What Does DNA Do?

What Does DNA Do?

• Most people—although they may know the basic ideas—don't know what DNA really does and how it does it.
• What we know:
  • DNA is the hereditary material, as shown by Griffith, Avery, McCarty, McLeod, Hershey, Chase, and Chargaff.
  • The structure of DNA, due to the breakthrough work of Franklin, Wilkins, Watson, and Crick.
  • One of the key results from this work is that DNA can “easily” be copied, which is important for hereditary material.
• However, we still don't know the exact answer to the question, “What does DNA do?”

First Insight into the Function of Genes

• In 1909, Archibald Garrod, a British medical doctor, studied the disease alkaptonuria.
  • Symptoms: urine turns black when exposed to air because alkapton cannot be broken down.
  • Garrod noticed that this condition was common in certain families, so he assumed it must be an inherited condition.
• Garrod's interpretation:
  • Genes dictate phenotypes through enzymes that catalyze specific chemical processes in the cell (in this case, the breakdown of alkapton).
  • Therefore, according to Garrod, an inherited disease is one in which a person is unable to synthesize a particular enzyme, which in turn disrupts a metabolic pathway.
  • He called these diseases “inborn errors of metabolism.”

Unknown source

One Gene, One Protein

• A big breakthrough experiment on how genes function.
• In 1941, George Beadle and Edward Tatum at Caltech studied mutations in common bread mold (Neurospora crassa).
  • Some X-ray–induced mutations would create strains of mold that could not grow on minimal media.
  • When specific extra nutrients are provided, the mold grows normally.
  • The mutant strains lack the enzymes to produce those extra nutrients.
    • There were three enzymes involved in their experiment, each of which was crucial for synthesizing the amino acid arginine.
    • The different classes (types) of mutants created were each deficient in a certain enzyme, and adding a different nutrient in each case would allow the mutant to grow normally.
    • This showed offered a direct link between a single mutation and a single enzyme (the “one gene–one enzyme hypothesis”).
• Conclusion: a mutation in one gene affects the abilities of one specific enzyme to perform necessary tasks.
• An animation is available at the DNA Learning Center.

Figure 17.1, page 295, Campbell's Biology, 5th Edition

The Current View of Genes

• Until recently, the belief has been that genes code for proteins.
  • This is based Beadle and Tatum's experimental results for which they won the Nobel Prize in the late 1950s.
• The human genome has been decoded:
  • An organism's genome is the whole hereditary information that is encoded in its DNA (or RNA).
  • This includes both the genes and the non-coding sequences.
  • In the human genome, approximately 20 000–25 000 genes are estimated to exist.
• Comparing this to the human proteome, a discrepancy is found:
  • An organism's proteome is the amalgamation of all of its proteins, i.e. the protein products of the genome.
  • In the human proteome, there are approximately 400 000 proteins (some estimates say as high as 1 million).
• Since there is not a one-to-one correspondence between genes and proteins, some genes must be producing multiple different proteins.

DNA to Protein: A Brief Overview of Protein Synthesis

• Transcription: the DNA message is transcribed (copied) by RNA.
• RNA Processing
  • The RNA transcript (copy) is modified and leaves the nucleus as mRNA.
  • The ends of the RNA are altered.
  • Portions of the RNA are removed.
• Translation: the genetic code is translated from mRNA, producing a protein in the cytoplasm.

The Genetic Code

• DNA has four nucleotides, which must code for 20 different amino acids.
  • It must use three nucleotides per amino acid (called a triplet code).
  • One nucleotide could only code for 4¹ = 4 amino acids, and two could only code for 4² = 16.
  • Therefore, since 4³ = 64 > 20, it must use three—even though this is way more than the 20 needed.
• Since DNA comes as a double stranded molecule, the question is often asked: “Which strand carries the code?”
  • Both do!
  • Sometimes one strand carries the code and sometimes the other strand carries the code. There is no set pattern.
  • The strand that carries the genetic information is called the template. Therefore, the mRNA is complementary to the DNA sequence on the template strand.

Complementary Bases

• When the genetic code is transcribed (copied) from the template strand into mRNA, it is not identical to the template strand, it is complementary.
  • If the DNA template strand has a G, the mRNA complement has a C.
  • If the DNA template strand has a C, the mRNA complement has a G
  • If the DNA template strand has an A, the mRNA complement has a U.
  • If the DNA template strand has a T, the mRNA complement has an A.
  • Therefore, if the DNA says ACC, the mRNA says UGG.

From Codons to Amino Acids

• The genetic code was cracked in the 1960s.
• 64 codons
  • 61 code for actual amino acids.
  • 3 are stop codons.
    • These define the end of a protein-coding sequence (they are like a period at the end of a sentence).
    • They do not code for an amino acid.
  • There is 1 start codon.
    • The start codon is AUG, which codes for the amino acid methionine (it is like the way we capitalize the first letter in a sentence to indicate the start of a sentence).
• The code is redundant.
  • Multiple codons for the same amino acid.
  • These sequences only differ in the third base.

^* This is a start codon for prokaryotes only.