Select the incorrect statement. (a) transposition of an Alu element requires tra

Question

Select the incorrect statement. (a) transposition of an Alu element requires transcription of its DNA to single-stranded RNA, which is then reverse transcribed to DNA (b) because retrotransposons within the L1 H family have a poly(A) region, they are thought to ha originated from an ancient mRNA (c) DNA sequences classified as VNTRs and STRs arose from misaligned crossing-over during synapsis of homologous chromosomes (d) a LINE is longer than a SINE and an individual VNTR is longer than an individual STR (e) a similarity between LINEs/SINEs and VNTRs/STRs is that both undergo retrotransposition

Explanation / Answer

Ans :-) (a) Mobile DNA elements(Alu elements), ranging from hundreds to a few thousand base pairs in length, originally were discovered in eukaryotes, they also are found in prokaryotes. The process by which these sequences are copied and inserted into a new site in the genome is called transposition.

The transpostion of Alu elements fall under two categories:-(1) those that transpose directly as DNA and (2) those that transpose via an RNA intermediate transcribed from the mobile element by an RNA polymerase and then converted back into double-stranded DNA by a reverse transcriptase. Mobile elements that transpose through a DNA intermediate are generally referred to as transposons. (As discussed below, this term has a more specific meaning in reference to bacterial mobile elements.) Mobile elements that transpose to new sites in the genome via an RNA intermediate are called retrotransposons because their movement is analogous to the infectious process of retroviruses.

The statement (a) is a correct statement.

(b) Transcripts from PolyA non-LTR retrotransposons generated by RNA polymerase II (pol II), such as LINE-1, SVA and the mRNA that gives rise to processed pseudogenes have the added advantage that the polyadenylation machinery will enzymatically add 100 to 200 adenosines to the 3 end. This process ensures that the transcripts of these elements will consistently have this important feature for retrotransposition. However, SINEs are transcribed by RNA polymerase III (pol III) and do not normally undergo polyadenylation as mRNAs do.

This explanation prooves that the poly A tail present in the retrotransposons is orginated from the ancient mRNAs, it is synthesized by the poly adenylation machinery and the above statement (b) is an incorrect statement.

(c) VNTRs and STRs are synthesized because of the recombination or replication error during the DNA synthesis. This is why VNTRs are variable in every PCR cycle.

The above statement (c) is a correct statement.

(d) Non-LTR retrotransposons move through a somewhat different RNA-mediated event, discussed below. Up to several kb in length, the non-LTR retrotransposons are commonly referred to as long interspersed elements (LINEs). Structurally, they contain an internal promoter for RNA polymerase II, a 5 untranslated region (UTR), two open reading frames (ORFs), and a 3 terminal polyadenylation site. The size of the first discovered line was approximately 6.4 kb.

Non-LTR interspersed elements is the nonautonomous retrotransposons. These are short interspersed elements (SINEs), which are approximately 80–400 bp in length and require activities encoded by the autonomous retrotransposons and/or the host for their mobility . Unlike LINEs, SINEs have an internal promoter for RNA polymerase III and a 3 A-rich tract. The most prominent SINEs are the human Alu elements and the rodent B1 elements.

The statement (d) is a correct statement.

(e)SVA elements are a family of non-autonomous retroelements within the primate lineage. They originally were named “SINE-R” elements, with the “R” indicating a sequence of retroviral origin. However some researchers identified a composite retroelement during studies of the retinitis pigmentosa (Rp) gene, and renamed this retroelement “SVA” after its three main components, SINE-R, VNTR and Alu. Over half of the SVA elements in primate genomes are full-length and they exhibit hallmarks of retroelements, indicating that they likely are mobilized in trans by target primed reverse transcription using the endonuclease and reverse transcriptase activities encoded by L1 elements. In general, SVA elements flanked by TSDs (target site duplications), terminate in a poly (A) tails, and can become truncated and/or inverted during their integration into the genome. Individual SVA elements also may vary in size as a result of polymorphisms in the VNTR.

So all the elements LINEs/SINEs and VNTRs/STRs all undergo retrotransposition.

The statement (e) is a correct statement.

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