Fundamentals of Genetics, Evolution, and DNA Technologies

Historical Landmarks in Genetics and Genomics

$1865$ : Gregor Mendel's work on the laws of inheritance was published.
$1900$ : Mendel's laws were rediscovered.
$1913$ : The first genetic linkage map was developed.
$1944$ : DNA was identified as the genetic material.
$1953$ : The double helix structure of DNA was discovered by James Watson and Francis Crick, with contributions from Rosalind Franklin and Maurice Wilkins.
$1961$ : The genetic code was deciphered.
$1966$ : The genetic code was fully cracked by Marshal Nirenberg and others.
$1972$ : The first recombinant DNA molecule was created by Paul Berg.
$1977$ : Frederick Sanger developed DNA sequencing (Sanger Sequencing) techniques.
$1982$ : The first recombinant DNA drug (insulin) was approved by the FDA.
$1983$ : Kary Mullis developed the Polymerase Chain Reaction (PCR) technique (awarded Nobel Prize in Chemistry in $1993$ ).
$1990$ : The Human Genome Project (HGP) was officially launched in the United States by the National Institutes of Health (NIH) and the Department of Energy (DOE).
$1995$ : The first bacterial genome (Haemophilus influenzae, $1.83\,Mb$ ) was sequenced and published in Science.
$1996$ : The sequence of the first human chromosome ( $22$ ) was completed.
$1999$ : The first human chromosome ( $22$ ) sequence was published.
$2000$ : A draft version of the human genome sequence was announced by President Bill Clinton and Prime Minister Tony Blair.
$2003$ : The Human Genome Project was completed, covering $99\%$ of the genome with an error rate of less than $1$ in $10,000$ bases.
$2013$ : The first multi-gene sequencing diagnostic test for tumor profiling was introduced in the NHS.
$2015$ : The first patients in the $100,000$ Genomes Project received diagnoses.
$2017$ : Release of genome-wide genotype data for $500,000$ UK Biobank participants.
$2020$ : Routine use of Whole Genome Sequencing (WGS) began in the NHS.
$2022$ : A truly complete ("telomere-to-telomere") human genome sequence was generated.

Understanding the Genome

Etymology and Definition: * The term "Genome" was coined in $1920$ by Professor Hans Winkler, a Professor of Botany at the University of Hamburg. * Winkler's original proposal defined the "Genom" as the haploid chromosome set which, along with the pertinent protoplasm, specifies the material foundations of the species. * Modern definition: A genome is an organism’s complete set of DNA, including all of its genes.
Human Genomic Organization: * The human genome is spread over $46$ chromosomes ( $23$ pairs). * Inheritance: $23$ chromosomes are inherited from the mother (egg) and $23$ from the father (sperm).
Cellular Categories: * Somatic Cells: These make up most of the body and contain two genome copies (diploid). * Germ Cells: Egg and sperm cells. They start with two copies but end with one genome copy each after meiosis, prior to fertilization.
Gene Expression: The genome contains tens of thousands of genes ( $ca. 24,000$ ). Genes are only "switched on" or expressed when needed.

DNA Structure and Chemistry

Composition: DNA stands for Deoxyribonucleic Acid. It is a two-stranded molecule with a double helix shape.
Nucleotide Components: Each nucleotide consists of: 1. A sugar molecule (deoxyribose). 2. A phosphate group. 3. A nitrogenous base.
Bonding: * Glycosidic bond: Connects the nitrogenous base to the sugar molecule. * Phosphodiester bond: Attaches the phosphate group to the sugar molecule, forming the "backbone."
Nitrogenous Bases: Categorized as Heterocyclic, Aromatic organic compounds. * Purines: Two-carbon nitrogen ring bases; larger than pyrimidines. Examples include Adenine ( $A$ ) and Guanine ( $G$ ). Catabolism end product is uric acid. Primarily synthesized in the liver. * Pyrimidines: One-carbon nitrogen ring bases. Examples include Thymine ( $T$ ) and Cytosine ( $C$ ). Catabolism end products are ammonia and carbon dioxide. Synthesized in a variety of tissues.
Directionality and Polarity: * Strands have a beginning and end designated as $5'$ (five prime) and $3'$ (three prime). * Strands run antiparallel: one runs $5'$ to $3'$ (sense strand) and the other runs $3'$ to $5'$ (antisense strand).

DNA Replication

Definition: The process by which a cell makes an identical copy of its genome before division.
Semi-Conservative Nature: Each new DNA molecule consists of one original (old) chain and one newly synthesized chain.
Process Steps: 1. Unwinding: The enzyme Helicase "unzips" the double helix by breaking hydrogen bonds between complementary bases ( $A$ with $T$ , $C$ with $G$ ). 2. Replication Fork: The separation creates a "Y" shape called a replication fork. The separated strands act as templates. 3. Primer Binding: An enzyme called Primase produces a short RNA piece called a primer, which binds to the strand to mark the starting point for synthesis. 4. Leading Strand Synthesis: Oriented $3'$ to $5'$ toward the fork. DNA Polymerase adds nucleotides continuously in the $5'$ to $3'$ direction. 5. Lagging Strand Synthesis: Oriented $5'$ to $3'$ away from the fork. Replicated discontinuously. Numerous RNA primers are bound at various points. 6. Okazaki Fragments: Chunks of DNA added to the lagging strand in the $5'$ to $3'$ direction. 7. Primer Removal: Exonuclease strips away RNA primers. The resulting gaps are filled with complementary nucleotides. 8. Proofreading: The new strand is checked for sequence mistakes. 9. Ligation: The enzyme DNA Ligase seals the DNA sequence into two continuous double strands.

Polymerase Chain Reaction (PCR)

Definition: A laboratory technique used to make millions to billions of copies of a specific segment of DNA.
Components and Primers: * Uses short synthetic DNA fragments called primers (typically $18$ to $25$ nucleotides long). * Primers target unique sequences to identify specific parts of the genome, such as a gene.
Workflow: 1. Denaturation: The reaction mixture is heated to break hydrogen bonds, separating the two DNA strands. 2. Annealing: Temperature is lowered to allow primers to bind to the selected segment. 3. Synthesis (Extension): DNA polymerase synthesizes new strands complementary to the templates. 4. Repetition: Multiple rounds (typically $20$ to $30$ cycles) amplify the segment exponentially.

Gel Electrophoresis

Function: Separates DNA fragments based on size and charge.
Mechanism: * DNA has a negatively charged phosphate backbone due to bonds between oxygen and phosphorus atoms. * An electric current is run through an agarose gel containing the DNA. * DNA molecules move toward the positive electrode ( $+$ ). * Speed and Size: Smaller fragments travel through the gel pores faster than larger fragments, allowing separation into distinct bands.

Recombinant DNA (rDNA) Technology and Cloning

Concept: Combining DNA from different sources in a lab to manipulate or isolate segments of interest.
Enzymatic Tools: * Restriction Enzymes: DNA-cutting enzymes that recognize specific target sequences. They produce either "staggered" cuts (sticky ends with single-stranded overhangs) or blunt ends. * DNA Ligase: A DNA-joining enzyme that links matching DNA ends together.
DNA Cloning Process: 1. A target gene is inserted into a circular DNA vector called a plasmid. 2. Transformation: The process of introducing DNA into a cell. Bacteria are often used for storage and rapid replication. 3. Heat Shock: High temperatures are used to change the bacterial membrane, enabling the plasmid to enter. 4. Selection: Antibiotics are used to select only the bacteria that successfully carry the plasmid. 5. Expression: Bacteria may be induced to express the gene and produce proteins.

Genes and Epigenetics

Gene Definition: A segment of DNA carrying information for a specific protein or RNA. * Alternative Splicing: A single gene can specify more than one protein. * Count: Humans have approximately $24,000$ genes.
Central Dogma: DNA is transcribed into messenger RNA (mRNA), which is then translated by ribosomes into proteins.
Epigenetics: Changes in gene activity that do not involve alterations to the underlying DNA sequence. Influenced by diet, behavior, and environment (e.g., pollution). * DNA Methylation: Methyl marks added to DNA bases to repress gene activity. * Histone Modification: Molecules attach to the "tails" of histone proteins, altering how tightly DNA is wrapped and thus affecting activity.

DNA Sequencing Technologies

First Generation: Sanger Sequencing

Inventor: Frederick Sanger (“Father of Genomics”).
Methodology: Known as the Chain-Termination Method.
Key Requirements: * Single-stranded DNA template. * Primer (short oligonucleotide) to provide a free $3'$ hydroxyl ( $OH$ ) group. * DNA Polymerase. * Normal deoxynucleotides ( $dATP$ , $dTTP$ , $dGTP$ , $dCTP$ ). * Dideoxynucleotides ( $ddNTPs$ ): Chain-terminating sugars ( $2',3'$ dideoxyribose) that lack the $3'$ $OH$ group required for further addition.
Mechanism: A small amount of one $ddNTP$ (e.g., $ddATP$ ) is added. Termination occurs at different positions across different strands, creating a "family" of fragments of varying lengths.
Automated Sequencing: Modern versions use fluorescently labeled $ddNTPs$ (each base a different color). These are separated by capillary electrophoresis and detected by a laser, with data fed directly to a computer.

Second Generation: Next-Generation Sequencing (NGS/Illumina)

Terminology: Whole-genome massively parallel sequencing.
Mechanism: Based on "Sequencing by Synthesis" ( $SBS$ ).
Workflow: 1. Fragmentation: DNA is broken into small pieces ( $75-400\,bp$ ). 2. Library Prep: Adapters are ligated to fragments. 3. Flow Cell Attachment: Fragments attach to a high-density flow cell. 4. Clonal Amplification: Bridge amplification creates millions of clusters (each cluster is a clone of a fragment). 5. Imaging: Fluorescently labeled nucleotides are incorporated; a high-resolution camera captures images of the flow cell to identify the base color.
Output: Generates hundreds of millions to billions of short sequencing reads (typically $75-150$ bases).

Third Generation: Long-Read Sequencing

Characteristics: Sequences single molecules without the need for PCR amplification.
Platforms: * Pacific Biosciences (PacBio): Uses Zero-Mode Waveguides ( $ZMW$ ). Produces extremely long reads ( $14,000$ to $40,000\,bp$ ). Can detect base modifications/epigenetics. * Oxford Nanopore Technologies: Detects changes in electrical signal/current as a single DNA or RNA molecule passes through a nanopore. Capable of sequencing $10,000$ bases per second in real-time.

Genomic Projects and Precision Medicine

The Human Genome Project (HGP): * Collaborative international consortium (US, UK, France, Germany, Japan, China). * Duration: $1990\text{--}2003$ ( $13$ years). * Cost: $\$3\,billion$ ( $2.7\,billion$ reported in some contexts). * Methods: Used Hierarchical Shotgun Sequencing.
100,000 Genomes Project (Genomics England): * Participants: NHS patients with rare diseases (and their families) and cancer patients. * Goal: Aimed to transform NHS care by offering diagnoses and creating a genomic medicine service.
Genome UK Strategy ( $2020$ ): Three core pillars: 1. Diagnosis and Personalized Medicine: Incorporating genomics into routine care. 2. Prevention: Using genomics for predictive and preventative care. 3. Research: Supporting fundamental and translational research.
Variation: A single base difference is known as a Single Nucleotide Polymorphism (SNP), also called a Single Nucleotide Variant or Single Base Mutation.
Personalized Medicine: Technology now allows a human genome to be sequenced in a few days for less than $\pounds1,000$ . The primary remaining challenge is data analysis.