Nucleic Acids: Sequencing and Manipulation

4. Nucleic Acids

4.1 Nucleotides

Nucleotides are the basic building blocks of nucleic acids, such as DNA and RNA. Each nucleotide consists of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and one or more phosphate groups.

• Nitrogenous bases: Purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; uracil in RNA).

• Functions: Nucleotides serve as energy carriers (e.g., ATP), signaling molecules, and monomers for nucleic acid synthesis.

4.2 Nucleic Acid Structure

Nucleic acids are polymers of nucleotides linked by phosphodiester bonds. DNA is typically double-stranded and forms a double helix, while RNA is usually single-stranded.

• Directionality: Sequences are always written from the 5' to 3' end.

• Base pairing: Adenine pairs with thymine (or uracil in RNA), and guanine pairs with cytosine via hydrogen bonds.

4.3 Nucleic Acid Function

Nucleic acids store and transmit genetic information. DNA encodes the instructions for protein synthesis, while RNA acts as a messenger and functional molecule in gene expression.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into proteins by ribosomes.

4.4 Nucleic Acid Sequencing

Nucleic acid sequencing is the process of determining the precise order of nucleotides within a DNA or RNA molecule.

• Nucleic acid sequence: The succession of letters (A, T, G, C for DNA; A, U, G, C for RNA) indicating the order of nucleotides.

• Sequencing direction: Sequences are presented from the 5' end to the 3' end.

Nucleic Acid Sequencing Chain-Terminator Method (Dideoxy Method)

The chain-terminator or Sanger method is a classical technique for DNA sequencing, developed by Frederick Sanger and improved by Leroy Hood and colleagues.

• Key principle: Utilizes dideoxynucleotide triphosphates (ddNTPs) as chain terminators.

• Process:

  1. Four separate reaction mixtures are prepared, each containing a DNA fragment, DNA polymerase, a primer, large amounts of regular dNTPs, and a small amount of a single ddNTP (ddATP, ddTTP, ddGTP, or ddCTP).

  2. ddNTPs are randomly incorporated, terminating DNA synthesis at specific bases and generating fragments of varying lengths.

  3. Fragments are separated by gel electrophoresis and visualized (historically by x-ray film, now often by fluorescence).

  4. The sequence is read from the gel, bottom to top (5' to 3').

Mechanism: The 3'-hydroxyl group of the growing DNA strand reacts with an incoming dNTP to form a new phosphodiester bond. ddNTPs lack a 3'-OH, preventing further elongation.

Equation:

$\text{(DNA strand)}_{n} + \text{dNTP} \rightarrow \text{(DNA strand)}_{n+1} + PP_i$

When ddNTP is incorporated:

$\text{(DNA strand)}_{n} + \text{ddNTP} \rightarrow \text{(DNA strand with ddNTP at 3' end)}$

Automated DNA Sequencing

• Modern sequencing uses fluorescently labeled ddNTPs, allowing all four reactions to be performed in a single tube and detected by automated sequencers.

• Fragments are separated by capillary electrophoresis, and a laser detects the fluorescent tags, generating a chromatogram.

Steps:

1. Double-stranded DNA is separated.

2. DNA is incubated with DNA polymerase, primer, dNTPs, and labeled ddNTPs.

3. Polynucleotide fragments terminate at positions corresponding to each nucleotide.

4. Fragments are separated and detected based on size and fluorescence at the 3' end.

4.5 Manipulating DNA

Recombinant DNA technology, also known as molecular cloning or genetic engineering, enables the isolation, amplification, and modification of specific DNA sequences.

• Steps to express DNA of interest in cells:

  1. A DNA fragment is generated by restriction enzyme digestion, PCR, or chemical synthesis.

  2. The fragment is inserted into a vector (e.g., plasmid) containing sequences necessary for replication.

  3. The recombinant vector is introduced into host cells (transformation) and replicated.

  4. Cells containing the desired DNA are identified and selected.

Cloning

• Definition: Production of multiple identical organisms or DNA molecules derived from a single ancestor.

• In suitable hosts (e.g., E. coli or yeast), large amounts of inserted DNA can be produced.

• Cloning provides materials for further studies and allows gene expression analysis under controlled conditions.

Cloning Vectors

• Plasmids: Small, circular DNA molecules used as vectors. They replicate autonomously and often carry antibiotic resistance genes.

• Plasmid vectors are constructed for laboratory use to be small, replicate easily, and contain multiple cloning sites (restriction sites for DNA insertion).

• Selectable markers (e.g., Ampr for ampicillin resistance, lacZ for β-galactosidase activity) are used to identify successful clones.

Vector Type	Key Features	Selectable Markers
Plasmid	Small, circular, replicates in bacteria, multiple cloning sites	Ampr, lacZ
Bacteriophage	Infects bacteria, can carry larger inserts	Phage plaque formation
Cosmid	Hybrid of plasmid and phage, large inserts	Antibiotic resistance

Restriction Enzymes and DNA Ligase

• Restriction enzymes cut DNA at specific sequences, generating "sticky ends" that facilitate ligation.

• DNA ligase covalently joins the sugar-phosphate backbones of DNA fragments.

• Using the same restriction enzyme for vector and insert allows precise excision and insertion of DNA.

Transformation and Selection

• Recombinant plasmids are introduced into bacteria by transformation.

• Cells are plated on agar containing antibiotics (e.g., ampicillin). Only transformed cells survive.

• Blue/white screening with X-Gal and IPTG distinguishes recombinant from non-recombinant colonies:

Colony Color	lacZ Status	Insert Present?
Blue	Intact	No
White	Disrupted	Yes

Transgenic Organisms/Mice

• Genes can be overexpressed or knocked out in model organisms (e.g., mice) to study gene function.

• Transgenic mice are created by microinjecting cloned DNA into fertilized eggs, which are then implanted into a foster mother for stable integration.

Example: Creating a knockout mouse to study the effect of a specific gene deletion on development or disease.

Additional info: These techniques are foundational for modern biotechnology, genetic engineering, and biomedical research, enabling advances in gene therapy, agriculture, and medicine.