hereditary nonpolyposis colorectal cancer

[doctorF]

When performing DNA sequencing from a colonic polyp excised from a 28-year-old woman with hereditary nonpolyposis colorectal cancer, a high rate of guanine-thymine pairing is found. The functioning allele in this patient can correct this mispairing. Which property identifies the appropriate base to remove?
A. A multi-protein complex nicks the mutated strand
B. The mutated strand is methylated
C. A multi-protein complex nicks the parent strand
D. The mutated strand lacks methylation
E. The mutated strand is marked by bulky distortions

[Pixie]

answer is D...

[khushboo]

D. The mutated strand lacks methylation..

[Seetal]

yes D. because ur aim is to identify the mutated base pairs not how to remove it.

[devareddy]

guys explain it in detail pls

[inder]

Originally Posted by devareddy

guys explain it in detail pls

Yeah no problem.
When mismatch repair occurs (right after DNA synthesis in the G2 phase), it is improtant for the endonuclease to recognize which of the two strands which make up the DNA served as the template (thus has the correect sequence) and which strand was newly synthesized (thus may have defective nucelotides) the way it does this is by DNA methylation.... the template strand contains many methylated adenine residues... this methlyation inhibits the interaction of the endonuclease with that strand. Newly synthesized strands lack this methylation feature and thus the endonuceleases are free to interact with this strand and remove the mismatched nucleotides. Hope that helps

[doctorF]

ryt inder the correct answer is D.

In HNPCC, one allele involved in mismatch repair is damaged, leading to a gradual accumulation of mismatches like G-T pairings or A-C pairings. Daughter strands aren’t methylated until after DNA replication, so the unmethylated strand is the daughter strand. Bulky distortions are a hallmark of UV damaged bases, such as thymine dimers, and are recognized by nucleotide excision repair, not mismatch repair.

[mahmud]

Originally Posted by doctorF

ryt inder the correct answer is D.

In HNPCC, one allele involved in mismatch repair is damaged, leading to a gradual accumulation of mismatches like G-T pairings or A-C pairings. Daughter strands aren’t methylated until after DNA replication, so the unmethylated strand is the daughter strand. Bulky distortions are a hallmark of UV damaged bases, such as thymine dimers, and are recognized by nucleotide excision repair, not mismatch repair.

Hi i just want to know did u get these methylation part of the explanation in kaplan notes coz i just finished bio chem for the first time and i read hnpcc also but didn't get them or i have missed it..... pls let me know

[inder]

If you do have lipincott biochem 4th edition... the explaination regarding this topic is found on page 411

[devareddy]

thank u inder now i understood............. mahmud i too felt the same way i just completed biochem and its not there in the notes........inder every body is telling kaplan and fa are more than nough but if questions like this appear what should we do i'm doin only kaplan and fa and for path goljan reply me

[inder]

based on what I have read in the different forums from step 1 takers and some of my classmatees who have taken the exam... they have said first aid and UW was more than enough for the biochem section. I have not taken the exam yet so please take my advice like any other with a grain of salt.

[doctorF]

i even know ppl who had around 60-70% xam from questions regarding pathologies and bchems and micro relating to cell biology and genetics.... so many of my elder friends advice me to read cell biology from high yield bkz its detailed there... kaplan doesnt seem to be enough for many of the things!!! but many ppl even say that they didnt have even 10% bchem or cell bio in their xam depends on ur luck!!!

[Seetal]

Originally Posted by doctorF

i even know ppl who had around 60-70% xam from questions regarding pathologies and bchems and micro relating to cell biology and genetics.... so many of my elder friends advice me to read cell biology from high yield bkz its detailed there... kaplan doesnt seem to be enough for many of the things!!! but many ppl even say that they didnt have even 10% bchem or cell bio in their xam depends on ur luck!!!

yes.. i guess its all luck. questions are always jumbled up n the same question is NEVER repeated twice (just asked in different ways of different details are asked regarding the one question).

so let's just hope for the best aye?

[mahmud]

Originally Posted by inder

If you do have lipincott biochem 4th edition... the explaination regarding this topic is found on page 411

thanks every one for the suggestions...............devareddy, thanks for the support .i guess i will first finish kaplan, first aid and uw. then if i can't improve my score i will study something else. .............is it okay or shud try other plan?

[doctorF]

Originally Posted by Seetal

yes.. i guess its all luck. questions are always jumbled up n the same question is NEVER repeated twice (just asked in different ways of different details are asked regarding the one question).

so let's just hope for the best aye?

i tell u wat... a few are repeated!!!

[Seetal]

Originally Posted by doctorF

i tell u wat... a few are repeated!!!

lol..... awesome!