Central Dogma, Mutations, and Molecular Genetics: Study Guide

Central Dogma & Gene Regulation

Structure of Nucleotides and Double-Stranded DNA

• Nucleotides are the building blocks of nucleic acids (DNA and RNA). Each nucleotide consists of a phosphate group, a five-carbon sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base (adenine, thymine, cytosine, guanine in DNA; uracil replaces thymine in RNA).

• DNA is typically double-stranded, forming a double helix structure stabilized by hydrogen bonds between complementary bases (A-T and G-C).

• The two strands are antiparallel, meaning one runs 5' to 3' and the other 3' to 5'.

• 5' end of a DNA strand has a free phosphate group; 3' end has a free hydroxyl (-OH) group.

DNA Replication

• DNA replication is the process by which a cell copies its DNA before cell division.

• It is semiconservative: each new DNA molecule consists of one parental and one newly synthesized strand.

• Leading strand is synthesized continuously in the 5' to 3' direction.

• Lagging strand is synthesized discontinuously, forming short segments called Okazaki fragments, which are later joined by DNA ligase.

• Enzymes involved:

  • DNA polymerase: Synthesizes new DNA strands by adding nucleotides to a primer.

  • Helicase: Unwinds the DNA double helix.

  • Primase: Synthesizes RNA primers.

  • Ligase: Joins Okazaki fragments on the lagging strand.

RNA Transcription

• Transcription is the synthesis of RNA from a DNA template.

• RNA polymerase binds to the promoter region of DNA to initiate transcription.

• The resulting RNA transcript is complementary to the DNA template strand.

• In eukaryotes, the primary transcript (pre-mRNA) undergoes processing: addition of a 5' cap, poly(A) tail at the 3' end, and removal of introns (non-coding regions), leaving only exons (coding regions).

Translation and the Genetic Code

• Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Start codon: AUG (codes for methionine) signals the beginning of translation.

• Stop codons: UAA, UAG, UGA signal the end of translation.

• Codon: A sequence of three nucleotides on mRNA that specifies an amino acid.

• Anticodon: A sequence of three nucleotides on tRNA complementary to the mRNA codon.

• Monocistronic mRNA: Encodes a single protein (common in eukaryotes).

• Polycistronic mRNA: Encodes multiple proteins (common in prokaryotes).

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (the sequence of DNA).

• Phenotype: The observable characteristics or traits of an organism, resulting from the expression of the genotype.

Microbial Genetics

Mutations and Mutagens

• Mutation: A heritable change in the nucleotide sequence of DNA.

• Mutagens: Physical or chemical agents that increase the rate of mutation (e.g., UV radiation, chemicals).

• UV rays can cause thymine dimers, leading to errors during DNA replication.

• Types of mutations:

  • Point mutations: Change in a single nucleotide (substitution, insertion, deletion).

  • Frameshift mutations: Insertions or deletions that alter the reading frame.

  • Missense, nonsense, and silent mutations: Depending on the effect on the protein sequence.

Genetic Transfer in Bacteria

• Plasmid: Small, circular, double-stranded DNA molecules found in bacteria, independent of chromosomal DNA.

• Origin of replication: Specific sequence where plasmid replication begins.

• Plasmid transformation: Uptake of naked plasmid DNA by a bacterial cell from the environment.

• Bacterial conjugation: Transfer of genetic material between bacteria via direct contact, often involving a pilus (a tube-like structure).

• Transduction: Transfer of bacterial genes by viruses (bacteriophages).

• Replica plating: Technique to transfer colonies from one plate to another to identify mutants.

Biotechnology

Polymerase Chain Reaction (PCR)

• PCR is a technique used to amplify specific DNA sequences in vitro.

• Principles of PCR:

  1. Denaturation: Heating to separate DNA strands.

  2. Annealing: Cooling to allow primers to bind to target sequences.

  3. Extension: DNA polymerase synthesizes new DNA strands from primers.

• Usefulness of PCR: Used in diagnostics, cloning, forensics, and research to detect and amplify DNA.

Microarray

• Microarray: A laboratory tool used to detect the expression of thousands of genes at once by hybridizing labeled cDNA to a grid of DNA probes.

• Allows comparison of gene expression between different samples or conditions.

Table: Types of Mutations

Type of Mutation	Description	Effect on Protein
Missense	Single nucleotide change results in a different amino acid	May alter protein function
Nonsense	Single nucleotide change creates a stop codon	Results in truncated, usually nonfunctional protein
Silent	Change does not alter amino acid sequence	No effect on protein
Frameshift	Insertion or deletion shifts the reading frame	Usually produces nonfunctional protein

Key Equations

• DNA Synthesis Direction: $5' \rightarrow 3'$

• Central Dogma: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

• PCR Amplification: $N = N_0 \times 2^n$ Where $N$ is the number of DNA molecules after $n$ cycles, $N_0$ is the initial number.

Examples and Applications

• Example of Mutation: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

• Application of PCR: Detecting pathogens in clinical samples by amplifying their DNA.

• Use of Microarray: Comparing gene expression in healthy vs. cancerous cells.