Transcription


Transcription is the process by which a DNA sequence is copied into an RNA sequence

There are three main types of RNA generated via transcription

  • Messenger RNA (mRNA):  An RNA copy of a gene - it codes for the production of a protein (via translation)
  • Transfer RNA (tRNA):  A clover-leaf shaped RNA molecule which transfers a specific amino acid to the ribosome during translation
  • Ribosomal RNA (rRNA):  A component of the ribosome, it is responsible for the ribosome's catalytic activity


Structure of a Gene

A gene is a sequence of DNA which is transcribed into RNA and contain three main parts:

  • Promoter:  Responsible for the initiation of transcription (in prokaryotes, a number of genes may be regulated by a single promoter - this is an operon)
  • Coding Sequence:  The sequence of DNA that is actually transcribed (may contain introns in eukaryotes)
  • Terminator:  Sequence that serves to terminate transcription (mechanism of termination differs between prokaryotes and eukaryotes)


Process of Transcription

Transcription is the process by which a DNA sequence (gene) is copied into a complementary RNA sequence and involves a number of steps:

  • RNA polymerase binds to the promoter and causes the unwinding and separation of the DNA strands
  • RNA nucleotides bind to their complementary bases on the DNA strand (uracil pairs with adenine, cytosine pairs with guanine)
  • RNA polymerase covalently binds the nucleotides together, synthesising an RNA strand in a 5' - 3' direction
  • At the terminator, RNA polymerase and the newly formed RNA strand both detach from the DNA template, and the DNA rewinds
  • Many RNA polymerase enzymes can transcribe a DNA sequence sequentially, producing a large number of transcripts
  • Post-transcriptional modification is necessary in eukaryotes


Overview of Transcription


Transcriptional Regulation

Transcriptional regulation is the change in gene expression levels by altering the rate of transcription

This regulation can be achieved in one of two main ways:


Transcription Factors:

  • Transcription factors are proteins that are needed to facilitate the binding of RNA polymerase to the promoter
  • Without transcription factors, RNA polymerase cannot be released to transcribe the mRNA


Regulatory Proteins:

  • These proteins bind to non-coding sequences near the promoter and regulate transcription
  • Reppressors impede the movement of RNA polymerase along the gene's coding sequence, and thus impede transcription
  • Activators bind to enhancer sites near the promoter in order to increase the affinity of RNA polymerase for the promoter


The binding of these regulatory agents may be mediated or controlled by internal or external chemical signals (such as hormones)

The expression of different genes in different cells leads to differentiation of cell structure and function, resulting in specialised tissues 


Post-Transcriptional Modification

In eukaryotes, RNA transcripts must be processed in order to produce mature mRNA

Processing involves three main steps:

  • Capping:  A 5'-methylated cap is added to provide protection against digestion and enable recognition by translational machinery
  • Polyadenylation:  A 3'-poly(A) tail is added to provide protection against digestion and facilitate the export of the mRNA from the nucleus
  • Splicing:  Non-coding intervening sequences (introns) are removed to leave the sequences that will be expressed (exons)


Splicing removes introns to make mature mRNA


Occasionally, exons may also be spliced to generate different mRNA sequences from a single gene  (alternative splicing)

This allows for the formation of multiple distinct polypeptides from a single gene sequence