DNA, RNA, and Protein Synthesis

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  • DNA is a polynucleotide - multiple nucleotides joined together.
  • Each nucleotide is made up of a pentose sugar, a phosphate group, and a nitrogenous base.
  • The sugar in DNA is a deoxyribose sugar.
  • Each nucleotide has the same sugar and phosphate, but the bases vary.
  • Four possible bases: Gaunine, Adenine, Thymine and Cytosine.
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  • DNA nucleotides join together to form polynucleotide strands.
  • The nucleotides join up between the phosphate group of one nucleotide and the sugar of another, creating a sugar-phosphate backbone.
  • Two DNA polynucleotide strands join by hydrogen bonding between the bases.
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  • Genes are sections of DNA found on chromosomes.
  • Genes code for polyopeptides/proteins
  • Proteins are made from amino acids.
  • Different proteins have a different number and order of amino acids.
  • each amino acid is coded for by a triplet codon in a gene.
  • Different sequences of bases code for different amino acids
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RNA and mRNA

DNA molecules are found in the nucleus of the cell.

  • Organelles for protein synthesis are found in the nucleus.
  • DNA is too large to move out of the nucleus so a section is copied into RNA.
  • This process is called transcription.
  • RNA is a single polynucleotide strand.
  • RNA replaces Thymine with Uracil.
  • RNA leaves the nucleus and joins with a ribosome.
  • This process is called translation.


  • Made in the nucleus.
  • Three adjacent bases called a codon.
  • Carries the genetic code from the nucleus to the cytoplasm.


  • Found in the cytoplasm.
  • Amino acid binding site
  • Carries amino acids to the ribosomes.
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  • RNA polymerase attaches to the DNA double helix at the beginning of a gene.
  • Hydrogen bonds between the DNA strands break, separating the strands.
  • One strand is used as a copy for mRNA
  • RNA polymerase lines up free RNA nucleotides alongside the template strand.
  • mRNA ends up being a complementary strand of the DNA strand.
  • RNA nucleotides are joined together forming an mRNA molecule.
  • RNA polymerase moves along the DNA, separating the strands and assembling the mRNA strand.
  • Hydrogen bonds reform between the DNA strands, coiling back into a double helix.
  • When RNA polymerase reaches a stop codon, mRNA production is stopped
  • The mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm.
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  • Occurs at the ribosomes in the cytoplasm
  • The mRNA attaches itself to a ribosome and tRNA carries amino acids to the ribosome.
  • a tRNA molecule with an anticodon complementary to the first codon on the mRNA attaches by complimentary base pairing.
  • A second tRNA molecule attaches itself to the next codon.
  • The two amino acids are attached by a peptide bond. the first tRNA molecule moves away.
  • This process continues, producing a polypeptide chain of amino acids, until there's a stop codon.
  • The polypeptide chain moves away from the ribosome.
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The lac Operon - Lactose Absent

  • The regulator gene is expressed and the repressor protein is synthesised. I has two binding sites, one that binds to lactose and one tht binds to the operator region.
  • the repressor protein binds to the operator region. in doing so it covers part of the promoter region, where RNA polymerase normally attaches.
  • RNA polymerase cannot bind to the promoter region so the structural gene cannot be transcribed into mRNA.
  • Without mRNA these genes cannot be translated and the enzyme Beta-Galactosidase and lactose permease cannot be synthesised.
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The lac Operon - Lactose Present

  • Lactose / inducer molecules bind to the other site on the repressor protein. This causes the molecules of repressor protein to change shape so that its other binding site cannot bind to the operator region. The repressor dissociates from the operator region.
  • This leaves the promoter region unblocked. RNA polymerase can now bind to it and initiate the transcription of mRNA for genes Z and Y.
  • The operator - repressor - inducer system acts as a molecular switch. It allows transcription and subsequent translation of the structural genes, Z and Y, into the lac enzymes, Beta - Galactosidase and lactose permease.
  • As a result, E. coli bacteria can use the lactose permease enzyme to take up lactose from the medium into their cells. They can then convert the lactose into glucose and galactose using the Beta - Galactosidase enzyme. The sugars can then be used for respiration, thus gaining energy from lactose.
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Body Plans

  • A body plan is the general structure of an organism - the particular way the organism is arranged.
  • Proteins control the development of the body plan.
  • The proteins that control the body plan development are coded for by homeotic genes.
  • Homeotic genes have regions called homeobox sequences that code for a part of the protein called the homeodomain.
  • The homeodomain binds to specific sites on the DNA, enabling the protein to work as a transcription factor.
  • The proteins bind to DNA at the start of the developmental genes, activating or repressing transcription and so altering the production of proteins involved in the development of body plan.
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  • Apoptosis is a highly controlled process of programmed cell death.
  • Once apoptosis has been triggered the cells are broken down in a series of steps.
  • The cell produces enzymes that break down important cell components such as proteins in the cytoplasm and DNA in the nucleus.
  • As the cell's contents are broken down it begins to shrink and break up into fragments.
  • The cell fragments are engulfed by phagocytes and digested. 
  • Apoptosis is involved in the development of body plans - mitosis and differentiation create the bulk of body parts and then apoptosis refines the parts by removing the unwanted structures.
  • Examples of apoptosis would include the development of fingers and toes, frog development, and excess neurones in the nervous system.
  • All cells contain genes that code for proteins that promote or inhibit apoptosis.
  • during development, genes that control apoptosis are switched on and off in appropriate cells, so that some die and the correct body plan develops.
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Protein Activation

  • cAMP activates some proteins by altering their shape.
  • Some proteins produced by protein synthesis need to be activated in order for them to work.
  • Protein activation is controlled by molecules - hormone and sugars.
  • These molecules work by binding to cell membranes and triggereing the production of cAMP inside the cell.
  • cAMP then activates proteins inside the cell by altering their 3D structure.

Protein Kinase A PKA

  • PKA is an enzyme made of four subunits.
  • When cAMP isn't bound, the four units are bound together and are inactive.
  • When cAMP binds, it causes a change in the 3D structure, releasing the subunits.
  • PKA is now active.
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Mutations - 1

  • Any changes to the nucleotide sequence of DNA is called a mutation.
  • Substitution - One base is swapped for another.
  • Deletion - One base is removed.
  • Insertion - One base is added.
  • Duplication - One or more bases are repeated.
  • Inversion - A sequence of bases is reversed.
  • The order of the DNA bases in a gene determines the order of the amino acids in a particular protein. If a mutation occurs in a gene, the primary structure of the protein it codes for could be altered.
  • This may change the final 3D shape of the protein, preventing it from working properly.
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Mutations - 2

  • Mutations can be neutral, beneficial, or harmful.


  • The mjutation changes a base in the triplet, but the amino acid that the triplet codes for doesn't change. This happens because some amino acids are coded for by more than one triplet.
  • The mutation produces a triplet that codes for a different amino acid, but the amino acid is chemically similar to the original, so it functions like the original amino acid.
  • The mutated triplet codes for an amino acid not involved with the function of the protein.


  • These have an advantageous effect on the organism.
  • An exaple of this would be bacterial resistance to antibiotics, which is advantageous to the bacteria.


  • These have a disadvantageous effect on the organism.
  • An example of this is cystic fibrosis.
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