The Language of Life Basic Concepts in Human Genetics Jim Evans MD, Ph.D ASTAR Judges’ Science School Chapel Hill, NC 19 November 2012

“We have discovered the secret of life” February 28, 1953

All living things are composed of cells: Skin Brain Liver Blood etc……….

Cells are composed of tiny protein “machines”…. cytoplasm nucleus channels antibodies Structural components

Enzymes cut and glue chemicals to form other chemicals...

Proteins are long intricately folded chains….. Stylized Channel Antibody More Accurate

The Long Folded Chains that are proteins are composed of amino acids…. cys met gly tyr

The Order of Amino Acids Dictates How the Chain Will Fold, and Thus its Shape and Function There are 20 different amino acids used by living organisms Each has slightly different chemical characteristics The only difference between two proteins with radically different shapes (and thus functions) is the order of their amino acids With 20 amino acids there is a virtually infinite set of differing orders (and thus shapes) possible

The only difference between this membrane channel…. and this antibody…. is the order of amino acids which make up the protein chain

So, we can now rephrase our First Question….. How do we make specific proteins in order to carry out the tasks of life? (i.e. how do we build all the necessary tiny “machines”) Becomes… How do we direct the order of amino acids so as to make all the necessary proteins that are needed for a living organism?

DNA is the repository of information in the cell DNA It is by “reading” DNA that a cell makes the specific proteins necessary for life. Protein RNA

HOW does DNA direct (encode) the synthesis of specific proteins?... DNA directs (“encodes”) the order of each protein’s amino acids By virtue of the order of its own 4 building blocks, the nucleotides: “A”, “T”, “G”, and “C”

ASCO The DNA Double Helix Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Bases Base pair Sugar phosphate backbone

First, the cell makes a copy of one of the DNA strands….. A T G C G T C A G This is the “messenger” RNA and acts as a “tape” that will direct the construction of an individual protein To make a protein...

“Adapter” molecules (called tRNAs) exist in the cell which bind to the messenger RNA and line-up the proper amino acids for construction of the protein chain VAL ARG HIS

One end of an "adapter” molecule in the cell recognizes and binds to a specific triplet in RNA….. VAL ARG HIS The other end of an “adapter” molecule binds to a specific amino acid (the building blocks of proteins)….. The Genetic Code is read in triplets...

A T G C G T C A G VAL ARG HIS Growing Protein Chain mRNA

Thus, DNA directs the production of Proteins The order of nucleotides (DNA’s basic building blocks) dictates the order of amino acids in protein chains The order of amino acids in a protein dictates how that protein chain will fold and therefore what its structure and function will be (i.e. the shape of the protein “machine”)

A Gene is simply a segment of DNA that directs the synthesis of one particular protein... Gene #1Gene #2Gene #3 Your “Genome” is simply all of your DNA – all the genes and all the in-between material

ASCO DNA is Packaged in the Cell’s Nucleus Cell Nucleus Chromosomes Gene Protein Adapted from Understanding Gene Testing, NIH, 1995

One Central Question Remains How does DNA manage to pass its information on to the next generation?

The answer lies in the structure of DNA …. The DNA Double Helix Adenine (A) Thymine (T) Cytosine (C) Guanine (G) ASCO Bases Base pair Sugar phosphate backbone

The Double Helix is held together by the binding of its bases... Guanine (G) Cytosine (C) Always binds to Adenine (A) Always binds to Thymine (T) AGCTATGC TCGATACG

Thus, if one strand of the DNA double helix is present, the other can be reconstructed precisely... AGCTATGC TCGATACG

Passing Along the Information By virtue of being a double helix, each DNA molecule contains a copy of itself To make a new cell (or a sperm or egg and thus the next generation), the cell separates the two strands of DNA and copies each, forming a new identical double helix One copy is sent to each “daughter” cell Thus, each new cell (or new generation) receives the exact same information necessary for propagation of life

A G C T A T G C T C G A T A C G T C G A T A C G A G C T A T G C A G C T A T G C T C G A T A C G AGCTATGC TCGATACG A G C T A T G C T C G A T A C G A G C T A T G C T C G A T A C G The double helix is unzipped Copies of each strand are made The cell divides and sends one copy of each double helix to each new cell

Predictive Genetic Testing The most important goal of Medicine is to heal However, another important function of medicine is to tell the future Patients are interested in their prognosis even when we have no treatments to offer Genetics is providing us with a better “crystal ball” –But our predictions remain cloudy and fraught with the potential for problems

The BRCA1 Gene Normally Encodes a Protein that Regulates Cell Growth Cell Nucleus Chromosomes BRCA1 Gene BRCA1 Protein which keeps cell growth in check

A Mutated BRCA1 Gene Produces a Faulty Protein Which Allows Cells to Grow too Quickly Cell Nucleus Chromosomes BRCA1 Gene A non-functional BRCA1 Protein which cannot keep cell growth in check Mutation An absent or non-functional BRCA1 protein which cannot keep cell growth in check

Cancer Risks for a woman who carries a mutation in BRCA1 Probable small increased risk of other cancers (eg, prostate in male carriers) Life time risk of breast cancer ~85% 10 year risk of a second breast cancer ~35%-70% Life time risk of ovarian cancer ~50%

Polymorphisms Common DNA sequence changes –Which typically do not destroy protein function but may alter it Functional but altered protein Functional protein Polymorphisms are common and contribute to common diseases Exist as both susceptibility factors and as protective factors

Polymorphisms underlie part of our susceptibility to common disease Risk of lung cancer is increased with smoking, but a significant majority of smokers avoid it. –polymorphisms in GST-1 Alcohol increases risk of cirrhosis, but a minority of alcoholics develop cirrhosis –polymorphisms in Epoxide Hydrolase Susceptibility and natural history of TB and HIV are influenced by genetic factors –polymorphisms in n-RAMP and CCR5 Outcome of head trauma is highly variable –apo E polymorphisms