Chapter 16 a

Genomics & Transposition

Part 1: Transposable Elements

Transposable elements (TEs) are mobile genetic elements that can move within and between genomes. They play a significant role in genome structure, evolution, and disease. This section covers the types, mechanisms, and impacts of TEs.

• Transposable Elements (TEs): DNA sequences that can change their position within the genome, sometimes creating or reversing mutations and altering the cell's genetic identity.

• Selfish Genes: TEs are often referred to as 'selfish genes' because they propagate themselves within the genome, sometimes at the expense of host fitness.

• Genome Composition: TEs can constitute a large fraction of the genome, often more than protein-coding genes. In humans, ~45% of the genome is made up of TEs.

• Replication Mechanisms: TEs can replicate via RNA (retrotransposons) or DNA (DNA transposons) intermediates.

• Disease Association: TE insertions can disrupt gene function and cause diseases.

Types of Transposable Elements

TEs are classified into two major classes based on their transposition mechanism.

• Class I TEs (Retrotransposons): Move via an RNA intermediate. The process involves transcription, reverse transcription, and integration into a new genomic location. This is a 'copy-and-paste' mechanism, increasing the TE copy number.

• Class II TEs (DNA Transposons): Move via a DNA intermediate. The process involves excision and reintegration, typically a 'cut-and-paste' mechanism, which does not increase copy number unless replication occurs.

Comparison Table: Retrotransposons vs. DNA Transposons

Feature	Retrotransposons (Class I)	DNA Transposons (Class II)
Intermediate	RNA	DNA
Mechanism	Copy-and-paste	Cut-and-paste
Enzymes	Reverse transcriptase, integrase	Transposase
Copy Number Change	Increases	Usually unchanged
Similarity to Viruses	Similar to retroviruses (but lack env gene)	Not virus-like

Retrotransposons and Viruses

• Retrotransposons share structural similarities with retroviruses, including long terminal repeats (LTRs) and genes for reverse transcriptase and integrase.

• Key Difference: Retrotransposons lack the env gene, which is required for viral particle formation and infection.

• Examples: copia (Drosophila), Ty (yeast), L1 (human).

Integration of TEs into the Genome

TEs integrate into the genome, often creating flanking direct repeats and terminal inverted repeats at the insertion site.

• Flanking Direct Repeats: Short, identical sequences generated during TE insertion due to staggered cuts in the target DNA.

• Terminal Inverted Repeats: Sequences at the ends of the TE that are inverted complements of each other.

Mechanism of TE Integration

1. Transposase or integrase makes staggered cuts at the target site.

2. TE is inserted into the gap.

3. DNA polymerase fills in single-stranded gaps, creating direct repeats.

TE Insertions and Gene Disruption

TE insertions can disrupt gene function in several ways:

• Altering gene expression: Insertion near promoters can affect transcription.

• Disrupting reading frames: Insertion within exons can cause frameshift mutations.

• Disrupting splicing: Insertion within introns or splice sites can affect mRNA processing.

• No disruption: Some insertions occur in non-coding regions without functional consequences.

Alu Insertions and Human Disease

Alu elements are a type of SINE (Short Interspersed Nuclear Element) that can cause disease when inserted into genes.

Locus	Distribution	Subfamily	Disease
CaR	Familial	Ya4	Hypocalcemia, neonatal severe hyperparathyroidism
NF1	De novo	Ya5	Neurofibromatosis
PROGINS	About 50%	Ya5	Linked with ovarian carcinoma
BRCA2	De novo	Y	Breast cancer
GK	T	Y	Glycerol kinase deficiency
Additional info:	Table includes other loci and diseases such as XSCID, hemophilia, and complement deficiency.

Historical Context: Barbara McClintock

• Barbara McClintock discovered transposable elements in maize, earning the Nobel Prize in Physiology or Medicine in 1983.

• Her work predated the discovery of DNA as hereditary material and its structure.

Part 2: Introduction to Genomics

Genome Structure and Gene Density

Genomes vary widely in size and structure, but gene number does not always correlate with genome size. Gene density is highest in bacteria and archaea.

• Gene Density: Number of genes per megabase (Mb) of DNA. Bacteria/archaea have high gene density; eukaryotes have lower density due to non-coding and repetitive DNA.

• Genome Size: Can range from millions to billions of base pairs.

• Chromosome Organization: Genomes can be circular (prokaryotes) or linear (eukaryotes), with varying numbers of chromosomes.

Comparison Table: Genome Size and Gene Density

Organism	Genome Size (Mb)	# Genes	Genes/Mb	Introns/Gene
Escherichia coli	4.64	4,200	905	0
Saccharomyces cerevisiae	12.2	6,607	542	0.05
Arabidopsis thaliana	119	27,249	200	4
Drosophila melanogaster	180	13,937	82	2
Homo sapiens	3,101	20,790	6.7	9.0

Genome Annotation and Functional Genomics

• Experimental Annotation: Identifying genes and functional elements using laboratory techniques.

• Computational Annotation: Using bioinformatics tools to predict gene locations and functions.

• Functional Genomics: Genome-wide assays (e.g., RNA-seq) to study gene expression and function.

Genome Evolution

• Rearrangement: Genomes evolve through structural changes such as inversions, translocations, and duplications.

• Duplication: Gene and segmental duplications contribute to genetic diversity and evolution.

• Genetic Variation: Genomics enables the study of variation within species, important for population genetics and disease research.

Part 3: Genome Sequencing and Assembly

Sequencing and Analyzing Genomes

Modern genomics relies on high-throughput sequencing and computational analysis to assemble and interpret genomes.

1. Whole Genome Shotgun Sequencing: Fragmenting the genome, sequencing each piece multiple times, and assembling the sequences into a contiguous genome.

2. Experimental Annotation: Laboratory-based identification of genes and regulatory elements.

3. Computational Annotation: Bioinformatics prediction of gene structure and function.

4. Functional Experimentation: Testing gene function through experiments.

5. Evolutionary Analysis: Comparing genomes to study evolutionary relationships.

PCR and DNA Sequencing

• PCR (Polymerase Chain Reaction): Amplifies specific genomic regions exponentially using cycles of denaturation, annealing, and extension.

• DNA Sequencing: Uses modified nucleotides to determine the order of bases in DNA. Sanger sequencing is a classic method.

Equation for PCR Amplification:

$N = N_0 \times 2^n$

Where $N$ is the number of DNA copies after $n$ cycles, and $N_0$ is the initial number of template molecules.

Whole Genome Shotgun Sequencing

• Genome is fragmented into small pieces.

• Each fragment is sequenced multiple times (10-50x coverage).

• Short reads are assembled into contiguous sequences (contigs).

• Mate-pair information is used to join contigs across gaps.

Challenges: Repetitive Sequences

• Repetitive DNA (microsatellites, transposable elements) complicates assembly because identical sequences cannot be uniquely assigned.

• Sequencing larger fragments helps overcome these challenges by providing context to join contigs.

Metagenomics

• Metagenomics: Sequencing DNA from environmental samples to study microbial communities, including those that cannot be cultured in the lab.

• Reveals the vast diversity of microbial genes compared to human genes.

Summary

• Transposable elements are major components of genomes, influencing structure, function, and evolution.

• Genomics integrates sequencing, annotation, and functional analysis to understand genetic information.

• Technological advances such as PCR, shotgun sequencing, and metagenomics have revolutionized genetic research.