3.4.1 DNA, genes and chromosomes

• Gene- a section of DNA found on a particular position which carries the code for the synthesis of a specific polypeptide
• Chromosome- a linear DNA molecule wrapped around histone proteins found in the nucleus, which becomes visible and condensed in prophase of cell division
• DNA (deoxyribonucleic acid)- a polynucleotide that forms instructions for the synthesis of proteins found within organisms, contains the pentose sugar deoxyribose
• Eukaryotic DNA- long, linear DNA molecule which is associated with histone proteins
• Prokaryotic, mitochondrial and chloroplast DNA- short, circular DNA molecule which isn't associated with histone proteins
• The sequence of DNA bases in a gene determines the sequence of amino acids.
• Triplet- a sequence of three DNA bases which codes for a specific amino acid
• The Genetic Code:

  • Only a few amino acids have one triple code
  • Many amino acids have between 2 and 6 triplets each
  • Most amino acids are coded by more than one triplet (degenerate code)
  • The code is always read in one direction on the DNA/ RNA
• Stop codons- do not code for amino acids and is the end of the polypeptide
• Degenerate code- a genetic code where some amino acids might be encoded by more than one codon
• Universal- each triplet codes for the same specific amino acid in all organisms except mitochondria
• Non-overlapping- successive triplets are read in order and each nucleotide is part of only one triplet codon
• Non-coding DNA- a lot of eukaryotic DNA does not code for polypeptides, however non-coding DNA is not present in prokaryotes
• Introns- a non-coding sequence of DNA which separates exons within the sequence, usually cut out before protein synthesis and found only in eukaryotes
• Exons- a coding sequence of DNA which expresses genes leading to polypeptide formation, codes for mRNA, amino acids, rRNA and tRNA