7.2 Transcription and gene expression

By the end of this sub- unit you should be able to:

  • State that transcription occurs in a 5’ to 3’ direction.
  • Explain how eukaryotic cells modify mRNA after transcription.
  • Explain how splicing of mRNA increases the number of different proteins an organism can produce.
  • Outline that the promoter is an example of non-coding DNA with a function.
  • Describe the regulation of gene expression by proteins that bind to specific base sequences in DNA.
  • Describe how nucleosomes help to regulate transcription in eukaryotes through acetylation and methylation of histones.
  • Outline the effect of DNA methylation on genes.
  • Analyse changes in DNA methylation pattern from data provided.
  • Explain how the environment of a cell and of an organism has an impact on gene expression.

Lesson One: Transcription.

Objectives: learn how genes are modified post-transcription before translation. Appreciate how more than one protein can be made from the same gene. Learn some of the roles of the non-coding sections of DNA. Learn how the environment may affect gene expression (epigenetics).

Keywords: 

  • transcription: the synthesis of messenger RNA from the antisense strand, in the nucleus of a cell. 
  • exons: the sections of DNA that are used in translation 
  • introns: the sections of DNA that are cut out and not used in translation (memory trigger: Introns are In-between the coding DNA, Exons are Essential for translation)
  • coding DNA is used to make proteins, non-coding is not used to make proteins.
  • Spliceosome: the enzyme that cuts out the introns and binds the axons together
  • promoter: non-coding DNA that RNA polymerase binds to in order to start transcription (binding site for RNA polymerase
  • operator: non-coding DNA that is next to the promoter, and if blocked by a repressor, stops the RNA polymerase from binding to the promoter.
  • regulator gene: a gene that makes a represser that may bind to an operator and stop the binding of RNA polymerase to the promoter, effectively blocking transcription.
  • Repressor: a protein that can bind to the operator and block transcription (made by the gene regulator).
  • Lac operon: a gene that makes lactase, in prokaryotes.

Transcription

OSC_Microbio_11_03_TxnElong

 

  • Transcription happens in the nucleus. A single stranded complimentary copy of the antisense strand is made, indirectly copying the sense strand.

Q) Explain why transcription copies the antisense strand, and not the sense strand?

 

  • Transcription can only happen in the 5′ to 3′ direction. This is like DNA replication.

Q) Compare and contrast DNA and RNA polymerase?

  • Sections of non-coding DNA are cut out out by enzymes called spliceosomes. These introns are not needed in translation. The coding DNA sections are called exons, and they are stuck together to form the processed mRNA which will go to a ribosomes to be translated. (memory trigger: Introns are In-between the coding DNA, Exons are Essential for translation)
  • Depending on the way that the exons are stuck together, you may end up with different proteins. This is because it is the sequence of the codons that decides the sequence of amino acids. If you change the order that the exons are stuck together, you change the resulting polypeptide.
image credit: NIH science education

image credit: NIH science education

Switching genes off and on

By understanding the operator, promoter, and gene regulator, we can see how the gene for lactase can be switched on only when needed. The famous is example is that in prokaryotes, the gene to make lactase is switched on only when lactose is present. This was found through studying the gene that makes lactase in prokaryotes (the gene is called lac operon).

1) Lactose is not present:

  • Gene regulator synthesises the repressor.
  • Repressor binds to the operator, next to the promoter
  • The promoter is blocked by the repressor, and RNA polymerase cannot bind to it
  • Transcription is blocked
  • No lactase is synthesised (as it is not needed)

2) Lactose is present:

  • Gene regulator synthesises the repressor
  • The repressor binds to lactose, inactivating the repressor
  • The repressor can no longer bind to the operator
  • The promoter site is no longer blocked by the repressor
  • RNA polymerase may bind to the promoter
  • Transcription occurs
  • Lactase is synthesised (as it is needed).

watch the images in this youtube video of a slide presentation to get a visual explanation of this.

 

Epigenetics – or how the environment can influence gene expression

There are several mechanisms known for how environmental factors can affect gene expression.

We will study:

  1. methylation (when a methyl group binds to histones, de-activating a gene)
  2. acetylation (when an acetyl group binds to histones, increasing transcription)

Why histones?

In eukaryotes, DNA is associated with histone proteins, in that the DNA is coiled around histone proteins to form nucleosomes. Modification of the histones can make it easier or harder for the DNA to uncoil, which is necessary for transcription to occur.

Memory trigger: Methylation is Minimising to gene expression. Acetylation is Activating

1. Methylation:

Methylation is considered to be highly important. A methyl group (CH3), binds to the histone ‘tail – end of the protein molecule’, causing it to change and make uncoiling the DNA more difficult. This effectively reduces transcription by switching off the genes that have been methylated.

Skill: review slides 17-20 in the powerpoint below to practice analysing methylation patterns in chromosomes

note hypomethylated means less methylated than before while hypermethylated means more methylated than before.

task review the DBQ on p.358 in the oxford book 

2. Acetylation is the binding of an acetyl group (CH3CO) to a histone ‘tail’. Unlike methylation, this causes activation of the gene as it makes it easier to unwind the DNA.

Shocking fact: During your life, methylation and acetylation processes resulting from your lifestyle will enhance and repress the function of various genes in your body cells. Scientists have found evidence that 1% of those changes will actually be passed on to your offspring. In other words, some environmental effects can be inherited. 

A study based in highland Kenya found that mothers who moved to high altitudes, before giving birth gave birth to babies who already had the elevated haemoglobin levels that they had acquired as their bodies adapted to the reduced oxygen levels in the lower pressure atmosphere.

 

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