Topic 4

• What are the two main types of DNA based on cellular structure?

  Eukaryotic and prokaryotic DNA
• What are the similarities and differences between eukaryotic and prokaryotic DNA?


  Similarities:

  • Nucleotide structure is identical.
  • Nucleotides are joined by phosphodiester bonds.
  • Mitochondrial/chloroplast DNA is similar to prokaryotic DNA.

  
  

  Differences:

  • Eukaryotic DNA is linear; prokaryotic DNA is circular.
  • Eukaryotic DNA is bound to histone proteins; prokaryotic DNA is not.
  • Eukaryotic DNA contains introns; prokaryotic DNA does not.
  • Eukaryotic DNA is longer.
• What is the definition of a gene?


  A length of DNA that codes for a single polypeptide.
• What is the definition of a locus?


  The fixed position of a gene on a DNA molecule.
• What is meant by the term triplet?


  A sequence of 3 DNA bases which codes for a specific amino acid.
• What do the terms universal, non-overlapping, and degenerate mean in relation to the genetic code?

  • Universal: The same specific base triplets code for the same amino acids in all living things.
  • Non-overlapping: Base triplets don’t share bases; each base is read only once.
  • Degenerate: Most amino acids are coded for by more than one base triplet.
• What is a start codon?


  A base triplet at the start of an mRNA molecule that is the initiation signal for translation.
• What is a stop codon?


  A base triplet at the end of an mRNA molecule that signals when translation should be terminated.
• What is meant by the term intron?


  Portions of DNA within a gene that do not code for a polypeptide.
• What is meant by the term exon?


  Portions of DNA that code for specific amino acids.
• What is meant by the term genome?


  The complete set of genes in a cell.
• What is meant by the term proteome?


  The full range of proteins that a cell/organism is able to produce.
• Describe the structure of an mRNA molecule.


  • A single stranded complementary copy of a gene.
  • Can be thousands of nucleotides long.
  • Contains uracil instead of thymine.
• Describe the structure of a tRNA molecule.


  • Single stranded polynucleotide.
  • Folded into a clover shape held by hydrogen bonds.
  • Has an anticodon that binds to complementary codons on mRNA.
  • Has a specific amino acid binding site.
• What is transcription?


  Formation of mRNA molecules from the DNA that makes up a particular gene.
• Where does transcription take place in eukaryotes?


  In the nucleus.
• Where does transcription take place in prokaryotes?


  In the cytoplasm.
• Describe the process of transcription.


  1. DNA helicase breaks hydrogen bonds between DNA strands.
  2. Strands separate, unwind, and expose bases.
  3. One DNA strand acts as a template strand.
  4. Free RNA nucleotides pair with exposed bases.
  5. RNA polymerase joins nucleotides until a stop codon is reached.
  6. Pre-mRNA is spliced to remove introns, forming mature mRNA.
  7. mRNA leaves the nucleus through a nuclear pore.
• What is translation?


  The assembly of proteins using the sequence of codons on mRNA.
• Where does translation take place?


  In ribosomes in the cytoplasm.
• Describe the process of translation.


  1. mRNA attaches to a ribosome at the start codon.
  2. A tRNA with an anticodon complementary to the start codon attaches to mRNA.
  3. A second tRNA attaches to the next codon on mRNA.
  4. tRNAs carry specific amino acids determined by their anticodon.
  5. Adjacent amino acids are joined by a condensation reaction forming a peptide bond using ATP.
  6. tRNA is released after amino acid is joined to polypeptide.
  7. Ribosome moves along mRNA until stop codon is reached.
  8. Polypeptide chain is released from ribosome.
• Why is ATP required in the synthesis of proteins?


  To provide energy for the bond between the tRNA and the amino acid and for the formation of peptide bonds.
• How do two different amino acids differ from each other?


  They have a different R group.
• Why might a change in the DNA base sequence of a protein result in a change in the structure of the protein?


  • It would change the amino acid sequence/primary structure.
  • This could change the position of hydrogen and disulphide bonds.
  • This would alter the tertiary structure of the protein.
• How does a gene code for a protein?


  • Genes consist of a sequence of bases.
  • Each three bases (triplets) codes for one amino acid.
  • The sequence of bases determines the sequence of triplets.
  • This determines the sequence of amino acids, which equals the primary structure of the protein.
• What is an allele?


  A different form/version of a gene.
• What is a mutation?


  A change in the base sequence of DNA.
• What are two types of mutations?


  1. Substitution
  2. Deletion
  3. Addition
• Explain how mutations can result in a non-functional enzyme.

  • Mutations cause a change in the base sequence of DNA.
  • This results in a change in the sequence of triplet codes.
  • This causes a change in the sequence of amino acids and the primary structure.
  • This results in changes in hydrogen, ionic, and disulphide bonds.
  • Therefore, the substrate is no longer complementary, and enzyme-substrate complexes cannot form.
• Explain why a mutation doesn’t necessarily cause a change to the polypeptide that it codes for.


  • A mutation may result in the substitution of one base in the mRNA with another.
  • The genetic code is degenerate.
  • The changed codon may still code for the same amino acid.
  • Therefore, the polypeptide will be unchanged and there will be no effect.
• Explain why a deletion/addition mutation is more likely to result in a change to a polypeptide than a substitution mutation.


  • Deletion/addition mutations cause a frame shift in all the base triplets after the mutation.
  • All the codons downstream of the mutation are affected.
  • Most/all of the amino acids coded for by these codons will be different, significantly affecting the tertiary structure.
  • In substitution mutations, only one base is affected, so only a single codon is changed.
  • The effect on the polypeptide is likely to be less severe as only one amino acid will be different.
• Why are errors in DNA replication more dangerous than errors in transcription?


  • Errors in DNA replication may be inherited.
  • They will have a permanent effect on the whole organism.
  • Errors in transcription usually only affect specific cells, are temporary, and not inherited.
  • Therefore, they are less damaging.
• What are mutagenic agents?


  Agents that increase the rate of mutation.
• Give examples of mutagenic agents.


  1. Ultraviolet radiation
  2. Ionising radiation
  3. Certain chemicals
  4. Some viruses
• What are chromosome mutations?


  Changes in the structure or number of whole chromosomes.
• What is meant by the term ‘non-disjunction’?


  A failure of the chromosomes to separate properly during meiosis.
• What happens if non-disjunction occurs during meiosis I and meiosis II?


  • If it happens during meiosis I, all 4 daughter cells will have abnormal numbers of chromosomes.
  • If it happens during meiosis II, only half the daughter cells will have an abnormal chromosome number.
• What is the difference between a chromosome and a chromatid?


  A chromosome is a DNA double helix associated with histone proteins; a chromatid is one of the two identical copies of DNA connected at the centromere.
• What does meiosis produce?


  4 genetically different, haploid daughter cells from one parent cell.
• What does diploid mean?


  Cells that contain two copies of each chromosome.