Lesson 19: Genomics
19.1 Genomics Overview and Mapping
Genomics is the branch of biology focused on mapping, annotating, and analyzing whole genomes. It integrates classical and molecular genetics to understand the structure, function, and relatedness of genomes. Technological advances have dramatically increased understanding of genomes:
Mid-1970s: First genes isolated.
Mid-1990s: First complete bacterial genome, Haemophilus influenza, sequenced.
2003: Human genome sequenced.
Today: Entire genomes (billions of nucleotide base pairs) can be sequenced in hours.
Genome Maps
Maps in genomics have varying levels of resolution:
Genetic Maps:
Derived from recombination frequency and linkage analysis.
Provide relative locations of genes or genetic markers.
Physical Maps:
Provide precise positions in the genome, often at the nucleotide level.
Use landmarks within the DNA sequence as markers (e.g., enzyme cutting sites, small nucleotide segments).
Show the absolute location of a marker.
Distances between markers are measured in base pairs (bp) or kilobase pairs (kbp).
Restriction Maps: One of the earliest types, created by digesting DNA with restriction enzymes to map cut sites. These allow researchers to cut and remove specific DNA pieces for study.
Chromosome Maps: Low-resolution maps used for studying whole chromosomes or translocating genes. They use various staining techniques and are viewed with a microscope, allowing researchers to view entire chromosomes or genomes at a glance.
Sequence Tagged Sites (STS): Small, unique DNA stretches amplified using PCR that occur at only one location in the genome. Researchers use STSs to identify DNA fragment locations and piece together fragments by analyzing overlapping STS sites, organizing them into a contiguous sequence (contig).
19.2 Genome Sequencing
Advancements in sequencing technology have made it possible to sequence large genomes quickly and affordably. All sequencing methods rely on the polymerase chain reaction (PCR), electrophoresis, and chain-terminating nucleotides.
Chain-Terminating Nucleotides (Dideoxynucleotides)
Lack the \text{-hydroxyl group}, preventing further phosphodiester bond formation and thus terminating DNA synthesis.
Early manual sequencing, developed by Fred Sanger, utilized multiple replication reactions, which was time-consuming.
Automated Sequencing
Involves a template strand of unknown sequence subjected to PCR.
Four different dideoxynucleotides, each labeled with a different color fluorescent tag, are incorporated.
DNA fragments are separated in a single capillary tube by electrophoresis; smallest fragments migrate fastest.
A laser and photo-detector identify the fluorescent dideoxynucleotides, determining the sequence.
Next-Generation Sequencing (NGS)
Developed recently, offering speed, affordability, and the ability to sequence larger DNA fragments.
The DNA sample is fragmented, and single-stranded fragments are attached to a solid surface.
Multiple copies of each fragment are created via PCR.
Synthesis begins with DNA polymerase, incorporating special reversible chain-terminating nucleotides that fluoresce.
These reversible terminators allow each fragment to be sequenced multiple times, greatly increasing efficiency.
Genome Assembly
Most genomes are too large for single-step sequencing, requiring fragmentation, sequencing, and then assembly into a contig by matching overlapping sequences.
Clone-Contig Method:
Genome fragmented into large DNA pieces (clones).
Clones are ordered based on physical mapping landmarks (e.g., STS sites).
Clones are then broken into smaller, sequencesable fragments.
These fragments are sequenced and assembled to create clone sequences, which are then combined into a large contiguous segment of DNA (contig).
Shotgun Method:
Does not rely on genetic or physical maps.
Whole genome is fragmented into manageable pieces for sequencing.
Computer software assembles all pieces based on overlapping nucleotide regions.
Both methods were used for the Human Genome Project.
Human Genome Project
The first draft was published in 2000, a result of over years of collaboration between the U.S. government and Celera.
Utilized genetic maps, clone-contig, and shotgun sequencing methods.
A surprising finding was that the human genome contained only about genes, far fewer than the predicted , indicating that organismal complexity is not solely a function of genome size or gene number.
19.3 Genome Annotation
Genome annotation assigns information, such as the number, location, and type of genes, and their contribution to phenotype, to genomic sequences. This information is typically stored in searchable online databases.
Open Reading Frame (ORF): A potential gene sequence, characterized by a start codon, a series of amino acids, and a stop codon.
GenBank: A prominent online database that serves as an annotated collection of publicly available DNA sequences.
Basic Local Alignment Search Tool (BLAST): Used by researchers to compare an unknown gene sequence to those in databases, providing insights into its potential function if similar known sequences are found.
Coding Sequences: DNA sequences that produce protein or functional RNAs (e.g., transfer RNA, ribosomal RNA); these are annotated in online databases.
Non-Coding DNA
A significant portion of DNA, especially in eukaryotic genomes, does not code for proteins; it can comprise as much as \text{ of the human genome}.
Categories include introns, structural DNA, simple sequence repeats, non-coding RNA involved in gene regulation, segmental duplications, pseudogenes, and transposable elements (DNA sequences that can move and often copy themselves).
ENCODE Project (Encyclopedia of DNA Elements)
Conducted from 2003 to 2012, this collaborative effort aimed to identify all functional elements in the human genome.
Conclusion: The project suggested that \text{ of DNA in the human genome is functional}.
Definition of "Functional" DNA: In addition to traditional coding regions, ENCODE researchers also considered sequences with reproducible "biological activity" as functional. Examples include DNA methylation, chromatin modification, and sites cut by DNase enzymes.
Criticisms of ENCODE's Definition:
Critics argue that "biological activity" alone is insufficient, as DNA replicated by DNA polymerase would also be considered active.
Merely locating enzymatic activity is not proof of a regulatory function or gene product.
Evolutionary biologists argue that only elements selected through evolutionary pressure and providing a benefit to the organism should be considered functional.
Despite controversy, the ENCODE project substantially contributed to human genome mapping.
19.4 Genome Analysis
Genome analysis is concerned with identifying the role or purpose of DNA elements in the genome. Primary approaches include comparative genomics, functional genomics, and proteomics.
Comparative Genomics
Uses information from one genome to learn about a second genome.
Synteny: Refers to the conserved arrangements of segments of DNA in related genomes, derived from comparing physical maps and sequence data.
Researchers use syntenous regions to predict gene function, locate similar genes, or infer evolutionary relatedness (e.g., between grains like rice, sugarcane, corn, and wheat, which diverged about million years ago).
Functional Genomics
Uses biotechnology to highlight the connection between genotype and phenotype by characterizing RNA products and proteins encoded by genes.
Broken into three categories:
Study of all RNA molecules (the transcriptome).
Study of all proteins (the proteome).
Study of protein interactions.
Transcriptome Analysis
DNA Microarrays: Tools enabling researchers to determine which genes are expressed at a particular location or time. Each dot on a chip represents a known gene, with colors indicating expression levels. Requires designing a chip with "known" genes.
RNA Sequencing (RNA-seq): Uses next-generation sequencing to capture all mRNA transcripts at a given time, providing comprehensive information on gene expression beyond just suspected genes.
Note: mRNA expression does not guarantee protein translation due to post-translational regulation.
Proteomics
The study of the proteome, or the collection of proteins encoded in the genome.
More challenging to analyze than the transcriptome due to alternative splicing and post-transcriptional modifications.
Mass Spectrometry (Mass-spec):
Proteins are isolated, separated, and cut into peptides by protease enzymes.
Peptides are separated, ionized, and their charge-to-mass ratio is calculated and compared to databases for protein identification.
Can also fragment peptides to determine component amino acids.
Protein Microarrays: Similar to DNA microarrays, but antibodies are applied to the chip to identify proteins in a sample.
Bioinformatics
The application of computer programming, mathematics, and modeling to analyze large amounts of biological data.
Applied to proteomics for rapid identification of proteins.
Used to predict protein structure from DNA and amino acid sequences based on chemical properties and interactions.
Applications and Ethical Considerations of Genomics
Synthetic Biology: Considered the next frontier, aiming to engineer organisms to solve problems (e.g., enhancing biofuel production, cleaning environmental disasters).
In 2010, scientists successfully constructed and inserted a synthetic bacterial genome into Mycoplasma mycoides.
Diagnostics: Identification of genes responsible for genetic disorders.
Forensics: Genomic sequencing and STS mapping to identify unknown remains or pathogens (e.g., reconstructed 1918 Spanish Influenza virus, SARS-CoV-2 research).
Ethical Concerns: Misuse of technology and ownership of gene sequences.
2013 Supreme Court ruling: Biotech company Myriad Genetics could not patent natural gene sequences (e.g., for breast cancer genes) but could patent synthetic sequences created using them.