— EUKARYOTIC TRANSCRIPTION Process: DNA → pre-mRNA → (processing) → mature mRNALocation: Nucleus — 1. Initiation Phase - Transcription starts when the enzy
--- 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 help the protein get its final shape , proper location in the cell, and biological activity . --- Quick Memory Aid: --- DNA REPLICATION – STEP BY STEP DNA replication is the process by which a cell makes an exact copy of its DNA before division. It is semi-conservative , meaning each new DNA molecule has one old (parental) strand and one new (daughter) strand . --- GENERAL DNA REPLICATION STEPS (Eukaryotes) --- Step 1: Initiation – DNA Unwinding What happens: The enzyme DNA helicase attaches to the origin of replication (a special DNA sequence). Helicase breaks hydrogen bon