DNA, Gene Expression, Mutations, Gene Regulation, and Biotechnology: Exam 4 Study Guide

DNA: Structure, Function, and Discovery

History of DNA Discovery

The discovery of DNA as the genetic material involved several key experiments and scientists. Understanding these milestones is essential for appreciating modern molecular biology.

• Frederick Griffith (1928): Demonstrated transformation in bacteria, suggesting a "transforming principle."

• Avery, MacLeod, McCarty (1944): Identified DNA as the transforming principle.

• Hershey and Chase (1952): Used bacteriophages to confirm DNA as the genetic material.

• Watson and Crick (1953): Proposed the double helix structure of DNA.

DNA Structure and Nucleotides

• DNA is a double-stranded molecule composed of nucleotides.

• Nucleotides: Each consistso0of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• Four Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Base-Pairing Rules: A pairs with T; C pairs with G.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself during cell division.

• Enzymes Involved:

  • DNA helicase: Unwinds the DNA double helix.

  • DNA polymerase: Synthesizes new DNA strands.

• Leading and Lagging Strands:

  • Leading strand: Synthesized continuously.

  • Lagging strand: Synthesized in short fragments (Okazaki fragments).

• Direction of Synthesis: DNA is synthesized in the 5’ to 3’ direction.

• Proofreading Mechanisms: DNA polymerase checks and corrects errors during replication.

Example: During replication, DNA polymerase adds nucleotides to the growing strand, matching A with T and C with G.

Gene Expression: Transcription and Translation

Genes and Their Function

A gene is a segment of DNA that codes for a specific protein or functional RNA.

Transcription and Translation

Gene expression involves two main processes: transcription and translation.

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

  • Initiation: RNA polymerase binds to the promoter region.

  • Elongation: RNA polymerase synthesizes the RNA strand.

  • Termination: RNA polymerase releases the completed RNA molecule.

• Translation: mRNA is decoded to synthesize a protein.

  • Initiation: Ribosome assembles at the start codon.

  • Elongation: tRNA brings amino acids to the ribosome; peptide bonds form.

  • Termination: Ribosome reaches a stop codon and releases the polypeptide.

RNA vs. DNA

• RNA: Single-stranded, contains ribose sugar, uses uracil (U) instead of thymine (T).

• DNA: Double-stranded, contains deoxyribose sugar, uses thymine (T).

• Base-Pairing in RNA: A pairs with U; C pairs with G.

mRNA and tRNA

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosome.

• tRNA (transfer RNA): Brings amino acids to the ribosome; contains anticodons complementary to mRNA codons.

Codons and Anticodons

• Codon: A sequence of three mRNA nucleotides that codes for an amino acid.

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

Amino Acids and the Genetic Code

• Amino acids: Building blocks of proteins; there are 20 standard amino acids.

• Reading the Amino Acid Table: Use the mRNA codon to determine which amino acid is specified.

Example: The mRNA codon AUG codes for methionine (start codon).

Mutations: Types and Outcomes

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function.

• Deletions: Removal of one or more nucleotides.

• Insertions: Addition of one or more nucleotides.

• Substitutions: Replacement of one nucleotide with another.

  • Silent mutation: No change in amino acid.

  • Missense mutation: Changes one amino acid.

  • Nonsense mutation: Creates a stop codon, truncating the protein.

Example: A missense mutation in the hemoglobin gene causes sickle cell anemia.

Epigenome and Gene Regulation

The Epigenome

The epigenome refers to chemical modifications to DNA and histone proteins that affect gene expression without changing the DNA sequence.

• DNA methylation: Addition of methyl groups to DNA, often silencing genes.

• Histone modification: Alters chromatin structure and gene accessibility.

Gene Regulation Mechanisms

• Epigenetic/Transcriptional Controls:

  • DNA Packing: Chromosome condensation can silence genes.

  • Chromosome Inactivation: Example: X-chromosome inactivation in females.

  • Regulatory Proteins:

    • Operons: Gene clusters in prokaryotes regulated together.

    • Transcription Factors: Proteins in eukaryotes that control gene expression.

• Post-transcriptional Controls:

  • Alternative Splicing: Different combinations of exons produce multiple proteins from one gene.

  • Introns: Non-coding regions removed from pre-mRNA.

  • Exons: Coding regions retained in mature mRNA.

  • miRNAs (microRNAs): Small RNAs that interfere with mRNA translation.

  • Protein Activation/Breakdown: Proteins may be activated or degraded after translation.

Example: The lac operon in Escherichia coli is regulated by the presence of lactose.

Biotechnology: Tools and Applications

Domestication

Domestication is the process by which humans select and breed organisms for desired traits. Examples include maize (corn) and dogs.

Applications of Biotechnology

• Genetic modification of crops for improved yield and resistance.

• Medical applications such as gene therapy and pharmacogenomics.

• Production of biofuels using genetically engineered organisms.

Biotechnology Tools

• Gel Electrophoresis: Separates DNA fragments by size using an electric field.

• PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences.

  • Equation for PCR amplification: $N = N_0 \times 2^n$ (where $N$ is the final number of DNA molecules, $N_0$ is the initial number, and $n$ is the number of cycles)

• Southern Blotting: Detects specific DNA sequences in a sample.

• Recombinant DNA Technology: Combines DNA from different sources to create new genetic combinations.

Genetically Modified Organisms (GMOs)

• GMOs: Organisms whose genomes have been altered using biotechnology.

• Methods include gene insertion, deletion, or modification.

• Examples: Bt corn, Roundup Ready soybeans.

Genome Mapping and Sequencing

• Genome Mapping: Locating genes and markers on chromosomes.

• Whole-Genome Sequencing: Determining the complete DNA sequence of an organism.

  • Sanger Method: Uses chain-terminating nucleotides to sequence DNA.

Applications of Genomics

• Predicting disease 2q based on genetic markers.

• Tailoring drug treatments to individual genetic profiles (pharmacogenomics).

• Producing biofuels using engineered microbes.

Summary Table: Biotechnology Tools and Their Uses

Tool	Main Purpose	kj Example
Gel Electrophoresis	Separates DNA fragments by size	DNA fingerprinting
PCR	Amplifies DNA sequences	Detecting pathogens
Southern Blotting	Detects specific DNA sequences	Gene identification
Recombinant DNA Technology	Combines DNA from different sources	Creating GMOs
Sanger Sequencing	Determines DNA sequence	Whole-genome sequencing

Additional info:

• Review videos on the epigenome, maize domestication, and GMOs for applied understanding.

• For exam preparation, practice reading amino acid tables and interpreting biotechnology data.