Transposons and RNA: Key Aspects and Functions
Transposons
Transposons are segments of DNA that can move or “jump” from one chromosomal location (donor site) to another, either within the same chromosome or to a different one (target site). The insertion of a transposon into an essential gene could be lethal to the cell, so this movement is closely regulated and usually infrequent. Transposons are molecular parasites that passively replicate within the host cell’s chromosome. In some cases, transposons carry genes that are useful to the host cell.
Bacteria have two types of transposons:
- Insertion sequences: Contain only the sequences required for transcription and genes for proteins that promote the process.
- Complex transposons: Contain one or more genes in addition to those required for transcription, such as antibiotic resistance genes.
Replicative transcription involves the transposon being fully replicated, leaving a copy behind at the donor site. Eukaryotic transposons have structural similarities to bacterial transposons and sometimes use similar transcription mechanisms.
General Aspects of RNA
Most RNA molecules perform their functions as single strands. All RNA molecules are formed from the permanent information stored in DNA during the transcription process. An enzymatic system converts a segment of double-stranded DNA into an RNA chain. There are many types of RNA, including:
- tRNA: Transfer RNA (reads genetic information)
- mRNA: Messenger RNA (encodes amino acid sequences)
- rRNA: Ribosomal RNA (forms part of ribosomes)
Replication copies the entire chromosome, while transcription is more selective. Regulatory sequences indicate the beginning and end of DNA segments to be transcribed.
RNA Polymerase
RNA polymerase elongates the RNA chain by adding ribonucleotides to the 3′-hydroxyl end. It uses only one of the two DNA strands as a template, copying it in the 5′ to 3′ direction. U residues are inserted into the RNA to pair with A residues in the template DNA. Unlike DNA polymerase, RNA polymerase does not need a primer to initiate synthesis. Initiation occurs when RNA polymerase binds to specific sequences called promoters. The DNA double helix unwinds, forming a transcription bubble. The movement of the enzyme causes positive supercoils in the DNA. The two DNA strands have different roles.
RNA polymerase lacks an active site with 3′ to 5′ exonuclease proofreading activity, so the error rate of transcription is greater than that of chromosomal DNA replication.
Transcription Initiation
RNA polymerase binds to specific DNA sequences called promoters. The initiation process consists of two main steps: promoter binding and initiation. RNA polymerase first binds to the promoter, forming a closed complex, and then an open complex. Transcription then begins.
Differences Between Transcription and Replication
Transcription and replication are two distinct processes:
- Transcription: An enzymatic process where the genetic information contained in DNA is used to specify a complementary base sequence in an mRNA chain.
- Replication: The synthesis of a duplex DNA molecule identical to the parental duplex DNA.
Replication copies the entire chromosome, while transcription is more selective. The enzymes that carry out these processes are different. Unlike RNA polymerase, DNA polymerase requires a primer for synthesis and only initiates when it binds to promoters.