Advanced Cell Biology. Lecture...
Transcript of Advanced Cell Biology. Lecture...
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Advanced Cell Biology. Lecture 13
Advanced Cell Biology. Lecture 13
Alexey Shipunov
Minot State University
February 13, 2012
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Advanced Cell Biology. Lecture 13
Outline
Questions and answers
DNADNA replicationDNA reparation
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Advanced Cell Biology. Lecture 13
Outline
Questions and answers
DNADNA replicationDNA reparation
![Page 4: Advanced Cell Biology. Lecture 13ashipunov.info/shipunov/school/biol_250/2011_2012/lec_250-13.pdf · Advanced Cell Biology. Lecture 13 DNA DNA replication Telomerase I Lagging strand](https://reader030.fdocuments.us/reader030/viewer/2022041101/5edb6c84ad6a402d6665a328/html5/thumbnails/4.jpg)
Advanced Cell Biology. Lecture 13
Questions and answers
Previous final question: the answer
Why cells use RNA as DNA replication primers?
I Primers should be used for starting nucleotide chain,therefore they should be nucleic acids
I Primers contain errors and should be removed, thereforethey should be distinguishable from the rest of chain,therefore, they should be RNA instead of DNA
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Advanced Cell Biology. Lecture 13
Questions and answers
Previous final question: the answer
Why cells use RNA as DNA replication primers?
I Primers should be used for starting nucleotide chain,therefore they should be nucleic acids
I Primers contain errors and should be removed, thereforethey should be distinguishable from the rest of chain,therefore, they should be RNA instead of DNA
![Page 6: Advanced Cell Biology. Lecture 13ashipunov.info/shipunov/school/biol_250/2011_2012/lec_250-13.pdf · Advanced Cell Biology. Lecture 13 DNA DNA replication Telomerase I Lagging strand](https://reader030.fdocuments.us/reader030/viewer/2022041101/5edb6c84ad6a402d6665a328/html5/thumbnails/6.jpg)
Advanced Cell Biology. Lecture 13
Questions and answers
Meselson-Stahl experiment (1958), again
How to rule out two other hypotheses?
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNADNA replication
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA helicases
I DNA helicases are natural zippersI They use energy of ATP to untangle the double helixI Single-strand binding protein (SSBP) associates with DNA
strand to prevent re-forming base pairs
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Single strand binding protein, SSBP
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Sliding clamp (DNA clamp)
I Keeps DNA polymerase attached to the templateI Form a ring around different DNA polymerasesI Most are trimers of PCNA proteins
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Human DNA clamp protein (trimer of PCNA)
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA sliding clamp movie
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA replication complex
1. DNA: old helix, leading strand, lagging strand2. DNA polymerases3. DNA helicases4. SSBP5. DNA clamp6. Primase7. DNA ligase
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA replication machine (simplified)
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA replication machine (more realistic)
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA replication sewing machine movie
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
DNA replication in general movie
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Telomerase
I Lagging strand cannot reach the end of DNA moleculeI Every replication cycle chromosome lost parts of
telomeres from its endsI To prevent a loss on meaningful DNA fragments,
telomerase extends chromosome with new telomeresequences
I In humans, telomeres are several thousands of TTAGGGsequences
I Telomeres are also recognizable ends of chromosomes
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Telomerase and telomere
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Telomere movie
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Advanced Cell Biology. Lecture 13
DNA
DNA replication
Telomere theory of aging
I In humans, telomerase is only active in germ cells, stemcells and certain white blood cells
I Olovnikov (1971) postulated that lost of DNA ends willeventually stop division of cells and may stimulatesenescence of cells
I There is a strong support of this hypothesis with some celltypes (e.g., blood vessels wall cells); however, mice withknocked-out telomerase gene do not show significantlyless lifespan
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
DNADNA reparation
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Single nucleotide mutations
I SN mutations (deletions and substitutions) may damageseverely the function of protein
I In sickle-cell anemia, substitution turns Glu to Val andmodify structure of hemoglobin and then red blood cells
I However, people with partly modified red blood cells areless susceptible to malaria
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Sickle-cell anemia
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Mutation theory of ageing and/or cancer
I Accumulation of mutations will result in a constant loss offunctions
I Cells will either degrade or start to go out of control(cancer)
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
DNA mismatch repair
I Normally, error rate of DNA polymerase + proofreading is≈10−7
I DNA mismatch repair proteins decrease it to ≈10−9
I They react on DNA conformation deviations; recognizenewly synthesized strand by nicks, and remove wrongfragments which are later replaced with DNA polymeraseand ligase
I Some cancers are results of mutations in DNA mismatchprotein genes
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
DNA mismatched repair system
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Depurination and deamination
I Depurination is detaching A and G from its sugar; it resultsin deletion*
I Deamination is C to U conversion; it results in substitution*
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Depurination and deamination
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Final question (1 point)
What is the difference between deletion and substitution?
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Final question (1 point)
What is the difference between deletion and substitution?
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
Summary
I DNA replication system is a multienzyme complexI Ends of chromosomes are constantly shortening and
extending with new telomeresI DNA suffers from multiple damaging events; multiple
reparation systems are trying to lower mutation risks
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Advanced Cell Biology. Lecture 13
DNA
DNA reparation
For Further Reading
A. Shipunov.Advanced Cell Biology [Electronic resource].2011—onwards.Mode of access: http://ashipunov.info/shipunov/school/biol_250
B. Alberts et al.Essential Cell Biology. 3rd edition.Garland Science, 2009.Chapter 6.