DNA, Genes, and Chromosomes: Core Concepts of Heredity

Fundamentals of Genetics and Heredity

Deoxyribonucleic Acid (DNA)

DNA is the inherited material that carries genetic information in all living organisms, passed down from generation to generation. A DNA molecule is composed of nucleic acid nucleotides.

Nucleotides are formed by three essential components:

• A phosphate group.
• A pentagonal ring sugar (Deoxyribose).
• A nucleobase (nitrogenous base).

In DNA, there are four basic types of nitrogenous bases: Cytosine (C), Guanine (G), Thymine (T), and Adenine (A).

DNA Replication: Ensuring Genetic Continuity

The information contained within the sequence of DNA bases is the foundation of inheritance for every species. This information must be transmitted intact from cell to cell to ensure the continuity of that species.

When a cell prepares to divide, the two chains of DNA separate. DNA polymerases are enzymes that synthesize complementary nucleotides, forming a new strand.

The result is two identical DNA double helices. Each new helix retains one strand from the initial molecule (the template) and one newly synthesized strand. This process is known as semiconservative replication.

Chromosomes and Karyotypes

Chromosomes

DNA is packaged into structures called chromosomes, which typically have a rod-like shape. Each chromosome consists of a single DNA molecule tightly associated with many proteins.

Each species has a specific number of chromosomes. Human beings have 23 pairs of chromosomes (46 total): 23 inherited from the father and 23 from the mother.

These pairs are called homologous chromosomes and carry the same type of hereditary information.

Karyotype

A karyotype is the complete set of chromosomes of an organism, typically organized, sorted, and matched by size and shape.

Genetic Terminology

Gene

A gene is a specific fragment of DNA that encodes the synthesis of a protein. The various combinations of the four nucleobases are essential for encoding the synthesis of proteins that characterize every living being and allow for biological function and life.

Gametogenesis

Gametogenesis is the biological process of forming sex cells (gametes). It involves producing a reproductive cell (e.g., sperm or egg) from a precursor cell, typically containing 46 chromosomes in humans.

Genotype and Phenotype

The genotype is the specific genetic constitution of an organism concerning a particular character or trait.

A specific character (or trait) is determined by the two genes (alleles) located on homologous chromosomes.

The phenotype is the observable manifestation of the genotype—the attributes that can be observed due to the underlying genetic information.

Example: Blood Group Determination

The human blood group is determined by a single gene that may occur in three forms, or alleles: A, B, and O.

• Alleles A and B are codominant.
• Allele O is recessive.