Notes
DNA Mutations
Sections
DNA Mutations
Heritable changes in a sequence of DNA that arise by chance and may or may not cause phenotypic changes in the cell
CAUSES OF MUTATIONS
- Replication errors
- Spontaneous damage
- Mutagens
Mutagens
Agents such as radiation or chemicals that damage DNA and promote mutations
CENTRAL DOGMA
- DNA template encodes information in codons
- Template transcribed to produce RNA transcript
- Ribosome translates RNA transcript to produce amino acid
Codon
Groups of three consecutive nucleotides that are translated to amino acids.
Wobble position
3rd nucleotide in a codon; base change in last position can generate same amino acid
TYPES OF MUTATIONS
Substitution: single nucleotide replaced
- Point mutations
- Nonsense mutation: generates stop codon
- Missense mutation: generates incorrect amino acid
- Silent mutation: generates same amino acid
Deletion: single nucleotide removed
- Frame-shift mutation
Insertion: single nucleotide inserted
- Frame-shift mutation
FIDELITY OF REPLICATION
- Frequency of replication errors very low (1 in 10^9 nucleotides added)
- DNA polymerase increases fidelity:
i. Selects for correct nucleotide
ii. Proofreads (3' to 5' exonuclease)
CLINICAL CORRELATION
Sickle-cell anemia
- Missense mutation (adenine to thymine) changes 3D structure of hemoglobin beta-subunit
- Produces valine instead of glutamate
- Results in sickle-shaped red blood cell instead of biconcave
- Confers malaria-resistance: parasite cannot grow in mutated hemoglobin
Full-Length Text
- Here we will learn the types of mutations that can occur in DNA and their biochemical origins.
- To begin, start a table to learn the definition of a mutation.
- Denote that mutations are heritable changes in a sequence of DNA that arise by chance and may or may not cause phenotypic changes in the cell.
- Denote that mutations result from:
- Replication errors, which are mistakes that occur as DNA is replicated.
- Spontaneous damage, which is damage to a nucleotide or nucleotides that can induce mutations.
- Mutagens, which are agents such as radiation or chemicals that damage DNA and promote mutations.
- Denote the three key types of mutations:
- Substitution, in which a single nucleotide is replaced.
- Deletion, in which a single nucleotide is removed.
- Insertion, in which a single nucleotide is inserted into the sequence.
Let's illustrate these, now.
- First, draw a single strand of DNA with four nucleotides and label the 5 prime and 3 prime ends.
- Label the nucleotides as follows: adenine (A), guanine (G), thymine (T), cytosine (C).
Now, we'll draw three arrows for the three types of mutations.
- Substitution, redraw the DNA strand but replace the guanine with a thymine.
- Deletion, again redraw the DNA strand but this time delete the guanine. We only have three nucleotides now, instead of four.
- Insertion; draw a final DNA strand with five nucleotides. Show that a thymine inserts between the guanine and thymine nucleotides of our original strand.
- These mutations can lead to changes in the RNA transcript and the resulting amino acid.
Let's review the central dogma before we move on.
- Write that DNA encodes information in codons, which are groups of three consecutive nucleotides that are translated to amino acids.
- Return to our diagram and circle the first three nucleotides in our DNA strand: A-G-T.
- Indicate that they are transcribed to produce the RNA transcript: UCA.
- Label UCA a codon, and indicate that a ribosome translates it to produce the amino acid serine.
- Each codon can be translated into an amino acid or a start/stop codon, which signal the initiation or termination of translation.
With this in mind, let's illustrate the possible consequences of mutations.
- Show that the substitution in our diagram is a nonsense mutation.
Why? Let's illustrate this now.
- First, draw our mRNA transcript: U-A-A.
- We leave out the last nucleotide because it is not part of this codon.
- Then, show that a ribosome translates U-A-A to STOP, thus terminating our polypeptide.
- In nonsense mutations, the substitution generates a stop codon.
Next, missense mutations, in which a new amino acid is translated.
- Again, redraw the nucleotides from our original DNA strand, but this time replace guanine with another adenine: A-A-T-C.
- Draw the resulting mRNA transcript: U-U-A.
- Indicate that a ribosome translates U-U-A to leucine instead of serine.
- As a clinical correlation, write that sickle-cell anemia is a life-threatening disease in which a missense mutation produces a change in the three dimensional structure of hemoglobin, particularly in the beta-subunit.
- The mutation must occur in both copies of the beta-globin gene in order for this structural change to occur.
- Show that normally, red blood cells have a biconcave shape.
- Indicate that in sickle-cell anemia, an adenine nucleotide in the normal gene for beta-globin is substituted with a thymine.
- This single-nucleotide change produces the amino acid valine instead of glutamate, which changes the structure of hemoglobin.
- Show that this in turn generates the "sickle-shaped" red blood cells after which this disease is named.
- However, write that individuals with sickle-cell disease are also resistant to malaria because the parasite that causes it cannot grow in hemoglobin.
Now, the last possible outcome of a substitution: silent mutations, in which the same amino acid is translated despite the mutation.
- This time, let's substitute the third nucleotide in our codon, thymine, with a guanine.
- Draw the resulting DNA strand: A-G-G-C
- Now, the mRNA transcript: U-C-C
- And finally the resulting amino acid: serine.
- Finally, indicate that such silent mutations can occur because of "wobble-position" in codons.
- Write that the "wobble-position" is the third nucleotide in a codon, and refers to the fact that a base change in this last position often produces the same amino acid.
Finally, let's illustrate the consequences of deletions and insertions by drawing the resulting mRNA transcripts for each of these mutations.
- For deletion: U-A-G, which translates to a Stop codon.
- For insertion: U-C-A.
- Show that the codon U-C-A translates to serine, the same amino acid!
- However, return to our original DNA strand and transcribe the final nucleotide C.
- Show that the next codon in our sequence would start with guanine.
- Now, return to our deletion and insertion mutations and transcribe the remaining nucleotides.
- Write that for the deletion mutation, the first nucleotide of the second codon in our original DNA strand is now the last nucleotide of the first codon.
- For the insertion mutation, indicate that the next codon would start with A-G, instead of C.
- Now, label both of these mutations "Frame-shift" mutations; unlike substitutions, they affect all the subsequent codons on a DNA strand.
- Now, label the substitution mutations "point-mutations" to distinguish them from frame-shift mutations.
- Now, recall that mutations are generally caused by replication errors or mutagens.
- Finally, write that DNA has a high "Fidelity of Replication."
- The frequency of errors that occur during replication is very low: about once in 109 nucleotides added to a parent strand!
Why is this?
- Write that DNA polymerase increases fidelity in two key ways:
- It selects for the correct nucleotide. It undergoes a conformational change when it binds the correct nucleotide that makes its polymerase activity more energetically favorable.
- It can proofread; it has 3 prime to 5 prime exonuclease activity. When it finds a mistake, it can hydrolyze the incorrect nucleotide in the 3 prime to 5 prime direction and replace it with the correct one.