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HBIO2 > Genetics
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Nucleic Acids - The Key to Life

  • 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
  • Differ from proteins as it has phosphorus and NO sulphur
  • Made up of several chains of nucleotides
  • DNA and RNA are types of nucleic acids


  • Sugar-phosphate backbone (ensures stability of the molecule)
    • Pentose sugar
      • Deoxyribose in DNA
      • Ribose in RNA
    • Phosphate group
  • Organic bases
    • Purines (double rings of C and N - bigger)
      • Adenine
      • Guanine
    • Pyrimidines (single ring of C and N - smaller)
      • Thymine in DNA only
      • Uracil in RNA only
      • Cytosine

Deoxyribonucleic Acid (DNA)

  • Made up of 2 separate chains of nucleotides hold together by base pairing
    • Connected by weak hydrogen bonds
    • Can easily be opened for replication
    • Adenine-Thymine has 2 H-bonds
    • Cytosine-Guanine has 3 H-bonds
  • DNA normally twist into a helix (coil) / forms a double helix
    • Makes the molecule compact (store a lot in small space)
    • Protects from damage as base pairs are facing inwards
  • Both chains of DNA are
    • Directional → according to the attachment between sugars and phosphate group
    • Antiparallel → essential for gene coding and replication

Semi-Conservative Replication of DNA

  • Semi-conservative replication: each DNA strand acts as a template for the formation of a new strand
  • Happens during interphase S of the cell cycle
  • Unwinding
    • Enzyme DNA helicase separates 2 strands of DNA by breaking hydrogen bonds
    • Strands are separated a little at a time (not all at once!)
    • This creates a replication fork which moves along the strand
  • Free DNA molecules join up to exposed bases by complementary base pairing
    • Adenine with Thymine (A=T 2-H bonding)
    • Cytosine with Guanine (CΞG 3-H bonding)
  • For the new 5' to 3' strand
    • Enzyme DNA polymerase catalyses the joining of the separate nucleotides
    • New strand is completed "all in one go"
  • For the 3' to 5' strand
    • DNA polymerase produces short sections of strand
    • 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

Ribonucleic Acid (RNA)

  • Ribose instead of deoxyribose
  • Single chain (shorter than DNA)
    • Can pass through nucleus into cytoplasm
  • Base difference
    • Uracil instead of thymine
    • Adenine, guanine, and cytosine are the same
  • Messenger RNA (mRNA) carries the code from DNA that will be translated into an amino acid sequence
  • Transfer RNA (tRNA) transfers amino acids to their correct position on the mRNA strand

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

DNA and Inheritance

  • Cell metabolism: reactions inside cells
  • Metabolic pathway: sequence of chemical reactions
  • Alleles: different forms of the same gene
  • Gene: 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

Alleles and Genes

  • Humans have 46 chromosomes
    • 22 of them are paired up as homologous chromosomes
    • Females have an additional homologous pair of sex chromosomes (XX)
    • Males have an X and Y sex chromosome
  • Pair of homologous chromosomes
    • One of the pair is inherited from the mother, one from the father
    • Gene is a small section of DNA that codes for a specific characteristic
      • Hair colour,
      • Eye colour, ...
    • Found on both pairs of chromosomes at the same locus (position)
    • A gene can have different alleles (forms)
      • Brown eyes, blue eyes, ...
      • Black hair, brown hair, ...
    • This influences the phenotype
  • Multiple alleles
    • Human ABO blood group is controlled by the gene called immunoglobulin I
    • The immunoglobulin gene has 3 alleles IA, IB, I0
    • These alleles code for antigen A, B, neither A/B, respectively
    • Only 2 alleles can be present → IAIB is codominant, I0 is recessive

Genes Control Phenotype


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

Cystic Fibrosis

  • Revision from Unit 1 Section 3.1.3(c)
  • Autosomal recessive disorder
  • Mutation of the CFTR gene on chromosome 7
  • Deletion of 3 bases / allele is missing 3 nucleotides
  • Those nucleotides code for the amino acid phenylalanine
  • Phenylalanine is missing from the CFTR protein
  • Faulty CFTR protein cannot control the opening of chloride channels in the cell membrane

Phenylketonuria (PKU)

  • Autosomal recessive disorder
  • Gene mutation in DNA/gene coding for the enzyme phenylalanine hydroxylase
  • Phenylalanine hydroxylase is not produced
  • Amino acid phenylalanine cannot be converted to the amino acid tyrosine
  • Tyrosine
    • Necessary to produce the pigment melanin
    • Patients are fair-haired, fair skinned and blue eyed (phenotype)
  • Phenylalanine
    • Accumulates in the blood and causes irreversible brain damage
    • Found in most food that contains proteins
    • Treated by avoiding food that contains phenylalanine (diet low in protein)
    • Levels in blood are regularly measured by GP
  • All babies are screened shortly after birth to prevent learning difficulties


Defect (PKU):