The key difference between capsid and nucleocapsid is that capsid is the protein coat that surrounds nucleic acid of the virus particle while nucleocapsid is the capsid together with the nucleic acids of a virus.
Viruses are infective particles composed of nucleic acid molecules covered with protein capsids. They are very tiny particles which can be seen only under the electron microscope. Furthermore, viruses are intracellular obligate parasites. Hence, they cannot multiply without a host organism or a living cell. Viruses show both living as well as non-living characteristics. A virus particle consists of two main components: the viral genome and protein coat. Protein capsid covers or surrounds the viral genome while nucleocapsid refers to the capsid with the genome inside.
CONTENTS
1. Overview and Key Difference
2. What is Capsid
3. What is Nucleocapsid
4. Similarities Between Capsid and Nucleocapsid
5. Side by Side Comparison – Capsid vs Nucleocapsid in Tabular Form
6. Summary
What is Capsid?
The capsid is one of the two major parts of a virus. It can be defined as the protein shell that surrounds the genome of the viral particle. Therefore, the capsid is a protein cover. It consists of several oligomeric structural subunits of proteins called protomers. Several protomers (5 to 6) collectively make the individual protein subunits of the protein capsid known as capsomeres. Capsomeres are organized in a precise and highly repetitive pattern around the nucleic acid. These capsomeres are the smallest morphological units of the capsid that are only visible under the electron microscope. A single virion has a large number of capsomeres.
Figure 01: Capsid
Protein capsid can be arranged in different shapes. There are three basic shapes as helical, icosahedral or polyhedral and complex arrangement. The majority of viruses have helical or icosahedral capsid structures. Some viruses, especially bacteria infecting viruses (bacteriophages), have complicated capsid structures. Capsomeres are organized in a spiral manner in helical viruses. In icosahedral viruses, capsomeres are arranged in 20 equilateral triangular faces.
Protein capsid performs several functions. It protects the genetic material of the virion particle. It also aids in transferring virus particles between host organisms. Moreover, capsid aids in specificity and viral infectivity since it has spikes. Spikes are glycoprotein protrusions which can bind with certain receptors on the host cell.
What is Nucleocapsid?
A virus comprises of an outer protein coat and an inner nucleic acid core. The nucleocapsid is the combination of the capsid and nucleic acid core. The inner nucleic acid core contains either RNA or DNA, but not both DNA and RNA. Moreover, DNA can be single-stranded or double-stranded. Similarly, RNA can be single-stranded or double-stranded.
Figure 02: Nucleocapsids
Structurally, a viral genome is packaged inside symmetric protein capsid. Therefore, the capsid, together with the genome, forms the nucleocapsid of a virus particle. Naked viruses have only nucleocapsids. However, enveloped viruses have a lipid bilayer called envelope surrounding the nucleocapsid.
What are the Similarities Between Capsid and Nucleocapsid?
- Capsid and nucleocapsid are two structural components of a virus.
- The capsid is one of the two parts of the nucleocapsid.
- Both structures are very important for viral survival and infectivity.
What is the Difference Between Capsid and Nucleocapsid?
The capsid is the protein cover that protects and surrounds the viral genome. Meanwhile, the nucleocapsid is the term that refers to the viral genome and protein capsid together. So, this is the key difference between capsid and nucleocapsid. Moreover, nucleocapsid includes the viral genome, while capsid does not include a viral genome. Therefore, this is also a significant difference between capsid and nucleocapsid.
Summary – Capsid vs Nucleocapsid
A virus is an obligate parasite. It consists of genetic material (DNA or RNA) enclosed in a protective protein coat called a capsid. Both nucleic acid and capsid form the nucleocapsid. Some viruses have an envelope surrounding the nucleocapsid. Naked viruses have only the nucleocapsid; they do not possess an envelope. Thus, this is the summary of the difference between capsid and nucleocapsid.
Reference:
1. Gelderblom, Hans R. “Structure and Classification of Viruses.” Medical Microbiology. 4th Edition., U.S. National Library of Medicine, 1 Jan. 1996, Available here.
2. “Structure of Viruses.” Boundless Microbiology. Lumen, Available here.
Image Courtesy:
1. “Helical capsid” By Arionfx – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Nucleocapsids” By Y tambe – Own work (jawiki) (CC BY-SA 3.0) via Commons Wikimedia