This video we're going to begin our lesson on viroids. First, we need to recall from some of our previous lesson videos that viroids, like viruses, are acellular obligate intracellular parasites. However, unlike viruses, which are made of proteins, nucleic acids, and sometimes lipids with enveloped viruses, viroids are only made of nucleic acid, not proteins, not lipids. More specifically, viroids are only made of a single short strand of RNA. This single short strand of RNA tends to form a closed ring, forming a ring structure. Viroids are only known to infect and cause disease in plant cells. Like plant viruses, these viroids need to enter the host plant cell via wound sites within the plant that can be caused by physical damage, like an insect causing physical damage. Viroids replicate using a host cell's RNA dependent RNA polymerase, an enzyme similar to the replicase of RNA viruses. Viroids do not encode for any proteins; instead, they act similarly to small interfering RNAs or siRNAs, capable of blocking the expression of plant genes, causing disease. If we look at our image below, we can get a better understanding of these viroids and viroid infections. Viroids consist of a single strand of RNA that is circular. In this case, you can see it creates the structure that you see here. The viroid can enter a plant cell via wounds. Here, you can see a healthy plant, but once it is infected with viroids, it can become an unhealthy plant with a disease caused by viroids. Notice that viroids do not get translated. Instead, they act similarly to siRNAs, where they block host mRNA translation, preventing gene expression of plant genes, leading to disease. The viroid siRNA generated from the viroid can block the host mRNA, preventing the translation of the host mRNA. As a result, these host proteins will not be expressed, which can lead to plant diseases. There are many questions about viroids that still exist. Continued research is ongoing every day to solve these issues and answer those questions about viroids. This here concludes our brief lesson on viroids and how they are acellular obligate intracellular parasites that are only made of RNA. I'll see you all in our next video.
18. Viruses, Viroids, & Prions
Viroids
18. Viruses, Viroids, & Prions
Viroids - Online Tutor, Practice Problems & Exam Prep
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concept
Viroids
Video duration:
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Video transcript
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Problem
ProblemViroids characteristically are composed of:
A
ssRNA.
B
dsRNA.
C
ssDNA.
D
dsDNA.
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Problem
ProblemViroids are:
A
Small fragments of naked dsDNA that infect plant cells.
B
Small fragments of naked dsRNA that infect plant cells.
C
Small fragments of naked ssRNA that infect plant cells.
D
Small fragments of naked ssRNA that infect animal cells.
E
Small fragments of naked ssDNA that infect animal cells.
F
None of the above.
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