BackDNA, RNA, and Genetic Information: Structure, Replication, and Applications
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DNA Structure and Function
DNA as a Nucleic Acid
DNA (deoxyribonucleic acid) is the hereditary molecule of all living organisms. It stores genetic information and passes it from one generation to the next. DNA's structure—a double helix made of repeating nucleotides—allows it to carry, copy, and mutate biological information.
DNA is one of two nucleic acids (the other is RNA).
Main function: information storage.
DNA is a polymer, meaning it is made of repeating monomers.
The monomers of DNA are nucleotides.
Chromosomes
DNA molecules coil and fold into long fibers called chromosomes.
Human body cells contain 46 chromosomes.
Each chromosome contains one long DNA molecule, which is compacted to organize the DNA.
The Double Helix
DNA is a double helix: two strands twisted together like a candy-cane spiral.
The two strands are held together by hydrogen bonds between bases.
Universality
DNA's chemical structure is the same in all living organisms—bacteria, plants, humans, etc.
Nucleotides: Parts of a Nucleotide
Each nucleotide has three parts:
Sugar: Deoxyribose (a 5-carbon sugar)
Phosphate group: Negatively charged, forms part of the DNA backbone
Nitrogenous base: Four types:
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
Base Structure
A and G are purines – double-ring structures.
C and T are pyrimidines – single-ring structures.
DNA Alphabet
All genetic information is encoded using this four-letter code: A, T, C, G.
Polynucleotide Structure
A polynucleotide is a long chain of nucleotides.
DNA contains two polynucleotide strands wound together to form the helix.
Backbone: Made of alternating sugar and phosphate groups.
Backbone is identical in all DNA across all life.
Sequence
The order of bases along the strand makes each organism unique.
Base Pairing Rules
Bases form specific pairs via hydrogen bonds:
A pairs with T
G pairs with C
Because of base-pair rules, the sequence on one DNA strand determines the sequence on the opposite strand.
Example: If one strand reads A T G C C A, the complementary strand reads T A C G G T.
DNA Replication
Semi-Conservative Replication
DNA replication ensures that genetic information is faithfully passed to new cells. The double helix unwinds, each strand serves as a template, and enzymes build new complementary strands. Each new DNA molecule contains one original strand and one new strand; DNA replication is semi-conservative.
After replication, each new DNA molecule consists of:
1 original (parent) strand
1 newly synthesized strand
Example:
Original DNA: Left: T G A A G G G C A; Right: A C T T C C C G T
After replication:
Copy #1: original left + new right
Copy #2: original right + new left
The Process of DNA Replication
Double Helix Is Unwound
Helicase binds at origins of replication.
Helicase unwinds the double helix by breaking hydrogen bonds.
This forms a replication bubble where bases are exposed.
New Strands Are Synthesized
Each separated strand is bound by DNA polymerase:
DNA polymerase reads the template strand, adds complementary nucleotides (A↔T, C↔G), and builds the new DNA strand.
Another protein helps hold DNA in place during replication.
DNA Fragments Are Joined
DNA polymerase produces short fragments (especially on the lagging strand).
DNA ligase fuses these fragments.
The final product is a continuous, complete DNA molecule.
Fun Fact: DNA replication inside a cell occurs at about 50 nucleotides per second!
DNA vs. RNA: Nucleic Acids Compared
Key Structural Differences
Both DNA and RNA are polymers of nucleotides, but they differ in:
Number of Strands
DNA: Double-stranded, forms the classic double helix.
RNA: Single-stranded, often shows as a single ribbon.
Sugar Component
DNA: Deoxyribose (missing an oxygen at carbon 2).
RNA: Ribose (has an OH group at carbon 2).
Nitrogenous Bases
DNA: A, G, C, T (thymine contains a CH3 group).
RNA: A, G, C, U (uracil replaces thymine; uracil is similar to thymine but has H instead of CH3).
Summary Table: DNA vs. RNA
Feature | DNA | RNA |
|---|---|---|
Strands | Double-stranded | Single-stranded |
Sugar | Deoxyribose | Ribose |
Bases | A, T, C, G | A, U, C, G |
The Flow of Genetic Information: Central Dogma
DNA → RNA → Protein
The Central Dogma of biology describes how genetic information flows from DNA to RNA to protein. DNA stores genetic information, RNA carries the message, and ribosomes use that message to build proteins.
DNA directs the production of RNA (transcription).
RNA directs the production of proteins (translation).
Proteins create traits (e.g., eye color, hair color, blood type, cholesterol levels, enzyme activity).
Replication, Transcription, and Translation
Replication: DNA → DNA
Transcription: DNA → RNA
Translation: RNA → Protein
Transcription (Making RNA from DNA)
Transcription is the process of copying information from DNA to RNA, specifically mRNA, which carries the message to be used in protein production.
Occurs in the nucleus of eukaryotic cells.
Steps:
RNA polymerase binds the promoter ("Start Here").
RNA polymerase opens the DNA double helix and uses one strand as a template.
RNA polymerase builds the RNA strand one nucleotide at a time, following base-pair rules (A→U, T→A, C→G, G→C).
Transcription ends when RNA polymerase reaches a terminator.
RNA processing (in eukaryotes): Introns are removed, exons are spliced together, and a cap and tail are added.
Translation (RNA Becomes Protein)
Translation is the process by which a molecule of mRNA is used to build a protein.
Occurs in the cytoplasm, inside ribosomes.
Requires three types of RNA:
mRNA – carries the message
tRNA – brings amino acids
rRNA – part of the ribosome structure
Each ribosome reads mRNA three bases at a time (a codon), each codon corresponds to one amino acid.
tRNA has two important ends:
Anticodon: a three-base sequence that pairs with an mRNA codon
Amino acid attachment site: holds the correct amino acid
Genetic Code:
A codon = 1 amino acid
Some amino acids are encoded by multiple codons (e.g., AAA and AAG both = lysine)
Start codon: AUG (methionine)
Stop codons: UAA, UAG, UGA (signal translation ends)
Translation Phases
Initiation: Ribosome assembles at the start codon of mRNA.
Elongation: Ribosome reads each codon, tRNA brings the correct amino acid, polypeptide grows.
Termination: Ribosome reaches a stop codon, releases the completed polypeptide.
Gene Regulation and Expression
Gene Regulation in Action
Gene regulation determines which genes are turned on or off, controlling which proteins are produced in a given cell. This allows cells to specialize and perform different functions.
Gene Type | Intestinal Cell | Nerve Cell | White Blood Cell |
|---|---|---|---|
Glucose-digesting enzyme | Active | Active | Active |
Antibody gene | Not active | Not active | Active |
Lactase gene | Active | Not active | Not active |
Hemoglobin gene | Not active | Not active | Active |
X Chromosome Inactivation
In female mammals, one of the two X chromosomes in each body cell is highly compacted and mostly inactive.
This ensures that males (one X) and females (two Xs) have the same number of active X genes.
The inactive X chromosome forms a Barr body, visible under a microscope.
Points of Gene Regulation
Transcription Control: Most genes in eukaryotic cells are initially "off." Transcription factors (proteins) must bind to DNA to initiate transcription.
RNA Processing Control: After transcription, RNA may be spliced and modified. A cap and tail are added. Alternative splicing can produce different mRNAs from the same gene. Small RNAs (microRNAs) can bind to mRNA to prevent translation.
Protein Control: Regulation can occur during translation or after the protein is made (activation, modification, degradation).
Signal Transduction Pathways and Gene Expression
Signal Transduction
Cells send signal molecules that bind to receptor proteins on target cells. This triggers a signal transduction pathway, a chain of relay molecules that can result in gene regulation (genes turned on or off, producing mRNA and proteins).
Cell-to-Cell Signaling in Embryonic Development
Development requires careful coordination of cell division and differentiation.
Neighboring cells send chemical signals to guide organ formation and body organization.
Homeotic genes produce signals that turn other genes on or off, establishing body structure (head, tail, limbs, etc.).
Mutations and Their Effects
Key Concept
A mutation is any change in the genetic information of a cell or virus. Mutations can be beneficial, harmful, or neutral. They are the raw material for evolution, but they often reduce fitness.
Causes of Mutations
Spontaneous mutations: Occur during DNA replication errors.
Mutagens: Physical or chemical agents that damage DNA (e.g., UV radiation, X-rays, tobacco chemicals).
Types of Mutations
Point Mutations: Change a single nucleotide in DNA.
Silent mutation: No change in protein.
Nonsense mutation: Changes an amino acid codon to a stop codon, producing a truncated, usually defective protein.
Missense mutation: Changes an amino acid codon to a different amino acid, often producing a different and defective protein.
Example: Sickle-cell disease: A single nucleotide change (A → T) in the hemoglobin gene.
Frameshift Mutations: Occur when nucleotides are inserted or deleted, shifting the reading frame of mRNA codons.
Loss of Gene Expression Control and Cancer
Key Concept
The cell cycle control system normally controls cell division. Mutations in genes that regulate the cell cycle can lead to uncontrolled cell growth, forming tumors. If tumor cells spread, this condition is called cancer.
Proto-Oncogenes and Oncogenes
Proto-oncogenes: Normal genes that code for proteins regulating the cell cycle.
Oncogenes: Mutated versions of proto-oncogenes that cause uncontrolled cell division.
Mutations in tumor-suppressor genes may produce faulty proteins, making cells ignore "stop" signals.
Summary Table: Growth Factors
Condition | Protein Behavior | Result |
|---|---|---|
Normal growth factor gene | Stimulates division appropriately | Cell cycle regulated |
Mutated growth factor gene | Overactive; stimulates division inappropriately | Tumor forms |
Tumor-Suppressor Genes
Normally produce proteins that inhibit cell division.
Mutation disables the gene → loss of growth inhibition → uncontrolled division.
Example: p53 gene: More than 50% of human tumors have a mutation in this tumor-suppressor gene.
Cancer: Out-of-Control Cell Growth
Progression of Cancer
Mutation of Proto-Oncogenes: Normal genes that regulate cell growth mutate into oncogenes, causing uncontrolled division.
Tumor Formation: The mutated cell and its descendants multiply to form a tumor.
Spread (Metastasis): Cancer cells move from their original site to other parts of the body.
Types of Tumors
Tumor Type | Spread Ability | Notes |
|---|---|---|
Benign | No | Usually not fatal; can disrupt organs |
Malignant | Yes | Can metastasize; considered cancer |
Cancer Prevention and Treatment
Some mutations are inherited or occur spontaneously—cannot be prevented.
Most are caused by carcinogens (environmental agents promoting cancer).
Lifestyle choices that reduce risk:
Not Smoking
Sun Protection
Healthy Diet
Regular Exercise
Regular Screenings
Cancer Treatment:
Surgery: Removes tumors
Radiation Therapy: Targets and damages cancer cell division
Chemotherapy: Drugs disrupt cell division throughout the body
Genetic Engineering and Laboratory Manipulation of DNA
Genetic Engineering
Genetic engineering is the direct manipulation of genes for practical purposes, often to produce pharmaceuticals, improve crops, or study gene function.
Biotechnology has been around since prehistoric times (e.g., selective breeding, yeast in bread-making).
DNA Technology
DNA technology refers to a set of methods for studying and manipulating genetic material.
Recombinant DNA: DNA molecules created by combining sequences from two different sources.
Restriction Enzymes
Restriction enzymes are proteins that cut DNA at specific nucleotide sequences.
Each enzyme recognizes a specific short DNA sequence and cuts exactly at that location.
DNA from different sources cut with the same enzyme can be joined, allowing gene splicing and cloning.
Cloning a Gene
Gene Isolate: Identify and isolate the gene that produces the desired protein.
Insertion into a Plasmid: Insert the gene into a bacterial plasmid (circular DNA).
DNA Cloning: Bacteria replicate, producing large quantities of the gene and its protein product.
Laboratory Manipulation of DNA
Genomic Libraries
A genomic library is a collection of DNA fragments representing an organism’s entire genome.
Steps:
Cut the genome with restriction enzymes into fragments.
Insert each fragment into a plasmid (small circular bacterial DNA).
The resulting set of plasmids contains every part of the genome, making it possible to locate and manipulate any gene.
Nucleic Acid Probes
A nucleic acid probe is a molecule labeled with radioactive or fluorescent tags.
It binds to a DNA fragment with a matching sequence via base-pairing rules, helping researchers identify the gene of interest.
Gene Editing: CRISPR-Cas9
CRISPR-Cas9 allows scientists to edit specific genes in living cells.
How it works:
Guide RNA binds to the target DNA sequence.
The Cas9 protein cuts the DNA at the targeted location.
DNA repair enzymes fix the break, allowing researchers to insert, remove, or replace DNA.
DNA Synthesis
DNA can be created from scratch using automated DNA synthesizers.
Machines can produce sequences with several hundred nucleotides, which can be joined to make longer DNA molecules.
Genetically Modified Plants and Animals
Genetically Modified Organisms (GMO)
GMO: An organism that has acquired one or more genes by artificial means.
Transgenic organism: An organism that carries a gene from another species (e.g., a plant with a bacterial gene).
Applications: Transgenic GMOs are one of the most widespread uses of genetic engineering.
Transgenic Plants and Animals
Method for plants:
Insert a desired gene into a plasmid.
Use the plasmid to transfer the gene into the plant’s genome.
Result: The plant expresses the new trait from the inserted gene.
Method for animals: Agricultural scientists are modifying food animals to make them healthier or more productive. Example: Fast-growing transgenic salmon (AquaAdvantage salmon) approved by the FDA in 2015.
Pharmaceutical GMOs can produce medically useful human proteins.
Safety and Ethical Concerns
Pros:
No evidence that GMOs pose special risks to humans or the environment.
Can be more nutritious and grow in diverse habitats.
Safety measures exist; more rigorous research is needed.
Cons:
Potential unknown hazards to health or the environment.
GMOs could escape into the wild, affecting ecosystems.
Widespread use could reduce biodiversity.
Polymerase Chain Reaction (PCR): Multiplying DNA Samples
PCR Technique
Definition: A laboratory technique used to target and copy a specific segment of DNA quickly and precisely.
Objective: Starting from a single DNA copy, PCR can produce billions of copies in just a few hours.
Uses:
Determining DNA profiles (forensics, ancestry analysis)
Studying relationships among organisms
Screening for disease-causing genes
PCR Steps
Thermal cycler: The DNA sample is repeatedly heated and cooled.
Denaturing DNA: Heating and cooling denatures the double strand.
DNA polymerase: Synthesizes a new complementary DNA strand.
Targeting and Copying DNA
Primers: Short (15–20 nucleotides) single-stranded DNA molecules that bind to sequences at each end of the target region.
DNA polymerase binds to the primers and synthesizes new DNA strands using free nucleotides.
Including the correct primers and reagents, large amounts of DNA for a specific region can be produced for further experimentation.
DNA Profiling: STR Analysis and Gel Electrophoresis
DNA Profiling
Definition: A set of laboratory techniques used to determine whether two DNA samples come from the same individual.
Short Tandem Repeats (STRs): DNA regions where a short nucleotide sequence is repeated consecutively.
Key Features:
Locations and sequences of STR sites are identical in all humans.
The number of repeats varies among individuals, making STRs highly useful for identification.
STR Analysis
Purpose: Compare lengths of STR sequences at 13 predefined sites in the human genome.
Process:
Extract DNA from samples.
Use PCR to amplify the STR regions.
Compare the lengths of repeats between samples.
Significance: Variability in STR lengths ensures that no two people (except identical twins) share the same DNA profile at all 13 sites.
Gel Electrophoresis
Purpose: Visualize and compare STR lengths.
How it works:
DNA samples are loaded into a gel.
An electric current separates DNA (smaller fragments travel farther than larger ones).
Each lane shows several STR patterns.
Interpretation: Matching bands indicate shared STR lengths; mismatched bands indicate different individuals.
Whole Genomes Can Be Sequenced
Genome and Genomics
Genome: The complete set of genetic material in an organism or virus.
Genomics: The scientific study of genomes and their interactions.
History: Genomics began in 1995 when scientists sequenced the genome of Haemophilus influenzae, a disease-causing bacterium.
The Human Genome Project
Completion: In 2003, researchers sequenced virtually all human genes.
Key Findings:
Humans have 22 shared chromosomes + XY sex chromosomes.
Humans have 21,000 genes within 3 billion DNA nucleotides.
Human DNA similarity: Two humans of the same sex share >99.5% identical DNA.
Gene content and expression: Only 1.5% of human DNA codes proteins.
Protein-coding sequences: 24% of DNA helps control gene but does not encode proteins.
Repetitive DNA: 59% of DNA consists of repeated sequences.
Whole-Genome Shotgun Method
Definition: A set of techniques used to sequence an entire genome rapidly.
How it works: Combines multiple methods to generate large amounts of genomic data efficiently.
Proteomics
Definition: The study of the complete set of proteins encoded by a genome.
Significance: Proteins perform the essential tasks in cells, so understanding all human proteins (~100,000) is critical, especially since the number of proteins far exceeds the number of genes (~21,000).
Gene Therapy: Aims to Cure Genetic Diseases
What is Gene Therapy?
Definition: Gene therapy is the alteration of a person’s genes to treat or cure a disease.
Purpose: Targets diseases caused by mutations in a single gene.
Status: Proven to work in theory and has cured some patients, but overall effectiveness and safety remain limited.
Obtaining a Normal Gene
Isolation: The normal gene is isolated from an individual.
RNA Conversion: Enzymes convert the DNA gene into an RNA version.
Delivery Vehicle: The RNA gene is combined with an infectious but harmless retrovirus.
Result: A recombinant retrovirus carrying the normal gene, ready for therapy.
Injecting the Normal Gene
Target Cells: Bone marrow cells are preferred because they multiply continuously and produce the normal protein for a long time.
Goal: Cure the patient, potentially permanently, with a single injection.
Case Study: Severe Combined Immunodeficiency (SCID)
Disease: Caused by the absence of an enzyme required for immune function. SCID patients must remain in protective isolation (“bubbles”).
Results of Therapy:
Since 2002, 22 children have been cured using gene therapy.
Complication: 4 developed leukemia, and 1 died due to the retrovirus-related complication.
Implication: Gene therapy shows promise, but safety remains a major concern.
Additional info: These notes cover topics from Ch. 6 DNA: The Molecule of Life, Ch. 5 Chromosomes and Inheritance, and related chapters on genetic engineering, gene expression, and cancer, as outlined in a General Biology college course.
