The molecular instructions for all known forms of life are stored in deoxyribonucleic acid, or DNA. This complex molecule serves as the instruction manual for the development, functioning, growth, and reproduction of every organism. The study of this genetic material reveals how traits are passed down through generations and how the mechanics of a cell are orchestrated.
The Structure of DNA
DNA is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. This structure is often visualized as a twisted ladder, where the sides are formed by an alternating sugar-phosphate backbone. The rungs of this ladder are formed by pairs of nitrogenous bases, which are held together by hydrogen bonds.
Each strand is made up of simpler units called nucleotides, consisting of a phosphate group, a deoxyribose sugar, and one of four nitrogen-containing bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). The two strands are held together by complementary base pairing, where Adenine always bonds with Thymine, and Guanine always bonds with Cytosine.
The two strands in the double helix run in opposite directions, an orientation described as antiparallel. In eukaryotic cells, the extensive DNA molecule is highly organized and packaged into structures called chromosomes, which are located inside the cell nucleus. A typical human cell contains approximately three billion base pairs of DNA organized into 46 chromosomes.
Defining the Gene
A gene represents a specific segment of the DNA sequence that holds the instructions for a functional product. This functional product is often a protein, but it can also be a functional RNA molecule. The gene is the unit of heredity, influencing a particular characteristic in an organism.
The process by which the information in a gene is used to create this product is called gene expression. This process involves two main steps: transcription and translation. During transcription, the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule. This mRNA then leaves the nucleus to be used as a template for protein synthesis.
The sequence of the bases (A, T, C, G) within a gene translates into a specific sequence of amino acids, which are the building blocks of proteins. The cell’s machinery reads the bases in triplets, where each three-base sequence, or codon, codes for a particular amino acid. The resulting protein is then responsible for orchestrating nearly every function of the cell, such as forming structural components or acting as enzymes.
Mechanisms of Inheritance
The transmission of genes from parents to offspring is the basis of inheritance. Most organisms inherit two copies of every gene, one from each parent, because the genetic material is contained in paired chromosomes. Different versions of the same gene are known as alleles.
During the formation of reproductive cells, or gametes, these paired alleles must separate, a principle known as the law of segregation. This separation occurs during meiosis, ensuring that each gamete—sperm or egg—receives only one allele for each trait. When fertilization occurs, the gametes unite, and the offspring receives a pair of alleles, one from each parent, restoring the two-copy state.
An organism’s combination of alleles is its genotype, while its observable characteristics are its phenotype. In many cases of inheritance, one allele is dominant and the other is recessive. A dominant allele will produce its trait whether the individual has one copy or two.
The recessive trait is only observed when an individual inherits two copies of the recessive allele. Genes for different traits located on separate chromosomes also assort independently during gamete formation, meaning the inheritance of one trait does not influence the inheritance of another.
Mutations and Genetic Diversity
A mutation is an alteration in the nucleic acid sequence of the genome. These changes can occur spontaneously due to errors during DNA replication or cell division, or they can be caused by external factors like radiation. The simplest type of change is a point mutation, which involves the modification of a single base pair within a gene.
Insertions or deletions involve adding or removing one or more nucleotides from the DNA sequence. If an insertion or deletion is not a multiple of three bases, it can cause a frameshift, dramatically altering the subsequent amino acid sequence in the protein product. These changes in the gene sequence can result in a protein that is non-functional or has an altered function.
While some mutations can lead to disease, such as a single base-pair substitution causing sickle cell anemia, they are also the ultimate source of all genetic variation. Mutations provide the raw material upon which evolutionary forces, like natural selection, can act.