Protein Synthesis & Repression

What is transcription? 

process where DNA is transcribed to make RNA 📝

Where does transcription occur in eukaryotes vs. prokaryotes?

Eukaryotes=Nucleus; Prokaryotes=Cytoplasm

What are the 3 steps of transcription?

Initiation 🟢, Elongation 🟡, and Termination 🔴.

What is the “promoter” region called and what is its sequence?

TATA Box (TATAAA) 

What are the 3 steps of transcription initiation? 🟢 

• RNA polymerase binds to TATA Box Promoter

• helps unwind DNA H+ bonds to form transcription bubble

• RNA synthesized by complementary base pairing

How is RNA synthesized during initiation?

using complementary base pairing of free nucleotides

thru DNA template (e.g., TATAAA → AUAUU).

Q: What do upstream and downstream mean in transcription?

• Upstream = left of promoter;

• Downstream = right (direction RNA polymerase moves, 5’→3’).

What are the 3 steps of transcription elongation? 🟡

• RNA polymerase moves downstream,

• it adds nucleotides to the 3’ end→ elongates mRNA transcript

• previous DNA rewinds back to double helix 

What are the 3 steps of transcription termination 🔴

• RNA Polymerase reaches terminator sequence on DNA 

• mRNA & RNA polymerase released

• DNA helix re-winds 

What happens specifically to mRNA in termination? 🔴

it is released, so mRNA leaves nucleus to attach to a ribosome

What is translation?

process where mRNA is translated into amino acid chain (protein).

Where does translation occur?

at the ribosome in the cytoplasm

Define a codon and give an example 

• A triplet of mRNA bases that code for 1 amino acid

• ex: DNA’s TAC= mRNA’s AUG 

• How many mRNA bases code for one amino acid?

• What holds them together? 

• 3 bases = 1 codon = 1 amino acid.

• Peptide bonds 

What is the start codon and what amino acid does it code for?

• AUG signals start of translation

• codes for Methionine.

What are the 3 stop codons? What is their function?

• UAA, UAG, and UGA signal end of translation

How many total codons exist in the genetic code?

64 triplet codons.

Why is the genetic code described as “redundant”/“degenerate”?

bc multiple codons can code for the same amino acid.

How many ‘sense’ codons & what do they code for

61 sense= amino acids

How many ‘nonsense’ codons & what do they code for

3 nonsense= stop signals

What does DNA hold for protein synthesis? 

DNA has template to make RNA  

How is mRNA transcribed, & what does it do

• mRNA is transcribed from DNA template

• carries genetic code to ribosomes to create protein

Q: What are the 2 main functions of a ribosome?

• to read the mRNA sequence & 💌

• assemble proteins from amino acids bound to tRNA 🧩

What are the three ribosome sites and their functions?

• A site: Holds tRNA that carrying the next amino acid.

• P site: Holds tRNA carrying the growing polypeptide chain.

• E site: where tRNA exits ribosome 

How does the ribosome facilitate translation?

pairs tRNA anticodons with mRNA codons,

linking amino acids together.

What are the 2 roles of tRNA?

• carries specific amino acid to ribosome, 

• matches it to correct codon on mRNA sequence 

Describe how tRNA looks/structure (3)

• Single RNA strand ~80 nucleotides

• Hydrogen bonds twist into 3D molecule 

• L-shaped (🍀)

Q: What are constitutive genes?

housekeeping genes always on & expressed at fixed rate

What are the two main types of regulated genes, only expressed when needed

• inducible genes 

• repressible genes 

What is catabolite repression? What does it prevent? 

• mechanism of preferring/metabolizing glucose for energy efficiency

• prevents expression of other genes to not waste energy 

• What is an operon?

• Where mostly found

near _ to act as a 

• A cluster of genes

• mostly in prokaryotes

• near promoter, acts as a switch using bound protein  

What is a regulatory promoter gene?

DNA segment that can turn the promoter and the operator on/off, controlling gene expression 

What is the default state of the Lac operon?

OFF (when lactose is absent).

What acts as the inducer in the Lac operon?

Allolactose (form of lactose).

How does allolactose induce the Lac operon? (5 steps)

1. Allolactose binds to active repressor protein

2. changes its shape= inactivates it

3. inactive protein can’t bind to operator.

4. Bc RNA polymerase free to bind to promoter

5. so it does & transcribes lacZ, lacY, lacA (enzymes that break down lactose)

What is the default state of the Trp operon?

ON (when tryptophan is absent).

How does tryptophan regulate the Trp operon? (4)

1. Tryptophan binds to inactive repressor protein,

2. changes its shape= activates it,

3. so activate protein binds to operator 

4. blocks transcription of genes E-D-C-B-A (make tryptophan)

What happens when glucose is absent but lactose is present? (4) 

• cAMP levels rise,

• cAmp binds to CAP (complex)

• complex helps RNA polymerase bind

• → Lac operon turns on, allowing lactose metabolism.

What happens to the Lac operon when glucose is present? (4) 

• cAMP levels drop,

• prevents cAMP–CAP complex 

• RNA polymerase cannot bind

• → Lac operon stays off.