DNA and Genetics: Understanding Heredity and Molecular Biology
The Genetic Revolution
The discovery of DNA’s molecular structure marked the beginning of a revolution in the field of genetics and reproduction.
Genetics: Understanding Biological Inheritance
Genetics is the study of everything related to biological inheritance, genes, and their expression in organisms. Born at the beginning of the 20th century with Mendel’s experiments in plant hybridization, it investigates how hereditary characteristics are transmitted across generations. Mendel established a pattern and formulated three laws. The many avenues that opened up for research on Mendelian inheritance led to several conclusions in the first half of the 20th century:
- The unit of inheritance is called a gene.
- Genes are passed according to defined rules or laws.
- They are located on chromosomes, structures composed of a substance in the cell nucleus.
Sex is determined by genes or chromosomes in most living organisms. No one knew what genes were made of, so Griffith developed experiments that proved the existence of a “transforming principle” that passed from some bacteria to others, turning innocuous bacteria into pathogens. These experiments were conducted with bacteria that cause pneumonia. It was subsequently concluded that the “transforming principle” consisted of DNA (genes).
Terms Used in Genetics
- Genotype: The genetic constitution of an individual, referring to a character or a whole set of genes (AA, aa).
- Phenotype: The external expression of the genotype (green, rough).
- Allele: One of the variants that a gene may have (character).
- Homozygous: An individual who carries two identical alleles.
- Heterozygous: An individual who carries two different alleles.
- Locus: The location on the chromosome where each gene is located.
The Transmission of Characters
The transmission of characters takes place during the process of reproduction and constitutes the biological heritage. During transmission, each parent transmits genes across the chromosomes of their gametes. The descendants will be the result of all the crossings that may occur.
The Mendelian model considers that each genetic trait is determined by a pair of alleles, and genes are passed to offspring according to the following laws:
1st Law: The Law of Uniformity
The first filial generation, resulting from the crossing of two pure lines, is uniform.
2nd Law: The Law of Segregation
In the second generation, the type not stated in the first reappears because the factors are segregated in the gametes and meet in the offspring.
3rd Law: The Law of Independent Assortment
The features of different factors follow the above rules, without one influencing the other.
DNA: The Blueprint of Life
Locked in the nucleus (in eukaryotic cells), DNA contains the information with instructions for developing the characteristics of living things. By the mid-20th century, it was suspected that the carrier molecule was a type of nucleic acid, but it was in 1953 when scientists Watson and Crick proposed a model for its structure. From that moment, there was a significant development of research in this field.
Monomers and Polymers
- Monomer: A small molecule that binds to other identical molecules to form larger molecules.
- Polymer: A macromolecule composed of various monomers.
Nucleic Acids
Nucleic acids are polynucleotides, polymers of nucleotides, comprising:
- A nitrogenous base: adenine (A*), cytosine (C), guanine (G*), thymine (T1*), and uracil (U2). [* are only in DNA. 1 is only in DNA, and 2 is only in RNA]
- A pentose: ribose (in RNA, it has one more OH than DNA) or deoxyribose (in DNA, it has one less OH than RNA).
- A phosphate group.
The Structure of DNA
- A DNA strand consists of numerous nucleotides. The sequence of nucleotide bases in DNA constitutes the genetic information.
- The Watson and Crick model describes a double helix formed by two parallel chains or strands held together by the bases through hydrogen bonds.
- The bases of the two strands are complementary (A-T / C-G).
DNA Replication
Phase 1: Replication begins with the unraveling and separation of the two chains.
Phase 2: Each string is doubled independently by coupling nucleotides [A-T / C-G].
Phase 3: The final result is two new double helices that are an exact copy of the starting molecule.
Example: AGC-GAT -> AGC-TCG-CTA GAT -> AGC-GAT and GCT-CTA
The Expression of Genetic Information
The central dogma of molecular biology states that genetic information flows from DNA to protein with the intermediation of RNA: mRNA (messenger RNA), rRNA (ribosomal RNA), and tRNA (transfer RNA).
The transfer of information, or protein synthesis, occurs in two stages: transcription and translation (DNA – mRNA – tRNA – Proteins). (Shrine: new copy of DNA; transcription: DNA to RNA; translation: RNA to protein).