🔹 Eukaryotic Transcription & Translation

--- ##### **EUKARYOTIC TRANSCRIPTION** Process: DNA → pre-mRNA → (processing) → mature mRNALocation: **Nucleus** --- ##### **1. Initiation Phase** - Transcription **starts** when the enzyme **RNA polymerase II** needs to **attach to the DNA** at the **start of a gene**. - It looks for a special DNA sequence called the **promoter**. - Inside this promoter, there's a key signal called the **TATA box**, usually found **about 25 bases before** the actual starting point of transcription. - But RNA polymerase II **can’t bind to the DNA by itself** — it needs **help**. - First, a helper protein called **TFIID** comes in. Inside TFIID is a special subunit called **TBP** (**TATA-binding protein**) — this is the one that **sticks to the TATA box**. - After TFIID binds, more helper proteins join in — these are other **transcription factors** like **TFIIA** and **TFIIB**. - Together, all these proteins and RNA polymerase II form a group called the **pre-initiation complex (PIC)**. - This complex makes sure that transcription **starts at the correct point** on the DNA. --- ##### **2. Elongation Phase** - Once the complex is ready, the **DNA opens up** at the transcription start site — this creates a **bubble** where transcription happens. - **RNA polymerase II** begins reading the **template DNA strand** from **3' to 5' direction**. - It builds the **new RNA strand** in the **5' to 3' direction**. - It adds free RNA building blocks called **nucleoside triphosphates (NTPs)** — these are:**ATP**, **UTP**, **GTP**, and **CTP**. - These are added **one by one**, matching the DNA bases (A with U, T with A, G with C, C with G). - As RNA polymerase II moves along the DNA:It **unwinds the DNA ahead**. - It **closes it back behind**, keeping the bubble moving forward. --- ##### **3. Termination and Processing** - Transcription **ends** when RNA polymerase II reaches a **termination signal** — this is a special sequence in the RNA: **AAUAAA** (called the polyadenylation signal). - At this point, the **pre-mRNA** is released — but it's **not ready** to be used yet. - It needs to go through **processing steps** to become **mature mRNA**:**5' Cap Addition**:A **7-methylguanosine cap** is added to the **5' end**. - This **protects** the RNA and helps it **bind to ribosomes** later. - **3' Poly-A Tail Addition**:A long chain of **adenine nucleotides (poly-A tail)** is added to the **3' end**. - This gives **stability** and helps in **translation efficiency**. - **Splicing**:**Introns** (non-coding parts) are **cut out**. - **Exons** (coding parts) are **joined together**. - The final **mature mRNA** now leaves the **nucleus** and goes to the **cytoplasm** for translation. --- **EUKARYOTIC TRANSLATION** Process: mRNA → proteinLocation: **Cytoplasm (on ribosomes)** --- ##### **1. Initiation Phase** (Simplified) - Translation **starts** when the **small ribosomal subunit (40S)** binds to the **5' cap** of the mature mRNA. - This step needs help from **initiation factors** called **eIFs**. - The 40S subunit **moves along the mRNA** from **5' to 3' direction**, looking for the **start codon (AUG)**. - AUG codes for the amino acid **methionine** — this marks the true starting point. - A special **initiator tRNA**, carrying methionine, binds to the AUG codon by **base-pairing** its **anticodon** to the mRNA **codon**. - Once everything is aligned, the **large ribosomal subunit (60S)** joins. - Together, they form the full **80S ribosome**, which has **3 sites**:**A site** – where new tRNA with amino acid enters. - **P site** – where the growing protein chain is held. - **E site** – where empty tRNA exits the ribosome. --- ##### **2. Elongation Phase** - The ribosome reads the mRNA **codon by codon**. - For each codon:A matching **aminoacyl-tRNA** enters the **A site**. - A **peptide bond** forms between the new amino acid (in A site) and the existing chain (in P site).This is done by **peptidyl transferase**, an enzyme built into the ribosome. - The ribosome **shifts forward** by one codon (**translocation**).The tRNA in **P site moves to E site** and exits. - The tRNA in **A site moves to P site**, carrying the growing protein. - This cycle repeats, adding one amino acid at a time to the chain. --- ##### **3. Termination and Post-translational Processing** - Translation **ends** when the ribosome reaches a **stop codon** — **UAA, UAG, or UGA**. - These codons **don’t code** for any amino acid. - Instead, special proteins called **release factors (eRFs)** bind to the **A site**. - These trigger the **release** of the finished **polypeptide chain** from the ribosome. - The ribosome then **breaks apart**, ready for another round of translation. - The new protein may **still need processing** before it's fully functional:**Folding** (with help from **chaperone proteins**). - **Cleavage** of signal sequences. - **Chemical modifications**:**Phosphorylation** - **Glycosylation** - **Acetylation** - These steps