The enzymes DNA gyrase, which introduces negative supercoils into DNA, and DNA topoisomerase I, which removes negative supercoils in DNA, are required to prevent topological stresses from building up in the course of transcription and replication. 

TLS Online TPP Program

#Id: 6704

#Unit 3. Fundamental Processes

The mechanism of pre-tRNA splicing differs in three fundamental ways from the mechanisms used by self-splicing introns and spliceosomes.

First, splicing of pre-tRNAs is catalyzed by proteins, not by RNAs. Second, a pre-tRNA intron is excised in one step thatentails simultaneous cleavage at both ends of the intron. Finally, hydrolysis of GTP and ATP is required to join the two tRNA halves generated by cleavage on either side of the intron.

TLS Online TPP Program

#Id: 6705

#Unit 3. Fundamental Processes

The snoRNAs are 60 to 300 nucleotides long. Many are encoded within the introns of other genes and cotranscribed with those genes. Each snoRNA includes a 10 to 21 nucleotide sequence that is perfectly complementary to some site on an rRNA.

TLS Online TPP Program

#Id: 6706

#Unit 3. Fundamental Processes

There are two classes of snoRNPs involved in rRNA processing in nucleolus.  The box H/ACA snoRNPs are involved in pseudouridylylation, and box C/D snoRNPs function in 2-O-methylations.

TLS Online TPP Program

#Id: 6707

#Unit 3. Fundamental Processes

Cajal bodies are centers of RNP-complex assembly for spliceosomal snRNPs and other RNPs

TLS Online TPP Program

#Id: 6708

#Unit 3. Fundamental Processes

Conversion of specific uridine residues to pseudouridine and the addition of methyl groups to the 2′-hydroxyl groups

of specific riboses, that are important for the proper assembly and function of snRNPs in pre-mRNA splicing. 

These modifications occur in Cajal bodies, where they are directed by a class of snoRNA-like guide RNA molecules called scaRNAs (small Cajal body–associated RNAs)

TLS Online TPP Program

#Id: 6709

#Unit 3. Fundamental Processes

Cajal bodies are sites of reassembly of the U4/U6/U5 tri-snRNP complex from the free U4, U5, and U6 snRNPs released during the removal of introns from mRNAs