MBB lec 20 DNA damage/repair

what qualifies as permanent mutations

when mistakes escape repair

what qualifies as a mutation? is DNA damage a mutation?

-DNA damage itself it NOT YET a mutation

-its only a mutation if the damage is not repaired before dna replication

is a mutation a permanent change in dna sequence

yes

are all mutations harmful

no, but most are

-can rarely be advantage (drive adaptation or evolution)

list the 2 groups of mutations, and their subgroups

1. Point mutation (SUBSTITUTION)

• transversion and transition

2. Frameshift mutation (INSERTION & DELETION)

what are point and frameshift mutations each caused by

point (substitution): caused by:

-deamination

-alkylation

-oxidative damage

-UV radiation damage (downstream effect)

frameshift (insertion/deletion) caused by:

-DNA intercalating agents (like ethidium Br for dna gel visualization)

collectively, insertion and deletions area referred to as:

indels

transition vs transversion. what kind of mutations these occur in

-point mutations

-TRANSITION: purine replace another purine (A→G) or pyrimidine replace another pyrimidine (C→T)

-TRANSVERSION: purine replaced by pyrimidine (vice versa)

when do transition/transversion substitutions occur

when dna damage changes H bonding pattern of a base

often, base modification from substitutions are quickly repaired by ______. but if not repaired, and dna poly dont catch the mistake, what happens?

-repaired by cellular DNA repair enzymes

-if NOT repaired, change will be introduced into the daughter strand and become permanent (MUTATION)

-this might result in a different amino acid put into protein

2 base modification processes and briefly describe

1. Deamination (REMOVE amino group from base)

2. Alkylation (ADD methyl or ethyl groups to base)

explain deamination of adenine and cytosine

-adenine becomes hypoxanthine (mimics G, so induces mutation from A-T to G-C)

-cytosine becomes uracil (change C-G to T-A mutation after replication)

what repairs deaminated bases

base excision repair (take out incorrect base, replace w/ correct base)

describe what happens in alkylation, what alkylating agents do, and which base this commonly occurs on

-alkylating agents ADD methyl or ethyl groups to dna bases (disrupts H bond)

-common on GUANINE, forming O6-methylguanine (pairs w/ T instead of C)

-after replication, GC to AT mutation

how is alkylation repaired

-by using O6-alkylguanine DNA alkyltransferase (MGMT)

-removes methyl from base

what causes insertions/deletions

chemical mutagens called Intercalation agents (flat aromatic molecules intercalate between adjacent base pairs on dna, their presence can lead to insertion/deletion of one OR MORE base pairs)

whats the intercalating agent used in the lab to visualize dna on agarose gels (becomes fluorescent in uv light)

ethidium bromide

what accounts for 10% of all dna damage

UV & other ionizing radiation

what does uv radiation damage cause, and what does this create

-can cause adjacent thymine bases to form covalent bonds, creating thymine dimers

what are thymine dimers created by and what do they do

-created when uv radiation cause thymines to form cov bonds

-DISTORT dna helix (this disortion (kink) blocks polymerization machinery), which BLOCK replication/transcription

what repair methods for uv radiation damage in bacteria/lowereukaryotes/plants vs in humans

-bacteria/lower euk/plants: photo-reactivation (direct repair)

-humans dont have photolyases, so: nucleotide excision (and sometimes base excision repair)

what is the skin disease that has mutations in many nucleotide-excision repair genes (responsible for correcting thymine dimers)

Xero-derma pigmentosum (XP)