Table of Contents
Overview: From Traits to Molecules
“Heredity” means that offspring resemble their parents. The molecular foundations of heredity explain how this happens at the level of molecules inside cells.
Key ideas in this chapter:
- There is a physical carrier of hereditary information.
- This carrier is a molecule that can be copied with high fidelity.
- The information in this molecule is encoded, stored, and read out to build functional molecules (especially proteins).
- Errors and changes in this molecule lead to variation, which is essential for evolution.
Later chapters (e.g. on the structure of DNA, the genetic code, RNA, and from gene to protein) will go into detail. Here we focus on the general molecular principles that make heredity possible.
What Makes a Molecule Suitable as a Hereditary Carrier?
For a molecule to serve as the basis of heredity in a cell, it must meet several requirements:
- Capacity to store large amounts of information
- It must be built from repeating units (monomers) that can be arranged in many different sequences.
- Different sequences must be able to represent different hereditary “messages”.
- Stability and chemical robustness
- It must be stable enough under physiological conditions to keep information over the lifetime of the cell or organism.
- It must resist random breakdown, yet be chemically reactive enough that cells can copy and process it when needed.
- Copyability (replication)
- The molecule must be able to serve as a template for its own duplication.
- Copying must be highly accurate, but not absolutely perfect. Small copy errors (mutations) generate genetic variation.
- Accessibility of information
- Cells must be able to read (express) the information in a controlled way.
- “Reading” should be directional and organized (beginning, middle, end), enabling regulated gene activity.
- Universality and compatibility
- To allow universal inheritance, the same basic type of molecule and the same general rules of reading should apply across organisms.
In modern life on Earth, this role is played primarily by deoxyribonucleic acid (DNA). In some viruses, ribonucleic acid (RNA) serves as the main hereditary material.
Nucleic Acids as Information-Carrying Polymers
Monomers and Polymers
The molecules that store hereditary information, nucleic acids, are polymers: long chains built from many small units called nucleotides.
Each nucleotide consists of three parts:
- A sugar (deoxyribose in DNA, ribose in RNA)
- A phosphate group
- A nitrogenous base (A, T, G, C in DNA; A, U, G, C in RNA)
By linking nucleotides together in different linear sequences, nucleic acids can store information in a way similar to letters forming different words. The order of the bases along the chain carries the genetic instructions.
Directionality and the Sugar–Phosphate Backbone
Nucleic acid strands have a direction:
- One end is called the 5′ (five-prime) end
- The other end is called the 3′ (three-prime) end
This directionality arises from the orientation of the sugar–phosphate backbone. Cellular processes that copy or read nucleic acids proceed in a specific direction (usually 5′ → 3′).
The backbone itself is:
- Chemically stable, protecting the base sequence.
- Repetitive (sugar–phosphate–sugar–phosphate...), while the bases provide the variable information.
This separation between a constant structural framework (backbone) and a variable information-bearing part (base sequence) is a key design principle of the hereditary molecule.
Complementarity and Template-Based Copying
A central principle of heredity is complementary base pairing:
- Certain bases pair specifically with each other through weak hydrogen bonds.
- In DNA, A pairs with T, and G pairs with C.
- In RNA, A pairs with U, and G pairs with C.
Because of this, knowing the sequence of one strand allows you to predict the sequence of its partner. This is called complementarity.
This has two crucial consequences:
- Template function
Each strand can serve as a template for the synthesis of a new complementary strand. This is the molecular basis for semiconservative replication (covered later): when DNA is copied, each daughter molecule contains one old (template) strand and one newly synthesized strand. - Error checking
Because only specific pairings are energetically favored, cells can detect and correct many mismatches. This contributes to the high fidelity of heredity.
Complementarity also underlies other key processes, such as transcription and base-pairing between DNA and RNA during gene expression, which will be explained in later chapters.
From Information Storage to Biological Function
DNA sequences are not active by themselves; they become biologically meaningful when they are interpreted by the cell’s molecular machinery.
Important high-level ideas:
- A gene is typically defined as a functional unit of heredity, usually a segment of DNA that contains information to build a specific RNA or protein.
- Proteins, in turn, perform most structural, catalytic, and regulatory roles in the cell.
- Thus, genetic information stored in nucleic acids is ultimately expressed as biological function, primarily through proteins.
The flow of genetic information in modern cells is often summarized by the “central dogma”:
$$
\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}
$$
- DNA serves as the long-term storage of genetic information.
- RNA acts as a versatile intermediary and sometimes as a functional molecule itself.
- Proteins are typically the final products that carry out most cellular functions.
The precise steps of this flow (transcription, translation, etc.) and their variations are covered in later chapters.
Genetic Information as a Coded Message
The information in DNA and RNA is encoded in the sequence of bases. This is analogous to how letters encode words and sentences.
Key conceptual aspects of this genetic code:
- A small alphabet (A, T, G, C in DNA; A, U, G, C in RNA) is combined into triplets (groups of three) to specify individual amino acids.
- The code is:
- Specific: each triplet (codon) corresponds to a particular amino acid or a signal such as “start” or “stop”.
- Redundant: multiple codons can encode the same amino acid.
- Largely universal: with minor exceptions, the same code is used by almost all known organisms, emphasizing the common origin of life.
At the molecular level, heredity therefore depends on:
- A digital-like encoding of information in discrete units (bases).
- A translation system that interprets this code to assemble amino acids into proteins.
The detailed structure of the genetic code and its consequences will be examined in the dedicated “Genetic Code” chapter.
DNA and RNA: Division of Labor
Although both DNA and RNA are nucleic acids, they play different roles in the molecular basis of heredity.
General division of labor:
- DNA
- Serves mainly as long-term information storage.
- Usually double-stranded, which enhances stability and enables repair using the complementary strand.
- Replicated with high accuracy when cells divide.
- RNA
- Acts mainly as a working copy or intermediate.
- Often single-stranded and generally less stable than DNA.
- Exists in various functional types (messenger, catalytic, structural, regulatory), which collectively connect stored genetic information to cellular activities.
In some viruses, RNA itself functions as the primary genetic material; in those systems, RNA must fulfill both storage and functional roles, with specialized replication mechanisms.
Variation and the Molecular Basis of Evolution
Heredity requires faithful copying of genetic information, but evolution requires change. At the molecular level, these two demands are balanced as follows:
- High-fidelity replication and repair keep most genetic information unchanged across generations.
- Occasional changes in nucleotide sequence (mutations) occur because:
- Copying is not perfectly accurate.
- DNA can be damaged by chemical agents, radiation, or other factors.
- These changes alter:
- The sequence of bases in DNA.
- The RNA and protein products derived from that DNA.
- Ultimately, the traits of the organism.
Thus, the molecular foundations of heredity simultaneously ensure:
- Continuity: most features are preserved from one generation to the next.
- Variability: new variants arise, providing raw material for natural selection and evolution.
The different kinds of variation and mutation, and their biological consequences, are covered in later genetics chapters.
Molecular Heredity Across the Tree of Life
Despite the enormous diversity of living organisms, the molecular principles of heredity are strikingly similar across all known life forms:
- All cellular life uses DNA (with the same four bases) as its primary genetic material.
- The basic mechanisms of replication, transcription, and translation are strongly conserved.
- The genetic code is nearly universal.
- Many important genes (for example, those involved in energy metabolism or cell division) show clear similarities between bacteria, plants, animals, and fungi.
This shared molecular toolkit reflects a common evolutionary origin and underlines that modern biology views heredity as a unifying molecular process across the biosphere.
Summary of Core Molecular Principles of Heredity
- Heredity is grounded in nucleic acids (DNA, and in some systems RNA) that store, copy, and transmit genetic information.
- Nucleic acids are linear polymers of nucleotides whose base sequence encodes information.
- Complementary base pairing allows accurate template-based copying and underlies the stability and repair of hereditary information.
- The information in DNA is interpreted through RNA to produce proteins, which execute most biological functions.
- The genetic code defines how nucleotide sequences are translated into amino acid sequences.
- Replication fidelity ensures stable inheritance, while mutations introduce variation, linking molecular heredity to evolution.
- The fundamental molecular mechanisms of heredity are shared by all known life, reflecting a common origin.