Test 3: Gene Expression

Introduction to DNA Technologies

  • Explanation of the inability to use phone during class

  • Clarification regarding the absence of a test before spring break due to insufficient coverage of topics

DNA Replication and PCR (Polymerase Chain Reaction)

  • Overview of DNA replication previously discussed

  • Introduction of PCR as an in vitro DNA replication method that has revolutionized molecular biology

    • Key processes involved in PCR: maturation, annealing, and extension

    • PCR amplifies DNA by billions of times, generating billions of copies

Visualizing Amplified DNA

  • Traditional methods of visualizing DNA (comparison to coffee brewing)

  • Explanation of agarose gel electrophoresis:

    • Samples are placed in wells on a rectangular sheet (gel)

    • Buffer is added and an electric current is applied

    • DNA's negative charge causes it to move toward the positive charge

    • Larger pieces remain near the wells while smaller pieces migrate further down

    • Usage of a DNA ladder (known DNA sizes) to determine base pair sizes of samples

    • Example base pairs: 1000bp, 500bp, 200bp

    • Diagnostic applications to match expected DNA sizes

Practical Applications in Clinical Settings

  • Example of using PCR for diagnosing infections

    • Historical context: traditional culturing methods for Neisseria gonorrhoeae

    • Issues with culturing due to the organism’s fragility

    • Failure to report due to the organism’s death before culturing

    • Transition to using PCR allows detection of both live and dead DNA

    • Visual representation of positive results on an agarose gel for gonorrhea

Techniques for DNA Detection

  • Introduction of real-time PCR using fluorescent tags (fluorochromes)

  • Visual representation of increasing fluorescence indicating amplification in real-time

  • Issues encountered:

    • Background fluorescence and over-amplification leading to false positives

    • COVID-19 context: widespread positive results due to excessive PCR cycles (greater than 40 cycles)

    • Recommendation to limit PCR cycles to under 30 for reliability

Advanced Applications and Forensics

  • Description of rapid PCR testing in hospitals for detection

    • Example of pediatric cases examined in Manhattan Valley County hospitals

    • Use of various primers specific for detecting multiple pathogens simultaneously

  • Forensic uses of PCR in famous cases (ex: O.J. Simpson)

    • Initial skepticism due to improper sample collection and contamination concerns

Gene Expression Overview

  • Transition from DNA replication to gene expression processes: transcription and translation

  • Definitions:

    • Gene expression = transcription + translation

    • Distinction: replication refers to making more copies of DNA, expression involves using the genetic code to produce proteins

Transcription Process

  • Diagram depicting gene expression:

    • Conversion of DNA (template) to RNA (messenger RNA) leading to protein synthesis

  • Two types of genes:

    • Constitutive genes: continuously expressed (housekeeping genes)

    • Inducible genes: expressed only when needed (dependent on environmental conditions)

    • Illustration of gene expression variations based on stimuli

Types of RNA Involved in Transcription

  • Overview of different types of RNA:

    • Messenger RNA (mRNA): carries the code for polypeptide chains

    • Definition: mRNA is also called the transcript, the product of transcription

    • Transfer RNA (tRNA): carries amino acids to the ribosome during translation

    • Structure: clover-leaf configuration with the anticodon region and the amino acid attachment site

    • Ribosomal RNA (rRNA): structural component of ribosomes

    • MicroRNA (miRNA): small RNAs that regulate gene expression by inhibiting translation

Process of Transcription in Bacteria

  • Specifics related to transcription:

    • Uses a single RNA polymerase enzyme

    • Initiation at promoter regions:

    • Definition: Promoter includes consensus sequences (-10 and -35 regions)

    • Importance of sigma factor in recognizing and binding to promoter regions

  • Mechanics of transcription:

    • Synthesizes RNA in a 5' to 3' direction

    • Distinctions between template strands (antisense) and coding strands (sense)

Further Considerations in Transcription

  • Significant points regarding RNA synthesis:

    • No introns present in prokaryotic genes

    • Recognition of start and stop sites critical for terminating transcription

    • Processes can overlap with translation in prokaryotic cells; polyribosomal complexes may form

Gene Regulation and the Transcriptome

  • Example with Streptococcus pyogenes (causes strep throat)

  • Explanation of how gene expression changes in different environments (plate media vs. throat)

  • Importance of understanding gene regulation to develop therapeutics for infections and diseases

Translation Process Overview

  • Introduction to the translation process:

    • Involves translation initiation with ribosomal subunits, tRNA, and mRNA

    • Shine-Dalgarno sequence as ribosome binding site

  • Steps of translation:

    • Key details on how the ribosome assembles on mRNA and the elongation of new polypeptides

    • Importance of codons and their recognition by tRNA during synthesis

Translation Mechanics

  • Sequence of codons leading to polypeptide synthesis:

    • Total of 64 codons including 3 stop codons (UAA, UAG, UGA)

    • First codon (AUG) generally denotes start of translation with special roles in protein synthesis

  • Explanation of the dynamics of amino acid bonding and ribosome movement during translation

  • Complexities of translation including initiation complex formation, elongation, and termination

Conclusion and Encouragement for Questions

  • Addressing students for questions and thoughts regarding the presented material before moving on to further topics

  • Introduction of visual aid videos to clarify complex processes in transcription and translation