What Does DNA do

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Chapter 17

• Lecture 1
• What Does DNA do?

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What Does DNA do?

• Most people when asked the question will give an
  answer like
• Its the code of life.
• Its the hereditary material.
• Its the material that makes up your genes.
• I encourage you to probe people and ask them,
  How does it work? If you do this with your
  parents tonight you might get a lot of blank
  stares. Check it out- see how your parents do
  with this question!!
• Now I will ask you the same question
• We know DNA is the hereditary material thanks to
  Griffith, Avery, McCarty, Macleod, Hershey,
  Chase, Chargaff, followed by the breakthrough
  work of Franklin, Wilkins, Watson, and Crick.
• One of the key results from this work is that DNA
  can easily be copied.
• This is important if it is the hereditary
  material!
• However, we still need to ask the important
  question What does DNA do?

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First Insight into the Function of Genes (1)

• Date 1909
• Place England
• Researcher Archibald Garrod (British medical
  doctor)
• Disease Alkaptonuria
• Symptoms Urine turns black when exposed to air.
• Observation Garrod noticed that this condition
  was common in certain families and assumed it
  must be an inherited condition.

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First Insight into the Function of Genes (2)

• Hypothesis Genes dictate phenotypes (physical
  manifestations of genes) through enzymes that
  catalyze specific chemical processes in the cell.
  
• Therefore, according to Garrod, an inherited
  disease is one in which a person is unable to
  make a particular enzyme which in turn messes up
  a metabolic pathway.
• He called these diseases Inborn Errors of
  Metabolism

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One Gene, One Protein

• Big breakthrough experiment on how genes function
• Date 1941
• Researchers Beadle and Tatum
• Location Caltech
• Organism Common Bread Mold (Neurospora)
• Conclusion They discovered that a gene codes for
  an enzyme (protein).
• Learn this experiment on your own
• Figure 17.1 in book
• Lesson 16 at the DNA Learning Center (Animation)

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Current View of Genes

• Basic Dogma until recently Genes code for
  proteins
• Based on Beadle and Tatums experimental results
  for which they won the Nobel Prize in the late
  1950s
• Human Genome has been decoded!!
• Genome total number of genes in an organism
• Human Genome Approximately 20,000- 25,000 genes
• Proteome total number of proteins in an organism
• Human Proteome Some estimates- 1 million human
  proteins!
• What is going on?
• There is obviously not a one to one
  correspondence between genes and proteins
• Some genes are doing double and triple duty, etc.

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DNA to Protein A Brief Overview of Protein
Synthesis

• Three steps
• 1. Transcription
• DNA message is copied
• The DNA message is transcribed (copied) by RNA
• 2. RNA Processing
• RNA transcript (copy) is modified and leaves the
  nucleus as mRNA
• Ends of the RNA are altered
• Portions of the RNA are removed
• 3. Translation
• The genetic code is translated from mRNA and a
  protein is produced in the cytoplasm

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Genetic Code

• If DNA codes for proteins we have a problem
• DNA has 4 nucleotides which must code for 20
  different amino acids
• The code cannot be 1 nucleotide for 1 amino acid
• The code cannot be 2 nucleotides for 1 amino acid
  since there are only 42 possibilities (16)
• It must be 3 nucleotides for 1 amino acid.
• However that means there are 43 64 possible code
  words. This is way more than the 20 we need!!
• Since DNA comes as a double stranded molecule the
  question is often asked Which strand carries the
  code?
• The answer is- both!
• The strand that carries the genetic information
  is called the template.

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Complementary Bases

• When the genetic code is transcribed (copied)
  from the template by the mRNA, it is not
  identical, it is complementary.
• Think of the term, complementary like this. . .
• When you say two people complement one another
  they seem to go together perfectly- one supplies
  what the other needs and vice versa.
• This is how it is with the DNA template and the
  mRNA complement.
• So, if you have the bases ACC on the DNA, you get
  the complementary bases UGG on the mRNA.

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Genetic Code

• The Genetic Code was cracked in the 1960s
• 64 codons
• 61 code for actual amino acids
• 3 are stop codons (do not code for an amino acid)
• 1 is a start codon
• Codes for the amino acid methionine
• Code is redundant
• The redundant code only differ in the last base
  of the triplet