7.1 Essential ideas

7.1.2 Transcription and gene expression

  • At the beginning of transcription, RNA polymerase unzips the double helix and binds to one of the DNA strands at a site known as the promoter.
  • The template strand is read in the 3’ to 5’ direction, so that the mRNA transcript is assembled in the 5’ to 3’ direction.
  • As RNA polymerase moves along, free nucleotides are attracted to their complement on the template strand at the 3’ end of the growing mRNA transcript.
  • Most eukaryotic genes contain coding sequences and non-coding, intervening sequences. At the end of transcription, the mRNA molecule contains the same coding and non-coding sequences, in the same order, as the DNA sense strand.

Figure 7.1.2a – Coding and non-coding (template) strands of DNAFigure 7.1.2a – Coding and non-coding (template) strands of DNA

Regulation of transcription by nucleosomes

  • Transcription of specific genes can be switched on or off, so that cells can optimise their production of specific proteins when and where they are needed most.

Figure 7.1.2b – Chemical modification of nucleosomes regulates transcription Figure 7.1.2b – Chemical modification of nucleosomes regulates transcription 

  • Histones have lysine residues that project outwards from the nucleosome. These can be modified by addition of acetyl groups.
  • Adding acetyl groups to histone tails loosens the packaging of DNA and allows the initiation of transcription by RNA polymerase, effectively switching the gene on.
  • On the other hand, direct methylation of DNA results in tighter packaging of nucleosomes. When nucleosomes are tightly packed, transcription factors and RNA polymerases are blocked access, and the gene is effectively switched off.

Regulation of gene expression by proteins

  • Some proteins regulate the rate of gene expression by binding to specific base sequences of DNA.

Figure 7.1.2c – Overview of transcription factorsFigure 7.1.2c – Overview of transcription factors

Protein class

Binding sequence

Action

Transcription factors

Regulatory region of the promoter (TATA box)

Initiate RNA polymerase binding

Activator

Enhancer

Increase rate of transcription

Repressor

Silencer

Decrease rate of transcription

The impact of the environment on gene expression

  • An organism’s environment, as well as the environment of a cell, can influence gene expression. For example, morphogens are cellular signalling proteins that play a role in the pattern of development of an embryo.
  • During development, gene expression is affected by the concentration gradients of morphogen in the embryo.

Figure 7.1.2d – The development of the neural tube in the correct orientation depends on concentrations of two morphogensFigure 7.1.2d – The development of the neural tube in the correct orientation depends on concentrations of two morphogens

  • Similarly, gene expression can be regulated by the external environment.
  • For example, the gene responsible for colouration in Siamese cats and Himalayan rabbits is not expressed at normal body temperatures. The gene is switched in areas of the body where the temperature is lower. This results in the phenomenon known as point colouration.

Post-transcriptional modification in eukaryotes

  • When RNA polymerase reaches the termination sequence of DNA, transcription ends – the pre-mRNA molecule is released into the nucleus and the DNA rewinds.

Figure 7.1.2e – Comparing primary mRNA with mature mRNAFigure 7.1.2e – Comparing primary mRNA with mature mRNA

  • In order to protect the pre-mRNA from degradation, a protective cap is added to the 5’ end, and a tail is added to the 3’ end.
  • Following the addition of the 5’ cap and the poly(A) tail, non-coding regions of the transcript, called introns, are excised, and the coding regions, called exons, are spliced together to form mature mRNA.

Animation of mRNA splicing and the role of the spliceosome

  • Protein complexes made of enzymes and small nuclear ribonucleic proteins (snRNPs) are responsible for the removal of introns and splicing of exons to form mature RNA.

Figure 7.1.2f – Alternative splicing increases the number of different proteins an organism can produceFigure 7.1.2f – Alternative splicing increases the number of different proteins an organism can produce

  • When a gene has multiple exons, it may be spliced in a number of different ways, resulting in a different amino acid sequence and protein structure. This is called alternative splicing.

Figure 7.1.2g – Point colorationFigure 7.1.2g – Point coloration
Why is the fur of a Siamese cat’s ears, nose, tail and feet darker than the rest of its body?

The lac operon was the first system of gene expression regulation by a repressor protein to be understood in prokaryotes

Language help

  • The coding strand has the same code (except for the substitution of uracil for thymine) as the mRNA transcript. The template strand is NOT the coding strand.
  • The promoter is a non-coding sequence of DNA located near a gene that acts as a binding site for RNA polymerase.

Figure 7.1.2h – Transcription and translation in eukaryotesFigure 7.1.2h – Transcription and translation in eukaryotes

Figure 7.1.2i – ProkaryotesFigure 7.1.2i – Prokaryotes
Transcription and translation in prokaryotes

Concept help