Tripura PSC Exam Botany Notes: Genetics: Transcription

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What is Transcription?

Copying a gene as RNA

DNA-directed RNA synthesis from a gene

What is a gene?

  • A sequence of DNA that is transcribed from specific start to specific stop base sequences.
  • Beadle and Tatum, working with the eukaryote mold Neurospora crassa, concluded that one gene codes for one protein.

But what about genes that code for RNA՚s like rRNA and tRNA?

  • A gene is a sequence of DNA that is transcribed into a single RNA as defined by specific start and stop sequences of bases.
  • A cistron is synonymous with a gene. A polycistronic RNA results from the transcription of an operon. DNA replication, transcription and translation.

what՚s an operon?

  • A genetic unit containing several genes with related functions: The bacterial operon for lactose (milk sugar) metabolism contains 3 genes coding for 3 different proteins.
  • An operon is transcribed as a single unit, a polycistronic messenger RNA (mRNA) that codes for more than one gene product.

Name 4 types of RNA. What are their functions?

  • mRNA, messenger RNA that is translated into protein
  • rRNA, ribosomal RNA that, together with ribosomal proteins, forms a structural scaffold for the translation of mRNA, the ribosome
  • tRNA, transfer RNA, a specific carrier of amino acids
  • snRNA, small nuclear RNA involved in processing of mRNA in the nucleus

What is the major transcription enzyme?

  • RNA polymerase, a DNA-directed RNA polymerase
  • RNA synthesis is 5՚ to 3
  • substrates are ribonucleoside triphosphates (to make ribonucleic acid)
  • begins at the promoter, 5՚ end of the gene
  • processivity is very high, proceeds to 3՚ end of gene without stopping or falling off the gene
  • proof reading by precise Watson-Crick base pairing, A = U and G = C
  • Regulation of transcription of a gene is at the 5՚-end of the gene at region (s) termed operators
  • Transcription of some genes is constitutive = housekeeping genes
  • Transcription of other genes is in response to a stimulus = inducible genes

What are exons and introns?

  • exons are coding regions
  • introns are non-coding regions of the mRNA transcript
  • exons and introns are found in most, but not all, eukaryote genes
  • introns have to be spliced out before the mRNA is translated
  • splicing is by snRNA՚s acting as enzymes, or ribozymes, an example of the catalytic function of RNA

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