AP Biology - Unit 6 Study Guide
AP Biology • Unit 6

Gene Expression & Regulation

Exam Weight: 12–16% • Topics 6.1 – 6.8
6.1

DNA and RNA Structure

DNA and RNA are nucleic acids that store and transmit genetic information.

The Central Dogma
DNA
→
Transcription
RNA
→
Translation
Protein
DNA Double Helix Structure A T G C T A C G A T G C 5' 3' Nucleotide Components PO₄ Phosphate Sugar Deoxyribose (DNA) Base A, T, G, C Hydrogen Bonds A-T: 2 bonds | G-C: 3 bonds (G-C pairs are stronger)
Adenine (A) Thymine (T) Guanine (G) Cytosine (C)
DNA vs RNA
FeatureDNARNA
SugarDeoxyriboseRibose
StrandsDoubleSingle
BasesA, T, G, CA, U, G, C
PairingA-T, G-CA-U, G-C
Types of RNA
mRNA

Carries code to ribosome

tRNA

Brings amino acids; anticodon

rRNA

Makes up ribosomes

AP Tip: In RNA, Uracil (U) replaces Thymine (T).
6.2

DNA Replication

DNA replication copies DNA before cell division. It is semiconservative (each new DNA has one old + one new strand).

DNA Replication Fork Helicase (unwinds) DNA Pol III DNA Pol III Primase RNA Primer 5' → 3' 5' → 3' Leading Strand Lagging Strand Key Enzymes Helicase - unwinds Primase - RNA primer DNA Pol III - adds nt RNA Primer Replication Fork Okazaki Fragments (joined by Ligase)

Semiconservative replication: Each new DNA molecule contains one original (blue) strand and one newly synthesized (red) strand.

Key Enzymes
EnzymeFunction
HelicaseUnwinds double helix
PrimaseMakes RNA primers
DNA Polymerase IIIAdds nucleotides 5'→3'
DNA Polymerase IReplaces primers with DNA
LigaseJoins Okazaki fragments
Leading Strand

Synthesized continuously toward the fork

Lagging Strand

Synthesized in Okazaki fragments away from fork

Critical: DNA Pol ONLY adds to the 3' end. Synthesis is always 5'→3'.
6.3

Transcription and RNA Processing

Transcription copies DNA into RNA. It occurs in the nucleus.

Transcription Process 3' 5' Template Strand 5' 3' RNA Polymerase 5' Cap mRNA (growing) Direction of transcription Promoter Terminator 1. Initiation RNA Pol binds promoter DNA unwinds 2. Elongation Reads template 3'→5' Builds mRNA 5'→3' 3. Termination Reaches terminator mRNA released
Three Stages
Initiation

RNA Pol binds promoter

Elongation

Reads 3'→5', builds 5'→3'

Termination

Reaches terminator, releases

RNA Processing in Eukaryotes Pre-mRNA E1 Intron E2 Intron E3 Processing Mature mRNA 5' Cap E1 E2 E3 AAAA... 3' 5' Cap • Modified guanine • Protects mRNA • Ribosome binding Poly-A Tail • 100-250 adenines • Protects from enzymes • Aids nuclear export Splicing • Introns removed • Exons joined • By spliceosome
RNA Processing (Eukaryotes)
5' Cap

Protects; helps ribosome bind

Poly-A Tail

Protects; aids export

Splicing

Introns out, exons joined

Memory Trick: "EX-ons are EX-pressed, IN-trons stay IN the nucleus"
6.4

Translation

Translation converts mRNA into protein at ribosomes.

Translation at the Ribosome 5' 3' AUG GCU UGG CAA UAA Large Subunit (60S) Small Subunit (40S) E P A CGA Met Ala Trp Gln GUU 5' → 3' START STOP Codon (mRNA) Ribosome Sites A Aminoacyl (incoming) P Peptidyl (chain) E Exit (empty tRNA) Anticodon: Complementary sequence on tRNA that pairs with mRNA codon Reading: Ribosome moves along mRNA 5'→3', reading codons in sequence
The Genetic Code
Start: AUG

Methionine

Stop: UAA, UAG, UGA

No amino acid

Properties

Universal, Redundant

Ribosome Sites
A site

Aminoacyl - new tRNA enters

P site

Peptidyl - holds growing chain

E site

Exit - empty tRNA leaves

Codon vs Anticodon: Codons on mRNA; anticodons on tRNA (complementary, antiparallel).
6.5

Regulation of Gene Expression

Cells control when, where, and how much of a gene product is made.

Levels of Regulation

Epigenetic → Transcriptional → Post-transcriptional → Translational → Post-translational

The Lac Operon No Lactose = OFF Promoter Operator lacZ lacY lacA Repressor (active) RNA Pol BLOCKED! No transcription Lactose Present = ON Promoter Operator lacZ lacY lacA Repressor (inactive) Lac RNA Pol mRNA When No Lactose: • Repressor binds operator • RNA Polymerase blocked • No enzymes produced When Lactose Present: • Lactose binds repressor • Repressor releases operator • Transcription proceeds Negative Regulation The repressor protein normally BLOCKS transcription. Lactose acts as an INDUCER that removes the block.
The Lac Operon
No Lactose = OFF

Repressor blocks operator

Lactose = ON

Lactose binds repressor, releases operator

AP Tip: Lac operon = negative regulation. Lactose is the inducer.
6.6

Gene Expression and Cell Specialization

All cells have the same DNA, but express different genes. This is differential gene expression.

Cell Differentiation & Stem Cells Totipotent (Zygote) Any cell type + placenta Pluripotent (ES Cells) Any body cell type Multipotent (Adult SC) Related cell types only Neuron Muscle Blood Cell Differential Gene Expression Same DNA in all cells Different genes turned ON/OFF = Different cell types & functions Transcription Factors Proteins that bind DNA Activate or repress genes Control cell identity
Differentiation

Transcription factors turn specific genes on/off → specialized cells

Stem Cells

Totipotent → Pluripotent → Multipotent

Key Point: Cell identity = which genes are expressed, controlled by transcription factors.
6.7

Mutations

A mutation is any change in DNA sequence.

Types of Point Mutations Original: AUG GAA UUU → Met - Glu - Phe Silent: AUG GAG UUU → Met - Glu - Phe Same amino acid! Missense: AUG GUA UUU → Met - Val - Phe Different amino acid Nonsense: AUG UAA --- → Met - STOP Truncated protein! Frameshift Mutations Original: AUG GAA UUU GGC Insertion (+C): AUG CGA AUU UGG C ALL codons shifted! Severity Ranking Frameshift > Nonsense > Missense > Silent
Point Mutations
TypeEffect
SilentSame amino acid (no effect)
MissenseDifferent amino acid
NonsenseCreates STOP codon → truncated protein
Frameshift Mutations

Insertions or deletions (not multiples of 3) shift the reading frame, changing ALL codons after the mutation.

AP Tip: Frameshifts are usually MORE harmful than substitutions.
6.8

Biotechnology

Biotechnology manipulates DNA for human benefit.

PCR (Polymerase Chain Reaction) 1. Denature 95°C → 2. Anneal 55-65°C Primer Primer 3. Extend 72°C 2 copies! Exponential Amplification Cycle 1 2 → Cycle 2 4 → Cycle 3 8 → Cycle n 2ⁿ → 30 cycles ~1 billion! Taq Polymerase: Heat-stable DNA polymerase from thermophilic bacteria
Gel Electrophoresis − + Large Small Migration How It Works DNA is negatively charged Moves toward + electrode Smaller = travels farther Larger = stays near top

Separates DNA by size: Smaller fragments travel farther through the gel matrix toward the positive electrode.

Key Techniques
TechniquePurpose
PCRAmplify DNA (denature→anneal→extend)
Gel ElectrophoresisSeparate DNA by size
Restriction EnzymesCut DNA at specific sequences
TransformationInsert DNA into bacteria
CRISPR-Cas9Precise gene editing
Applications

Medicine (insulin), Agriculture (GMOs), Forensics, Research

Ethics

Germline editing, GMO safety, Privacy, Enhancement

AP Tip: Know purposes: PCR=amplify, Gel=separate by size, CRISPR=edit precisely.
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