Genetic information

• guanine is complementary to cytosine thymine is complementary to adenine.
• What is the process of DNA replication called?

  Semi-conservative replication
• What are the steps involved in the semi-conservative replication of DNA?

  1. DNA helicase breaks hydrogen bonds between bases.
  2. Strands separate, exposing bases.
  3. DNA nucleotides are attracted to complementary bases.
  4. Hydrogen bonds form between nucleotides and bases.
  5. DNA polymerase joins nucleotides by condensation reaction.
  6. Phosphodiester bonds form between adjacent nucleotides.
  7. Two identical DNA molecules coil to form helixes.
• What enzyme breaks the hydrogen bonds between DNA bases during replication?

  DNA helicase
• What happens to the DNA strands when DNA helicase acts on them?

  The strands separate and expose the bases on each strand
• What attracts DNA nucleotides to the exposed bases during replication?

  Complementary bases
• What forms between DNA nucleotides and their complementary bases?

  Hydrogen bonds
• What is the role of DNA polymerase in DNA replication?

  It joins together the nucleotides
• What type of reaction does DNA polymerase use to join nucleotides?

  Condensation reaction
• What type of bond is formed between adjacent nucleotides during DNA replication?

  Phosphodiester bond
• What happens to the DNA molecule as DNA polymerase moves along it during replication?

  Two identical molecules of DNA are formed
• What is the final structure of the DNA after replication?

  Two helixes
• contrast eukaryotic DNA with prokaryotic DNA   


  prokaryotic DNA found in the cytoplasm of bacteria and is non-linear (circular) and is shorter whereas eukaryotic DNA is found in the nucleus is longer and is associated with histones and contains introns
• The start codon is composed of the same three bases in the same order (AUG). This means that as well as triggering the start of translation, the start codon always codes for the same amino acid - methionine.
• Describe the process of translation.

  ribosome in the rough endoplasmic reticulum attach to the mRNA strand

  tRNA molecule with a complementary anticodon binds to a codon on the mRNA strand, bringing an amino acid with it

  followed by a second tRNA molecule, complementary to the next codon on the mRNA strand, binding to this codon

  Each tRNA molecule carries an amino acid

  ribosome joins the amino acids together using a condensation reaction, which forms a peptide bond between amino acids repeated until a polypeptide chain of amino acids is formed
• ribosome roles in translation

  assemble amino acids into specific proteins by facilitating tRNA binding
• the role of a tRNA molecule in translation

  contain anti codons specific to the codon on mRNA strand and carry amino acids
• translation


  the mRNA attaches itself to a ribosome and the tRNA brings amino acids . A tRNA molecule that has a complementary anticodon for the first codon on the amino acid binds to it . Another tRNA that is complementary for the the second codon binds . The amino acids they carry are joined together by the ribosome.This continues until the ribosome meets a codon called a ‘stop’ codon and this signals for translation to stop and the polypeptide to detach (All the active processes in transcription and translation occur with the energy released from the breakdown of ATP into ADP + Pi)
• What is a gene mutation?

  A gene mutation is a change in the base sequence of a chromosome
• What is a mutagenic agent?

  A mutagenic agent is an outside factor that increases the rate of mutations.
• what is a mutation and how can it be caused?

  a mutation is caused by any change to the base sequence of DNA

  mutations are caused by incorrect bases added during DNA replication
• explain how primary structure can sometimes be formed despite a mutation in base sequence of DNA

  mutation does not alter primary sequence

  genetic code is degenerate meaning the mutation can still produce the same amino acid

  this type of mutation is known as a base mutation
• 1-and 2- base deletions completely alter polypeptide sequence downstream of the mutation , whereas 3-base deletions do not

  this is because DNA is non-overlapping , this means each nucleotide is part of one triplet so a mutation to one or two bases will change the triplet
• explain why a deletion mutation is usually more serious than a substitution mutation

  a deletion mutation causes a frameshift , which changes order of nucleotides

  this changes the sequence of amino acids coded for and therefore changes tertiary structure of the protein

  on the other hand a substitution mutation only affects one triplet as the genetic code is degenerate different codons can code for the same amino acid therefore sequence of amino acids may not be different therefore leaving the tertiary structure unchanged
• peptide bonds are formed in the translation process by adjacent amino acid joining together by a condensation reaction this is done through the hydrolysis of ATP and the enzyme peptidyl transferase which catalyses the formation of peptide bonds during protein synthesis
• What is the process of DNA replication called?

  Semi-conservative replication
• What role does DNA helicase play in DNA replication?

  It breaks hydrogen bonds between DNA bases
• What happens to the DNA strands when helicase breaks the hydrogen bonds?

  The two strands separate, exposing bases
• Where are DNA nucleotides found during replication?

  In the nucleus
• How do DNA nucleotides interact with the exposed bases on the strands?

  They are attracted to complementary bases
• What forms between DNA nucleotides and complementary bases?

  Hydrogen bonds
• What is the role of DNA polymerase in DNA replication?

  It joins nucleotides to form phosphodiester bonds
• What type of reaction forms phosphodiester bonds between nucleotides?

  Condensation reaction
• What happens as enzymes move along the DNA molecule during replication?

  Two identical DNA molecules form
• What structure do the two identical DNA molecules coil to form?

  Two helixes
• the process of natural selection

  1. new alleles from a gene are created by random mutations
  2. if new allele increases chances of survival in the environment they are more likely to reproduce
  3. the reproduction passes the advantageous allele to the next generation
  4. over many generations new allele increases in frequency among the population
• state 1 role of nitrogen

  one of the most important roles of nitrogen is to create an organism’s DNA

  nitrogen is taken from an organism’s environment , just like other essential substances such as carbohydrates and lipids
• describe the process of translation

  1. ribosome in the rough endoplasmic reticulum attach to the mRNA strand
  2. tRNA molecule with a complementary anticodon binds to a codon on the mRNA strand, bringing an amino acid with it
  3. followed by a second tRNA molecule, complementary to the next codon on the mRNA strand, binding to this codon
  4. Each tRNA molecule carries an amino acid
  5. ribsome moves along the mRNA strand
  6. ribosome joins the amino acids together using a condensation reaction,
  7. forming a peptide bond between amino acids repeated until a polypeptide chain of amino acids is formed
• what is directional selection ?

  • when one of the extreme traits has the selective advantage
  • occurs when there is change in environment
  • the modal trait changes
  • an example of directional selection is antibiotic resistance
• what is stabilising selection ?

  • the modal trait has the selective advantage
  • occurs when there is no change in the environment
  • the modal trait remains the same
  • standard deviation increases as extreme traits decreases
  • an example of stabilising selection is human birth weights