Which ends of eukaryotic mrna are protected from degradation

The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcription occurs in three main stages: initiation, elongation, and termination. The region of unwinding is called a transcription bubble. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter.

In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all Figure 9. Transcription always proceeds from one of the two DNA strands, which is called the template strand. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it Figure 9.

Termination Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that result in RNA polymerase stalling, leaving the DNA template, and freeing the mRNA transcript. On termination, the process of transcription is complete.

In a prokaryotic cell, by the time termination occurs, the transcript would already have been used to partially synthesize numerous copies of the encoded protein because these processes can occur concurrently using multiple ribosomes polyribosomes Figure 9. In contrast, the presence of a nucleus in eukaryotic cells precludes simultaneous transcription and translation. The newly transcribed eukaryotic mRNAs must undergo several processing steps before they can be transferred from the nucleus to the cytoplasm and translated into a protein.

Interestingly, the same enzymes are responsible for the actual degradation of the mRNA independent of the pathway taken see figure. Our understanding of enzyme function is, however, limited to a static 3 dimensional fold of one of the many conformations that these proteins can adopt.

To fully understand how molecular motions lead to catalytic activity a complete picture of the protein dynamics is required. In addition, the catalytic activity of these enzymes must be tightly regulated to prevent premature degradation of mRNA and to ensure maximum activity as soon as an mRNA has been identified as a substrate. As such, intermolecular interactions that modulate catalytic activity, e.

Figure 1: Simplified mechanism of mRNA decay. In eukaryotes, pre-rRNAs are transcribed, processed, and assembled into ribosomes in the nucleolus, while pre-tRNAs are transcribed and processed in the nucleus and then released into the cytoplasm where they are linked to free amino acids for protein synthesis. The four rRNAs in eukaryotes are first transcribed as two long precursor molecules. Enzymes then cleave the precursors into subunits corresponding to each rRNA.

In bacteria, there are only three rRNAs and all are transcribed in one long precursor molecule that is cleaved into the individual rRNAs. Some of the bases of pre-rRNAs are methylated for added stability. The eukaryotic ribosome is composed of two subunits: a large subunit 60S and a small subunit 40S. The bacterial ribosome is composed of two similar subunits, with slightly different components.

Each different tRNA binds to a specific amino acid and transfers it to the ribosome. Mature tRNAs take on a three-dimensional structure through intramolecular basepairing to position the amino acid binding site at one end and the anticodon in an unbasepaired loop of nucleotides at the other end. There are different tRNAs for the 21 different amino acids. Most amino acids can be carried by more than one tRNA. Structure of tRNA : This is a space-filling model of a tRNA molecule that adds the amino acid phenylalanine to a growing polypeptide chain.

The amino acid phenylalanine is attached to the other end of the tRNA. In archaea and eukaryotes, each pre-tRNA is transcribed as a separate transcript. Multiple nucleotides in the pre-tRNA are chemically modified, altering their nitorgen bases. On average about 12 nucleotides are modified per tRNA. But over other modifications can occur. A significant number of eukaryotic and archaeal pre-tRNAs have introns that have to be spliced out.

Introns are rarer in bacterial pre-tRNAs, but do occur occasionally and are spliced out. After processing, the mature pre-tRNA is ready to have its cognate amino acid attached.