University:
Kentucky State University
Course:
CHE 315 | Biochemistry
Academic year:

2024
Views:

403

Pages:

13
Author:

salcignofi13

TRANSCRIPTION-THE MECHANISM 42 General description • Transcription is the process of making RNA from DNA • One of the two strands of the DNA double helix acts as a template for the synthesis of an RNA molecule 43 Complementary sequences • As in DNA replication, the nucleotide sequence of the RNA chain is determined by the complementary base-pairing between incoming nucleotides and the DNA template • The RNA chain produced by transcription is also known as the transcript. 44 Enzyme and substrate • The enzymes that perform transcription are called RNA polymerases. • Like the DNA polymerase, RNA polymerases catalyze the formation of the phosphodiester bonds between two nucleotides. • The growing RNA chain is extended in the 5-to-3 direction. • The substrates are nucleoside triphosphates (ATP, CTP, UTP, and GTP). 45 46 Stages of transcription • Initiation: it starts when RNA polymerase recognizes a control segment (called promoter) in DNA that precedes the first nucleotides that is transcribed. • Elongation: RNA polymerase moves down the DNA segment. • Termination: Transcription ends when the RNA polymerase reaches a stop sequences. 47 Promoters • These are DNA sequences that have these features: – They are located before genes – They are RNA polymerase-binding sites – They contain regulatory sites (binding sites of transcription factors) 49 Gene • The part of DNA that is transcribed into RNA. 50 Introns and exons • Not the whole of mRNA is used to make a protein. • The part that is used to make a protein is called an exon. • Exons are interrupted by sequences known as introns, which are removed. 51 RNA splicing • The intron sequences are removed from the newly synthesized RNA through the process of RNA splicing. 52 Alternative splicing • The transcripts are spliced in different ways to produce different mRNAs and different proteins. – These are known as protein isoforms. • The process is known as alternative splicing. 53 Types of RNA • Coding RNA (messenger RNA (mRNA)) that codes for proteins – 2% of human DNA (genome) • Noncoding: it is not used to make proteins – ~70% of human genome – They have various functions. 54 MicroRNA or miRNA (an example) • MicroRNA molecules are produced by transcription as double-stranded RNA that gets processed into single-stranded RNA. One strand is taken to a complementary site on a targeted mRNA. • Once bound, it leads to either mRNA degradation or inhibition of translation. • Protein level is reduced. Primary microRNA (transcription from DNA) 55