DNA - a Primer

DNA is the molecule of heredity.  All of your genes are coded in your DNA.  A gene is simply a blueprint for the manufacture of a protein.  Since all of your body functions are regulated by one or more enzymes, the DNA contains all the information necessary to create the systems required to keep your body operational.

The structure of DNA was a great mystery, but two scientists, James Watson and Francis Crick, compiled data from many sources and created the first model of DNA structure

• DNA is composed of Four Nucleotides, Adenine (A), Guanosine (aka Guanine, G), Thymine (T) and Cytosine (C)

• Adenine and Guanine are  Purines - they are larger and have a double ring
• Thymine and Cytosine are Pyrmidines - they are smaller and  have a single ring
• The Nucleotides are arranged in an anti-parallel double helix with Adenine pairing with Thymine and Guanine pairing with Cytosine via hydrogen bonds
• Antiparallel - DNA has a directionality (which is written as 5' to 3').  If you laid out a strand of DNA on a table, one strand of DNA will be 5' to 3' from left to right.  The other strand of DNA will be 5' to 3' from right to left.
• Anytime  you manufacture DNA, you must always make it in a 5' to 3' direction (the 5' and the 3' refer to the carbon of the sugar that the binding functional group is bound to)
• A purine always pairs with a pyrmidine (A-T has two H-bonds, G-C has three H-bonds)

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RNA - a molecule similar to DNA

RNA, or ribonucleic acid, is similar to DNA.

• However, instead of having bases of ATCG, RNA has bases of AUCG.
• The T is replaced with Uracil (abbreviated as U)
• RNA is also less stable that DNA

• Proteins are found in all body systems
• All enzymes are proteins
• Proteins are chains of small molecules called amino acids
• Amino acids are linked together like beads on a string.
• The final protein is usually curled up in a ball

DNA Replication

Important enzymes in DNA replication:

• Helicase - unwinds DNA
• Topoisomerase - prevents supercoiling of DNA
• DNA Polymerase - uses one strand of DNA as a template to make the complementary strand

A portion of the double helix is unwound by a helicase. A molecule of DNA polymerase binds to one strand of the DNA and begins moving along it in the 3' to 5' direction, using it as a template for assembling a leading strand of nucleotides and reforming a double helix.   Because DNA synthesis can only occur 5' to 3', a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens. This molecule must synthesize discontinuous segments of polynucleotides (called Okazaki fragments). Another enzyme, DNA ligase then stitches these together into the lagging strand

From DNA to Protein

• This is one of the fundamental concepts of molecular biology
• All visible phenotypes (and those you don't see too!) are the result of the actions of enzymes, which are protein catalysts that regulate all body functions
• Any alteration of this process may affect the formation of proteins, hence the expression of the phenotype
• Transcription - the synthesis of RNA under the direction of DNA
• Translation - the actual synthesis of a protein, which occurs under the direction of mRNA

• DNA opens up
• An enzyme, RNA polymerase, makes a complementary copy of DNA called mRNA
• mRNA contains the code for the protein
• mRNA leave the nucleus and attaches to a ribosome
• A ribosome is structure which acts like a factory - proteins are manufactured here

• At the ribosome, the mRNA is "read" and the message is decoded.
• The "language" used in the mRNA are a series of 3-letter sequences called codons
• The genetic code is redundant (64 combinations, only 20 aa's)

• Example: CCC, CCU, CCA, CCG all code for Proline
• Example: AUG CAU UAC UAA
• Codes for: Met---His---Tyr---Stop