Mutations in Replication
289-291
What did you learn in your reading?
• Who performs mismatch repair?
• Why does DNA constantly have to be
maintained?
• What is excision repair?
• What is the end-replication problem?
• Who fixes it?
Randoms…
• Haploid cell (sperm/egg) have about 3 billion
base pairs
• Approximately 100-200 million base pairs per
chromosome
• Initial pairing errors are 1 in 10,000
• At the end of the completed strand, only 1 in 1
billion have errors
• There are 2 types of repair…
Mismatch Repair vs Damage Repair
• DNA polymerase
proofreads and fixes
mistakes before the
strand is complete
• Many other proteins
also perform mismatch
repair
• Reactive chemicals,
radioactive emissions,
X-rays, and UV rays are
constantly (potentially)
harming DNA
• Cell continuously
monitors and repairs
Excision Repair
• Nuclease: cuts
damaged portion
• DNA polymerase
adds new
nucleotides
• Ligase seals DNA
back up
Xeroderma
pigmentosum
• Disorder in the
excision-repair
enzyme
• Skin cells mutations
are not corrected
after UV damage
Replication Problem
• DNA polymerase can only add nucleotides to
the 3’ end of a preexisting polynucleotide
• So….. Where primer is removed a gap remains
unfilled
• If it would stay this way, each new cell would
be shorter and shorter until no DNA would
remain
• **Only a problem in eukaryotes…why??
Telomeres and Telomerase
• Telomere: special nucleotide sequence at ends of
DNA
– TTAGGG is typical human repeating telomere
• Telomerase: enzyme that catalyzes the
lengthening of telomeres with the help of RNA
strand
• Job of telomere:
– Protect the genes from being eroded from generation
to generation of replication
– Prevent the ends from activating cell death
(unnecessarily)
Telomerase
• http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/an
imation19.html
Telomeres and Aging
• http://player.discoveryeducation.com/index.cf
m?guidAssetId=AB274EB9-EDF1-40F0-91225B996E8E08E0&blnFromSearch=1&productco
de=US
Sickle Cell and Malaria
• http://www.pbs.org/wgbh/evolution/library/01/
2/quicktime/l_012_02.html
• How can a mutation be harmful in one
environment and helpful in another?
• Why should a mutation persist if it kills people?
• Why are there more people with sickle cell
disease in one part of the world than in other
parts?
Cracking the Code
• http://video.pbs.org/video/1841308959/
• 15:27 (time to start clip)
• How can a mutation in a single DNA base affect
the production of normal proteins?
• How are mutations passed on to offspring?
• What does it mean to be a carrier of a disease?
• What is the pattern of inheritance for diseases
carried by recessive genes?
• What is Tay Sachs?
Cracking the Code
• http://www.teachersdomain.org/resource/tdc
02.sci.life.gen.modification/
• What are some possible future uses of genetic
technology?
• If future technology allows it, do you think
parents should be able to choose specific
characteristics for their babies?
• Do the benefits of new genetic tools outweigh
the risks?