AP Biology Unit 6.2 Study Notes

Unit 6.2 Learning Objectives

  • By the end of this unit, you should be able to:

    • Describe the types of interactions that regulate gene expression.

    • Explain how the location of regulatory sequences relates to their function.

    • Explain how the binding of transcription factors to promoter regions affects gene expression and the phenotype of the organism.

    • Explain the connection between the regulation of gene expression and phenotypic differences in cells and organisms.

    • Explain the use of genetic engineering techniques in analyzing or manipulating DNA.

Chapter 15 Concept 1: Gene Regulation Overview

  • Gene expression is the process by which information from a gene is used to synthesize a functional gene product.

    • Functional gene products can be proteins or functional RNA (tRNA, rRNA, etc.).

  • Cells regulate the extent of gene expression, known as gene regulation.

    • Some genes are expressed more than others; some may not be expressed at all.

  • Dynamic Nature of Gene Expression

    • Gene expression varies due to signal transmission within and between cells and environmental stimuli.

Components of Gene Regulation

  • Regulatory genes: DNA sequences coding for regulatory proteins or small regulatory RNA (sRNA): RNA pieces binding to mRNA to activate/inhibit translation.

  • Regulatory sequences: Noncoding DNA sequences to which regulatory proteins bind.

    • Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.

Regulatory Proteins

  • Control of Gene Activity: Regulatory proteins can increase or decrease gene transcription.

    • Negative control: Involves repressors that block transcription by binding to DNA.

    • Positive control: Involves activators that stimulate transcription by binding to DNA.

Chapter 15 Concept 2: Gene Regulation in Bacteria

  • Prokaryotic Gene Regulation: Prokaryotes conserve resources by expressing proteins only when needed, usually at the transcription level.

    • Example: Bacteria regulate enzymes for nutrient synthesis based on environmental availability.

Operons

  • A set of functionally related genes under coordinated control by a single on-off switch known as an operator.

    • An operator is a short stretch of DNA positioned within the promoter, managing RNA polymerase access.

    • Together with the promoter and controlled genes, they comprise an operon.

Negative Control of Operons

  • An operon can be turned off by a repressor protein that binds to the operator, preventing transcription.

  • Repressor proteins are coded by regulatory genes with their own promoters.

  • Types of negative control: repressible operons and inducible operons.

Repressible Operons

  • Typically active but can be turned off by activating a repressor.

  • A corepressor activates the repressor protein, allowing it to bind to the operator.

  • Example: The trp operon synthesizes tryptophan. Presence of tryptophan acts as a corepressor, turning off the operon, conserving energy.

Inducible Operons

  • Usually inactive, can be turned on by inactivating the repressor.

  • An inducer binds to the repressor, inhibiting it, allowing transcription.

  • Example: The lac operon codes for enzymes to catabolize lactose. Presence of lactose increases allolactose, an inducer, which inactivates the repressor, activating the operon.

Operons and Metabolism

  • Inducible operons are involved in catabolic pathways, synthesizing digestive enzymes.

    • Active only when the substrate is present (inducer).

  • Repressible operons are involved in anabolic pathways, coding for synthesizing enzymes.

    • Activity stops when the end-product is abundant (corepressor).

Positive Control of Operons

  • Some operons experience positive control by an activator that enhances transcription.

  • Example: cAMP receptor protein (CRP) promotes transcription when bound to RNA polymerase.

  • Many operons utilize both negative and positive regulatory mechanisms, exemplified in the lac operon with varying glucose levels and CRP activity.

Continuous Gene Expression

  • Certain genes are continuously expressed, often those coding for essential components used by the cell, such as ribosomal genes.

Chapter 15 Concept 3: Gene Regulation in Eukaryotes

  • Eukaryotic Gene Regulation: Essential for multicellular organisms, where all cells possess the entire genome, but different expression leads to specialization.

Chromatin Structure

  • Eukaryotic DNA is structured into chromatin, DNA and protein complexes.

    • DNA wraps around histones (groups of eight protein molecules), appearing as nucleosomes.

    • Degree of chromatin coiling determines active (euchromatin) vs. inactive (heterochromatin) regions.

Regulation of Chromatin Structure

  • Chemical modifications (like histone acetylation) impact chromatin structure and gene expression.

    • Histone acetylation loosens chromatin, promoting transcription initiation.

    • DNA methylation adds methyl groups to DNA bases (like cytosine), compacting chromatin, leading to reduced transcription and potential long-term gene inactivation during differentiation.

Epigenetic Changes

  • These changes modify phenotype expressions without altering genotype.

Regulatory Sequences

  • Found with most eukaryotic genes, regulatory sequences regulate transcription by binding proteins.

    • Promoters: Bind to RNA polymerase.

    • Operators: Bind repressors (bacteria).

    • Enhancers: Bind activators (eukaryotes).

Promoters in Eukaryotic Genes

  • Many contain the TATA box, rich in adenine and thymine.

  • Eukaryotic promoters bind regulatory proteins (transcription factors) to RNA polymerase.

Transcription Factors

  • General transcription factors are necessary for all protein-coding gene transcription.

  • Specific transcription factors regulate particular genes; they can be activators (increasing expression) or repressors (decreasing expression).

    • Activators bind enhancers; repressors bind silencers.

    • Combinations of transcription factors determine gene expression levels.

Coordination of Gene Expression

  • Eukaryotic genes each have individual promoters and regulatory sequences.

  • Genes that function together often share identical regulatory sequences, allowing recognition by the same transcription factors for simultaneous transcription.

Post-Transcriptional Regulation

  • Regulation post-transcription modifies gene expression rapidly in response to environmental changes.

  • Examples include:

    • Alternative RNA splicing: Different mRNAs formed from one gene through varying intron combinations.

    • RNA interference (RNAi): Small RNA molecules (microRNA) inhibit gene expression by binding mRNA, leading to translation inhibition and degradation.

Chapter 24 Concept 1: Increasing Genetic Variation

  • Mutations serve as the ultimate source of genetic variation in all life forms.

  • In eukaryotes, variation can arise from sexual reproduction (segregation, independent assortment, crossing over). In prokaryotes, it comes from horizontal gene transfer: transformation, transduction, conjugation, transposition; and in viruses via recombination.

Genetic Variation in Eukaryotes

  • Sexual reproduction leads to new combinatory DNA in zygotes formed from parent sperm and egg cells.

Genetic Variation in Prokaryotes

  • Prokaryotes reproduce asexually, but can gain genetic variation through horizontal gene transfer:

    • Transformation: Uptake of nude DNA from the environment.

    • Transduction: Introduction of foreign DNA by bacteriophages.

    • Conjugation: DNA is transferred between cells via direct contact using pili.

    • Transposition: Movements of DNA segments within or between DNA molecules.

Chapter 17 Concept 1: Viruses

  • Viruses are infectious particles composed of genetic material and protein coats, categorized by their genetic structure (ssDNA, dsDNA, ssRNA, dsRNA).

    • Capsid: Protein shell of a virus. Bacteriophages infect bacteria and replicate only in host cells.

Lytic and Lysogenic Cycles

  • Lytic cycle: Viral life cycle culminating in host cell death through replication.

  • Lysogenic cycle: Viral DNA integrates into the host cell’s chromosome and replicates without causing cell death.

Flow of Genetic Information in Retroviruses

  • A retrovirus inserts its RNA genome into host DNA, using reverse transcriptase to convert RNA to DNA. This DNA may integrate into the host genome, leading to new progeny viral assembly.

Genetic Variations in Viruses

  • Rapid evolution in viruses; mutations arise from replication mistakes, especially in RNA viruses lacking proofreading mechanisms.

  • Recombination increases variation when similar viruses infect the same cell, merging genetic materials.

Chapter 13 Concept 4: Biotechnology

  • Biotechnology techniques allow for DNA and RNA analysis and manipulation, including:

    • Polymerase Chain Reaction (PCR)

    • Gel Electrophoresis

    • Bacterial Transformation

    • DNA Sequencing

Polymerase Chain Reaction (PCR)

  • Rapid amplification of DNA samples, requiring:

    • DNA template

    • Primers

    • DNA nucleotides

    • Taq polymerase: Temperature-stable enzyme.

  • Basic steps: 1. Denaturation (96°C): Separate DNA strands. 2. Annealing (55°C): Primers bind to template strands. 3. Extension (72°C): Taq polymerase synthesizes new DNA. 4. Repeat.

Gel Electrophoresis

  • DNA samples cut with restriction enzymes before running through a gel.

    • DNA is negatively charged and moves toward the positive electrode. Smaller fragments travel further.

  • Applications: Determine alleles and parental lineage by examining fragment patterns.

Bacterial Transformation

  • Introduction of DNA into bacterial cells utilizing recombinant DNA plasmids.

DNA Sequencing

  • Determines nucleotide order using the Sanger sequencing technique with dideoxynucleotide triphosphates (ddNTPs) that halt elongation.

    • Differentially labeled ddNTPs allow for sizing fragments post-gel electrophoresis to decipher the sequence.