DNA Structure, Function, and Genetic Code
DNA: Structure and Characteristics
1) DNA is composed of two chains coiled together to form a double helix.
2) Each chain is made up of units called nucleotides. In DNA, there are four types of nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). Each nucleotide consists of a phosphate, a deoxyribose sugar, and a nitrogenous base.
3) The two chains are united through their nucleotides in the following way: adenine binds to thymine (A-T), and guanine binds to cytosine (G-C). The number of adenines is equal to the number of thymines, and the number of guanines is equal to the number of cytosines.
Functions of DNA
1) Carry the genetic information. The order of DNA bases contains the genetic information of an organism. With this language of only four letters, long chains can form a huge number of sequences that will be unique to each species and each organism.
2) Transmit information from one cell to its daughter cells during cell division. Before a cell divides, it produces DNA replication, based on the complementarity of the two chains.
3) Direct the functioning of the cell. The expression of DNA determines the characteristics of a cell.
Genetic Code
The genetic code is the language that converts the order or sequence of nitrogenous bases of DNA into the amino acid (AA) order of a protein. In DNA are written, in a special code or language, instructions to form a living being; that is, the characteristics of the cell and therefore the organism are written. This is because DNA directs the synthesis of key cellular molecules: proteins, as they have:
1) Structural function.
2) Enzymatic function. Enzymes are proteins involved in all biochemical reactions of the cell.
The DNA code consists of four letters that are the nitrogenous bases. Every three bases, or triplet, represent or order an amino acid that joins another to form a protein. The order or sequence of the bases determines which amino acid is placed.
DNA Replication
1) Separation of the two chains of the DNA double helix.
2) Synthesis of new strands. Each of the original chains serves as a template for a new complementary strand: in front of an A, a T is placed; in front of a G, a C is placed, and vice versa.
3) The process continues until the two strands are copied, resulting in two final DNA molecules, each consisting of an original DNA strand and a new one.
Protein Synthesis
Transcription (DNA to RNA)
Transcription is the process of transferring information encoded in DNA into RNA. RNA is produced through a polymerase. When a gene is transcribed, the two DNA strands separate, one of which serves as a template for RNA synthesis, placing the RNA nucleotides complementary to DNA: G versus C, C versus G, A versus T, and U versus A (there is no T in RNA). The RNA that is formed and carries the instructions for the synthesis of a protein is called messenger RNA (mRNA).
Types of RNA
There are three types of RNA formed by transcription from a DNA fragment:
1) Messenger RNA (mRNA): Carries the instructions, the message, from the DNA in the nucleus to the ribosomes for the synthesis of a protein.
2) Transfer RNA (tRNA): Transports amino acids and places them in the order marked by the mRNA.
3) Ribosomal RNA (rRNA): Forms the ribosomes where proteins are synthesized.
Translation (RNA to Protein)
Translation is the process by which the fragment of mRNA that has left the nucleus is read or translated in the ribosomes, and a protein is synthesized. It occurs in the ribosomes. Every three bases (or codon) of mRNA are identified as one distinct amino acid. With the four nitrogenous bases of RNA, 64 triplets are formed, of which 61 code for the 20 amino acids, and 3 are termination codons. Another RNA molecule involved is tRNA.
tRNA Characteristics:
1) Have a one-dimensional structure like a cloverleaf.
2) The anticodon is the set of three bases that represent the point of contact with the mRNA. Each tRNA has a unique anticodon, which is complementary to the mRNA codon.
3) Each tRNA molecule has between 75-80 nucleotides.
4) For each amino acid, there is at least one tRNA.
Translation Steps:
1) The mRNA binds to the ribosome, where the AUG codon is located, which is the initiation codon in the genetic code.
2) A tRNA whose anticodon is complementary to AUG binds, and the corresponding amino acid is methionine.
3) The ribosome moves along the mRNA. The second amino acid, with its corresponding tRNA whose anticodon is complementary to the second codon of mRNA, is located in the ribosome, and its amino acid joins the first one by a peptide bond.
4) The tRNA that has released its amino acid returns to the cytoplasm to look for another amino acid.
5) The ribosome moves down the mRNA chain until it finds a stop codon (UAA, UAG, or UGA).
Mutations
Mutations are changes in DNA. Gene mutations are those that alter the nucleotide sequence of a single gene. This base change can also cause changes in the protein encoded by that gene. There are two types of gene mutations:
1) Mutation by substitution of one nucleotide for another: Only affects one amino acid. Since each amino acid is coded by more than one triplet, it is possible that the new triplet codes for the same amino acid, and the protein does not change. This is a silent mutation. If the amino acid changes, the effect on the protein can be very variable.
2) Mutation by a shift in reading frame: These are caused by the insertion or loss (deletion) of any nucleotide. In this case, all the triplets from the point of mutation are changed, producing a shift in the reading frame that will result in an entirely new, inactive protein fragment.
According to the affected cells:
1) Germline mutations: Affect the cells that produce gametes.
2) Somatic mutations: Affect any other cell. They occur spontaneously or are caused by mutagens such as radiation, X-rays, etc.
Definitions
DNA (Deoxyribonucleic Acid): Carries the genetic information that determines the properties of a living being and is passed from parents to offspring.
Nucleotides: Units that make up a strand of DNA.
Chromatid: Each of the longitudinal strands of a duplicated chromosome.
Genome: All the genetic information in the DNA of an organism.
Gene: A DNA fragment that carries information for the synthesis of a protein.
Transgenic Organisms: Organisms that have suffered the alteration of their genome by the introduction of an exogenous gene.
Cloning: The process by which identical copies of an organism are obtained and developed asexually.
Recombinant DNA (rDNA): Molecules manipulated outside living cells by modification of DNA segments.