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  • Created on: 02-12-12 09:18

Structure of DNA

  • Nucleic acids are the information molecules of the cell.
  • The information takes the form of a code in the molecules of DNA.
  • Parts of the code are copied into one form of RNA.
  • Both DNA and RNA are polymers.
  • The single units are called mononucleotides. Each mononucleotide has three parts – pentose sugar, nitrogen-containing base and phosphoric acid.
  • There are two types of n-containing bases found in nucleic acids: Purine and Pyrimidines.
  • The purine bases have two n-containing rings. The purines are Adenine and Guanine.
  • The pyrimidine bases have only 1 n-containing ring. The pyrimidines are Thymine and Cytosine.
  • The phosphate group makes the mononucleotides and hence the nucleic acids, acidic.
  • The sugar, the base and the phosphate group are joined together by condensation reactions, with the elimination of two water molecules, to form the nucleotide.
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Structure of DNA

  • A DNA molecule is made up of 2 polynucleotide strands twisted around each other, with the sugar and phosphates forming the backbone of the molecule, and the bases pointing inwards and pairing up in specific ways to result in the joining of the two strands. The two strands are antiparallel and they are known as the 5' and 3' according to the number of the carbon atom in the pentose sugar to which the phosphate group is attached in the first nucleotide of the chain
  • Mononucleotides link together by condensation reactions to form polynucleotide strands. The sugar of one nucleotide bonds to the phosphate of the next by a phosphodiester bond, so polynucleotides always have a hydroxyl group at one end and a phosphate group at the other.
  • To form DNA, A and T join together via 2 H-bonds. C and G join together via three H-bonds.
  • Phosphate groups – Hydrophilic
  • Bases – Hydrophobic.
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Protein Synthesis


  • Takes place in the nucleus
  • DNA double helix unwinds, and H-bonds between the bases break
  • The sequence on the template (antisense) strand is used in the production of an mRNA molecule
  • This mRNA is built up from free nucleotides which line up alongside the DNA template strand
  • Every triplet code on DNA gives rise to a complementary codon on mRNA.
  • RNA polymerase is the enzyme involved in this process of synthesizing mRNA.
  • The messenger RNA molecule now leaves the nucleus through a pore in the nuclear envelope.
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Protein Synthesis


  • Takes place on ribosomes. These are small organelles made of ribosomal RNA and protein.
  • A transfer RNA molecule carries an amino acid on one end and has three bases (anticodon) attached at the other end and these pair with complementary bases on the messenger RNA codon.
  • The amino acids carried by the tRNA join by means of peptide bonds.

DNA as the genetic code:

  • A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
  • The genome is all the genes in an individual or species.
  • A codon is a sequence of three base pairs on the DNA or RNA.
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DNA replication

  • When the DNA replicates, the two strands of the DNA molecule 'unzip' along the line of hydrogen bonds and unravel. This is brought about by the enzyme helicase. The strands act as templates for the new DNA strands.
  • The exposed bases attract free DNA nucleotides and hydrogen bonds form between matching pairs. DNA polymerase and DNA ligase link adjacent nucleotides together along their sugar-phosphate backbone.

Evidence for semi-conservative replication as derived from Meselson and Stahl's experiments:

  • They grew several generations of the bacteria E.coli in a medium containing only the heavy isotope of nitrogen. The bacteria grown in this medium took up the radioactive isotope to make the cell chemicals, including protein and DNA. After several generations all the bacterial DNA was labeled with 15N
  • The bacteria were then moved to a medium containing normal 14N. This meant all the new nucleotides incorporated into the replicated DNA were 'light' but the original DNA nucleotides were 'heavy'.
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DNA replication

  • They allowed the bacteria to divide and their DNA to replicate once. They then extracted and centrifuged the DNA. All the DNA had the same density, half-way between that of 15N- and 14N- containing DNA.
  • They ruled out the conservative model but they had to decide between fragmentary and semi-conservative. They extracted and centrifuged DNA after two rounds of replication, resulting in two bands – one medium and one light, confirming the semi-conservative replication model.
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Basics of inheritance

  • Phenotype – the physical and chemical characteristics that make up the appearance of an organism.
  • Genotype – genetic information passed on from the parents to their offspring.
  • Locus – the position of a gene on a chromosome.
  • Alleles – different version of a gene that produce variations of the characteristic.
  • Homozygous – if both of the alleles coding for a particular characteristic are identical. Homozygotes are referred to as true breeding.
  • Heterozygous – if the two alleles coding for a characteristic are different.
  • Dominant – alleles whose effect is expressed whether the individual is homozygous or not.
  • Recessive – alleles whose effect is expressed only when the organism is homozygous recessive.
  • Monohybrid cross – when one gene is considered at a time in a genetic cross.
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  • Mutations are changes in the arrangement of bases in an individual gene or in the structure of the chromosome, which change the arrangement of the genes on the chromosome.
  • The body has its own DNA repair systems. Specific enzymes cut out or repair any parts of the DNA strands that become broken or damaged.

There are three types of mutations:

  • Point mutations - These are caused by the miscopying of just one or a small number of nucleotides. These are either; Frame Shift mutations OR A change in the position of a base.
  • Chromosomal mutations - these are caused by changes in the positions of genes within the chromosomes.
  • Whole-chromosome mutations - these are caused by either the loss of an entire chromosome during meiosis or duplication in one cell by errors in the process.
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