Everything You MUST Know about Gene Expression (AP Bio Unit 6)

Main ideas / lessons by topic

1) Gene expression overview (Unit 6 framing)

The video motivates AP Bio exam prep by walking through the “flow” of genetic information:

• DNA → RNA (transcription) → mRNA → protein (translation)

It also promises coverage of:

• DNA/RNA structure & function
• DNA replication
• Genetic code
• Regulation of gene expression
  • Prokaryotes: operons
  • Eukaryotes: multiple layers of control
• Mutation & horizontal gene transfer
• Biotechnology tools:
  • recombinant DNA
  • gel electrophoresis
  • PCR
  • sequencing

2) DNA structure & function

DNA structure

• DNA is a double-stranded, helical molecule made of nucleotide monomers.
• Each nucleotide has:
  • a 5-carbon sugar (deoxyribose)
  • a phosphate group
  • one of four nitrogenous bases
• DNA has a sugar-phosphate backbone, while bases pair inside the helix.

Base pairing rules (must be memorized)

• A pairs with T
• G pairs with C

Strand orientation

• The two DNA strands are anti-parallel:
  • one strand is 5’ → 3’
  • the other is 3’ → 5’

Why DNA works as heredity

• Information storage: base order forms an informational code.
• Replicability: complementary base pairing allows copying.
• High fidelity: correct template-directed pairing supports accurate inheritance.
• Mutation: low-rate changes (spontaneous or environmental) allow variation for evolution.

DNA vs RNA

• DNA is the genetic material in most organisms (heredity).
• RNA can be hereditary material in some viruses (examples given: HIV, SARS-CoV-2).
• RNA functions in information transfer and protein synthesis, including:
  • mRNA, tRNA, rRNA
• In eukaryotes, RNA also helps regulate gene expression (e.g., splicing introns).

3) Prokaryote vs eukaryote genome organization (and plasmids)

Prokaryotes (bacteria/archaea)

• DNA is in looped/circular chromosomes (beginning and end connected).
• DNA is generally not wrapped around histones (“naked”).
• Genome sizes vary (roughly 100,000 to 10 million base pairs).

Eukaryotes

• DNA is in multiple linear chromosomes.
• DNA is wrapped around histones.
• Human example given: ~3.2 billion base pairs.

Plasmids

• Small extra-chromosomal DNA loops, mostly in bacteria.
• Roles:
  • participate in horizontal gene transfer (via conjugation)
  • often carry antibiotic resistance genes
• Used in genetic engineering for:
  • replicating DNA
  • expressing engineered genes in bacterial cells

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Detailed methodology / step-by-step processes

4) DNA replication (semiconservative model + key enzymes)

Big picture

• Replication uses each existing strand as a template.
• Semiconservative replication means:
  • each new DNA molecule has:
    • one conserved parental strand
    • one newly synthesized strand

Process model (AP Bio-level)

Initiation

• Helicase binds the origin of replication and separates strands by breaking hydrogen bonds.
• This creates a replication fork.

Elongation (role of major proteins)

• Single-strand binding proteins keep DNA from rewinding.
• Primase synthesizes short RNA primers.
• DNA polymerase
  • adds nucleotides to the 3’ end of the growing strand (builds sugar-phosphate bonds)
  • can only work starting from a primer
  • matches nucleotides based on base pairing rules using the template strand

Leading vs lagging strand

• Leading strand: relatively continuous synthesis following fork opening direction.
• Lagging strand: discontinuous synthesis because DNA polymerase can only extend 5’→3’.
• Lagging strand fragments are Okazaki fragments (each requires a primer).

Primer removal & completion

• DNA polymerase I removes RNA primers and replaces them with DNA.
• DNA ligase seals gaps between fragments using sugar-phosphate bonds, completing daughter DNA strands.

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5) Transcription (DNA → RNA)

Central dogma

• DNA makes RNA, RNA makes protein.
• Information flow:
  • DNA triplets (genes) → mRNA codons → amino acid sequence

What is a gene?

• A DNA sequence that codes for an RNA product, which ultimately leads to a protein (trait-determining unit).

Principal RNA types and functions

• mRNA (messenger RNA): carries instructions from DNA to ribosomes.
• rRNA (ribosomal RNA): structural/catalytic core of ribosomes; helps bind amino acids.
• tRNA (transfer RNA): brings specific amino acids to ribosomes via anticodon matching.
• Small RNAs exist (mentioned later for eukaryotic regulation).

Transcription steps

• Promoter marks where transcription begins.
• RNA polymerase binds promoter and:
  • reads the template strand in the 3’→5’ direction
  • synthesizes RNA in the 5’→3’ direction
• Transcription ends when RNA polymerase reaches a terminator and dissociates.

Template vs coding strand terminology

• Template strand = non-coding/minus/antisense strand: it is transcribed.
• Coding strand = sense/plus strand: its sequence matches mRNA except:
  • RNA uses U instead of T

Prokaryotic transcription feature

• No nucleus, so transcription products can be immediately translated.
• Multiple ribosomes may translate the same mRNA simultaneously (polysomes).

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6) Translation (mRNA → protein) using the genetic code

Genetic code concepts

• Translation uses codons, each made of 3 RNA nucleotides, coding for one amino acid.
• Code is nearly universal with minor exceptions.
• Redundant: multiple codons can encode the same amino acid.

How to read codons (example logic taught)

• Codon positions:
  • 1st base + 2nd base + 3rd base
• Often:
  • the first two bases strongly determine the amino acid
  • synonymous codons may differ at the third base

Key players

• mRNA: provides codon order.
• Ribosome: links amino acids into a polypeptide (primary structure).
• tRNA: has:
  • an anticodon that pairs with the mRNA codon
  • an amino acid attached to its binding site

Translation phases

Initiation

• Processed mRNA (no introns) leaves nucleus.
• Small ribosomal subunit binds mRNA and finds the start codon (AUG).
• tRNA with anticodon complementary to AUG binds; carries methionine.
• Large subunit joins; the first tRNA sits in the P site.

Elongation

• Next charged tRNA binds the A site (anticodon matches next codon).
• Ribosome forms a peptide bond between amino acids in P site and A site.
• Translocation: ribosome moves one codon forward.
• The used tRNA exits via the E site, and the cycle repeats.

Termination

• Ribosome reaches a stop codon.
• Stop codons do not have matching tRNAs.
• A release factor binds the stop codon, triggering ribosome dissociation and releasing the completed polypeptide.
• Polypeptide then folds into a functional protein.

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7) Regulation of gene expression

7a) Prokaryotic regulation: operons (Trip and Lac)

Core definition

• Operon = a cluster of genes transcribed as a single RNA with regulatory control elements.
• Operon parts:
  • Promoter: RNA polymerase binding site
  • Operator: repressor binding site
  • Regulatory gene: produces regulatory protein (often a repressor)

Trip operon (repressible operon; tryptophan)

• Function: structural genes encode enzymes for tryptophan synthesis.
• No tryptophan present:
  • repressor cannot bind operator → RNA polymerase transcribes structural genes → enzymes made
• Tryptophan present:
  • tryptophan binds the repressor (co-repressor) → repressor changes shape → binds operator → blocks RNA polymerase → transcription stops

Energy-saving rule: If tryptophan is present, don’t make it. Described as negative feedback: output (tryptophan) inhibits further transcription.

Lac operon (inducible operon; lactose)

• Function: structural genes make enzymes to digest lactose.
• Lactose absent:
  • repressor is in default shape → binds operator → blocks transcription → no lactose-digesting enzymes produced
• Lactose present:
  • lactose binds repressor → repressor shape changes → cannot bind operator
  • operator stays free → RNA polymerase transcribes structural genes → enzymes produced to break lactose down

Rule: If lactose is absent, don’t make enzymes to digest it. Negative feedback explanation in terms of system output:

• turning on transcription increases lactose digestion → lactose decreases → system turns off

Growth/lags concept (glucose vs lactose)

• Bacteria prefer glucose first.
• When glucose runs low, there is a lag while lac operon enzymes are produced.
• Once lactose runs out, growth can lag again until another nutrient is introduced.

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7b) Eukaryotic regulation: epigenetics, RNA processing, small RNAs

Major challenge

• Multicellular eukaryotes must express the right genes in different cell types even though most cells share the same DNA.

Epigenetics

• Epigenetics = reversible chemical modifications affecting gene expression without changing DNA sequence.
• Mechanisms mentioned:
  • DNA methylation: generally prevents transcription (gene silencing)
  • Histone modification (acetylation): loosens DNA packaging, allowing transcription
• Drives tissue differentiation; can sometimes be transmitted across generations (intergenerational transmission mentioned).

Transcription regulation in eukaryotes (big picture)

• Regulatory DNA sequences interact with proteins:
  • promoters
  • enhancers (increase transcription probability)
  • activators
  • DNA-bending proteins
  • mediator proteins
  • general transcription factors
• Goal: enable RNA polymerase to bind and initiate transcription.

Coordinated tissue expression via shared regulatory elements

• Same hormone signal can activate different genes depending on cell/tissue context.
• Example concept:
  • testosterone receptor acts as a transcription factor; target genes differ between neck skin vs muscle.

Introns, exons, and mRNA processing

• Introns: intervening sequences removed from pre-mRNA.
• Exons: expressed sequences that remain and are spliced into mature mRNA.
• Eukaryotic mRNA processing includes:
  • 5’ cap (GTP cap):
    • protects mRNA from degradation
    • helps nuclear export and ribosome binding
  • 3’ poly-A tail:
    • increases mRNA stability
    • delays breakdown

Alternative splicing (source of variation)

• Same pre-mRNA can be spliced in different ways to make different mRNA isoforms.
• Produces proteins with different combinations of functional domains.
• Increases phenotypic variation in eukaryotes; contrasted with prokaryotes (no intron-exon alternative splicing in the described context).

Small RNAs and microRNA (miRNA) regulation

• miRNAs regulate gene expression after transcription (posttranscriptional control).
• Process:
  • DNA codes for a miRNA → miRNA is processed to a mature form
  • miRNA binds an RNA silencing complex
  • Outcomes:
    • Perfect match to target mRNA → mRNA degraded
    • Partial match → translation inhibited (pausing)
• Overall: changes gene expression via RNA silencing.

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8) Mutation and evolution

8a) Mutation types

• Mutation definition: random change in DNA or chromosome.
• Point mutation: change in a single nucleotide.

Silent vs nonsense vs missense

• Silent: DNA changes but produces the same amino acid (genetic code redundancy).
• Nonsense: changes a codon into a stop codon → truncated protein.
• Missense: changes an amino acid to another.

Conservative vs non-conservative missense

• Conservative: similar chemical properties (less likely to disrupt function greatly).
• Non-conservative: major chemical change → likely affects protein structure/function.

8b) Frameshift mutations

• Caused by insertion or deletion of nucleotides not in multiples of 3.
• Shifts the reading frame, causing widespread wrong codons.
• Can produce extensive missense and/or early stop codons.

8c) Example disease: sickle-cell disease

• Caused by a missense mutation in hemoglobin.
• Key molecular change:
  • substitution altering charge/chemistry (nonpolar valine substituting for glutamic acid)
• Consequences:
  • hemoglobin molecules stick together under low oxygen
  • red blood cells sickle → cells become rigid and damage tissues

Inheritance:

• Recessive: phenotype expressed when homozygous
• Heterozygotes have sickle-cell trait

Context-dependent fitness:

• Heterozygote advantage against malaria.

8d) Mutations and fitness (context dependence)

• Mutation effects depend on environment:
  • Positive: improves survival/reproduction (higher fitness)
  • Negative: reduces fitness (e.g., sickle-cell anemia in typical contexts)
  • Neutral: no noticeable effect (e.g., noncoding changes or silent mutations)

8e) Germline vs somatic mutations

• Germline mutations
  • occur in gamete-producing cells
  • present in every body cell after conception
  • inherited and subject to natural selection
• Somatic mutations
  • occur in body cells during development/adulthood
  • not passed to offspring
  • examples: cancer, environment-triggered mutations (e.g., UV skin cancer)

8f) Horizontal gene transfer vs evolution input

• Mutations create raw variation; natural selection acts on variation to produce adaptation.

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9) Horizontal gene transfer (HGT)

Contrast

• Vertical gene transfer: parent → offspring (all or half of genome transmitted).
• Horizontal gene transfer: gene transfer between organisms (not offspring).

Mechanisms in bacteria (and beyond)

• Conjugation (bacterial “sex”)
  • cells use pilus to connect
  • plasmids can be copied and transferred
  • spreads genes such as antibiotic resistance
• Transformation
  • bacteria take up free DNA fragments from environment
  • fragments may incorporate into genome (can include plasmids)
  • foundation of some genetic engineering approaches
• Transduction (viral transfer)
  • viruses can mistakenly package host DNA into viral particles
  • infection transfers that DNA into new host cells
• Viral recombination
  • two viral variants infect the same host
  • viral and host genomes mix
  • recombination creates new viral strains
  • can contribute to emergence/pandemics (example concept: novel flu/SARS variants)

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10) Biotechnology: recombinant DNA and analytical tools

10a) Recombinant DNA & creating it artificially

Recombinant DNA definition

• DNA combined from more than one source.

Core lab method described

• Use restriction enzymes:
  • recognize restriction sites
  • cut DNA to create fragments with sticky ends (single-stranded overhangs)
• Use DNA ligase:
  • seals strands together by forming sugar-phosphate bonds
• Result:
  • recombinant plasmid containing an inserted gene

How this enables making a human protein in bacteria (insulin example)

• Steps outlined:
  • Extract plasmid from bacteria
  • Cut plasmid with restriction enzyme(s) to generate matching sticky ends
  • Cut out target human gene with the same restriction enzyme(s) (sticky ends become complementary)
  • Ligate to form recombinant plasmid
  • Introduce plasmid into bacteria via transformation
  • Engineered bacteria replicate and express the human protein

10b) Why introns must be removed for bacterial expression

• Bacteria can’t properly process eukaryotic introns.
• If introns aren’t removed, translation would fail (nonfunctional protein).
• Two described ways to remove introns before engineering:
  • reverse-engineer from a known amino acid sequence using genetic code
  • use mRNA from producing cells (already spliced) and convert to DNA using reverse transcriptase → cDNA (described as used with retrovirus-like enzyme)

10c) Gel electrophoresis / DNA fingerprinting

• Gel electrophoresis separates DNA fragments by size and/or electrical charge.
• DNA is negatively charged (moves away from negative electrode toward positive).
• Smaller fragments move faster/farther through porous gel.
• Used for:
  • restriction fragment analysis / DNA fingerprinting (e.g., forensics)

10d) Restriction mapping (problem-solving method)

• Plasmid map indicates restriction sites and distances (kb).
• Digest with an enzyme → number and sizes of fragments predicted.
• Predicted fragment sizes match gel lanes after electrophoresis.

10e) PCR (polymerase chain reaction)

• Purpose: