Which one of the following statements about Short Interspersed Nuclear Elements (SINEs) is TRUE?
SINEs represent a class of retrotransposons.
Short Interspersed Nuclear Elements, or SINEs, are significant components of many eukaryotic genomes. They belong to a class of repetitive DNA sequences known for their ability to replicate and insert into new genomic locations. Understanding the nature of SINEs involves looking at how they move within the genome and what machinery they use.
SINEs as Retrotransposons
Retrotransposons are a type of transposable element that move by using an RNA intermediate. This process, called retrotransposition, involves transcribing the DNA element into RNA, reverse transcribing the RNA back into DNA, and then inserting the new DNA copy into a new site in the genome. There are different types of retrotransposons, including LTR retrotransposons and non-LTR retrotransposons. Non-LTR retrotransposons include Long Interspersed Nuclear Elements (LINEs) and Short Interspersed Nuclear Elements (SINEs).
The statement "SINEs represent a class of retrotransposons" is TRUE because SINEs transpose through an RNA intermediate, which is the defining characteristic of retrotransposition. They are indeed classified as non-LTR retrotransposons.
SINEs Transposition Mechanism
Let's examine the other statements to see why they are incorrect:
- Statement 2: SINEs can transpose independently. This is FALSE. SINEs are non-autonomous retrotransposons. They do not encode the enzymes necessary for their own retrotransposition, such as reverse transcriptase and endonuclease. Instead, SINEs rely on the enzymatic machinery provided by autonomous retrotransposons, primarily LINEs, which reside in the same genome.
- Statement 3: SINEs can mobilize the neighboring LINE repeats. This is FALSE. SINEs cannot mobilize LINEs. In fact, it is the reverse: LINEs can provide the necessary proteins (like reverse transcriptase and endonuclease) that enable SINEs to transpose. SINEs are often transcribed and their RNA products are then recognized and processed by the LINE machinery for integration.
- Statement 4: SINEs are normally transcribed by RNA polymerase I. This is FALSE. Eukaryotic cells have different RNA polymerases responsible for transcribing different types of RNA. RNA polymerase I transcribes ribosomal RNA (rRNA). RNA polymerase II transcribes messenger RNA (mRNA), small nuclear RNA (snRNA), and microRNA (miRNA). RNA polymerase III transcribes transfer RNA (tRNA), 5S rRNA, and other small RNAs, including most SINEs. SINEs typically contain internal promoters that are recognized by RNA polymerase III.
Based on the analysis of each statement, only the statement that SINEs are a class of retrotransposons is accurate.
Important Questions from Genes and chromosomes
Readers of histone modifications include:
The amino acid side chains of the four histones in the nucleosome are subjected to remarkable variety of post-translation modifications such as phosphorylation, acetylation and methylation. Which one of the following post-translational marks on histone tails is usually associated with transcriptional repression?
In context of DNA methylation, which one of the following statements is FALSE?
The nucleosome is the fundamental subunit of chromatin in eukaryotes. Following statements are made about nucleosome:
A. Generally, a typical nucleosome contains ~ 200bp of DNA and two copies of each histone (H2A, H2B, H3 and H4)
B. 146 bp length of DNA per nucleosome core particle is strictly maintained across the organisms
C. The histone octamers are not conserved during/after replication, however, H32-H42 tetramers are.
D. Variants have been identified for all core histones except histone H3
E. While wrapping around the core histones, the structure of DNA is altered at the middle of the nucleosome core particle and exhibits increased number of base pairs per turn
Which one of the following combination of statements is most appropriate?
Following statements were made about transposons:
A. Transposons have inverted terminal repeats and their integration generates inverted repeats at the flanks of the target site in the host genome.
B. A composite transposon can transpose as a unit.
C. The transposition event may cause deletions or inversions or move a host sequence to a new location.
D. The transposition event may cause deletions or inversions but cannot move a host sequence to a new location.
E. Replicative transposition proceeds through cointegration.
Which one of the following represents the combination of the correct statements?
