Introduction to Genetics: From DNA to Gene Expression

Posted on Aug 20, 2024 in Biology

What is Genetics?

Genetics is the study of biological inheritance, genes, and their expression. Bateson and Johannsen proposed a set of concepts that formed the foundation of classical genetics, establishing conclusions such as:

  • The unit of inheritance is called a gene.
  • Genes are transmitted according to defined rules or laws.
  • Genes are located on chromosomes.
  • Sex is determined by genes or chromosomes in most living organisms.

Common Genetic Terms

  • Genotype: The genetic constitution of an individual’s character or their entire set of genes (e.g., AA, Aa, aa).
  • Phenotype: The external expression of the genotype; the observable characteristics of an individual (e.g., yellow, wrinkled).
  • Allele: Each of the variants that a gene may have. For each gene, an individual possesses a pair of alleles (e.g., A = dominant, a = recessive).
  • Homozygous: An individual with two identical alleles (e.g., AA, aa).
  • Heterozygous: An individual with two different alleles (e.g., Aa).
  • Locus: The specific location on a chromosome where a particular gene is situated.

Griffith’s experiments with pneumonia bacteria provided evidence for the existence of a “transforming principle,” later identified as DNA.

Transmission of Characters

The transmission of characters occurs during reproduction, passing from parent to offspring. This process, known as biological inheritance, involves the transmission of genes via chromosomes carried within gametes (sperm and egg cells). Gametes are formed through meiosis, where only one allele from each gene pair is passed on.

The Mendelian Model

The Mendelian model of inheritance, named after Gregor Mendel, posits that each character is determined by a pair of genes. These genes are transmitted to offspring according to the following laws:

  • Principle of Uniformity: The first filial generation (F1) inherits one allele from each parent, resulting in a uniform phenotype if one allele is dominant.
  • Principle of Segregation: In the second filial generation (F2), recessive phenotypes reappear because alleles segregate during gamete formation and reunite during fertilization.
  • Principle of Independent Assortment: Different characters are inherited independently of each other, meaning the inheritance of one trait does not influence the inheritance of another.

DNA: The Blueprint of Life

Every cell of every living organism contains deoxyribonucleic acid (DNA), a molecule that carries the genetic instructions for the development and functioning of that organism. Genes are segments of DNA.

James Watson and Francis Crick proposed the double helix model of DNA structure.

Nucleic Acids: The Building Blocks of DNA

DNA is a nucleic acid, a polymer made up of nucleotide monomers. Each nucleotide consists of:

  • A nitrogenous base: adenine (A), cytosine (C), guanine (G), thymine (T)
  • A pentose sugar: deoxyribose
  • A phosphate group

There are two types of nucleic acids:

  • Deoxyribonucleic acid (DNA): Contains deoxyribose sugar and the bases A, C, G, and T.
  • Ribonucleic acid (RNA): Contains ribose sugar and the bases A, C, G, and uracil (U).

DNA Structure and Replication

  • DNA consists of two strands of nucleotides linked together in a double helix shape, resembling a twisted ladder. The two strands are held together by hydrogen bonds between complementary bases (A with T, and C with G).
  • During cell division, DNA replicates itself in a process that ensures each daughter cell receives an identical copy of the genetic information. The replication process involves three main steps:
  1. Unwinding and separation of the two DNA strands.
  2. Each strand serves as a template for the synthesis of a new complementary strand, using free nucleotides.
  3. The result is two identical DNA double helices.

Gene Expression: From DNA to Protein

The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein. This process involves two main stages:

  • Transcription: Information encoded in DNA is transcribed into messenger RNA (mRNA), a molecule that carries the genetic code from the nucleus to the cytoplasm.
  • Translation: The mRNA sequence is translated into a specific sequence of amino acids, forming a protein. This process occurs at the ribosomes.

The Genetic Code

The genetic code is the set of rules by which information encoded within genetic material (DNA or RNA sequences) is translated into proteins by living cells.

  • The genetic code is universal, meaning it is the same in all living organisms.
  • A sequence of three nucleotides, called a codon, specifies one amino acid.
  • Some amino acids are encoded by multiple codons (degeneracy).
  • There are start and stop codons that signal the beginning and end of protein synthesis.

Recombinant DNA Technology

Recombinant DNA technology allows the transfer of genetic information between organisms. This technology has led to the development of transgenic organisms, which contain DNA from another species. For example, bacteria can be genetically engineered to produce human insulin.

Cloning the Insulin Gene in Bacteria

Recombinant DNA technology enables the cloning of genes, such as the insulin gene, into bacteria. This process involves inserting the desired gene into a vector, such as a plasmid, and introducing it into the bacteria. The bacteria then produce the protein encoded by the inserted gene.