Medical Genetics I: Introduction

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Transcription

Basics of Transcription

Transcription literally means making a written copy of something. In biology, transcription is the first step of gene expression in which a particular segment of DNA is copied into mRNA based on complementary base pairing. The template DNA strand is the one that is copied onto the RNA while the coding strand is the one that has the same sequence as the RNA. Transcription takes place in the nucleus.

Stages of Transcription

Transcription consists of 3 stages:

  1. Initiation
  2. Elongation
  3. Termination
Steps of transcription (Credit: Kelvinsong, CC BY-SA 4.0)

Initiation

  • TATA-box binding protein (TBP) binds to the TATA box (located in the promoter region. The promoter is the start site of transcription)
  • General transcription factors are recruited to the promoter
  • RNA polymerase binds to the transcription factors and TBP to form the pre-initiation complex
  • RNA polymerase is phosphorylated at the C terminal and transcription begins.

Elongation

  • After initiation, RNA polymerase moves along the DNA template adding nucleotides to the growing chain of RNA.
  • Nucleotides are added to the 3 prime end of the chain.

Termination

RNA polymerase reaches a transcription termination sequence in the DNA and the newly synthesized RNA is released.

mRNA modifications

These are modifications to the mRNA after transcription. They are done in the nucleus and help to prepare the mRNA for transport to the cytoplasm, for signaling to the ribosome and to protect it from cytoplasmic RNases.

  1. Splicing: The removal of the introns by a spliceosome
  2. 5′ cap: A methyl-guanosine group is added to the 5′ end of the mRNA
  3. 3′ tail: A poly-adenosine tail is added to the 3′ end of the mRNA
Steps of splicing (Credit: LadyofHats, CC BY-SA 4.0)

Regulation of Transcription

Regulation is done mainly by binding of an enhancer or repressor molecule to an activator or silencer region respectively of the DNA.

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