Transcription in detail

Transcription (Detailed Explanation) Transcription is the process by which genetic information stored in DNA is copied into RNA. It is the first step of gene expression and is essential for protein synthesis. Definition Transcription is the synthesis of RNA from a DNA template with the help of the enzyme RNA polymerase. Types of RNA Produced During transcription, different types of RNA can be formed: 1. mRNA (Messenger RNA) – carries genetic information from DNA to ribosomes. 2. tRNA (Transfer RNA) – brings amino acids during protein synthesis. 3. rRNA (Ribosomal RNA) – forms the structure of ribosomes. Location of Transcription In prokaryotes: occurs in the cytoplasm. In eukaryotes: occurs inside the nucleus. Components Required for Transcription 1. DNA template strand 2. RNA polymerase enzyme 3. Ribonucleotides (ATP, GTP, CTP, UTP) 4. Transcription factors (mainly in eukaryotes) Strands of DNA in Transcription DNA has two strands: Template strand (antisense strand) Used by RNA polymerase to synthesize RNA. Coding strand (sense strand) Has the same sequence as RNA except that thymine (T) is replaced by uracil (U) in RNA. Steps of Transcription 1. Initiation RNA polymerase binds to a specific DNA region called the promoter. The DNA double helix unwinds. One DNA strand acts as the template. Promoter A promoter is a DNA sequence that signals the starting point of transcription. Examples: TATA box in eukaryotes Pribnow box in prokaryotes 2. Elongation RNA polymerase moves along the template strand in the 3′ → 5′ direction. RNA is synthesized in the 5′ → 3′ direction. Complementary RNA nucleotides are added. Base Pairing Rules DNA A → RNA U DNA T → RNA A DNA G → RNA C DNA C → RNA G Example: DNA Template: 3′ – TAC GGA ATT – 5′ RNA Formed: 5′ – AUG CCU UAA – 3′ 3. Termination RNA polymerase reaches a terminator sequence. Transcription stops. Newly formed RNA is released. Transcription in Prokaryotes Only one RNA polymerase is present. Transcription and translation occur simultaneously. mRNA usually does not require processing. Transcription in Eukaryotes Eukaryotes have three RNA polymerases: RNA Polymerase Function RNA Polymerase I Synthesizes rRNA RNA Polymerase II Synthesizes mRNA RNA Polymerase III Synthesizes tRNA Post-Transcriptional Modifications In eukaryotes, the primary RNA transcript (hnRNA) is modified before becoming mature mRNA. 1. Capping Addition of methyl guanosine cap at the 5′ end. 2. Tailing Addition of poly-A tail at the 3′ end. 3. Splicing Removal of non-coding regions called introns. Coding regions (exons) are joined together. Difference Between Transcription and Replication Feature Transcription Replication Product RNA DNA Enzyme RNA polymerase DNA polymerase Primer Required No Yes Entire DNA copied? No Yes Occurs before Translation Cell division Importance of Transcription First step of protein synthesis. Controls gene expression. Helps cells produce required proteins. Essential for growth, metabolism, and repair. Central Dogma The flow of genetic information is: DNA → RNA → Protein DNA \rightarrow RNA \rightarrow Protein Diagram of Transcription Summary Transcription is the biological process in which RNA is synthesized from DNA by RNA polymerase. It occurs in three major steps: initiation, elongation, and termination. In eukaryotes, RNA undergoes processing such as capping, splicing, and tailing before becoming functional mRNA.