Nanogold at nonpermissive temperature. These vesicles were similar in size to other vesicles that accumulated in thesec18 mutant (Figure 11) (Kaiser and Schekman, 1990). Several arguments suggest that these are primary endocytic vesicles. First, these were the only labeled structures that we detected in sec18 spheroplasts at nonpermissive temperature, and they accumulated with time at nonpermissive temperature in the mutant spheroplasts. In wild-type cells at the same temperature all of the above described endocytic structures were labeled efficiently. If the positively charged Nanogold had reached the early endosome in the sec18 spheroplasts we would have detected it as we did in wild-type cells after similar incubation periods. Second, the sec18 block represents the first known postinternalization block in endocytosis. This was concluded from the following experiments. When α-factor was internalized in the sec12 mutant, it accumulated in biochemically defined early endosomes (Hicke et al., 1997). Detection of the α-factor receptor by immunofluorescence under similar conditions in sec12 cells revealed that the receptor was concentrated in peripheral, relatively large punctate structures. Similar experiments using the sec18 mutant showed accumulation of the α-factor receptor in smaller, peripheral dots by immunofluorescence easily distinguishable from the structures accumulated in sec12 cells. A double mutant (sec12 sec18) showed a sec18 phenotype. These data showed that the small, peripheral dots that accumulated in sec18 cells were epistatic to and preceded the large peripheral compartment, which fractionated like the biochemically defined early endosomes. The small peripheral dots seen in sec18 cells, therefore, are likely precursors of the early endosomes seen by immunofluorescence insec12 cells and must correspond to the vesicles seen here insec18 spheroplasts because they contain all of the internalized label. Third, mammalian Sec18p has been shown to associate with endocytic clathrin- coated vesicles, and N-ethylmaleimide, an inhibitor of Sec18p, blocks fusion of clathrin-coated vesicles (Woodman and Warren, 1991 Steel et al., 1996). Finally, the vesicles seen in sec18 spheroplasts resemble the first endocytic intermediates observed using wild-type spheroplasts at 15°C.

Fig. 11. Putative primary endocytic vesicle profiles. Profiles of putative primary endocytic vesicles were taken from sec18 spheroplasts incubated for 30 min at 32°C with positively charged Nanogold.

The molecular requirements for the internalization step of endocytosis in yeast show similarities to several different types of endocytic internalization seen in mammalian cells, but the similarities are not close enough to any of them to be certain of a true homology in mechanism (Riezman et al., 1996). For instance, mutations in clathrin affect the internalization of ligands in both cell types, but in yeast this effect is only partial. On the other hand, actin is absolutely required for endocytosis in yeast, whereas actin depolymerization in animal cells using cytochalasin D has been proposed to affect clathrin-dependent internalization from apical, but not basolateral, plasma membrane (Gottlieb et al., 1993 Jackmanet al., 1994). This does not rule out that actin is required for clathrin-mediated endocytosis at the basolateral surface because cytochalasin D may not depolymerize all cellular actin equally. In fact, a role for actin in all clathrin-mediated endocytosis has been proposed recently (Lamaze et al., 1997). Actin is also required for two other types of endocytic internalization in animal cells: induced internalization through caveolae (Parton et al., 1994) and phagocytosis (Greenberg et al., 1991). A clear distinction between the clathrin or caveolar uptake and phagocytic uptake is the size of the primary endocytic vesicles. For this reason it was important to identify the primary endocytic vesicle in yeast.

The size and regularity of the putative primary endocytic vesicles described here would be most consistent with their being derived through a coat-dependent mechanism, rather than a solely actin-based, phagocytic-like mechanism. The size of the primary endocytic structure determined by the latter mechanism depends upon the size of the particle being internalized, not upon the dimensions of an assembled coat structure, such as clathrin coats or caveolar coats. It is hard to imagine how actin could generate small, uniform vesicles independent of a coat protein. In yeast, no protein with clear sequence homology to caveolin is present therefore, a caveolin homologue apparently plays no role in this event. On the other hand, clathrin coats could participate in endocytic internalization because clathrin mutants show a 50% block in this step. One possible role for clathrin in endocytosis that would be consistent with the partial block could be the regulation of the size of the endocytic vesicle and/or the recruitment of receptors into internalization structures. The precise role of clathrin in the process of internalization will have to await further experimentation and would benefit greatly from the detection of the internalization structures. Hopefully, some of the endmutants that affect the internalization step of endocytosis will be useful for this.


Question 1.
In tamarind, the pinnate leaf is
(a) bipinnate
(b) tripinnate
(c) paripinnate
(d) imparipinnate

Question 2.
Artabotrys is a hook climber in which the hooks are modified
(a) inflorescence axis
(b) petiole
(c) roots
(d) stipules

Answer: (a) inflorescence axis

Question 3.
Exceptional roots of Cuscuta are
(a) haustorial
(b) coralloid
(c) mucorhizal
(d) all of the above

Question 4.
The petiole is swollen and spongy in
(a) Nepenthes
(b) Trapa
(c) Clematis
(d) all of the above

Question 5.
Which one of the following plants dose not have root – pockets
(a) Pistia
(b) Lemna
(c) Ficus
(d) Eichhornia

Question 6.
The largest petal overlaps the lateral ones in _________ aestivation.
(a) Papilionaceous
(b) Valvate
(c) Twisted
(d) Imbricate

Answer: (a) Papilionaceous
Explanation:
In pea and bean flowers, there are five petals, the largest overlaps the two lateral petals which in turn overlaps the two smallest anterior petals. This is called Vexillary or papilionaceous aestivation.

Question 7.
In Lathyrus, the leaves are modified into
(a) thorns
(b) cladodes
(c) tendrils
(d) spines

Question 8.

Label the parts of a monocot seed.
(a) A.Endosperm, B. Scutellum, C. Radicle, D. Coleoptile, E. Plumule
(b) A.Endosperm, B. Coleoptile, C. Scutellum, D. Radicle, E. Plumule
(c) A.Endosperm, B. Scutellum, C. Coleoptile, D. Radicle, E. Plumule
(d) A.Endosperm, B. Scutellum, C. Coleoptile, D. Plumule, E. Radicle

Answer: (d) A.Endosperm, B. Scutellum, C. Coleoptile, D. Plumule, E. Radicle
Explanation:

Question 9.
Potato tuber is an underground stem because
(a) it lacks chlorophyll
(b) it is swollen
(c) it possesses axillary buds
(d) it stores starch as reserve food material

Answer: (c) it possesses axillary buds

Question 10.
Floating roots are characteristic of
(a) Tinospora
(b) Jussiaea
(c) Viscum
(d) Vanda

Question 11.
Thors and spines are
(a) homologous organs
(b) analogous organs
(c) thron is homologous while spine is analogous
(d) spine is homologous while is analogous

Question 12.
Which of the following picture shows fibrous root system?

Question 13.
In which of the following plants stems do not store the food material
(a) potato
(b) ginger
(c) onion
(d) colocasia

Question 14.
In banana, the stem is
(a) underground only
(b) both underground and aerial
(c) absent
(d) aerial only

Answer: (a) underground only

Question 15.
Srem modified to perform the function of a leaf and having many internodes is called as
(a) phylloclade
(b) cladode
(c) offset
(d) phyllode

Question 16.
Certain plants called as runners can be easily propagated. This is due to the fact
(a) they are numerous in numbers
(b) they store the ready food
(c) they lie horizontally on the soil
(d) they can produce adventitious roots quite readily at the nodes

Answer: (d) they can produce adventitious roots quite readily at the nodes

Question 17.
The coloured part of a Bougainuillea flower is the
(a) corolla
(b) calyx
(c) bracts
(d) androecium

Question 18.
The axillary buds arise
(a) endogenously from the pericycle
(b) exogenously from the tissues of the shoot apex
(c) endogenously from the cambium tissues
(d) exogenously from the innermost layers of cortex

Answer: (b) exogenously from the tissues of the shoot apex

Question 19.
The arrangement of sepals or petals in floral bud is called
(a) Placentation
(b) Aestivation
(c) Bracteate
(d) Phyllotaxy

Answer: (b) Aestivation
Explanation:
The mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl is known as aestivation.

Question 20.
The term phyllotaxy is used to describe the
(a) type of ovary in a plant
(b) mode of arrangement of leaves
(c) type of roots
(d) arrangement of sepals and petals

Answer: (b) mode of arrangement of leaves

We hope the given NCERT MCQ Questions for Class 11 Biology Chapter 5 Morphology of Flowering Plants with Answers Pdf free download will help you. If you have any queries regarding CBSE Class 11 Biology Morphology of Flowering Plants MCQs Multiple Choice Questions with Answers, drop a comment below and we will get back to you soon.


The Molecular Biology of Coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.


BBLN-1 is essential for intermediate filament organization and apical membrane morphology

Epithelial tubes are essential components of metazoan organ systems that control the flow of fluids and the exchange of materials between body compartments and the outside environment. The size and shape of the central lumen confer important characteristics to tubular organs and need to be carefully controlled. Here, we identify the small coiled-coil protein BBLN-1 as a regulator of lumen morphology in the C. elegans intestine. Loss of BBLN-1 causes the formation of bubble-shaped invaginations of the apical membrane into the cytoplasm of intestinal cells and abnormal aggregation of the subapical intermediate filament (IF) network. BBLN-1 interacts with IF proteins and localizes to the IF network in an IF-dependent manner. The appearance of invaginations is a result of the abnormal IF aggregation, indicating a direct role for the IF network in maintaining lumen homeostasis. Finally, we identify bublin (BBLN) as the mammalian ortholog of BBLN-1. When expressed in the C. elegans intestine, BBLN recapitulates the localization pattern of BBLN-1 and can compensate for the loss of BBLN-1 in early larvae. In mouse intestinal organoids, BBLN localizes subapically, together with the IF protein keratin 8. Our results therefore may have implications for understanding the role of IFs in regulating epithelial tube morphology in mammals.

Keywords: BBLN BBLN-1 C. elegans bublin epithelial tube intermediate filaments intestine.

Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.


A morphology-based assay platform for neuroepithelial-like cells differentiated from human pluripotent stem cells

Cell morphology is recognized as an important hallmark of neural cells. During the differentiation of human pluripotent stem cells (hPSCs) into neural cells, cell morphology changes dynamically. Therefore, characterization of the morphology of cells during this period is important to improve our understanding of the differentiation and development of neural cells. General methods for the directed induction of hPSCs include the steps of multi-cellular aggregation or high-density cell culture, particularly at the early phase of neural differentiation, and therefore, the morphology of each differentiating cell is difficult to recognize. Here, we have developed a new method for the directed differentiation of neuroepithelial-like cells (NELCs) from hPSCs at a low cell density in an adherent monolayer culture, as well as an image-processing algorithm to evaluate the cell morphology of differentiating NELCs, in order to follow cell morphology during the differentiation of hPSCs into NELCs. Using these methods, the morphological transition of differentiating cells was observed in real time using phase contrast imaging and then quantified. Because cell morphology is also considered an inherent biological marker of neural cells cultured in vitro, this method is potentially useful to study the mechanisms underlying neural cell differentiation.


Watch the video: Viral Morphology (May 2022).