Long terminal repeats

Long terminal repeats (LTRs) are sequences of DNA that repeat hundreds or thousands of times. They are found in retroviral DNA and in retrotransposons, flanking functional genes. They are used by viruses to control of gene expression and integration. For example, a retrovirus genome might contain the following features:


 * LTR -- gag -- pol -- env -- LTR

The LTRs are partially transcribed into an RNA intermediate, followed by reverse transcription into complementary DNA (cDNA) and ultimately dsDNA (double-stranded DNA) with full LTRs. The LTRs then mediate integration of the retroviral DNA via an LTR specific integrase into another region of the host chromosome. Retroviruses such as Human Immunodeficiency Virus (HIV) use this basic mechanism.

quiz
{Long terminal repeats (LTRs) are sequences of DNA that ________ hundreds or thousands of times. + repeat - repeats - repeating - repeated
 * type="[]"}
 * Correct! LTRs are sequences that repeat multiple times.
 * Incorrect. The subject 'sequences' is plural, so the verb should be 'repeat'.
 * Incorrect. The verb form 'repeating' doesn't fit the sentence structure.
 * Incorrect. The sentence is in present tense, so 'repeated' is not appropriate.

{They are found in retroviral DNA and in retrotransposons, ________ functional genes. - finding - finds + flanking - flanked
 * type="[]"}
 * Incorrect. The verb form 'finding' doesn't fit the sentence structure.
 * Incorrect. The subject 'They' is plural, so the verb should be 'flank'.
 * Correct! The LTRs flank functional genes.
 * Incorrect. The sentence is in present tense, so 'flanked' is not appropriate.

{They are used by viruses to ________ of gene expression and integration. - control + take control - taking control - took control
 * type="[]"}
 * Incorrect. The preposition 'to' requires an infinitive form of the verb.
 * Correct! Viruses use LTRs to take control of gene expression and integration.
 * Incorrect. The verb form 'taking' doesn't fit the sentence structure.
 * Incorrect. The sentence is in present tense, so 'took' is not appropriate.

{The LTRs are partially transcribed into an RNA intermediate, followed by reverse transcription into complementary DNA (cDNA) and ultimately dsDNA (double-stranded DNA) with full ________. - LTR + LTRs - LTR's - LTRing
 * type="[]"}
 * Incorrect. The word 'LTR' is singular, but the context suggests a plural form.
 * Correct! The LTRs are transcribed and then integrated into dsDNA with full LTRs.
 * Incorrect. The possessive form 'LTR's' is not appropriate here.
 * Incorrect. 'LTRing' is not a valid term.

{The LTRs then mediate integration of the retroviral DNA via an LTR specific ________ into another region of the host chromosome. - integrase - integrases + integrase - integrating
 * type="[]"}
 * Incorrect. The word 'integrase' is singular, but the context suggests a plural form.
 * Incorrect. The word 'integrases' is plural, but the context suggests a singular form.
 * Correct! The LTRs mediate integration via an LTR-specific integrase.
 * Incorrect. The verb form 'integrating' doesn't fit the sentence structure.

{Retroviruses such as Human Immunodeficiency Virus (HIV) ________ this basic mechanism. - use + use - uses - using
 * type="[]"}
 * Incorrect. The subject 'Retroviruses' is plural, so the verb should be 'use'.
 * Correct! Retroviruses, like HIV, use this basic mechanism.
 * Incorrect. The subject 'Retroviruses' is plural, so the verb should be 'use'.
 * Incorrect. The verb form 'using' doesn't fit the sentence structure.