Table of Contents
Overview: What Makes Heredity Possible?
All living things resemble their parents because they inherit instructions for building and running a cell or organism. These instructions are stored in long molecules called nucleic acids. In almost all organisms, the main genetic material is DNA; in some viruses, it is RNA.
In this chapter, the focus is on what is special about nucleic acids that allows them to act as carriers of genetic information, without yet going into detailed structure (covered in “Structure of DNA” and “Ribonucleic Acid (RNA)”) or exactly how information becomes protein (in “From Gene to Protein”).
Key points:
- Which molecules are genetic material.
- How we know nucleic acids (especially DNA) are the carriers of heredity.
- What “information” means in a biological sense.
- Why nucleic acids are particularly suited to store, copy, and transmit this information.
What Counts as Genetic Material?
In cells:
- DNA (deoxyribonucleic acid) is the primary hereditary material in bacteria, archaea, and eukaryotes (plants, animals, fungi, protists).
- DNA is organized into chromosomes. In eukaryotes, it is found mainly in the nucleus; smaller DNA molecules also occur in mitochondria and chloroplasts.
- The DNA molecule is divided into functional units called genes.
In viruses:
- The genetic material can be DNA or RNA.
- It can be single-stranded or double-stranded.
- It may be linear or circular.
Despite these differences, all genetic material shares three essential functions:
- Storage of information (stable over time).
- Replication (copying before cell division or before virus reproduction).
- Expression (providing instructions for making RNA and proteins).
Nucleic acids are the only class of biological macromolecules that routinely perform all three of these functions.
Why DNA Was Not Always Obvious as the Genetic Material
Before nucleic acids were accepted as carriers of heredity, many biologists believed proteins must be genetic material because:
- Proteins are made from 20 different amino acids and show enormous variety.
- DNA was thought to be a simple, monotonous polymer and therefore “too simple” to carry complex information.
Several key experiments changed this view and showed that DNA is the genetic material in cells and many viruses. You do not need all experimental details here, but it is important to understand what kind of evidence convinced scientists.
Bacterial Transformation (Griffith and Avery)
Griffith’s observation
- Griffith worked with two strains of the bacterium Streptococcus pneumoniae:
- A virulent (disease-causing) “smooth” (S) form.
- A non-virulent “rough” (R) form.
- When he mixed heat-killed S cells (no longer able to cause disease) with living R cells, some R cells were converted into S cells.
- This meant that some “transforming principle” from dead S bacteria survived heating and changed the hereditary properties of the R bacteria.
Griffith did not know what the transforming principle was, only that it was heritable.
Avery, MacLeod, and McCarty’s conclusion
Later, Avery and coworkers isolated different types of molecules (proteins, DNA, RNA, etc.) from the heat-killed S bacteria.
- When they treated the extracts with enzymes that destroyed proteins or RNA, transformation still occurred.
- When they treated the extracts with enzymes that destroyed DNA, transformation stopped.
Conclusion: The transforming principle is DNA, not protein. In other words, DNA can change the genetic properties of a cell, which is a property expected of genetic material.
Bacteriophage Infection (Hershey–Chase)
Bacteriophages are viruses that infect bacteria and consist mainly of:
- A protein shell.
- DNA inside.
Hershey and Chase labeled:
- DNA with radioactive phosphorus.
- Protein with radioactive sulfur.
They let these labeled phages infect bacteria and then separated the empty phage coats from the infected cells.
- Most of the radioactive DNA entered the bacterial cells.
- Most of the radioactive protein stayed outside, attached to empty phage coats.
New phages produced inside the bacteria contained radioactive DNA but little or no radioactive protein.
Conclusion: It is the DNA, not the protein, that enters cells and directs the formation of new viruses. Therefore, DNA acts as genetic material.
RNA as Genetic Material in Some Viruses
In many viruses (e.g., some plant viruses, influenza virus, coronaviruses), RNA serves as the genetic material.
Evidence includes:
- Purified viral RNA alone can sometimes infect cells and produce new virus particles.
- The replication cycle of these viruses can be traced to copying of RNA into more RNA (or, in retroviruses, into DNA using reverse transcriptase).
Conclusion: Although DNA is the universal genetic material in cellular life, RNA can also carry genetic information, especially in viruses.
What Is “Information” in a Biological Sense?
When we say DNA or RNA carries information, we do not mean “information” in the everyday sense (like a message written in human language). Instead, genetic information is:
- A sequence of smaller units (called nucleotides).
- This sequence determines:
- The order of amino acids in proteins.
- Which RNAs are produced.
- When and where these molecules are produced.
A useful analogy is text made from letters:
- Alphabet: A small set of symbols (letters).
- Words: Specific sequences of letters.
- Sentences: Strings of words that convey meaning.
Similarly, in nucleic acids:
- There are 4 basic “letters” (the nitrogenous bases) in DNA, and 4 in RNA (slightly different set).
- Specific sequences of these bases correspond to genes and regulatory elements.
- The same set of four letters can produce a huge number of possible sequences.
This idea is formalized later in the “Genetic Code” chapter, but the key point here is: information is encoded in the linear order of nucleotides.
Why Nucleic Acids Are Good Carriers of Genetic Information
Several properties make nucleic acids particularly suited to their role.
1. They Are Long, Linear Polymers Built from a Small Set of Units
Nucleic acids are polymers: long chains made of repeating subunits (nucleotides). Each nucleotide includes:
- A sugar.
- A phosphate group.
- A nitrogenous base (the “letter”).
Because only four different bases are used, but can be combined in any order, nucleic acids can:
- Store very large amounts of information.
- Do so using a simple, universal chemical alphabet.
Even a relatively short DNA molecule of 1,000 base pairs can, in principle, have $4^{1000}$ different possible sequences—an astronomically large number.
2. They Can Be Exactly Copied
A crucial property of genetic material is the ability to be copied faithfully but not perfectly (allowing rare mutations).
The key to this lies in complementary base pairing:
- In DNA, certain bases pair specifically (details appear in “Structure of DNA”).
- Each strand of DNA can act as a template to build its complementary strand.
- This built-in pairing rule allows the sequence information on one strand to determine the complementary sequence.
Consequences:
- Replication: Before cell division, DNA can be duplicated so each daughter cell receives a copy.
- Stability of genetic information: The two strands back each other up. Damage on one strand can often be repaired using the other as a guide.
In RNA viruses, copying is performed by RNA-dependent polymerases, but the same principle applies: base pairing allows an existing strand to guide synthesis of a new strand.
3. They Are Chemically Stable Enough for Long-Term Storage, Yet Changeable
For heredity, the information must:
- Be stable over an organism’s lifetime.
- Still allow rare changes (mutations), which are the raw material for evolution.
DNA is:
- Chemically more stable than RNA (partly due to its sugar and double-stranded structure).
- Protected and compacted in chromosomes and, in eukaryotes, further shielded within a nucleus.
RNA is:
- Generally less stable, usually functioning as a short-lived intermediate (for example, messenger RNAs).
- But in RNA viruses and some cellular RNAs (e.g., rRNA, tRNA), it can also be sufficiently stable to act as a genetic or structural molecule.
Mutations—changes in the nucleotide sequence—occur through rare errors in replication or damage. Their consequences are treated in detail in the “Mutation” chapters.
4. They Can Be Read and Interpreted by Cellular Machinery
Nucleic acids are not just passive storage devices; their sequences can be “read” and used to:
- Direct the synthesis of RNAs (via transcription).
- Specify the sequence of amino acids in proteins (via translation).
Cells contain specialized enzymes and structures that interact with nucleic acids in highly specific ways:
- Polymerases copy nucleic acid sequences.
- Ribosomes and associated molecules interpret RNA sequences when building proteins.
- Regulatory proteins and RNAs recognize particular nucleotide sequences to control when genes are active.
Thus, nucleic acids form the core layer of information on which most cellular processes depend.
DNA vs. RNA in Information Storage
Although both are nucleic acids, DNA and RNA play somewhat different roles in heredity and information flow.
DNA: Long-Term Storage
Functions:
- Primary genetic material in cellular organisms.
- Long-term storage of hereditary information.
- Passed from generation to generation.
Features relevant to its role:
- Usually double-stranded with complementary base pairing.
- Often very long and packaged with proteins (e.g., histones in eukaryotes).
- Relatively chemically stable, making it suitable for preserving information over long timescales.
RNA: Versatile Information Molecule and Genetic Material in Some Viruses
Functions:
- In most cells, RNA acts mainly as an intermediate or functional form of information:
- Messenger RNA (mRNA): temporary copy of information from DNA.
- Ribosomal RNA (rRNA), transfer RNA (tRNA): structural and functional roles in protein synthesis.
- Various regulatory RNAs.
- In many viruses, RNA itself is the genome—the hereditary material that is copied and passed on.
Features relevant to its roles:
- Usually single-stranded, allowing it to fold into complex 3D structures (important for many functions).
- Less stable than DNA, which suits roles where information should be temporary (e.g., mRNA), but requires special strategies for long-term genome stability in RNA viruses.
Genes, Genomes, and Hereditary Information
A few key organizational concepts help put nucleic acids into context.
Gene
A gene is a specific segment of DNA (or RNA in RNA viruses) that contains the information needed to produce:
- A functional RNA molecule, and often
- A protein (in protein-coding genes).
Important points for this chapter:
- A gene is defined by its nucleotide sequence.
- Changing the sequence can change the gene’s function or activity.
- Different versions of the same gene (alleles) differ in their nucleotide sequences.
Detailed discussion of gene expression and protein synthesis belongs to the chapter “From Gene to Protein”.
Genome
A genome is the complete set of genetic material of an organism or virus.
- In bacteria and archaea:
- Usually one main circular DNA molecule plus optional smaller DNA circles (plasmids).
- In eukaryotes:
- Multiple linear DNA molecules (chromosomes) in the nucleus.
- Additional smaller genomes in mitochondria and, in plants and algae, chloroplasts.
- In viruses:
- One or more molecules of DNA or RNA.
This entire nucleic acid content, with all its sequences, constitutes the full hereditary information of that organism or virus.
Heredity: Passing Nucleic Acid Information to the Next Generation
Heredity consists of transmitting this genetic information from parent(s) to offspring:
- During cell division, DNA is replicated and each daughter cell receives a copy.
- In sexual reproduction, offspring inherit combinations of nucleic acid sequences from both parents.
- In viruses, the nucleic acid genome is packaged into new virus particles and transmitted to new host cells.
Because nucleic acids can be copied with high fidelity, the information they carry remains largely consistent across generations, with occasional changes that lead to genetic variation.
Nucleic Acids and the “Central Dogma” (Preview)
A widely used summary of information flow in biology is:
$$
\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}
$$
This emphasizes that:
- DNA is usually the long-term storage.
- RNA is usually the working copy or functional molecule.
- Proteins are the main executors of cellular functions.
There are known exceptions (for example, reverse transcription in retroviruses), but the key role of nucleic acids as carriers and transmitters of information remains central in all cases.
Detailed mechanisms of these processes will be covered in later chapters:
- “Ribonucleic Acid (RNA)”
- “Genetic Code”
- “From Gene to Protein”
- “Retroviruses Contradict the Central Dogma”
Summary
- Nucleic acids (DNA and RNA) are the molecules that carry genetic information in all known forms of life and in viruses.
- Evidence from classic experiments with bacteria and viruses showed that DNA (and in some viruses, RNA) is the hereditary material, not protein.
- Genetic information is stored in the sequence of nucleotides along a nucleic acid strand, analogous to letters in a text.
- Nucleic acids are ideal carriers of heredity because they:
- Are long polymers built from a small, universal set of units.
- Can be copied accurately using base-pairing rules.
- Are stable enough for long-term storage yet subject to rare, heritable changes.
- Can be read and interpreted by cellular machinery to produce RNAs and proteins.
- Genes are defined stretches of nucleic acid encoding functional products; the genome is the entire genetic content of an organism or virus.
- Through replication and transmission of nucleic acids, hereditary information is passed from one generation to the next, forming the basis of genetics.