genetics, classification & biodiversity

• DNA


  Carries genetic information
• RNA


  Transfers genetic information from DNA to ribosomes
• Components of a DNA nucleotide

  • Deoxyribose
  • Phosphate group
  • Adenine
  • Cytosine
  • Guanine
  • Thymine
• Components of an RNA nucleotide


  • Ribose
  • Phosphate group
  • Adenine
  • Cytosine
  • Guanine
  • Uracil
• Purine bases


  Adenine and guanine
• Pyrimidine bases


  Thymine, uracil and cytosine
• Phosphodiester bonds


  Bonds that join nucleotides together
• DNA molecule


  Double helix composed of two polynucleotides joined by hydrogen bonds between complementary bases
• RNA


  Relatively short polynucleotide chain
• Genetic code


  Order of bases on DNA, consisting of triplets of bases coding for particular amino acids (codons)
• Gene


  Sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain
• Locus


  Location of a gene
• Introns

  Non-coding sections of DNA
• Exons

  Coding regions of DNA
• Genetic code

  • Non-overlapping
  • Degenerate
  • Contains start and stop codons
• Each amino acid is coded for by one or more combination of triplets, resulting in 64 possible triplets
• Genes are separated by non-coding repeats of bases
• A change in the base sequence of DNA alters the amino acid sequence and the protein, which can have various effects
• Harmful mutations

  • Cystic fibrosis
  • Sickle cell anaemia
• Eukaryotic cells

  DNA molecules are long and linear, found in the nucleus and associated with histones to form chromosomes
• Prokaryotic cells

  DNA is short and circular, not associated with proteins
• Humans have 46 chromosomes in every cell, arranged into homologous pairs
• Homologous pair

  Two chromosomes that carry the same genes, but can carry different alleles
• Allele


  Alternative form of the same gene
• Transcription


  Process where a section of DNA is transcribed into mRNA in the nucleus
• Translation


  Process where mRNA, tRNA and ribosomes are involved in the synthesis of a polypeptide chain
• mRNA


  Long single strand created during transcription, with base sequence complementary to DNA
• tRNA


  Small molecule made up of around 80 nucleotides, with an amino acid attachment site and an anti-codon
• Transcription


  1. DNA uncoils and separates

  2. One DNA strand used as template to make mRNA

  3. Nucleotides line up by complementary base pairing and joined by phosphodiester bonds

  4. Stop codon reached, DNA rejoins
• Translation


  1. mRNA attaches to ribosome

  2. tRNA collects amino acids and carries them to ribosome

  3. Amino acids join by peptide bonds, tRNA detaches

  4. Process repeated until stop codon reached
• Ribosomes can join up to 15 amino acids per second, and up to 50 ribosomes can move along the same mRNA strand
• Substitution mutation


  One nucleotide in the DNA sequence is replaced by another
• Deletion mutation


  A nucleotide in the DNA sequence is lost, leading to a frameshift
• Polyploidy


  Changes in the whole set of chromosomes, resulting in more than two sets
• Non-disjunction


  Chromosomes fail to separate correctly in meiosis, resulting in gametes and zygotes with one more or one less chromosome
• Polyploidy


  • Many modern wheats
• Non-disjunction


  • Down's syndrome
• Meiosis


  • Produces genetically different haploid daughter cells through independent assortment of chromosomes and crossing over of chromatids
• Meiosis 1


  Homologous chromosomes pair up, crossing over may occur, cell divides with each daughter cell containing one chromosome from each homologous pair
• Meiosis 2


  Chromatids of each chromosome are separated, producing 4 haploid daughter cells