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Genes incorporate coded information which determines the metabolism of organism

  • Nucleic acids carry the genetic code that determines the order of amino acids in proteins
  • Genetic material stores information, can be replicated, and undergoes mutations
  • Differs from proteins as it has phosphorus and NO sulphur

DNA Deoxyribonucleic Acid
Nucleotides are smaller units of long chains of nucleic acids. Each nucleotide has
  • A pentose sugar (deoxyribose in DNA, ribose in RNA)
  • A phosphate group
  • An organic base which fall into 2 groups,
    • Purines (double rings of C and N - bigger)
      • Adenine or Guanine
    • Pyrimidines (single ring of C and N - smaller)
      • Thymine or Cytosine
    • Base pairing by weak hydrogen bonds
      • Adenine-Thymine 2 H- bonds
      • Cytosine-Guanine 3 H- bonds
  • Chains are directional according to the attachment between sugars and phosphate group
  • They are antiparallel which is essential for gene coding and replication
  • DNA molecule has 2 separate chains of nucleotides hold together by base pairing / DNA normally twist into a helix (coil) / forms a double helix

Ribonucleic Acid (RNA)
  • Ribose instead of deoxyribose
  • Single chain (shorter than DNA - lower molecular mass)
  • Base difference: Uracil instead of Thymine. Adenine, Guanine and Cytosine are the same
    1. Ribosomal RNA (rRNA)
      • Located in the cytoplasm - ER
      • Reads mRNA code and assembles amino acids in their correct sequence to make a functional protein (translation)
    2. Messenger RNA (mRNA)
      • Commutes between nucleus and cytoplasm
      • Copies the code for a single protein from DNA (transcription)
      • Carries the code to ribosomes in the cytoplasm
    3. Transfer RNA (tRNA)
      • In the cytoplasm
      • Transfer amino acids from the cytoplasm to the ribosomes

The 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 / code is degenerate. Some amino acids are coded by more than one codon
  • Same triplet code will give the same amino acid in virtually 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
    eg START   AUG    and   STOP   UAA UAG UGA

DNA Replication (Semi-Conservative Replication)
  • Happens during Interphase 'S'
  • Separate the strands, a little at a time to form a replication fork
  • Events:
    1. Unwinding / Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds
    2. Semi-conservative replication / each strand acts as a template for the formation of a new strand
    3. Free DNA molecules join up to exposed bases by complementary base pairing
      1. Adenine with Thymine (A=T 2 -H bonding)
      2. Cytosine with Guanine (CΞG 3 -H bonding)
    4. For the new 5' to 3' strand the enzyme DNA polymerase catalyses the joining of the separate nucleotides
      "All in one go" completed new strand
    5. For the 3' to 5' strand DNA polymerase produces short sections of strand but these sections have to be joined by DNA ligase to make the completed new strand. Specific base pairing ensures that two identical copies of the original DNA have been formed

Transcription: DNA to mRNA
  1. DNA in nucleus unzips - bonds break
  2. Single template strand of DNA used for mRNA (triplet on DNA = codon for amino acid on mRNA)
  3. Enzyme RNA polymerase joins nucleotides together
  4. Free RNA nucleotides are assembled according to the DNA triplets (A-U / C-G / T-A)
  5. mRNA bases are equivalent to the non-template DNA strand
  6. Start and stop codons are included
  7. 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
    Total number of bases in the DNA sense strand and total number of bases in the mRNA are different
  8. 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
  1. tRNA with anticodon carries amino acid to mRNA associated with ribosome
  2. "Anticodon - codon" complementary base pairing occurs
  3. Peptide chain is transferred from resident tRNA to incoming tRNA
  4. 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 are hydrogen-bonded to one another
  • Structure involved in translation
  1. Messenger RNA (mRNA)
    Carries the code from the DNA that will be translated into an amino acid sequence
  2. Transfer RNA (tRNA)
    Transfer amino acids to their correct position on mRNA strand
  3. Ribosomes
    Provide the environment for tRNA attachment and amino acid linkage

DNA and Inheritance
  • Reactions in cells is referred to as cell metabolism
  • A sequence of chemical reactions is called a metabolic pathway
  • Different forms of the same gene are alleles
  • A gene is the length of DNA that carries the code for a protein (enzyme)
    • Enzyme effect the cell's metabolism
    • Visible changes are described with the phenotype
  • The phenotype is influenced by the metabolic pathway
  • Therefore
    • DNA controls enzyme production
    • Enzymes control metabolic pathways
    • Metabolic pathways influence the phenotype of an organism

Gene Mutations
  • Deletion, reading frame shifts
  • Substitution, one base replaced by another
  • Duplication, repetition of part of the sequence
  • Addition, Addition extra base
  • Change in one or more nucleotide bases in the DNA
  • Change in the genotype (may be inherited)

Cystic Fibrosis - Defective Gene
  1. Mutation causes the deletion of 3 bases in DNA. One amino acid (phenylalanine) is not coded for in the
    Cystic Fibrosis Transmembrane Regulator CFTR protein
  2. Faulty CFTR protein cannot control the opening of chloride channels in the cell membrane
  3. Results in production of thick sticky mucus, especially in lungs, pancreas and liver
  4. Organs cannot function normally and infection rate increases

Phenylketonuria (PKU) - Defective Gene
  1. Gene mutation in DNA coding for the enzyme phenylalanine hydroxylase
  2. Phenylalanine hydroxylase not produced
  3. Amino acid phenylalanine cannot be converted to the amino acid tyrosine
  4. Tyrosine is necessary to produce the pigment melanin
  5. Phenylalanine collects in the blood and causes retardation in young children
  6. Managed by controlling diet to eliminate proteins containing phenylalanine
  7. Disease is tested by drops of blood taken from the baby


Defect (PKU):


References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]