Translation and TranscriptionBy George Googasian

TranscriptionTranscription is the process of making a specialized RNA molecule.

Transcription occurs in the nucleus of eukaryotes and in the cytosol of prokaryotes.

There are three stages in Transcription of RNA from a DNA template; initiation, elongation, and termination.

The Three Steps of Transcription

InitiationInitiation occurs when the enzyme RNA polymerase attaches to a specific region of the DNA. This attachment site is called the promoter region because it promotes transcription. It is located just before the segment of the DNA coding strand that will be transcribed. In eukaryotic cells, proteins known as initiation factors must be present for the RNA polymerase to attach to the promoter region.

ElongationThe second stage is Elongation of the RNA. RNA polymerase partially unwinds the DNA, exposing the strand of the gene. The enzyme moves along the DNA away from the promoter site as it builds an RNA molecule. The sequence of DNA nucleotides determines the sequence of the RNA chain. A single complementary strand of RNA, called a primary transcript, is made.

TerminationThe third stage of transcription is Termination. When RNA polymerase reaches the terminator region, or the end of the DNA to be transcribed, the enzyme and primary transcript are released from the DNA. This ends transcription.

RNA Processing

RNA ProcessingAll three types of RNA are processed in the nucleus of eukaryotes before they leave the nucleus. Enzymes add additional nucleotides and chemically modify of remove others. First, enzymes attach a cap of chemically modified guanine nucleotides to the starting end of the mRNA molecule. Next, other enzymes replace part of the opposite end with a tail of 100-200 adenine nucleotides. This addition is called a poly-A tail. The cap and the tail help protect mRNA from enzymes that break down nucleic acids. Generally, the longer the poly-A tail, the longer the life span of a particular mRNA. The tail also helps transport RNA out of the nucleus. The cap helps the mRNA attach to a ribosome and begin translation.

RNA Processing Cont.

The final step in mRNA processing involves removal of some internal segments of the RNA that do not code for protein. These are called introns. The parts of the transcript that remain and code for protein are called exons.

RNA SplicingThe process of removing introns and rejoining cut ends is called splicing. Splicing requires precise recognition of the site to be cut. Splicing enzymes recognize the sequence GU at one end of an intron and AG at the other end. This can happen in several ways. Protein enzymes catalyze the splicing of tRNA in yeast and many other eukaryotes. Catalytic RNAs splice RNAs produced by mitochondria, chloroplasts, and many unicellular eukaryotes. If introns are left in RNA, the consequences can be serious. In addition to splicing, an important step in the processing of tRNA is the chemical modification of several nucleotides and folding into a cloverleaf shape.

rRNA ProcessingRibosomal RNA is not involved in coding. The primary rRNA transcript is spliced and modified to produce mature rRNA molecules. These molecules bind to proteins to form the large subunits of ribosomes. Often several ribosomes bind to the same mRNA molecule, forming a polyribosome.

Translation

TranslationProtein synthesis translates the codon sequence of mRNA into the amino-acid sequence of a protein. This happens on ribosomes, where tRNA acts as a molecular adapter. One end of a tRNA molecule carries a specific amino acid. The corresponding anticodon is the opposite end of the tRNA molecule. In turn, the anticodon pairs with the mRNA codon that encodes this particular amino acid. Attachment of the correct amino acid to its tRNA molecule is called tRNA charging. Twenty different enzymes carry out the tRNA charging reactions. Each enzyme bonds a different amino acid to its matching tRNA.

Translation Cont.Charged tRNA, mRNA, and the growing polypeptide chain come together at specific binding sites on a ribosome. At these sites, tRNA anticodons base pair with mRNA codons. This positions the amino acids they carry so that they can bond to the growing polypeptide chain. Thus the sequence of codons dictates the amino-acid sequence.

Translation Cont. Cont.

One of the binding sites, the P site, holds the tRNA carrying the growing polypeptide chain. The A site holds the tRNA carrying the next amino acid to be added to the chain. Next to the P site is the exit site, or E site. An uncharged tRNA leaves the E site after its amino acid is added to the growing chain. During translation, the ribosome moves along the mRNA strand one codon at a time. Translation involves the three same stages; initiation, elongation, and termination, as transcription does. All three stages require enzymes that are part of the ribosome.

InitiationDuring initiation of transcription, the ribosome attaches at a specific site on the mRNA. This site is the start codon.

ElongationDuring elongation, peptide bonds join each amino acid with the next in sequence. A charged tRNA whose anticodon matches the next codon on the message enters the A site of the ribosome. This positions the amino acid it carries to form a peptide bond with the amino acid attached to the tRNA at the P site. When the bond forms, the polypeptide chain transfers to the tRNA at the A site. The entire ribosome moves down the mRNA to position the next codon at the A site. The uncharged tRNA leaves the E site. Now the tRNA that holds the growing polypeptide is at the P site. The A site is open and available for the next matching tRNa to bring in an amino acid.

TerminationTranslation terminates when a stop codon reaches the A site of the ribosome. Three codons can signal the end of translation; UAG, UAA, or UGA. No tRNA has an anticodon to complement these stop signals. Instead of a tRNA, a special protein known as a release factor binds to the stop codon in the A site. At this point translation stops and the tRNA releases the polypeptide. The ribosome lets go of the mRNA, the tRNA, and the release factor; then the two ribosomal units separate.

Translation ErrorsErrors sometimes occur during translation. The most common translation error results from misreading the nucleotide sequence. Initiation determines exactly where translation will begin. Starting from this point, the grouping of bases into codons is called the reading frame. If the start is shifted by one or two nucleotides in either direction, the frame changes. A different sequence of codons and amino acids will result. Some errors are due to splicing mistakes or changes in the DNA. If a nucleotide changes so that a codon becomes a stop codon, translation can terminate partway through the message. The result is a partial polypeptide. Insufficient amounts of a particular amino acid also can disrupt translation.