Notes

• gene - section of DNA on a chromosome coding for one or more polypeptides and functional RNA
• genes are located at a particular position on a DNA molecule, called a locus
• genetic code - three bases for each amino acid, called a triplet
• 64 possible triplets, 20 amino acids - some amino acids are coded for by more than one triplet
• degenerate DNA code - most amino acids are coded for by more than one triplet
• non-overlapping DNA code - each base in the sequence is read only once (bases 123456 are read as triplets 123 and 456)
• universal DNA code - each triplet codes for the same amino acid in all organisms
• in eukaryotes, much of the nuclear DNA does not code for polypeptides
• exons - a sequence of DNA that codes for an amino acid sequence
• introns - portions of DNA within a gene that do not code for a polypeptide, removed from pre-mRNA after transcription
• DNA in prokaryotes

  • short
  • circular
  • not associated with protein molecules
  • no chromosomes
• DNA in eukaryotes

  • long
  • linear
  • associated with proteins called histones
  • form chromosomes
• chromosomes - made up of two sister chromatids joined by a centromere
• homologous chromosomes - a pair of chromosomes, one maternal, one paternal, that have the same gene loci and therefore determine the same features
• diploid - a term applied to cells in which the nucleus contains two sets of chromosomes
• haploid - a term referring to cells that only contain a single copy of each chromosome