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HBIO4 > Metabolism
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Genetic Code

  • DNA codes for assembly of amino acids / forms a polypeptide chain (proteins - enzymes)
  • The code is read in a sequence of three bases called
    • Triplets on DNA (e.g. CAC TCA)
    • Codons on mRNA (e.g. GUG AGU)
    • Anticodons on tRNA (e.g. CAC UCA) - must be complementary to the codon of mRNA
  • Each triplet codes for one amino acid
  • Single amino acid may have up to 6 different triplets for it due to the redundancy of the code / some amino acids are coded by more than one codon (degenerate code)
  • Same triplet code will give the same amino acid in all organisms (universal code)
  • We have 64 possible combinations of the 4 bases in triplets, 43
  • No base of one triplet contributes to part of the code next to it (non-overlapping)
  • Few triplets code for START and STOP sequences for polypeptide chain formation
    • START: AUG
    • STOP: UAA, UAG, UGA

Protein Synthesis

Transcription: DNA to mRNA

  • DNA in nucleus unzips - bonds break
  • Single template strand of DNA used for mRNA (triplet on DNA = codon for amino acid on mRNA)
  • Enzyme RNA polymerase joins nucleotides together
  • Free RNA nucleotides are assembled according to the DNA triplets (A-U / C-G / T-A)
  • mRNA bases are equivalent to the non-template DNA strand
  • Start and stop codons are included
  • Introns (Non-coding) and exons (coding) DNA sequences are present in the primary mRNA transcript. Introns are removed before the mRNA is translated so that exons are only present in the mature mRNA transcript
  • [EXAM] Total number of bases in the DNA sense strand and total number of bases in the mRNA are different
  • mRNA moves into cytoplasm and becomes associated with ribosomes

Translation: mRNA to Protein via tRNA

Translation is the synthesis of a polypeptide chain from amino acids by using codon sequences on mRNA

  • tRNA with anticodon carries amino acid to mRNA associated with ribosome
  • "Anticodon-codon" complementary base pairing
  • Peptide chain is transferred from resident tRNA to incoming tRNA
  • tRNA departs and will soon pick up another amino acid

Requirement for Translation

  • Pool of amino acids / building blocks from which the polypeptides are constructed
  • ATP and enzymes are needed
  • Complementary bases that are hydrogen-bonded to one another
  • Messenger RNA (mRNA): carries the code from the DNA that will be translated into an amino acid sequence
  • Transfer RNA (tRNA): transfer amino acids to their correct position on mRNA strand
  • Ribosomes: provide the environment for tRNA attachment and amino acid linkage

Regulation of Transcription

  • Testosterone
    • Steroid hormones are lipid soluble → easily diffuse across cell membrane into cytoplasm
    • Binds to testosterone receptor found in cytoplasm of target cells
    • Can now enter the cell nucleus
    • Testosterone-Receptor complex binds to chromatin
    • Activates mRNA transcription
  • Acetylation
    • Acetyl group added to histone protein
    • Allows binding of RNA polymerase to DNA
    • Stimulates transcription
  • Methylation
    • DNA methylation inhibits transcription
    • See 3-4-2 Epigenetic Imprinting

 

DNA and Cancer

Tumours

  • Benign - does not spread from its origin (only increasing in size)
  • Malignant - this is cancer
    • Spread throughout the body invading other tissues and destroying them
    • Cells can break off from the primary malignant tumour
    • They can travel/spread via the lymph system
    • Cause secondary tumours at other places (metastasis)
    • Hard to find and difficult to remove (all of) them

Genes and Cancer

  • Tumour suppressor genes
    • Reduce normal activity by inhibiting cell division
    • Initiate cell death (apoptosis) if the cell’s DNA is damaged (due to mutations)
    • Mutation of this gene can cause a loss of its function
    • This will result in tumours
  • Proto-Oncogenes
    • Control cell division
    • Turn into oncogenes if they mutate and become overactive
    • RAS oncogenes
      • G-proteins are found on plasma membranes and enable cells to respond to growth factors
      • They are normally activated by enzymes found within the cell (GTPase)
      • Mutation of the RAS gene causes the production of GTPase deficient G-proteins
      • The mutated G-proteins remain active for longer causing tumours
    • Myc oncogenes
      • Myc gene produces a protein needed for normal cell division
      • Common mutation switches the myc proto-oncogene from chromosome 8 to 14
      • There, it acts as an oncogene / abnormal cell division / tumour
      • When both ras and myc oncogenes present together, malignant cells result
  • Methylation
    • Many methyl groups prevents gene transcription
    • Prevents transcription of tumour suppressor genes
    • Affected by the environment
      • Heavy metals
      • Pesticides
      • Smoking

Oestrogen and Breast Cancer

  • Most breast cancers contain oestrogen receptors
    • Oestrogen binds to receptors in cytoplasm
    • Stimulates cell division and cancer grows
  • Tamoxifen
    • Oestrogen antagonist → blocks oestrogen receptors
    • Prevents cell division but does not kill cells

Enzymes control and rate of reactions in cells

  • Reactions in cells is referred to as cell metabolism
  • A sequence of chemical reactions is called a metabolic pathway
  • DNA controls enzyme production
    • Enzymes control metabolic pathways
    • Metabolic pathways influence the phenotype of an organism
  • Function of enzymes
    • Break down of toxic products (build up could cause damage)
    • Production of a new product (melanin for pigmentation)

Inhibition

  • Slow down rate of reaction of enzyme when necessary (e.g. when temp is too high)
  • Molecule present in highest conc. is most likely to form an ES-complex
  • Competitive Inhibitors
    • Compete with substrate for active site
    • Shape similar to substrates / prevents access when bonded
    • Can slow down a metabolic pathway
  • Non-competitive Inhibitors
    • Chemical does not have to resemble the substrate
    • Binds to enzyme other than at active site
    • This changes the enzyme's active site and prevents access to it
  • Irreversible Inhibition
    • Chemical permanently binds to the enzyme or massively denatures the enzyme
    • Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death
    • Penicillin, the first of "wonder drug" antibiotics, permanently blocks pathways certain bacteria use to assemble their cell wall component (peptidoglycan)

End-product inhibition

  • Metabolic reactions are multi-stepped, each controlled by a single enzyme
  • End-products accumulate within the cell and stop the reaction when sufficient product is made
  • This is achieved by non-competitive inhibition by the end-product
  • The enzyme early in the reaction pathway is inhibited by the end-product