Genetic information

• describe the properties of the DNA molecules present in prokaryotic cells

  • short
  • circular
  • not associated with proteins
• describe the properties of the DNA molecules present in eukaryotic cells

  • very long
  • linear
  • associated with histones (a protein)
• how are chromosomes formed

  DNA + histones join together
• compare the DNA present in prokaryotic cells with the DNA present in eukaryotic cells

  prokaryotic cells:

  • short
  • circular
  • not associated with any proteins

  Eukaryotic cells:

  • very long
  • linear
  • associated with histones to form chromosomes

  Note - DNA in mitochondria and chloroplasts possess similar properties to the DNA in prokaryotic cells
• what is a gene

  • a sequence of DNA bases
  • Which code for the specific amino acid sequence of a polypeptide
  • and code for functional RNA e.g. ribosomal RNA and tRNAs
• what is meant by a locus

  • fixed position that a gene occupies
  • on a particular DNA molecule or chromosome
• what is a base triplet in DNA

  • a sequence of three nucleotide bases
  • that code for a specific amino acid
• what are codons

  base triplets in mRNA
• what is the genetic code

  the instructions contained in a gene that tell a cell how to make a specific protein
• what are three features of the genetic code

  • degenerate
  • non-overlapping
  • universal
• describe the 'degenerate' feature of the genetic code

  more than one triplet could code for the same amino acid
• describe the non-overlapping feature of the genetic code

  each base is only read once as part of a specific triplet
• describe the universal feature of the genetic code

  the same triplet codes for the same amino acids in all organisms
• what are introns

  • non-coding part of a gene
  • base sequences present within genes but do not code for amino acids
• what are multiple repeats

  • non-coding part of a gene
  • some of the base sequences present between genes
  • that consists of the same base sequence occurring again and again
• what are exons

  • coding part of a gene
  • base sequences in genes that code for amino acids
• describe how exons and introns are related to each other in a DNA sequence

  in the DNA sequence exons are separated by introns
• what is meant by the genome

  complete set of genes and non-coding DNA in a cell
• what is meant by the proteome

  the full range of proteins that a cell is able to produce
• what are the features of the structure of mRNA

  • it has linear structure
  • it possesses codons
• what are the features of the structure of tRNA

  • it has a clover leaf structure
  • possesses anticodons
  • contains hydrogen bonds
  • it has an amino acid binding site
• compare the structure of mRNA and tRNA

  mRNA:

  • linear structure
  • possess codons
  • no hydrogen bonds present
  • no amino acid binding site

  tRNA:

  • clover leaf structure
  • possesses anticodons
  • hydrogen bonds present
  • amino acid binding site present
• what is transcription

  the production of mRNA from DNA
• what's the difference in bases between DNA and mRNA

  mRNA uses Uracil instead of Thymine
• what is the key difference in the process of transcription in eukaryotes vs prokaryotes

  • in prokaryotes, transcription results directly in the production of mRNA from DNA
  • In eukaryotes, transcription results in the production of pre-mRNA
  • this is then spliced to form mRNA
• describe the process of transcription in eukaryotes

  • DNA helicase unwinds the double helix and breaks the hydrogen bonds between DNA bases
  • One strand will act as the template strand
  • free RNA nucleotides align by complementary base pairing (uracil is used instead of thymine)
  • RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds
  • this forms pre-mRNA
  • which is then spliced (removal of introns) to form mRNA
• how does splicing of pre-mRNA into mRNA work

  removes introns (non-coding DNA)
• what is translation

  the production of polypeptides from the sequence of codons carried by mRNA
• describe the process of translation

  • mRNA binds to ribosomes
  • tRNA anticodons bind to complementary codons on mRNA strand
  • tRNA brings a specific amino acid to the ribosome
  • amino acids are then joined together by peptide bonds using ATP
  • tRNA is released and the ribosome moves along the mRNA to form the polypeptide
• describe the role of a ribosome in the production of a polypeptide

  • acts as a binding site for mRNA
  • Allows tRNA with anticodons to bind
  • catalyses the formation of peptide bond between amino acids
• what is meant by gene mutations

  changes in the sequence of nucleotide bases in the DNA
• how can gene mutations occur

  can arise spontaneously during DNA replication
• what are the four different types of gene mutations

  • substitution
  • deletion
  • addition
  • duplication
• describe the substitution gene mutation

  • replacement of one or more bases
  • by one or more different bases
• describe the deletion gene mutation

  removal of one or more bases
• describe the addition gene mutation

  adding of one or more bases
• describe the duplication gene mutation

  where one or more bases is repeated
• describe why a mutation may cause a non-functional protein to be produced

  • mutation result in frame shift
  • which is the alteration in base triplets from the point of gene mutation (addition, deletion etc)
  • the sequence of amino acids is altered from the point of gene mutation and the protein formed is non-functional
• why do some substitution mutations not result in changes to the encoded amino acids

  • due to the degeneracy of the genetic code
  • which means the new base triplet could code for the same amino acid
• what are mutagenic agents used for

  to increase rate of mutation e.g. benzene, X-rays