Honors Biology 298 - Week 10

Information on Ames test

Mutation at the Molecular Level: DNA as a Target

as detected in the Ames test using bacteria that have a his defect (can’t grow without histidine), the ability to cause a mutation can enable bacteria to grow on histidine-free medium (histidine defect is corrected)

            at a molecular level, such mutations can involve

            one strain of the bacterium Salmonella is used to detect

                        such nucleotide substitutions in coding regions

                        1) missense mutations are single nucleotide changes

                        this substitution may or may not affect the function of gene products

                        ex’s:

                        2) sense mutations produce longer than normal proteins

ex:

3) nonsense mutations change codons that specify an amino acid

            deletions and insertions can range from mutations that involve

these types of mutagenic changes are emerging as a major cause of genetic

disorders

3) frameshift mutations

since codons consist of groups of three bases, adding or subtracting 1-3

bases

ex:  THE FAT CAT ATE HIS HAT

turns into:

this type of mutation leads to a dramatic change in the amino acid order

in frameshift mutations, the addition or deletion of a single base can cause large-scale

changes in the amino acid composition of the polypeptide

so how prevalent are mutations?         can all of them be corrected?        how corrected?

DNA Repair Mechanisms

fortunately, not every mutation that occurs results

if left uncorrected, both growing and nongrowing somatic cells might accumulate

in addition, the DNA in germ cells might incur far too many mutations

the correction of DNA sequence errors is crucial

How are errors corrected?

we (and also prokaryotes) have a number of genetically controlled systems to repair damage to DNA

thus, the measured rate of mutation reflects a balance between

much of what we know about these correction mechanisms comes from studying E. coli

this damage can either be:

1) single nucleotide changes

2) structural alterations or distortions

ex: formation of thymine dimers

ex: attachment of large chemical groups

ex: ionizing radiation or chemicals break phosphodiester bonds

these sites of damage are recognized by special enzymes or nucleases

actually accomplished by some of the same enzymes that are important in DNA replication

            ex: excision repair –

            3 steps:

            most of the damage is handled this way and each cell type probably has multiple

            excision repair systems

            people with xeroderma pigmentosa lack these protective repair

            ex: mismatch repair –

            have ways to distinguish “old” DNA from “new” DNA

mark template strand by methylation (-CH₃)

however, because the rate of background damage is so high

rates of DNA damage in a mammalian cell (37⁰C = body temperature)

certain genes are more susceptible to mutations

            ex: more than 300 mutations have been discovered in the cystic fibrosis

gene, CFTR

a defective CFTR protein causes less fluid to be added to the mucous secretions of

the affected cells

this causes the production of thick mucous which blocks airways in the lungs

certain mutations have more of an effect on the phenotype

what happens when there is too much damage in a cell?

would a cell continue on even with some level of damage?

it is thought that the growth of tumors is dependent on both

programmed cell death or apoptosis is a multistep process

            it is a controlled response to specific environmental stimuli (like what?)

            apoptosis is essential both to normal development and tissue maintenance

                        ex:

                        ex:

                        ex:

apoptosis is dependent on the proper regulation of both survival and death factors

            apoptosis can be thought of as a neat and tidy process

            apoptosis represents an evolutionarily conserved cellular response

it serves as a mechanism to prevent the proliferation of damaged cells

the failure of certain tumor cell types to undergo apoptosis

research is underway to investigate and understand the pathways controlling cell

death

            what about conditions where cellular degeneration results from a disease or

aging process

what about other types of factors that can stimulate apoptosis?

            1)

            cells deal with this by activating tumor suppressor genes like p53

what part of the cell cycle do think p53 acts in?

                        p53 controls cell cycle arrest

                        it holds cells in these stages

                        if damage is too much, p53 initiates programmed cell death

            2)

            3)

            4)

thus, apoptosis exists as a way to safeguard or prevent the persistence of damaged cells