Genetic Mutations

1. Different types of mutations2. Where mutations can occur3. Are mutations good or bad? 4. What causes mutations?

• A genetic mutation is an abnormal change in a chromosome.

Trisomy 21• We have already learned about

one type of mutation- Trisomy 21, in which a human has 3 copies of chromosome #21.

• This is a type of mutation where the error is in the number of chromosomes, not in the actual content of the genes.

• We are going to focus on mutations in the content of the genes.

Genetic Mutations

• Point Mutation• Addition• Deletion

Point Mutation• A point mutation is when a single base is

switched for a different base. • Example: DNA: AAA-TAT-GTG-CCA mRNA: UUU- AUA-CAC-GGU

If the first A in the mRNA instead was instead a C, that would be a point mutation.

mRNA w point mutation: UUU- CUA-CAC-GGU

What is the result of a point mutation?

• Use your charts:

• What AA chain would the normal mRNA produce : UUU- AUA-CAC-GGU ?

• What AA chain would the mutated mRNA produce : UUU- CUA-CAC-GGU ?

Phe-Ile-His-Gly

Phe-Leu-His-Gly

Test – Chapters 26+27 • Wednesday, 3/28 • Review sheet and updated website by

tomorrow.

When Mr. Rosen is Absent • Monday 3/19: Class• Tuesday 3/20: Class• Wednesday 3/21: Shakespeare competition• Thursday 3/22: You will have Dr. Marquard, who will be

doing an interesting lesson with you. He will be grading your participation and work, which will count towards your grade. You must bring a seed (of any type) to class on Thursday. This too will be graded.

• Friday 3/23: Review sheet / problems for the test• Monday 3/26: Dr. Marquard. • Tuesday 3/27: Mr. Rosen back – review for the test. • Wednesday 3/28: Test

Point mutations

• Generally, a point mutation will result in an amino acid chain that is different by one amino acid from what it is supposed to be.

• There are two exceptions to this rule: 1. A point mutation in the last letter of a codon2. A point mutation either adding or removing a

“STOP” codon

Error in last letter of a codon

• Example:Normal mRNA: UUU- AUA-CAC-GGUMutated mRNA: UUC- AUA-CAC-GGU

What will each one produce?

Both will produce Phe-Ile-His-Gly. Why is that? Because when the mutation occurs in the last letter of a codon, sometimes it does not change.

Errors affecting STOP codons

1. Errors can add an extra stop codon where they do not belong. This results in an AA chain with an extra division.

2. Errors can delete a stop codon where they do belong. This results in an AA chain that is longer than it is supposed to be.

Additions/Deletions• Addition mutations: Mutations in which a letter is added. • Deletion mutations: Mutations in which a letter is

deleted.

• These types of mutations generally have much more extreme effects on the AA chain because it ruins the normal the 3-codon reading frame.

• Example:• Normal: AAA-UUU-CCC-GGG• Mutant 1 : AAA-AUU-UCC-CGG-G • Mutant 2: AAU- UUC- CCG- GG

HW Sheet: Mutations

• Due tomorrow (3/20)

Are mutations good or bad?

Are mutations good or bad? • The vast majority of mutations are bad. They

result in disfunctional proteins that do not perform their jobs and usually hurt the mutated organism.

• There are very rare mutations that are actually beneficial.

• For example, many individuals in Africa have the recessive gene that causes their red blood cells to be moon-shaped (called sickle-cell). This makes them less effective at transporting oxygen, but happens to make the organism less vulnerable to malaria, a very common lethal disease.

• In this class, we will assume that all mutations are damaging.

Where do mutations occur?

• Mutations can occur in one of three locations: 1. In DNA transcription 2. DNA replication in mitosis3. DNA replication in meiosis

Mutation in DNA transcription

• These mutations are the least severe. • They will result in mutated mRNA, but the DNA

will remain the same.• Mutations in mRNA are the least severe. The

mutated mRNA will very likely produce a defective protein, however, after just one transcription, the mRNA is broken down. There will be a single defective protein, but that should not greatly affect the body’s functioning.

Mutations in DNA replication in Mitosis

• These mutations are more dangerous because once a cell has mutated in replication, its daughter cells will inherit the mistake and their daughter cells, etc.

• This can cause serious problems in a person, such as cancer or other diseases. [We will get back to cancer later]

• These mutations are not passed on to the next generation and will die when that individual dies.

Mutations in DNA replication in Meiosis

• A mutation in meiosis (the production of gametes) is, in a sense, the most severe mutation that is possible.

• This defective gamete might actually become a zygote and will pass on the mutation to the next generation. They will not, however, affect the actual person who develops the mutation.

• This zygote will develop with every cell in their body containing this defect.

• This is how defective copies of chromosomes (like the one that cause Tay Sachs, Huntington’s Disease, etc) began.

What causes mutations?

• Some mutations are caused my random mistakes in DNA pairing / replicating.

• Other mutations are caused by mutagens, things that increase the likelihood of mutation.

• Mutagens can be chemicals (such as tobacco smoke) or various types of light (X-ray, UV-light etc).

• We will look at the effects of X-rays.

X-rays (mutagen)• X-rays are one of the most common

mutagens because they are so common (medical, airport security, etc).

• X-rays are a very strong form of light that penetrates deep into the body (that is why it can take pictures of bones).

• One of the side-effects of X-rays is that it can affect a cell’s DNA and cause a mutation.

• This is why they will cover much of your body in a cloth that contains lead (which shields your body from the negative effects of the X-rays).

Cancer• Mutagens such as X-rays /UV rays

from sunlight can harm the DNA and cause it to malfunction.

• One of the outcomes is that a single cell that was supposed to stop dividing (and enter G0) failed to do so.

• This causes it to continue dividing out of control.

• These cells refuse to stop dividing and can spread throughout the entire body (malignant tumors).

Melanoma

Challenges of treating cancer1. The dangerous cells reproduce very rapidly2. The cancerous cells resemble normal body cells so

that the body’s immune system does not fight them off.

3. It is difficult to give medicine that will kill only the cancerous cells and not the person as well.

4. If even a single cancerous cell remains alive after treatment, it will continue multiplying rapidly and the cancer is not cured.

http://info.cancerresearchuk.org/cancerstats/incidence/age

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