The genetic code, nucleic acids, DNA and RNA<title> </head> <body> School Science Lessons<br> <a href="../biology/UNBiologyTable.html#UNBiologyTableH">Biology</a><br> 2025-10-10<br> (UNBiolGeneticCode)<br> <br> <a name="UNBiolGCH">Genetic code</a><br> Contents<br> <a href="#1.0H">1.0</a> Amino acids, DNA codons<br> <a href="#2.0H">2.0</a> Amino acids and their 64 codons<br> <a href="#3.0H">3.0</a> Nucleic acid, nitrogenous bases<br> <a href="#4.0H">4.0</a> Human cheek cells<br> <hr width="100%" size="2"><a name="1.0H">1.0</a> Amino acids DNA codons<br> The code for a amino acid is a 3 letter codon.<br> Most amino acids have more than one codon, e.g. phenylalanine has TTT and TTC.<br> End of chain has 3 codons.<br> The first letter in a codon is in position 1,<br> The second letter in a codon is in position 2<br> The third letter in a codon is in position 3.<br> After position 3 is END of chain.<br> <br> <a href="#AlaH">Alanine</a>, DNA codons<br> <br> <a href="#ArgH">Arginine</a>, DNA codons<br> <br> <a href="#AsnH">Asparagine</a>, DNA codons<br> <br> <a href="#AspH">Aspartic acid</a>, DNA codons<br> <br> <a href="#CysH">Cysteine</a>, DNA codons<br> <br> <a href="#GinH">Glutamine</a>, DNA codons<br> <br> <a href="#GluH">Glutamic acid</a>, DNA codons<br> <br> <a href="#GlyH">Glycine</a>, DNA codons<br> <br> <a href="#HisH">Histidine</a>, DNA codons<br> <br> <a href="#IleH">Isoleucine</a>, DNA codons<br> <br> <a href="#LeuH">Leucine</a>, DNA codons<br> <br> <a href="#LysH">Lysine</a>, DNA codons<br> <br> <a href="#MetH">Methionine</a>, DNA codons<br> <br> <a href="#PheH">Phenylalanine</a>, DNA codons<br> <br> <a href="#ProH">Proline</a>, DNA codons<br> <br> <a href="#SerH">Serine</a>, DNA codons<br> <br> <a href="#ThrH">Threonine</a>, DNA codons<br> <br> <a href="#TrpH">Tryptophan</a>, DNA codons<br> <br> <a href="#TyrH">Tyrosine</a>, DNA codons<br> <br> <a href="#ValH">Valine</a>, DNA codons<br> <hr width="100%" size="2"><a name="AminoacidsH">Amino acids</a> and their 64 codons<br> The genetic code is the relation between a DNA codon and an amino acid.<br> Note that some amino acids are coded by several codons, e.g. Alanine, and some by only two codons, e.g. Asparagine, and some by only one codon, e.g.Methionine.<br> <a name="AlaH">Ala</a> Alanine GCT, GCC, GCA, GCG<br> <a name="ArgH">Arg</a> Arginine CGT, CGC, CGA, CGG, AGA, AGG<br> <a name="AsnH">Asn</a> Asparagine AAT, AAC<br> <a name="AspH">Asp</a> Aspartic acid GAT, GAC<br> <a name="CysH">Cys</a> Cysteine TGT, TGC<br> <a name="GinH">Gin</a> Glutamine CAA, CAG<br> <a name="GluH">Glu</a> Glutamic acid GAA, GAG<br> <a name="GlyH">Gly</a> Glycine GGT, GGC, GGA, GGG<br> <a name="HisH">His</a> Histidine CAT, CAT<br> <a name="IleH">Ile</a> Isoleucine ATT, ATC, ATA<br> <a name="LeuH">Leu</a> Leucine TTA, TTG, CTT, CTC, CTA, CTG<br> <a name="LysH">Lys</a> Lysine AAA, AAG<br> <a name="MetH">Met</a> Methionine ATG<br> <a name="PheH">Phe</a> Phenylalanine TTT, TTC<br> <a name="ProH">Pro</a> Proline CCT, CCC, CCA, CCG<br> <a name="SerH">Ser</a> Serine TCT, TCC, TCA, TCG<br> <a name="ThrH">Thr</a> Threonine ACT, ACC, ACA, ACG<br> <a name="TrpH">Trp</a> Tryptophan TGG<br> <a name="TyrH">Tyr</a> Tyrosine TAT, TAC<br> <a name="ValH">Va</a>l Valine GUU, GUC, GUA, GUG<br> END of chain TAA, TAG, TGA<br> Position 1 is called the 5' end<br> Position 3 is called the 3' end<br> <hr width="100%" size="2">AAA<br> AAC<br> AAG<br> AAT<br> ACA<br> ACC<br> ACG<br> ACT<br> AGA<br> AGG<br> ATA<br> ATC<br> ATG<br> ATT<br> CAA<br> CAG<br> CAT<br> CAT<br> CCA<br> CCC<br> CCG<br> CCT<br> CGA<br> CGC<br> CGG<br> CGT<br> CTA<br> CTC<br> CTG<br> CTT<br> GAA<br> GAC<br> GAG<br> GAT<br> GCA<br> GCC<br> GCG<br> GCT<br> GGA<br> GGC<br> GGG<br> GGT<br> GUA<br> GUC<br> GUG<br> GUU<br> TAA<br> TAC<br> TAG<br> TAT<br> TCA<br> TCC<br> TCG<br> TCT<br> TGA<br> TGC<br> TGG<br> TGT<br> TTA<br> TTC<br> TTG<br> TTT<br> <br> <hr width="100%" size="2"><a name="3.0H">3.0</a> Nucleic acid nitrogenous bases<br> <br> A Adenine, 6-aminopurine, vitamin B4, (C5H5N5).<br> <br> G Guanine, 2-Amino-6-hydroxypurine, (C5H5N5O).<br> <br> C Cytosine, 4-Amino-2-hydroxypyrimidine, (C4H5N3O).<br> <br> T Thymine (only in DNA), 2, 4-Dihydroxy-5-methylpyrimidine, 5-Methyluracil, (C5H6N2O2).<br> or<br> <br> U Uracil (only in RNA), 2, 4-Dihydroxypyrimidine, 2, 4-Pyrimidinediol, (C4H4N2O2).<br> <br> <hr width="100%" size="2"><a name="4.0H">4.0</a> Human cheek cells<br> <a href="../images/9.3.67.gif">See diagram 9.3.67</a>: Human cheek cells.<br> Note the nucleus, cytoplasm, plasma membrane or plasmalemma, and granules.<br> You may have to seek approval to do this experiment, because saliva can carry disease.<br> Instead of taking cheek cells you can use prepared slides of cheek cells from a school laboratory supplier.<br> Cheek cells come from the stratified squamous epithelium tissue on the surface of the mucous membrane inside the cheek.<br> These flat, scale-like cells are shed constantly as the tissue is renewed so it is easy to obtain some for study by gently scraping the inside of the cheek.<br> This tissue is not keratinized so the surface cells are still living and have live nuclei, in contrast with shed epidermal cells.<br> Similar tissue lines the vagina.<br> Experiment<br> 1. Observe human epithelial cells from inside the cheek.<br> Use a clean toothpick to gently scrape the inside surface of the cheek.<br> Put the whitish scraping into a drop of water or 0.65% saline solution on a microscope slide.<br> Add a drop of stain, e.g. methylene blue or iodine solution, and apply a coverslip.<br> View under low power and high power.<br> Note the protoplasm containing a central nucleus and granular cytoplasm.<br> The outer boundary of the protoplasm is the plasma membrane.<br> Adjacent cells look like paving stones.<br> The nucleus and cytoplasm have a different refractive index, so note the interfaces between them.<br> In later experiments, compare the animal cell with the plant cell.<br> The animal cell has no cell wall.<br> 2. Isolate DNA from cheek cells<br> Warning! Some school systems do not allow any experiments using human cells or the cells of the students!<br> Dissolve half a teaspoon of salt in half a cup of water.<br> Add a little dishwashing liquid to break up the cells and release the DNA.<br> Put 25 mL of water in your mouth, but do not swallow it.<br> Move the water around your cheeks vigorously for 30 seconds to remove some cheek cells.<br> Spit the water into a clean cup.<br> Add 5 mL of this fluid to a 20 mL test-tube.<br> Add 2.5 mL of the salt and dishwashing liquid solution.<br> Put a stopper on the test-tube and move the test-tube up and down 3 times gently so that the contents do not form froth.<br> This movement breaks up the hundreds of cheek cells to release the DNA from the nucleus.<br> Gently pour into the test-tube a teaspoonful of ice cold ethanol that has been in a freezer for hours before the experiment.<br> Watch the point where the two layers meet.<br> Note the strands of DNA forming as cloudy filaments stretching up into the top ethanol layer.<br> DNA is not soluble in ethanol, so when the ethanol meets the DNA solution, it starts to precipitate form a DNA salt.<br> Use a glass hook or plastic tie wire to remove strands of DNA by slowly dipping up and down through the two layers.<br> Then gently invert the test-tube several times until the alcohol is mixed and the precipitated DNA will look like a small ball of white thread.<br> <hr width="100%" size="2"> </body> </html>