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Life Cycle of Eukaryotic and Viral Cells

Life Cycle of Eukaryotic and Viral Cells

Cells are the basic units of life in all living organisms. There are different types of cells, including eukaryotic and viral cells. Nonetheless, these cells perform similar functions: metabolism, synthesis of molecules, communication, and reproduction. At the same time, eukaryotic and viral cells exhibit different life cycles during reproduction and inheritance. Therefore, it is essential to understand the composition of each cellular structure (eukaryotic and viral), the life cycle of each cell, the differences and similarities between these two life cycles, and the advantages and disadvantages of these life cycles.

Composition of Eukaryotic Cells

The figure above by VanPutte et al. (2017) illustrates an idealized animal eukaryotic cell. The composition and functions of each cell organelle are as follows: the plasma membrane is the outer boundary of the cell. It comprises phospholipids, cholesterol, and proteins, which enable cellular communication. The plasma membrane also controls the entry and exit of substances into the cell. Next, the nucleus comprises nuclear pores, chromatin, which become chromosomes after cell division, and nucleoli, which contain ribosomal RNA and proteins. The nucleus controls all the cellular activities and chemical reactions, such as protein synthesis. Ribosomes contain RNA and proteins and form the sites for protein synthesis. Next, the rough endoplasmic reticulum (RER) contains membranous tubules, flattened sacs attached with ribosomes, while the smooth endoplasmic reticulum (SER) comprises membranous tubules, flattened sacs with no ribosomes. The function of RER is protein synthesis, while SER functions are lipid and carbohydrate manufacturing, detoxification of harmful substances, and calcium storage.

The Golgi apparatus is a flat membrane with sacs that modify, pack, and distribute lipids and proteins. Lysosomes contain digestive enzymes, and peroxisomes break down hydrogen peroxide and form the sites where lipids and amino acids are degraded, while proteasomes break down cytoplasmic proteins. Further, mitochondria are covered by two membranes and form the primary sites for ATP synthesis. Centrioles comprise parallel microtubules and serve as the site for microtubule formation. They also define cell polarity during cell division and form the base for cilia and flagella. Cilia transport substances over the cell surface, while flagella propel other cells like spermatozoa in humans. Lastly, microvilli contain microfilaments that increase the surface area for plasma membrane absorption and secretion of substances. They can also be modified to function as sensory receptors.

Composition of Viral Cells

Viral cells contain nucleic acids, which can either be DNA or RNA. They are covered by a protein coat, a capsid containing genomes and enzymes for synthesizing new virions (Parker et al., 2016). A capsid comprises protein subunits called capsomeres, which may contain various interlocking capsomere proteins to form the tightly packed capsid (Parker et al., 2016). A virus can be nonenveloped or enveloped.

The figure above illustrates a nonenveloped virus (Parker et al., 2016). Nonenveloped viruses, also called naked viruses, form from only nucleic acid and a capsid.

Conversely, enveloped viruses, illustrated in the figure above (Parker et al., 2016), comprise nucleic acid and a capsid covered by a lipid layer. A viral envelope encompasses a phospholipid membrane acquired from a host cell after the budding of the virion. In addition, protein structures extend from the capsid of some viruses called spikes. The tips of these spikes contain structures that enable a viral cell’s attachment and entry into a cell. For instance, the influenza virus hemagglutinin spikes (H) or the neuraminidase (N) influenza spikes enable the virus to disconnect from the host’s cellular surface and release new virions (Parker et al., 2016).

Life Cycle of Eukaryotic Cells

The cell cycle in eukaryotic cells begins with a stage called interphase. Interphase is divided into three phases: the G1 phase, the S phase, and the G2 phase. The G1 phase is the first gap, where the cell grows rapidly to form new molecules. In the S phase, DNA synthesis or DNA replication occurs. Notably, the chromosomes are duplicated (doubled), each comprising two sister chromatids. Lastly, in the G2 phase, the cell prepares for cell division. At this point, the cytoplasm comprises two centromeres, and the chromosomes duplicate but are still tightly packed into chromatin fibers (Simon et al., 2019).

The cell division phase is called the mitotic (M) phase. The M phase involves two additional stages called mitosis and cytokinesis. Notably, mitosis involves the division and even distribution of the nucleus, the nucleus contents, and the replicated chromosomes to form two daughter nuclei. Subsequently, cytokinesis involves the division of the cytoplasm and the cytoplasmic organelles. Accordingly, these two overlapping processes, mitosis and cytokinesis, yield two daughter cells with identical genetic compositions. Each daughter cell contains a nucleus, cytoplasm, cell organelles, and a plasma membrane (Simon et al., 2019).

Life Cycle of Viral Cells

The figure above by Ryu (2017) illustrates the life cycle of a viral cell, which involves three major stages: entry, genome expression and replication, and exit. Firstly, viral entry is divided into four steps: attachment, penetration, cytoplasmic trafficking, and uncoating (Ryu, 2017). Attachment is the first encounter between the virus and the host cell. It involves two host plasma membrane proteins: attachment factors and viral receptors. Attachment factors serve to hold the viral particles and facilitate interactions between the virus and the entry receptor. After the viral cell attaches to the target host cell, the next step is to penetrate the cytoplasm. Penetration mechanisms differ across enveloped and nonenveloped viruses. Notably, enveloped viruses utilize direct fusion or receptor-mediated endocytosis, while naked viruses utilize receptor-mediated endocytosis mechanisms.

Subsequently, after penetration, the next step is cytoplasmic trafficking. During this stage, the viral cell moves to a target site (cytoplasm or nucleus) within the host cell for genome replication. Accordingly, viruses that replicate within the cytoplasm require microtubule-assisted transport and receptor-mediated endocytosis as transport mechanisms. Conversely, viruses that replicate within the nucleus depend on microtubule-mediated transport. The next step after cytoplasmic trafficking is uncoating. As the viral cell approaches its target replication site, its genome becomes exposed to the host cell’s apparatus for gene expression, a process called uncoating (Ryu, 2017). Uncoating marks the end of the entry stage, and the virus-cell proceeds to the genome replication stage. Strategies for genome replication vary across different types of viruses; some are entirely dependent on host machinery, while others are entirely independent.

Next, the viral cell proceeds to the exit stage, which is divided into capsid assembly, release, and maturation (Ryu, 2017). Essentially, capsid assembly involves capsid assembly and genome packaging. Packaging is assisted by a packaging signal, a cis-acting element contained in a viral genome (Ryu, 2017). The viral capsid proteins recognize the packaging signal and selectively pack either RNA or DNA. Subsequently, the next step is releasing the viral particles. For nonenveloped viruses, particles are released via cell lysis of the host cell. Contrariwise, enveloped viruses utilize the exocytosis mechanism, also called budding, triggered by a peptide motif called the late (L) domain (Ryu, 2017). The last step in the viral life cycle is maturation. This process occurs in the extracellular environment after the release of the viral particle. This step is vital for viruses, such as picornaviruses and retroviruses, to acquire infection.

Comparing and Contrasting the Life Cycles of Eukaryotic and Viral Cells

The life cycle of eukaryotes and viruses differs in several ways. Firstly, a single eukaryotic life cycle produces only two genetically identical daughter cells, while a viral life cycle produces many viral particles. Secondly, eukaryotic cells do not require a host to reproduce. Conversely, viral cells must be within the target host cells to replicate; some depend entirely on the host’s cell machinery for replication, while others are independent. Nevertheless, these two life cycles are similar in that both begin with a single cell. Additionally, both life cycles involve DNA replication and expression. Lastly, both yield daughter cells and viral particles- eukaryotic life cycle and viral life cycle, respectively- carrying similar genetic composition to their parent cells.

Advantages and Disadvantages of Each of the Organism’s Life Cycles

The eukaryotic life cycle exhibits some advantages. These include rapid growth; rapid growth is attributed to the few resources and energy required for cell division. Besides, the cells do not require morbidity or assisted transport. On the other hand, viruses require morbidity during, for example, the penetration stage: enveloped viruses may depend on receptor-mediated endocytosis, while naked viruses depend on receptor-mediated endocytosis mechanisms (Ryu, 2017). Additionally, the viral life cycle may take more time for gene replication and expression, especially for viruses entirely dependent on the host’s mechanisms. However, completely independent viruses may take a shorter time.

References

Parker, N., Schneegurt, M., Tu, A., Forster, B., & Lister, P. (2016). Microbiology. OpenStax.

Ryu, W. (2017). Virus Life Cycle. Molecular Virology of Human Pathogenic Viruses, 31-45. https://doi.org/10.1016/b978-0-12-800838-6.00003-5

Simon, E., Dickey, J., Reece, J., & Burton, R. (2019). Campbell Essential Biology (7th ed.). Pearson Education, Inc.

VanPutte, C., Regan, J., Russo, A., Seeley, R., Stephens, T., & Tate, P. (2017). Seeley’s Anatomy & Physiology (12th ed.). McGraw-Hill Education.

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Question 


Read about Eukaryotic and Viral cells and their life cycle.

Describe the life cycle for Eukaryotic cells

Life Cycle of Eukaryotic and Viral Cells

Life Cycle of Eukaryotic and Viral Cells

Describe the life cycle of a viral cell.
Discuss the composition of each of the cellular structures and what their molecules are used for.
Compare and contrast the life cycles of each of the cells.
Identify any advantages or disadvantages of each of the organism’s life cycles.

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