Unit
- DNA double helix: The antiparallel strands of deoxyribonucleic acid are held together by hydrogen bonds between complementary base pairs: adenine with thymine and guanine with cytosine. This structure enables replication and transcription, forming the basis of genetic information storage.
- Central dogma: Information flows from DNA to RNA through transcription, then from RNA to protein through translation. This unidirectional pathway is fundamental to understanding how genetic modifications produce phenotypic changes.
- Genetic code: Triple codons of nucleotides specify twenty amino acids plus start and stop signals. The code is degenerate, meaning most amino acids are encoded by multiple codons, which allows silent mutations without altering protein sequence.
- Chromosome organization: DNA wraps around histone proteins to form nucleosomes, which compact into chromatin. This packaging regulates gene accessibility and is dynamically modified during cellular processes.
- Gene structure: Prokaryotic genes consist of promoter, coding sequence, and terminator. Eukaryotic genes additionally contain introns that are spliced out, and exons that remain in the mature mRNA.
- DNA replication: Semiconservative replication begins at origins of replication. Helicase unwinds the helix, DNA polymerase adds nucleotides in the 5' to 3' direction, and ligase seals Okazaki fragments on the lagging strand.
- Mutation types: Point mutations, insertions, deletions, and rearrangements alter DNA sequence. Understanding these is essential for designing targeted edits and interpreting sequencing data.
- Restriction enzymes: Bacterial endonucleases recognize specific palindromic sequences and cut DNA at precise locations. These enzymes are the primary tools for cutting DNA in genetic engineering experiments.
- DNA ligase: This enzyme catalyzes the formation of phosphodiester bonds between adjacent nucleotides. It is essential for joining DNA fragments during cloning and assembly.
- Plasmids: Extrachromosomal circular DNA molecules in bacteria replicate independently. They are commonly used as vectors to carry foreign genes into host cells.
- Polymerase chain reaction (PCR): A thermal cycling technique that amplifies a specific DNA region exponentially. It uses a thermostable DNA polymerase, primers flanking the target, and repeated cycles of denaturation, annealing, and extension to generate millions of copies from a single template.
- Gel electrophoresis: A method to separate DNA fragments by size using an agarose or polyacrylamide gel under an electric field. Smaller fragments migrate faster, allowing visualization of band patterns after staining with dyes like ethidium bromide or SYBR Safe.
- CRISPR Cas9 system: A genome editing tool derived from bacterial adaptive immunity. A guide RNA directs the Cas9 nuclease to a specific genomic sequence, creating a double strand break that can be repaired by non homologous end joining or homology directed repair for precise edits.
- Bacterial transformation: The process of introducing foreign DNA into bacterial cells, often using chemical competence or electroporation. Cells take up plasmid DNA, and selection with antibiotics identifies successful transformants, enabling propagation of recombinant molecules.
- Sanger sequencing: A chain termination method that uses dideoxynucleotides to generate fragments of varying lengths. Capillary electrophoresis separates these fragments, and the order of fluorescent tags reveals the nucleotide sequence, widely used for verifying cloned constructs.
Code Example
Python
# Simulate DNA transcription (DNA to RNA)
def transcribe(dna_seq):
complement = {'A':'U','T':'A','C':'G','G':'C'}
mrna = ''.join(complement[base] for base in dna_seq)
return mrna
dna = 'ATGCTACGT'
mrna = transcribe(dna)
print('DNA:', dna)
print('mRNA:', mrna)This Python function converts a DNA sequence (5' to 3') into its complementary mRNA transcript by replacing T with U. The output shows the result for a short test sequence.
Quick Revision
- DNA is a double helix with antiparallel strands and complementary base pairing.
- The central dogma describes information flow from DNA to RNA to protein.
- Restriction enzymes and DNA ligase are fundamental tools for cutting and joining DNA.
- Plasmids serve as common vectors for introducing foreign DNA into host cells.