DNA, genes and protein synthesis

• dna in prokaryotic cells

  dna molecules shorter, circular and arent associated with protein molecules - histones

  no chromosomes
• dna in eukaryotic cells

  dna molecules are longer, linear and occur in association with histones to form chromosomes

  chloroplasts and mitochondria also contain dna similar to prokaryotic cells
• chromosome structure 


  double helix wound around histones

  dna-histone complex coiled

  coil is looped and further coiled before being packed into chromosome

  = lots of dna condensed into one chromosome
• homologous chromosomes


  one of each pair derived from maternal chromosomes and other pair is derived from paternal chromosomes

  always 2 chromosomes that carry the same genes but not necessarily same alleles

  total number = diploid number

  == 46
• allele


  one of a number of alternative forms of a gene

  any changes in base sequence of gene produces a new allele = different amino acid sequence coded for = production of different polypeptide = different protein
• mRNA


  transfers dna code from nucleus to cytoplasm

  small enough to leave nucleus via nuclear pores and enter cytoplasm
• genome


  complete set of genes in a cell, including those in mitochondria and/or chloroplasts
• complete proteome

  full range of proteins produce by genome
• proteome


  proteins produced by a given type of cell under certain set of conditions
• RNA structure 


  polymer of mononucleotide sub-units

  forms single strand

  • pentose sugar ribose
  • one of organic bases - a , g , c and u
  • phosphate group
• messenger rna 


  long strand arranged in single helix

  base sequence determined by sequence of bases on length of DNA in transcription

  once formed, leaves nuclear via pores in nuclear envelope and enters cytoplasm, where it associated with ribosomes
• transfer rna


  relatively small molecule arranged in clover leaf shape

  single stranded

  opposite end of tRNA molecule is sequence of 3 organic bases = anticodon specific to one amino acid
• complementary base pairing in RNA

  guanine = cytosine

  adenine = uracil (RNA)

  adenine = thymine (DNA)
• protein synthesis - transcription

  enzyme causing two strands to separate and expose nucleotide bases

  nucleotide bases on one of two strands pair with free complementary nucleotides

  RNA polymerase moves along strand and joins nucleotides together = pre mRNA

  DNA strand rejoins behind RNA polyermase

  rna polymerase reaches stop codons and detached
• protein synthesis - pre-mRNA splicing

  eukaryotic cells

  splicing removes introns / non coding sections of DNA and introns are joined together = mature mRNA

  leave via nuclear pore and attracted to ribosomes
• protein synthesis - translation

  mRNA associates with ribosome at start codon

  tRNA with complementary anticodon sequences move to ribosome and pairs with mRNA codon carrying a specific amino acid

  tRNA molecule with complementary anticodon pairs with next codon

  ribosome moves along mRNA, bringing 2 tRNA molecules at a time

  two amino acids joined by peptide bond using enzyme and atp

  ribosome moves to third codon, first tRNA released from amino acid and is free to collect another amino acid

  continues until stop codon = complete polypeptide chain
• protein assembly 


  polypeptide coiled / folded = secondary structure

  secondary structure folded = tertiary structure

  different polypeptide chains and non-protein groups link to form quaternary structure