DNA

• Allele= a different version of a gene
• chromatid= one or two strands of a chromosome that are joined together by a single centromere prior to cell division
• chromatin= the material that makes up chormosomes. it consists of DNA and the protein histone
• chromosomes= a thread like structure made of protein and DNA by which hereditary information is physically passed from one generation to the next
• deletion= a gene mutation when a nucleotide is lose from a sequence thus affecting the codon in the location of the deletion and all subsequent codons
• exons= regions of a gene that code for a polypeptide
• gene= region of DNA at a specific locus on a chromosome that codes for a polypeptide and so contributes to the characteristics of an organism
• gene mutation= a change to one or more nucleotide bases in DNA resulting in a change in genotype which may be inherited
• introns= regions of a gene that do not code for a polypeptide. spiced out during mRNA processing
• locus= location of a gene on a chromosome
• mutagenic= a factor that can cause mutation, such as ionising radiation or certain organical chemicals
• substitution= a gene mutation where one nucleotide is replaced with another thus only affecting the codon in the location in the substitution, unless it produces a stop codon
• in prokaryotic cells, DNA molecules are short, circular and are not associated with proteins
• in the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones, together a DNA molecules and its associated proteins form a chromosome
• prokaryotic DNA:

  • is found freely in the cytoplasm (in a region called the nucleoid)
  • is naked (not bound with protein and doesnt make chromatin)
  • genomes are compact (contain little repetitive DNA and no introns)
  • contains extra-chromosomal plasmids
  • is circular in shape
• eukaryotic DNA:

  • is conatined within a nucleus
  • is bound to histone proteins
  • genomes contain large amounts of non-coding and repetitive DNA (including introns)
  • do not contain)
  • are linear in shape
• a start codon is AUG which codes for methionine
• the genetic code consists of three bases per codon which codes for an amino acid
• there are 64 possible combinations of triplets but only 20 different amino acids so some amino acids can be coded by more than one triplet
• a stop codon is UAA/UAG/UGA which do not code for any amino acid
• the genetic code is degenerate because there are more than enough codons to represent all possible amino acids
• tRNA has a clover leaf structure, anticodon arm, acceptor stem and D loop