7.2

• Epigenetic modifications

  • DNA methylation - addition of methyl group to CpG site, prevents transcription
  • Histone acetylation - addition of acetyl group, activates chromatin
  • Histone methylation - can activate or repress chromatin depending on position
• Transcription factors are proteins that bind to DNA
• What is a transcription factor?


  A protein that controls the transcription of gene so that only certain parts of the DNA are expressed, for example in order to allow a cell to specialise
• How did transcription factors work?


  They move from the cytoplasm into the nucleus, find the DNA near the target gene, and then either increase or decrease the rate of transcription
• Transcription factors bind to specific base sequences such as promoter sequences and enhancer sequences
• Promoter sequences


  Found upstream of the gene they act on – enable the binding of RNA polymerase and therefore promote transcription
• Enhancer sequences

  Regulate DNA activity by changing chromatin structure – making it more or less open to RNA polymerise.

  Open = active gene expression

  Closed = gene inactivity - transcription factors either stimulate or prevent transcription of the gene
• What is meant by epigenetic?

  Heritable and reversible modification to the DNA that do not involve changes to the nucleotide sequence
• Explain DNA methylation 


  Addition of a methyl (CH3) group to a CpG site (cytosine next to guanine) on DNA. Prevents transcription and affects histone structure to make more/less DNA accessible to RNA polymerase
• Explain histone modification

  • Acetylation - addition of an acetyl (COCH3) group, activates chromatin and allows transcription
  • Methylation - addition of a methyl group, can cause activation/inactivation of chromatin depending on the position of the lysine
• What is non-coding RNA?

  ncRNA affects transcription/modifies the product of transcription for example, ncRNA coats one X-chromosome, which superecoils and condenses to form the stable, inactive Barr body to maintain the balance of gene production
• What are introns?

  non-coding regions
• What are exons?

  coding regions
• How can splicing result in different products from a single gene?

  When is the exons are joined back together by enzymes, they can be joined in a variety of ways. Therefore several different versions of the RNA can be produced
• How are epigenetic‘s involved in cell differentiation?

  Epigenetic‘s control the amount of a cells DNA that is transcribed, and therefore ensures only the proteins needed for that specific cells function are produced
• What do transcription factors do?

  They are proteins which bind to regulatory regions of DNA and control DNA transcription
• What is RNA splicing?

  RNA splicing is a process by which non-coding regions of a pre-mRNA molecule, called introns, are removed and the remaining coding regions, called exons, are joined together to produce a mature mRNA molecule that can be translated into protein