BIO I - EXAM IV
🧠💥 1. DIRECTIONALITY (THE #1 TESTED THING)
🔥 THE 3 GOLDEN RULES
Polymerases READ → 3’ → 5’
Everything BUILDS → 5’ → 3’
Ribosome READS → 5’ → 3’
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🧬 DNA REPLICATION
DNA polymerase:
reads: 3’ → 5’
writes: 5’ → 3’
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📝 TRANSCRIPTION
RNA polymerase:
reads DNA: 3’ → 5’
builds RNA: 5’ → 3’
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🧫 TRANSLATION
Ribosome:
reads mRNA: 5’ → 3’
Protein:
built: N → C
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🚨 COMMON TRAPS
❌ NOTHING builds 3’ → 5’
❌ Ribosome does NOT read 3’ → 5’
❌ Don’t forget to label 5’ and 3’
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🧬💥 2. BASE PAIRING (2ND MOST TESTED)
DNA:
A ↔ T
G ↔ C
RNA:
A ↔ U
G ↔ C
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💡 KEY RELATIONSHIPS
mRNA = complementary to TEMPLATE
mRNA = same as CODING strand (but U instead of T)
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🚨 TRAPS
❌ Don’t match RNA to coding strand incorrectly
❌ Don’t forget U replaces T
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🧬💥 3. DNA REPLICATION
🔥 CORE IDEAS
Semi-conservative = 1 old + 1 new strand
Starts at origin (AT-rich)
Forms:
replication bubble
replication forks
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⚠ ANTIPARALLEL DNA
One strand: 5’ → 3’
Other: 3’ → 5’
👉 THIS causes everything
💥 LEADING vs LAGGING
Feature | Leading | Lagging |
Direction | continuous | discontinuous |
Fragments | ❌ | ✅ Okazaki |
Polymerase | 1 | 1 |
🚨 MUST KNOW
Okazaki fragments = lagging ONLY
Template strands = old DNA
At 1 fork:
👉 2 new strands, 2 polymerases
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📝💥 4. TRANSCRIPTION
🔥 WHAT IT DOES
DNA → RNA
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⚙ STEPS
Initiation → RNA pol binds promoter
Elongation → RNA grows
Termination → stops at terminator
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💡 KEY RULES
Promoter:
determines template strand
determines direction
❌ NOT transcribed
🧠 STRANDS
Strand | Role |
Template | used to make RNA |
Coding | looks like RNA (T→U) |
🚨 TRAPS
❌ Promoter is NOT transcribed
❌ RNA ≠ template strand
❌ RNA = complement of template
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🧫💥 5. TRANSLATION
🔥 FLOW
mRNA → protein
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🧬 CODONS
3 nucleotides = 1 codon
64 codons → 20 amino acids
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🧠 GENETIC CODE
Redundant (multiple codons same AA)
Unambiguous (1 codon = 1 AA)
Nearly universal
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⚙ RIBOSOME
Reads mRNA: 5’ → 3’
Builds protein: N → C
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🧬 tRNA
carries amino acid
has anticodon
binds codon via hydrogen bonds
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💥 RIBOSOME SITES
A = arrival
P = peptide
E = exit
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🚨 TRAPS
❌ Codons ≠ tRNA (they’re on mRNA)
❌ Anticodon = tRNA
❌ Binding = hydrogen bonds
⚡💥 6. MUTATIONS TYPES
Type | Effect |
Silent | no change |
Missense | diff amino acid |
Nonsense | STOP early |
Frameshift | everything shifts 😭 |
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💡 KEY IDEA
👉 Mutations can be:
harmful
neutral
beneficial
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🚨 TRAPS
Frameshift = BIGGEST impact
Silent = no protein change
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🧠💥 7. CENTRAL DOGMA
🔥 FLOW
DNA → RNA → Protein
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📍 LOCATIONS
Prokaryotes:
everything = cytoplasm
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Eukaryotes:
transcription = nucleus
translation = cytoplasm
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💡 DEFINITIONS
Gene = DNA sequence
Gene expression = making a functional product
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💀 FINAL 10-SECOND RECAP (READ BEFORE TEST)
Everything builds 5’ → 3’
Polymerases read 3’ → 5’
Ribosome reads 5’ → 3’
mRNA = complement of template
Leading = continuous
Lagging = fragments
Promoter = decides EVERYTHING
Codon = mRNA
Anticodon = tRNA
Frameshift = worst mutation
Strand | Description |
Template | used to make RNA |
Coding | same as RNA (T→U) |
Leading | continuous |
Lagging | fragments |
⚠ 4. PROMOTER = EVERYTHING (I’m not joking)
👉 Determines:
which strand is template
direction of transcription
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🚨 ALWAYS REMEMBER:
❌ promoter is NOT transcribed
❌ transcription is NOT always left → right
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🧪 5. COUNTING QUESTIONS (FREE POINTS IF YOU DON’T OVERTHINK)
They LOVE these:
At ONE replication fork:
2 new strands
2 DNA polymerases
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At ONE ribosome:
A, P, E sites
1 mRNA
multiple tRNAs cycling
💥 6. CODON LOGIC (THIS IS HOW YOU GET EASY POINTS)
You must instantly know:
codon = mRNA
anticodon = tRNA
3 bases = 1 amino acid
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🚨 SUPER TRAP:
They’ll give you:
DNA → ask for protein
👉 You MUST go:
DNA → RNA → protein
NOT skip steps
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⚡ 7. GENETIC CODE LOGIC (CONCEPT QUESTIONS)
You need to understand—not just memorize:
Redundant → multiple codons = same AA
Unambiguous → 1 codon = 1 AA
Conservative → similar codons = similar AA
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💡 PROF QUESTION STYLE:
“If a mutation changes the 3rd base, why might protein not change?”
👉 Answer: redundancy
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🧬 8. MUTATION IMPACT ORDER (VERY TESTABLE)
From LEAST → MOST severe:
👉 Silent < Missense < Nonsense < Frameshift 💀
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🧠 9. LOCATION QUESTIONS (EASY A POINTS)
Prokaryotes:
everything in cytoplasm
Eukaryotes:
transcription → nucleus
translation → cytoplasm
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🚨 TRAP:
“Where does gene expression occur?”
👉 Answer = both nucleus AND cytoplasm
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🚨 TRAP:
They swap the words in answer choices
👉 trying to catch you slipping
💡 YOUR FIX:
Always do this order:
Find promoter
Identify template
Apply direction (3’→5’ read)
Build RNA (5’→3’)
💀🔥 FINAL TRAPS YOU MUST DODGE
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🧬 1. “LEFT vs RIGHT” TRAP (THIS IS EVIL)
They’ll ask:
👉 “Which direction will transcription occur?”
💡 TRICK:
It is NOT always left → right
It depends on:
👉 promoter orientation
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🚨 RULE:
👉 RNA polymerase moves in the direction that lets it
read template 3’ → 5’
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🧠 2. “THEY FLIP THE DNA” TRAP
They will rotate or flip the strands like:
3’ —————— 5’
5’ —————— 3’
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🧪 3. “PROMOTER vs TERMINATOR” CONFUSIO
Feature | Role |
Promoter | START |
Terminator | STOP |
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🚨 TRAPS:
❌ promoter gets transcribed → WRONG
❌ terminator starts transcription → WRONG
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🧬 4. “REPLICATION vs TRANSCRIPTION” MIX-UP
They’ll give you a question like:
👉 “Which enzyme is working here?”
Choices:
DNA polymerase
RNA polymerase
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💡 YOU MUST IDENTIFY PROCESS FIRST:
DNA → DNA = replication
DNA → RNA = transcription
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⚠ 5. “HOW MANY?” QUESTIONS (SUPER COMMON)
At ONE replication fork:
2 new strands
2 polymerases
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🚨 TRAP:
Students say 4 polymerases
👉 WRONG (they count fragments 😭)
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🧫 6. “CODON vs ANTICODON” SWITCH TRAP
Term | Location |
Codon | mRNA |
Anticodon | tRNA |
🧬 7. “DNA vs RNA SEQUENCE” TRAP
They’ll give:
👉 DNA → ask for RNA
👉 RNA → ask for DNA
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🚨 COMMON MISTAKES:
forgetting U instead of T
not flipping direction
not making it complementary
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⚡ 8. “GENE EXPRESSION” TRAP
Question:
“What is gene expression?”
Choices:
DNA replication
RNA only
protein only
functional product
👉 ✅ Correct: functional product
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🚨 TRAP:
They try to make you pick “protein only”
👉 WRONG (RNA can be product too)
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🧠 9. “LOCATION CONFUSION” TRAP
Question:
“Where does gene expression occur in eukaryotes?”
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Correct:
👉 Nucleus + cytoplasm
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🚨 TRAP:
picking only nucleus ❌
picking only cytoplasm ❌
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🧬 10. “GENETIC CODE LOGIC” TRAP
They’ll ask conceptual:
👉 “Why doesn’t mutation always change protein?”
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You say:
👉 Redundancy
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⚠ 11. “READ vs WRITE” SWITCH TRAP
They LOVE this:
👉 “Which direction does enzyme READ vs WRITE?”
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ALWAYS:
read → 3’→5’
write → 5’→3’
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🚨 TRAP:
They flip wording to confuse you
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🧪 12. “WHAT HAPPENS AFTER TRANSCRIPTION?” TRAP
Correct:
👉 DNA strands re-form double helix
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🚨 TRAP:
“They stay separated” ❌
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🧬 13. “WHAT BINDS FIRST?” TRAP
In transcription:
👉 FIRST thing = proteins bind promoter