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Announcements

Lab reports: as printed in the X-linked cross lab write-up, you are expected to perform chi-square analysis on your data (both F1 and F2 for each cross - total of 4 chi-square tests); this will be basis for your discussion, ie. was there significant deviation between expected and observed ratios? New deadline: Fri. Nov. 8th at start of lecture.

Problem set 5 is graded and in folders in lab; avg. was 12/15. We’ll go over correct answers in lab this week; we’ll also take questions re. prob. set 6.

3. Ch. 17 rdg: skim pp.456-458; 460-461;


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Review of Last Lecture

  • Eukaryotic Gene Expression:

    it’s more complicated being multicellular

    2. The Promoter

    3. Enhancers

    4. Methylation

    5. Alternative splicing


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Outline of Lecture 28

I. Post-transcriptional gene regulations:

Alternative splicing

II. Classification of mutations

III. Detection of mutations in humans

IV. Different forms of mutations


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I. Post transcriptional gene regulation

If humans have approximately the same number of genes as a fruit fly, and we require more complex cellular functions (presumably with a larger number of proteins) - how do we accomplish this?

Making different forms of a protein from a single gene

Differential splicing of exons



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Alternative splicing

1. Chromosomal ratio activates txn of Sxl in females only

2. SXL controls splicing of tra-2 mRNA

3. Females: exon 2 (which has a stop codon) is removed via SXL

Males: exon 2 is not removed.

Males: no active TRA

Females: TRA is made.

5. TRA directs splicing of dsx mRNA in specific manner; in males default splicing occurs.


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II. Classification of mutations

A. Spontaneous vs Induced

B. Gametic vs Somatic

C. Phenotypic Effect:

  • Morphological, e.g. pea, fly traits

  • Biochemical, e.g. hemophilia, leu-

  • Behavioral, e.g. mating behavior in flies

  • Regulatory, e.g. in lac operon; important for evolution

  • Lethal, e.g. Huntington disease, Manx allele

  • Conditional, e.g. temperature-sensitive allele


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III. Detection of mutations in humans

- Pedigrees

- Dominant mutations easiest to detect

Where did original mutation occur?

Gamete of parent - gen.I

Probably not

Could it be an X-linked mutation?


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What is the Rate of Spontaneous Mutation?

  • Bacteriophage T2: 10-6 to 10-8 per gene replication.

  • E. coli: 10-5 to 10-10 per cell division.

  • Maize: 10-5 to 10-6 per gamete per generation.

  • Drosophila: 10-5 to 10-6 per gamete per generation.

  • Human: 10-5 to 10-6 per gamete per generation.


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Neutral vs. deleterious mutations

Most spontaneous mutations will occur in 95% of genome that does not encode genes - neutral mutations.

What is rate of deleterious mutations in humans?

1.6 deleterious genetic changes/individual/generation

Consequences to our species? Read short article by James Crow on page 465-466 in text.



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More types of mutations

  • Normal THE ONE BIG FLY HAD ONE RED EYE

  • Missense THQ ONE BIG FLY HAD ONE RED EYE

  • Nonsense THE ONE BIG ***

  • Frameshift THE ONE QBI GFL YHA DON ERE DEY

  • Deletion THE ONE BIG HAD ONE RED EYE

  • Insertion THE ONE BIG WET FLY HAD ONE RED EYE

  • Duplication THE ONE BIG FLY FLY HAD ONE RED EYE

  • Expanding Mutation

    • Generation 1 THE ONE BIG FLY HAD ONE RED EYE

    • Generation 2 THE ONE BIG FLY FLY HAD ONE RED EYE

    • Generation 3 THE ONE BIG FLY FLY FLY HAD ONE RED EYE


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Fragile X Syndrome: the Result of an Expanding Mutation Over Time

  • Mental retardation, characteristic facial structure.

  • CGG is present in 5’ UTR of FMR-1 gene:

    • wildtype: 6-54 repeats

    • carrier: 55-200 repeats

    • affected: > 200 repeats

  • > 200 repeats, DNA becomes methylated, inactivating the gene.

  • Function of protein still unclear.

  • Other examples: one form of muscular dystrophy, Huntington’s disease.


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Different forms of mutation Time

1. Tautomeric Shifts: spontaneous

2. Base Analogues: chemical

3. Alkylating Agents: chemical

4. Intercalating Dyes: chemical

5. Deamination: chemical

6. UV Radiation and Thymine Dimers

7. High-Energy Radiation (X rays, gamma rays, cosmic rays)

environmental


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  • Can’t be avoided - inherent in the chemistry of bases.

  • Always a transition: changing one purine (G or A) to the other purine, or one pyrimidine (C or T) to the other pyrimidine.

  • DNA proofreading minimizes the mutation rate due to tautomeric shifts. This involves 3’-5’ exonuclease activity in DNA polymerase.





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Formation of a TA to CG Transition TimeDuring DNA Replication

Transition is a purine replaced by different purine or pyrimidine

replaced by different pyrimidine.


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2. Base Analogues: DNA can Time

Incorporate 5-BU in place of Thymine

rare

common

Changes T-A pair > C-G pair. T > C, and A > G are both Transitions


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3. Alkylating Agents: Ethylmethane Time

Sulfonate (EMS) Alkylates Guanine

Note: changes a G-C pair into an A-T pair

(G > A is a transition, C > T is a transition)

Another example: mustard gases first used in WWI.


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4. Intercalating Dyes Cause Frameshifts Time

Intercalate themselves into the DNA double helix, distorting it,

and causing insertion or deletion during DNA replication or

recombination. Other examples: Ethidium Bromide, DAPI.


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5. Deamination is Caused by Nitrous Acid Time

(a) Causes: C -> T transition (and G -> A transition)

(b) Causes: A -> G transition (and T -> C transition). All.

Deamination can be spontaneous as well.


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6. Ultraviolet Radiation Cause Time

Thymine Dimers

Disrupts synthesis;

good for sterilization

of bacteria, bad for skin cancer.

260 nanometer

wavelength



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Effects of Ionizing Radiation Time

  • Causes either point mutations or breaks in phosphodiester bonds of DNA backbone.

  • If both strands broken, there can sometimes be repair in mammals through the double-strand break repair (DSB) system.

  • Dividing cells are more susceptible to therapeutic X-rays than non-dividing cells (radiation therapy for cancer).


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