From DNA to Protein: Understanding Gene Expression and Biotechnology
Reading and Translation of the Genetic Message
The Role of DNA, RNA, and Proteins
The intermediary between DNA and proteins is RNA. DNA contains the genetic information that determines the type of RNA and, subsequently, the type of proteins synthesized. RNA conveys this information to the sites of protein synthesis.
Transcription
Gene expression is the process by which a gene is read to produce an RNA molecule and then a protein. Genes begin their expression when, in the nucleus, they form an RNA molecule carrying the genetic message of the DNA.
Initiation of Transcription
Transcription occurs in several stages and involves enzymes. This enzymatic machinery reads the DNA sequence contained in a gene and synthesizes a complementary RNA molecule.
Maturation of RNA
The resulting RNA molecule undergoes some changes before exiting the cell nucleus. This molecule carries the genetic message from the nucleus to the cytoplasm and is therefore called messenger RNA (mRNA).
The first stage of transcription involves chromatin decompensation. In a relaxed state, the same enzymes involved in replication, such as gyrase and helicase, separate the DNA strands.
When the transcription factor has bound to a region near the gene, RNA polymerase starts reading the DNA and synthesizing a complementary mRNA molecule.
Function of RNA Polymerase
RNA polymerase synthesizes RNA. It begins RNA synthesis by reading the termination sequence, which comprises one of the following triplets: ATT, ACT, or ATC.
The Genetic Code
In living organisms, there are 20 amino acids. Each amino acid is coded by sequences of three nucleotides called codons in the mRNA.
Each codon contains the code for a single amino acid. Since there are only 20 amino acids, each amino acid can be encoded by more than one codon. For this reason, it is said that the genetic code is degenerate or redundant.
Some years ago, there was a dispute because some plant mitochondria encoded the amino acid tryptophan with the CGG codon, while the universal code assigned it to arginine. The problem was solved by the discovery of the “editing” process in 1985 in a protozoan. This process involves changes in the nucleotide sequence of the mRNA after it has been copied, with nucleotides being inserted, deleted, or replaced.
Maturation of mRNA
The original mRNA has two clearly defined areas:
- Introns: Segments that do not participate in protein synthesis. They are eliminated by restriction enzymes in the nucleus that act like scissors, cutting the mRNA at the correct location.
- Exons: Segments involved in protein synthesis.
The cutting of introns and splicing of exons results in a mature mRNA molecule shorter than the original but with translation capabilities.
Polyadenylation: This process involves adding a long sequence of adenine nucleotides called a poly-A tail. This tail acts as a signal to allow the export of mRNA to the cytoplasm.
Protein Synthesis (Translation)
When the mature mRNA leaves the nucleus, protein synthesis takes place through the process of translation.
Translation occurs on ribosomes, which move along the mRNA molecule containing the information for protein synthesis.
AUG: The codon encoding the amino acid methionine, found at the beginning of all proteins.
Mechanism of Translation
Initiation complex: The large subunit of the ribosome has two sites: the peptidyl (P) site and the aminoacyl (A) site.
tRNAs: These molecules bind to a specific codon through a region called the anticodon. Each tRNA also binds to a specific amino acid.
Catalytic site: This is the region where peptide bond formation takes place, leading to the creation of amino acid chains.
Mutations
Some mutations alter the structure of large pieces of DNA.
- Chromosomal mutations: Changes in the structure of chromosomes.
- Point mutations: Changes in a single nucleotide. Nucleotide replacements are called substitutions.
Many somatic mutations originate in cells and are transmitted to daughter cells but not to descendants. However, when mutations affect the genome of germ cells, they can be transmitted to offspring.
Mutagens
Mutagens cause direct damage to nucleotide sequences, leading to breaks or fractures in chromosomes.
They are classified into:
- Chemical agents
- Ionizing radiation
- Biological agents
Chemical agents are found in food and everyday substances such as dyes, etc. They modify DNA nucleotides.
Ionizing radiation comes from the sun and the earth. Prolonged exposure, such as excessive sun exposure, can cause mutations.
Biological agents include viruses and bacteria.
Biotechnology: The Manipulation of Genetic Material
Biotechnology is the intersection of technology and biology. It allows scientists to learn more about cellular processes, including heredity and gene expression.
Recombinant DNA
Recombinant DNA technology involves incorporating genes from one species into the genome of another.
Origin of Transgenic Organisms
The process begins with the isolation of the desired gene. Bacterial restriction enzymes are used, which are capable of recognizing and cutting short DNA sequences at specific locations. Once isolated, the gene needs to be multiplied. This is achieved by incorporating the isolated gene into a bacterial plasmid.
Applications of Recombinant DNA
If the gene is incorporated into a few cells of an adult organism, the foreign gene will be present in all daughter cells derived from those cells.
- Recombinant DNA technology has been used to improve plants and animals for human benefit.
- Medical applications: Treatment of diseases (gene therapy).
Benefits and Controversies of Transgenic Products
Transgenic products are artificial and their benefits are subject to ongoing debate.
Prions
Prions are proteins capable of denaturing other proteins. They can cause genetic diseases.