Genetic Expression: Transcription and Translation in Cells
Genetic Expression: Transcription and Translation
Transcription: The synthesis of RNA from DNA. A strand of RNA is a sequence of bases complementary to the original.
Translation: Passing on the information contained in the RNA to proteins.
These processes always occur in the cell.
Purpose: To control the operation of the cell (coordinating all cellular processes).
- “Mold string”: The strand that is transcribed.
- “Chain Information”: The strand complementary to the mold strand. It resembles the mRNA, but has U instead of T.
The start signal indicates when a chain begins in the mold.
Transcription in Prokaryotic Organisms
Initiation Phase
An RNA polymerase recognizes the promoter (start signal).
Elongation Phase
The RNA polymerase opens the double helix and starts placing complementary nucleotides (makes a copy). As it makes the copy, the helix re-spiralizes (the RNA polymerase progresses) and the mRNA enters the cytoplasm. Because the cell does not have a nuclear membrane, maturation is not required. The transcription and translation are simultaneous. The mRNA reaches the ribosome, where it matches the amino acids, forming proteins. The ribosome starts to translate at the 5′ end of the mRNA.
The transcribed strand (mold strand) is not always the same for all genes. The reading direction of the enzyme RNA polymerase is 3′-> 5′. Since the two chains are antiparallel, the RNA polymerases move in opposite directions, taking into account the spatial orientation (one strand to the right, the other to the left).
Termination Phase
When RNA polymerase reads the terminator signal for the gene, the copy and ribosomes continue translating tRNA and forming proteins.
Transcription in Eukaryotic Organisms
Initiation Phase
Before the promoter, there are sequences that allow for activators, a reaction that helps position it correctly, and silencer sequences, which prevent the reaction due to repressors.
Activators and repressors will bind to these sequences. This allows better control of the gene and its activity. These sequences are followed by the promoter and the DNA sequences to be transcribed must be above the RNA polymerase.
Elongation Phase
Differences:
- The initial tip adds a “brand”: methyl-GTP.
- There is no concurrent transcription and translation: ribosomes are in the cytoplasm (outside the nucleus). The translation will be in the cytoplasm, after the maturation of the pre-mRNA.
Termination Phase
Once transcribed, a poly-A tail is added to the 3′ end, which allows the pre-mRNA to leave the nucleus: the nuclear pores are proteins that allow entry or exit from the nucleus if a capture signal (poly-A) is present. The methyl-GTP is a signal for the ribosome when the mRNA is in the cytoplasm.
Maturation Phase
Maturation involves cutting pieces that have no useful information (introns), because they are useless in the pre-mRNA. Exons are snippets of useful information. This happens in the nucleus.
The Translation of Genetic Information
Translation: Synthesis of a protein, usually an enzyme, from the information contained in the mRNA.
0. Union of Specific tRNA with the Corresponding Amino Acid
This is a preliminary phase. The enzymes aminoacyl-tRNA synthetases unite the corresponding amino acid with the tRNA.
1. Introduction
Coupling between the two subunits of the ribosome, the mRNA, and the tRNA-Met, the start signal. It starts with the 5′ end of the mRNA. The codon marking the start signal is AUG. If there are nucleotides before it, they are ignored. In the central zone, the ribosome takes two triplets: the peptidyl site and the acceptor site.
2. Elongation
At the acceptor site comes the following tRNA. It provides the peptide bond between the two amino acids. The first tRNA (Met) leaves and the ribosome moves forward one triplet (ribosomal translocation).
These steps are repeated: the acceptor site is free, and the corresponding tRNA arrives.
3. Completion
When the ribosome encounters one of the termination triplets, the two subunits of the ribosome separate and the protein is released.
To Consider:
- It always begins at the 5′ end, when the start signal (AUG) is found, and progresses.
- When the first ribosome has been running for a while (it has already produced a piece of protein), another ribosome can be added, so that it makes another identical protein: mRNA can be read by more than one ribosome simultaneously.
- The life of an mRNA is 20-30 minutes. This allows for thousands of copies of the protein. It is then destroyed. This way, the production of proteins can be controlled.
Characteristics of the Genetic Code
- It is universal. There is the same relationship between codons and amino acids in all living beings.
- It is not ambiguous. Each codon is related to only one amino acid. It is always read from left to right and in the 5′-> 3′ direction (unidirectional).
- It is aligned with no overlaps. The codons are located one after the other and never share a nucleotide.
- The AUG codon (Met) is the start codon for translation. There are three codons that indicate the end of the translation: UAA (ochre), UAG (amber), and UGA (opal).
- Only methionine and tryptophan are encoded by a single codon.