7.1.3 Non-coding DNA

• DNA molecules are very long but only certain regions code for the production of polypeptides
  • These are called coding sequences
• In humans only 1.5% of the genome contains coding sequences
• The majority of a eukaryotic genome contains non-coding regions of DNA that do not code for polypeptides but have other important functions
• Non-coding gene regulatory sequences are involved in the control of gene expression by enhancing or suppressing transcription
• Non-coding sequences can produce functional RNA molecules like transfer RNA (tRNA)
• Introns are non-coding sequences of DNA found within genes of eukaryotic organisms
  • Different proteins can be produced from a gene depending on how introns are removed
• Telomeres are regions of repeated nucleotide sequences at the end of chromosomes that provide protection during cell division
  • The repeated sequence facilitates binding of an RNA primer at the end of the chromosome leading to synthesis of an Okazaki fragment
  • Without telomeres, DNA replication could not continue to the end of the DNA molecule and chromosomes would become shorter after every cell division
  • Nonetheless, telomeres shorten with age due to oxidative damage within cells
    • Loss of telomeres during ageing can be accelerated by smoking, exposure to pollution, obesity, stress and poor diet
    • Antioxidants in the diet are claimed to reduce the rate of telomere shortening

The RNA molecule produced from the transcription of a gene contains introns that must be removed before translation can occur.

• • DNA profiling (sometimes called genetic fingerprinting) enables individuals to be identified based on their DNA profiles
    
    • It can be used in forensic investigations or paternity testing
  • Short, non-coding regions of DNA called variable number tandem repeats (VNTRs) are analysed
    • The frequency that VNTRs are repeated is unique between different individuals
    • VNTRs are inherited and are similar in close relatives but different in unrelated people
    • Monozygotic (identical) twins inherit identical VNTRs so can’t be differentiated through profiling 
  • To compare the respective DNA profiles of individuals, different regions of DNA containing the VNTRs can be excised with restriction enzymes or amplified by PCR (Polymerase Chain Reaction)
  • The VNTR region for individuals will be a different size as they have different numbers of repeats
    
    • The resulting restriction fragment or amplified DNA will also be a different size
    • Different sized fragments will generate a unique DNA profile in gel electrophoresis

DNA profile of specific VNTRs from three individuals. Different VNTRs could be analysed simultaneously which would result in more bands in each column