BIO121: Detailed Study Notes on Sequencing and Evolutionary Biology Concepts

Sanger sequencing (also known as chain termination sequencing or dideoxy sequencing)
- Involves using four separate tubes, one for each nucleotide (A, T, C, G).
Shotgun sequencing
- A method where a large DNA molecule is fragmented into smaller pieces, which are then sequenced.
- Each fragment is sequenced, and the pieces are reassembled into a complete sequence.

Also known as sequencing by synthesis (SBS).
Utilizes a flow cell instead of multiple tubes for sequencing.
- A flow cell contains "lawns" of probes that can bind single-stranded pieces of DNA.
- Allows high-throughput sequencing of millions of DNA molecules simultaneously.
Bridge amplification affects the process:
- Instead of sequencing a molecule once, it sequences it thousands of times.
- Ensures error rates are significantly reduced (almost zero).
- Repeated cycles of amplification build up a dense cluster of identical DNA sequences on the flow cell, allowing for better accuracy.

In a three-dimensional space:
- Probes designed to bind to DNA are present.
- A piece of denatured double-stranded DNA binds to the probes, then replicates to form a bridge.
- This bridge folds, allowing synthesis of a new strand of DNA with more rounds of binding to probes.
- Continuous synthesis results in a massive number of single DNA molecules attached to probes on the flow cell.
Result:
- DNA molecules are amplified much faster and more effectively than in prior methods (e.g., Sanger).

After DNA amplification,
- The sequencing process involves adding labeled nucleotides (A, T, C, G) with different fluorescent colors.
- As nucleotides are incorporated into the growing DNA strand, a flash of light indicates which nucleotide was added.
- Can capture thousands of these flashes concurrently for massive parallel sequencing.
Sequencing multiple fragments allow for vastly more comprehensive information than traditional methods.

From the original Sanger sequencing:
- Human genome project took 13 years to sequence the first human genome.
With advances like Illumina sequencing,
- The same task can now be completed in two to three days, drastically reducing time and cost.
Further advancements led to nanopore sequencing.
- Requires no PCR (polymerase chain reaction) and can sequence DNA directly.
- Processes faster with an estimated sequencing time of about 6 hours for a human genome.

Processes: Direct read of DNA without amplification.
Method:
- Keep the original strand intact, which allows for immediate sequencing.
- The error rate is higher than other methods, making it still a developing technology.
Size and Accessibility: modern sequencing machines are considerably smaller and more accessible than earlier models.

Essential for stitching together sequences from millions of fragments to reconstruct a complete genome.
The nature of DNA fragmentation:
- DNA is not chopped uniformly.
- In typical sequencing applications, hundreds of thousands of copies of DNA fragments are sequenced which include overlaps.

Homology: Similar traits due to shared ancestry. E.g., humans and other primates.
Analogy: Similar traits due to convergent evolution; organism adapting to similar environments independently.

An exercise using BLAST involves finding genetic sequences to conduct phylogenetic analyses.
- Generating a phylogenetic tree can illustrate genetic relationships.
Importance of homologous characters in determining phylogeny, such as the presence of collagen in multiple species.

Clade: A group of species that includes an ancestral species and all its descendants.
Types of groups in cladistics:
- Monophyletic: Consists of an ancestor and all its descendants.
- Paraphyletic: Includes an ancestor and only some of its descendants; excludes unsuccessful descendants (e.g., Neanderthals).
- Polyphyletic: Includes species from different ancestors without their most recent common ancestor.

Shared Ancestral Characteristics: Traits that originated from an ancestor.
Shared Derived Characteristics: Traits unique to a clade, resulting from evolutionary changes that address specific environmental challenges.

Phylogenetic trees represent relationships but do not inherently indicate time scales.
Length of branches represents genetic changes and can be adjusted according to fossil data.

Used to estimate evolutionary time based on constant rates of mutation in some genes.
Estimation techniques include:
- Calibration using fossil data to link genetic variations and historical events.
Limitations:
- Evolution can occur at irregular rates due to various environmental factors and stresses.

Continuous advancements in sequencing will lead to quicker, more affordable genetic analysis.
Understanding molecular evolution helps provide insights into how genetic traits arise and adapt through time.