By C. Kohn, Waterford WI

 You now know that genes encode for specific traits like eye color, ear lobes, and milk production.  A gene is simply a section of DNA that creates the proteins responsible for a specific trait.  Genes are found in DNA; chromosomes are made of DNA

 DNA has several key components  A Phosphate Molecule  A Sugar Molecule  A Nitrogenous Base (A,T,G,C)

 The sugar and phosphate molecules comprise the ‘skeleton’ or ‘backbone’ of DNA  The nitrogenous base is used to encode the actual information on the gene needed to create the protein (a base is the C,G, T, or A)

 A nucleotide is a subunit (or building block) of DNA consisting of a base, a phosphate, and a ribose sugar. Nucleotide

 DNA has four nitrogenous bases  Adenine (A)  Thymine (T)  Guanine (G)  Cytosine (C)  All information encoded in DNA exists through different combinations of these four letters.

 The DNA bases always exist in the same kinds of combinations  A always pairs with T  G always pairs with C  “Great Combinations, Always Together”

 A-T and G-C combos must occur for two main reasons  1. This is the only way they will fit inside the framework of the DNA molecule  2. This is the only way that their binding sites will match up

 The bases are grouped into two categories  Pyrimidines  Purines

 Two pryrimidines would be too small to fit inside the structure of DNA  Two purines would be too big to fit inside the structure of DNA Too smallToo big

 C-G and T-A are also necessary because of binding sites  T and A have 2 binding sites  C and G have 3 binding sites ▪ They wouldn’t match up any other way

 Because of size, G and A would be too big together, and C and T would be too small together  Because of binding sites, G only matches up with C and T only matches with A

 Knowing these facts are HUGE!  This feature enables the structure of DNA to enable its function  In other words, because of the G-C, T-A combination, DNA can be read and replicated.

 DNA has 3 main components  A phosphate molecule  A ribose sugar  A Base (C,T,G,or A)  A phosphate, sugar, and base together is called a nucleotide, the building block of DNA  C-G and A-T are only possible because…  This is the only way they fit inside DNA  This is the only way their bonding sites match up

 To make a protein, we have to make a copy strand of DNA and send it to a ribosome  The copy strand is called mRNA

 While our genetic information is encoded in double-stranded DNA, copies of this information are encoded in single-stranded RNA.  RNA is a primitive version of DNA.  DNA and RNA are very similar; the key differences are that…  1. RNA can be single stranded  2. RNA replaces a T with a U (uracil)  3. Also, the sugar is slightly different (extra -OH molecule)

 The process of creating an mRNA copy of DNA is called Transcription.  Think of “transcript” of a TV program – it’s just a copy  Transcription has three stages:  1. Initiation – DNA is unwound by helicase enzyme and a polymerase enzyme binds to the DNA strand  2. Elongation – nucleotides are added by polymerase to the developing mRNA strand  3. Termination – polymerase and mRNA are released from the DNA strand; the strand is re-closed  Transcription involves two key enzymes:  Helicase: the enzyme the opens the DNA strand  Polymerase: the enzyme that creates the mRNA copy

CTGACTGACTGA GCTAGCTAGCTA CUGACGACGAUU Step 1: Helicase opens and unwinds the DNA strand Step 2: Polymerase adds a complementary base for each nucleotide Step 3: The newly created mRNA strand goes to a ribosome to be read Step 4: The DNA strand is closed and re-wound

 Transcription always occurs in a 5 > 3 direction.  The sugar molecule has 5 carbon atoms  The 5 th carbon atom is ‘inside’ the nucleotide, while the 3 rd carbon atom is at the ‘lower’ edge ▪ NOTE – there is no top or down in DNA, so use these terms carefully!  Just remember: 5 > 3 5 3

 Once an mRNA copy has been made, the next step is Translation.  Translation is when the information in the mRNA is ‘translated’ into the creation of a protein by a ribosome, or rRNA.

 The mRNA copy strand’s base letters are read in groups of three  E.g. if our mRNA strand was AUGGCAAAGGACCAU it would be read as AUG GCA AAG GAC CAU  Each group of three is called a codon.  i.e. AUG is a codon; GCA is a codon; etc.

 Each codon codes for a specific amino acid.  An amino acid is the building block of a protein  For example, GGG codes for Glycine  AUA codes for Serine  CUA codes for Leucine  Each codon will specific which amino acid is added next in order to create a protein

CUGACGACGAUU Arginine Serine Isoleucine Asparagine Arg Ser Iso Asp Protein

 Amino acids are determined by the strand of mRNA and brought to the ribosome by tRNA  tRNA will only bind to a complementary codon; e.g. ACG will bind the UGC–form of tRNA.

 A protein is a long string of amino acids.  The type of amino acids in a protein, and their order, determine the function of the protein  For example, insulin is shown here at the right  As you can see, it is simply a long chain of amino acids

The order and type of amino acids is the primary structure. The arrangement of amino acids will create either a helix spring or a pleated sheet. The combination of springs and sheets is the tertiary structure of a protein. The final functional protein is the quarternary structure.

Transcription occurs in the nucleus. Translation occurs in the ribosomes. DNA and mRNA are a part of transcription. mRNA, rRNA, and tRNA are a part of translation. Transcription involves making the mRNA copy of DNA. Translation involves using the mRNA copy to make a functional protein out of amino acids in the ribosome.