mutation and gene function

how can rare prototrophs (his+) be detected in a culture of auxotrophs (his-)?

(prototrophs would occur in the case of reverse mutation.) for his-, you need a minimal medium plus Histidine. to detect his+, you would put the culture on minimal medium only

point mutations alter

1 or a few base pairs

it is unusual to find examples of human pedigrees such as the one described that in effect represent a complementation test. the reason is that most genetic conditions in humans are rare, so it is unlikely that unrelated people with the same condition would mate. in the absence of complementation testing, what kinds of experiments could be done to determine whether a particular human disease can be caused by mutations at more than one gene?

Complementation tests on demand are not possible in humans. One way to tell if a trait can be caused by mutations in more than one gene is to map the mutations in several independent families. Mapping involves recombination analysis of the gene causing the mutant trait with other markers. If the mutations in different families map to different chromosomes or are far apart on the same chromosome, then they must be in different genes. If the mutations map close to each other on the same chromosome they may or may not be in the same gene.

can you think of a way to use forward mutation (his+ to his-) to test a compound for mutagenicity?

A wild-type (His+) strain could be grown in the presence of a potentially mutagenic compound (plus rat liver enzymes) and then plated for single colonies on minimal medium + histidine. Replica-plating on minimal medium (without a histidine supplement) would identify colonies that are His−; they would fail to grow.

normal sequence: G:C. after oxidative damage: GO:C. after replication: GO:A. after 2nd replication: T:A mutant sequence and GO:A. how many divisions did it take to cause this substitution of G:C to T:A?

2 divisions

DNA polymerase proofreading

3' to 5' exonuclease recognizes and excises mismatches. replication error is rare but does occur: 1 in 10^9 bp in vivo and 1 in 10^6 bp in vitro

how many different ways exist to mutate a base pair by substitution?

3. for example GC: AT, CG, or TA

how many genes control the eye color of fruit flies?

5 genes among the nine fly eye color mutations

it turns out that, even after taking into account the actual GC content of human DNA (42%) the frequency of CpG in human DNA is much lower than predicted by the calculation in the previous question. explain why

Any CpG dinucleotides in the genome that are not important functionally would mutate to TpG over time.

autism is a neurological disorder thought to be caused by mutant alleles of one or more genes. scientists had been wondering why the number of children diagnosed as autistic increased dramatically in a decade, from 1 in 500 in 2002 to 1 in 88 in 2012. researchers now think they might have found at least part of the answer: men are fathering children at later ages. a paper published showed a correlation between paternal age and the incidence of autism, the age of the mother was not a factor. how does this observation provide a possible explanation for the apparent increase in the rate of autism?

Because the germ cells that produce human sperm undergo mitosis continually, the older the father, the more mutations, on average, each of his sperm contains. Recall that the fixation of mutation requires DNA replication. Human females, on the other hand, are born with all (or most) of the eggs that they are ever going to have, so maternal age is not a factor in the accumulation of gene mutations. In addition, as described in the text, female germ cells underwent a far smaller number of mitoses than the germ cells of even a young, 20-year-old male. Because mutant alleles of a variety of genes are likely to promote autism, the correlation between autism and older fathers makes sense.

A researcher is studying coat color in mice. Wild-type fur in mice is brown. Three pure-breeding strains of mice with white fur have been isolated: milky, blanc, and weiss. White fur is a recessive trait in each strain. These mice are crossed in pairs and the progeny phenotypes are recorded. milky x blanc = all brown progeny milky x weiss = all brown progeny blanc x weiss = all white progeny

Blanc and weiss have mutations in the same gene; milky has a mutation in a different gene.

what is Huntington disease caused by?

CAG repeat expansion. polyQ disease. in normal people, there are 6-28 CAG repeats inside the polyQ coding region (ORF), people with Huntingtons have 36-120

(instapoll) which repair mechanisms rely on DNA sequence complementarity?

base excision repair, nucleotide excision repair, mismatch repair, and homologous recombination (not NHEJ)

are reactive oxygen species (e.g. superoxide O2-, hydrogen peroxide H2O2) spontaneous (naturally occurring) or induced by a mutagen or both? what is its effect on DNA? what is its repair mechanism?

both, spontaneous by cellular respiration. induced by irradiation and inflammation. oxidative damage of base and covalent crosslinks that distort the helix. BER for base damage, NER for crosslinks

x- rays and other ionizing radiation

breaks sugar-phosphate backbone, single or both strands causing SSB or DSB. can be repaired. if it is not repaired prior to replication, deletion can occur

is ionizing radiation (X rays, gamma rays, cosmic rays) spontaneous (naturally occurring) or induced by a mutagen or both? what is its effect on DNA? what is its repair mechanism?

can be either spontaneous or induced by mutagen. Single (SSB) and Double (DSB) strand breaks. BER for single strand breaks and HR/NHEJ for double strand breaks

is UV light spontaneous (naturally occurring) or induced by a mutagen or both? what is its effect on DNA? what is its repair mechanism?

can be either spontaneous or induced by mutagen. pyrimidine dimers distort the helix. NER

UV light

can crosslink adjacent pyrimidines (C and T) and distorts helix to cause DNA mismatch or stall of replication. produces thymine dimers. can be repaired

what can double strand break lead to and what are the repair mechanisms?

can lead to chromosomes rearrangements. repair mechanism: homologous recombination (S/G2 phases only). same mechanism of meiotic recombinations can repair DSB in mitotic cells using the sister chromatid repair mechanism: nonhomologous end joining. stitches any 2 broken ends, sometimes adds or deletes 1 or a few base pairs. (when not in S or G2 phases or HR is defective)

what are examples of accurate DNA repair systems?

complementarity dependent repair of damaged bases (BER), complementarity dependent repair of damaged nucleotides (NER), complementarity dependent repair of mismatched nucleotides (MMR), double strand break repair (NHEJ and HR)

You want to test a hydroxylating agent that adds an -OH group for mutagenicity by doing the Ames test. Hydroxylamine adds -OH to cytosine, and hydroxylated C pairs with A instead of G. There are different types of his- auxotrophs that you can choose from in order to detect reversion to his+ prototrophs in response to a compound under test. What type of his- mutants do you need to use for testing hydroxylamine in the Ames test?

Mutant with substitution of A-T (his+) with G-C (his-)

what interactions in proteins determine its structure?

covalent and noncovalent interactions.

what type of chemical bond are peptide bonds?

covalent bonds, strong

ROS damage cells by?

crosslinking to DNA, protein, or lipid. this contributes to aging

DNA can be mutated to have

damage or base sequence error. both of these can occur naturally or induced by mutation

what distinguishes each amino acid?

R side chain

When the his- Salmonella strain used in the Ames test is exposed to substance X, no his+ revertants are seen. However, when rat liver enzymes are added to the cells along with substance X, revertants do occur. Rat liver enzymes alone cannot induce reversion. How can you explain this result?

Rat liver enzymes converted substance X into a mutagen.

what is nucleotide excision repair (complementarity-dependent repair of damaged nucleotides)?

detects and removes DNA lesions/distortions. after exposure to UV, a protein complex scans for distortions to double helix (ex: thymine dimers) and makes a double nick on the damaged strand. DNA polymerase fills in the gap and DNA ligase seals the gap

what is base excision repair (complementarity-dependent repair of damaged bases)?

detects and removes damaged bases. an enzyme cleaves an altered nitrogenous base. another enzyme makes a single nick on the damaged strand. exonucleases remove nucleotides near the nick. DNA polymerase fills in the gap and DNA ligase seals the gap

what is DNA mismatch repair (complementarity dependent repair of mismatched nucleotides)

detects and repairs base mismatches and short insertion/deletion. a protein complex detects the mismatch and makes a nick nearby. nucleotides surrounding the mismatch are excised by exonuclease. DNA polymerase fills in the gap and DNA ligase seals the gap (intercalators)

20 different R groups with

different chemical properties. R group of amino acid is positively charged if basic (accept proton) or negatively charged if acidic (donate proton). uncharged amino acids can be polar (partially charged) or nonpolar (no net charge)

mutations that occur in somatic cells can lead to what?

diseases. cancer, neurodegeneration, and aging - but not heritable

what are N terminus and C terminus?

distinct ends of protein

mutations affecting phenotypes occurs at what rate?

even lower. 2-12 x 10^-6 mutations per gene per gamete. 27,000 human genes. .05-.30 mutations per gamete. 1 mutation in a gene in every 3-20 human gametes

what is a complementation test?

finds if mutations are in the same gene or different genes by doing pairwise crosses. mutation must be recessive, 2 parents must be homozygous for it. if 2 mutations are in the same gene, offspring will have the mutant phenotype. they will fail to complement. if 2 mutations are in different genes, offspring will have wild type phenotype. they will complement

(instapoll) which mutation would occur at a higher rate?

forward > reverse. There are many ways for mutations to change a wild-type allele into a mutant allele, but only one or a few ways to change a mutant allele into a wild-type allele.

N terminus of a protein contains what?

free amino group

C terminus of a protein contains what?

free carboxyl group

what do mutations tell us about gene function?

genes specify the identify and order of amino acids in a polypeptide chain, the sequence of amino acids in a protein determines its 3D shape and function, one gene=one protein is not entirely correct

what is a bacteria colony?

genetically identical bacteria cells. derived from a single cell that divided

what type of mutation can be passed to your offspring?

germ cell mutation

(instapoll) why do sperm of older men have more mutations than sperm of younger men?

germ cells of older men underwent more mitoses

mutations can occur in?

germ cells or somatic cells

what happens if DNA damage is unrepaired prior to replication?

has potential to generate mutations in somatic or germ cells and can cause dysfunction and disease

(instapoll) which one is the most accurate repair mechanism for DSB?

homologous recombination using sister chromatid for repair

(instapoll) 1 master plate without antibiotic. 3 replica plates with antibiotic. if the pattern on all 3 replica plates is identical, then the mutation that caused bacterial resistance must be?

spontaneous

are unstable nucleotide repeats spontaneous (naturally occurring) or induced by a mutagen or both? what is its effect on DNA? what is its repair mechanism?

spontaneous. insertion or deletion of the repeats by DNA pol. MMR only for small insertions or deletions

forward mutation

wild type to mutant a+ -> a-

what are prototrophs?

wild type. can grow on minimal media

can mutations not have any phenotypic effect?

yes. any change in DNA sequence is a mutation even if it has no effect on the phenotype

complementation testing requires that the 2 mutations to be tested are both recessive to wild type. suppose that 2 dominant mutations cause similar mutant phenotypes. how could you establish whether these mutations affected the same gene or different genes?

you can try to map the dominant mutations or analyze the DNA of candidate genes you think might be involved in the conditions.

is the black spot on the golden retriever's face a somatic or germline mutation?

somatic mutation

what can minimize aging?

antioxidants donate an electron to the free radicals unpaired electron and stabilize them

The term mutation refers to

any heritable changes in DNA regardless of their phenotypic effects.

suppose you are a toxicologist working in FDA and want to test chemical X for its mutagenic potential. assume X induces mutation at the rate of 10^-5 mutation per gene*cell division in bacteria that have 4,000 genes. 1. how many cells do you need to grow to detect a new mutation? 2. how many cells do you need to grow to detect a his mutation? 3. would you start out with a his- or his+ stock of bacteria?

1) 10^-5 mutation/gene*cell division * 4000 genes. cancel out gene. 1/25 mutation/cell division. find reciprocal 1/1/25= 25 cell division/mutation. 2^25 (2 because it doubles each time). 2) 10^-5 mutation/gene*cell division * x = 1 mutation/gene. x= 10^5 and then power of 2 so 2^100,000 cells.

(instapoll) a new ARG auxotroph has 2 mutations. it can only grow in the presence of arginine but also accumulates ornithine in media. what 2 genes are mutated in this auxotroph?

ARG-F and ARG-H

What level of protein structure could be affected by a nucleotide substitution that changes the identity of a single amino acid in a polypeptide?

All four levels of protein structure could be affected by a single amino-acid change.

aflatoxin B1 is a highly mutagenic and carcinogenic compound produced by certain fungi that infect crops such as peanuts. aflatoxin is a large molecule that chemically bonds to the base guanine in DNA to form aflatoxin guanine adduct. this adduct distorts the DNA double helix and blocks replication. what types of DNA repair systems are most likely to be involved in repairing the damage caused by exposure of DNA to aflatoxin B1?

DNA in most organisms is not exposed normally to high levels of aflatoxin B1.It is therefore unlikely that most cells would have evolved genes for enzymes that could directly remove the aflatoxin B1from guanine, or for a glycosylase enzyme that could specifically remove the adduced guanine-aflatoxin B1base (as in base excision repair). The nucleotide excision repair system is most likely to repair damage due to aflatoxin B1.This system would treat the adduct in a similar fashion to a thymine dimer, which is also a bulky group that distorts the double helix. Double-strand break repair by homologous recombination or nonhomologous end-joining is not involved because this mutagen does not produce double-strand breaks.

in human dna, 70% of cytosine residues that are followed by guanine (CpG dinucleotides) are methylated to form 5-methylcytosine. as shown in the following figure, if 5 methylcytosine should undergo spontaneous deamination, it becomes thymine. methylated CpG dinucleotides are hotspots for point mutations in human DNA. can you propose a hypothesis that explains why?

Deamination of 5-methylcytosinesproduces thymine (T); after replication this change could be fixed in the genome, producing C:G to T:A transitions. deamination of 5-methylcytosineswould produce T-G base pairs, and you would be correct inthinkingthat cells ought to have molecular mechanisms to correct these mismatches. In fact, cells in many organisms, including humans, have thymine glycosylase enzymes that remove Ts from these illicit T-G base pairs. However, thymine glycosylases are inefficient enzymes (much less efficient than uracil glycosylases), so errant Ts formed by deamination of 5-methylcytosines are repaired only rarely.

in general, why do mutations in genes cause or predispose to cancer?

Defect in DNA repair due to mutation in repair genes can increase the overall rate of mutation. With increase in the rate of mutation, there is a greater chance of mutating any gene, including genes that regulate cell division directly or indirectly. Therefore, as the rate of mutation increases, the likelihood of developing cancer also increases, and cancer becomes harder to treat over time as more mutations accumulate.

in humans, albinism is normally inherited in an autosomal recessive fashion. it has been observed that 2 albino parents can have several children, none of whom is an albino. interpret this inheritance pattern in terms of a complementation test.

Each parent must have been homozygous for recessive albinism-causing mutations in different genes. For example, one parent is aa BB and the other parent is AA bb. None of the children is albino because of complementation—all of them are Aa Bb. If the two mutations were in the same gene(for example, aa), then all the children would be albinos (aa).

given that the rate of forward mutation is so much higher than the rate of reversion, why does the ames test use the reversion rate to test for mutagenicity?

Even though the forward mutation rate is higher than the reversion rate, this screening process is much more labor-intensive than the selection that is employed when testing for revertants.

the physicist stephen hawking lived until 76 with ALS, a paralyzing neurodegenerative disease that is usually fatal at a much younger age. geneticists discovered that a major cause of ALS is the unusual expansion of a hexanucleotide repeat that lies within a gene called C9ORF72 at a location outside the gene's open reading frame. a single expanded allele is sufficient to cause ALS, but the reason the disease allele is dominant remains unclear. some experimental results support the theory that the allele makes a toxic RNA containing the expanded repeat. if this is correct, in what ways is the mutation similar to the mutant alleles that cause Huntington's or fragile X syndrome?

If the mutant C9ORF72 gene makes a toxic RNA that causes ALS, then the disease allele is dominant to the normal allele because even in the presence of the normal allele and normal gene product, the toxic RNA made by the mutant allele damages nerve cells. This scenario is similar to the reason why the HD allele is dominant to the HD+ allele, except that in the case of Huntington disease, it's the mutant protein product of the HD allele that is toxic. This potential mechanism of the ALS disease allele is similar to that of the fragile X mutation in FMR-1in that the expanded repeat is outside of the ORF.

in a genetics lab, kim and maria infected a sample from an e. coli culture with a particular bacteriophage. they noticed that most of the cells were lysed, but a few survived. the survival rate in their sample was about 1 x 10^-4. kim was sure the bacteriophage induced the resistance in the cells, while maria thought the resistant mutants probably already existed in the sample of cells they used. earlier for a different experiment they had spread a dilute suspension of e coli onto solid medium in a large petri dish, and after seeing that about 10^5 colonies were growing up, they had replica plated that plate onto 3 other plates. kim and maria decided to use these plates to test their theories. they pipette a suspension of the bacteriophage onto each of these replica plates. what should they see if kim is right? what should they see if maria is right?

Kim's hypothesis is that the bacteriophages are able to induce resistance in ~1 in every 104bacteria. If she is right, then several of the colonies on each of the replica plates (~10 if there are 10^5 bacteria on each plate) should contain resistant cells that will continue to grow after exposure to phage. These resistant cells would be generated only after the replica plating. If Kim is correct, resistant colonies will be distributed in random locations on the three replica plates. The number of colonies on the 3 plates should be roughly similar but are unlikely to be exactly equal. Maria's hypothesis is that the resistant cells are already present in the population of cells they plated on the original(master)plate. If she is right, then some of the colonies on the original plate (about 10 of them) should have already contained multiple resistant bacteria that will allow colonies to grow on the phage-treated replica plates. If Maria is right, resistant colonies will appear in identical numbers and at the same locations on all three of the replica plates.

how is it possible for fly strains to have different mutant phenotypes (eye color)?

The different mutant alleles of the white gene affect the function of the gene to different extents. The white mutant allele completely abolishes gene function resulting in no color. The other mutant alleles retain different levels of white gene function, and so the eyes of mutant homozygotes have some color, although the colors are lighter than the wild-type color. Because white, cherry, coral, apricot, and buff are all white-gene mutant alleles, no heterozygous combination (e.g. white/cherry) results in wild-type eye color—each pair of alleles fails to complement.

making the simplifying assumption that human DNA has an equal number of C:G and A:T base pairs, and that the human DNA sequence is random, how frequently in the human genome would you expect to find the base sequence CpG?

The frequency of each base is 1/4, and thus the expected frequency of CpG at a given location in the genome is the same as that of any dinucleotide: 1/4 × 1/4 = 1/16≈ 6.25%.

Cisplatin is a major anticancer drug that kills cancer cells by damaging their DNA and inducing apoptosis. ERCC1 is one of the genes involved in repair of DNA damage induced by cisplatin. A patient's response to cisplatin chemotherapy depends on the sensitivity of tumor cells to cisplatin-induced DNA damage. What is the relationship between the level of ERCC1 gene expression and susceptibility of tumor cells to cisplatin chemotherapy?

The level of ERCC1 gene expression in tumor cells is higher in patients who respond poorly to the cisplatin treatment.

when a particular mutagen identified by the Ames test is injected into mice, it causes the appearance of many tumors, showing that this substance is carcinogenic. when cells from these tumors are injected into other mice not exposed to the mutagen, almost all of the new mice develop tumors. however, when mice carrying mutagen induced tumors are mated to unexposed mice, all of the progeny are tumor free. why can the tumor be transferred horizontally (by injecting cells) but not vertically (from one generation to the next)?

The mutagen induced tumor-causing mutations in somatic cells, not in gamete-producing cells in the germ line. When the tumor cells (somatic) are injected into a new mouse, they will divide in an uncontrolled manner and cause a tumor to develop. The mutation that caused the original somatic cell to become cancerous is not present in the germ-line cells of the same mouse. Thus, cancer cannot be inherited in a Mendelian fashion.

the ames test uses the reversion rate (his- to his+) to test compounds for mutagenicity. is it possible that a known mutagen, like proflavin, would be unable to revert a particular his- mutant used in the ames test? how do you think the ames test is designed to deal with this issue?

Yes. specific mutagens can revert only particular types of mutations. Proflavin, for example, can revert only single-base insertions or deletions; it cannot revert nucleotide substitutions caused by mutagens such as base analogs. The Ames test deals with this issue by testing potentially mutagenic compounds for their ability to revert His−strains that have different types of mutations at the molecular level.

how do you find the number of genes involved in controlling traits for a given character?

a complementation test

what is mutation?

a heritable change (via mitotic and meiotic divisions) in the base pair sequence of DNA

what is a spontaneous mutation?

a mutation that arises naturally and not as a result of exposure to mutagens

what is replica plating?

a technique for reproducing the pattern of colonies on different plates

what is insertion?

adding 1 or a few base pairs

how is bacteria grown in the lab?

agar plates in petri dishes or suspension

what are somatic cells?

all cells of the body except germ (sex) cells

why do sperm have more point mutations than eggs?

human eggs form in fetal ovaries and it is thought that females are born with most if not all of the oocytes. a female zygote needs to undergo only 24 mitotic divisions to produce all of these oocytes she will have in her lifetime (0.5 million). human sperm forms in testes beginning at puberty and continues to form throughout his life. male germline cells undergo mitotic divisions continuously (23 rounds/year) to make enough sperm via meiosis in his lifetime (billions)

at what rate do mutations occur?

human mutation rate is low. 1 mutation in every 100 million bp, 3 billion bp per gamete, so 30 mutations per gamete and 60 mutations per zygote

what is an example of gain-of-function mutation?

huntington disease allele makes an abnormally shaped protein with an increased number of GLN (Q) amino acids.

what does the Ames test do?

identifies mutagens by measuring the rate of his- reversion to his+. utilizes REVERSE mutation. test 2 suspension groups: one group with the suspension of potential mutagen/carcinogen (suspension of his- mutant bacteria and rat liver enzymes), and one control group without the potential mutagen (suspension of his- mutant bacteria and rat liver enzymes) potential mutagen tested for the ability to cause mutation in bacteria. mixtures are plated onto mediums without histidine. there are many bacterial colonies and his- -> his+ revertants from group with potential mutagen and only few on control group

what do mutagens do?

increase the frequency of mutations

are intercalators (eg: proflavin) spontaneous (naturally occurring) or induced by a mutagen or both? what is its effect on DNA? what is its repair mechanism?

induced. insertion or deletion of 1 base pair by DNA pol. MMR

what are examples of mutagens?

intercalating agents insert between bases. can either insert or delete a single base pair during replication and can mutate anywhere in the genome. can be repaired. example: proflavin. intercalates into the double helix and disrupts DNA metabolism resulting in deletion or addition of a base pair. toxic in mammals so it is only used as a surface disinfectant or for treating wounds to prevent bacterial reproduction.

what are natural processed that can cause DNA damage?

ionizing radiation (x-rays and gamma rays from radioiostopes, cosmic rays. so strong can break covalent bonds in DNA) non-ionzing radiation (UV from the sun), oxidative damage (ROS produced during cellular respiration), or mistakes in DNA replication by DNA polymerase

which gene has a higher rate of mutation, small or large gene?

large gene, more nucleotides for mutations to occur in

what are proteins?

linear polymers of amino acids linked by peptide bonds. 20 different amino acids are the building blocks. Amino group: NH2 CHR group, carboxyl COOH group. COOH is acidic, NH2 is basic.

mutations depends on what?

meiotic/mitotic cell division. germ cell or somatic

you must provide growing bacteria with?

minimal medium and the correct pH or complete medium (supplemented with all 20 amino acids, 4 nucleotides, etc)

what does more cell division imply?

more opportunities for mutation

reverse mutation

mutant to wild type a- -> a+ (a+ is called revertant)

what are auxotrophs?

mutants that cannot complete a biosynthesis pathway. cannot grow on minimal media without a supplement

what is an induced mutation?

mutation arises as a response to mutagens

over a period of several years, a hospital kept track of the number of births of babies displaying achondroplasia, a rare condition caused by an autosomal dominant allele that results in dwarfism with abnormal body proportions. after 120k births, 27 babies had been born with achondroplasia. a physician was interested in determining how many of these dwarf babies resulted from new mutations and whether the mutations rate in this geographical area was higher than normal. he discovered that 4 of the dwarf babies had a dwarf parent. what is the apparent mutation rate of the achondroplasia gene in this population? it is unusually high or low?

of the achondroplasia births observed, 23/27 are due to new mutations because no family history of dwarfism exists. Achondroplasia is an autosomal dominant trait, so it will be expressed in the child that receives the mutant gamete. There were 120,000 births registered, and these were derived from 240,000 total gametes. The mutation rate = 23 mutant gametes/240,000 gametes = 9.5 ×10-5. This rate is somewhat higher than 2 to 12 ×10−6 mutations per gene per generation, which is the average mutation rate for genes in humans.

what are amino acids joined together by?

peptide bonds

which lowest level of structure directly determines the 3D structure of proteins?

primary

which interaction is present in the primary structure? secondary? tertiary and quarternary?

primary: covalent secondary: hydrogen (polar) tertiary and quarternary: polar and nonpolar interactions

what are the 4 levels of protein structure?

primary: linear sequence of amino acids joined together by peptide bonds secondary: localized folding by hydrogen bonds (in backbone- carboxyl and amino) tertiary: ultimate 3D structure of polypeptide quarternary: some proteins are composed of subunits, multimers. each subunit is a polypeptide encoded by a gene

mistakes made in DNA replication are rare due to?

proofreading function of DNA polymerase. a wrong base can be added but it recognizes it and fixes it.

how are reverse mutants detected? easier

rare his+ prototrophs can be detected in a culture of his- auxotrophs via SELECTION. start out with minimal medium + histidine and if you plate it on minimal medium only his+ revertants grow

how would you detect forward mutants?

rare his- auxotrophs can be detected in a culture of his+ prototrophs via a SCREEN. you take minimal medium + histidine and get both his+ and rare his-, you create a replica plate using only minimal medium to get his+ (whatever grows on the histidine supplemented plate but does not grow on the minimal medium plate are the forward mutants)

oxidative damage due to ROS

reactive oxygen species produced from cellular respiration damage DNA via oxidation of base or crosslinking (distorts helix). can be repaired

what is deletion?

removing 1 or a few base pairs.

what is subsitution?

replace one base pair with another.

is the black spot on the golden retriever's face a forward or reverse mutation if the wild type alleles are dominant?

reverse mutation B_ee to B_Ee

revertants, though rare, can be easily be detected by?

selection, do not have to use replica plating

how can a single amino acid change affect a protein?

sickle cell anemia: mutation occurs in polypeptide beta chain of hemoglobin. glutamic acid (charged) in normal individual and valine (uncharged, nonpolar) in those with sickle cell anemia. mutant hemoglobins form insoluble aggregates and will create a long fiber creating sickle shaped red blood cell.

what does slipped mispairing generate?

small deletions and insertions at repeat regions, causing microsatellite instability (MSI)

why are bacteria a good system for detecting rare mutations?

small size, rapid growth (divide every 20-30 minutes), very large numbers (in 15 hours, 1 cell becomes a billion), simple small genome, haploid (recessive mutations not hidden and do not require several cross and generations to uncover recessive mutations as in a diploid), inexpensive, can be frozen, humane

(instapoll) what type of point mutation could cause a single amino acid change like the sickle cell anemia one?

substitution

what are the 3 types of point mutations?

substitution, deletion, insertion

complementation tables

summarizes many complementation tests that reveal the number of genes. + means complementation, mutations in different genes. - shows no complementation, mutations in the same gene

what is replica plating useful for?

testing the origin of bacterial resistance mutation.

if the heritable change mutation causes is not repaired prior to DNA replication

the mutation replication. it is a race between DNA replication and repair. replication wins = mutation

what does a supplementation test determine?

the order of genes in a biochemical pathway. a biochemical pathway halts when an enzyme is missing. pathway can function by adding a missing component

what explanations can account for the pedigree of a very rare trait? how might you be able to distinguish between these explanations?

the trait is first seen in III-1 and shows a vertical pattern of inheritance between generations III and IV, suggesting a mutation that is dominant to wild type. the mutant gene cannot be X-linked because male IV-1 inherits it from his father. the trait is autosomal dominant yet not seen in the first 2 generations. 1st possibility: the mutant allele is incompletely penetrant. in this case, individuals from the first generation may have the mutant allele but do not display the trait. 2nd possibility: a mutation occurred in the germ line of individuals in generation 2, producing a gamete with the dominant autosomal mutation. how to discriminate: more info about this trait from a more extended family pedigree might indicate whether the trait is incompletely penetrant. if you could obtain DNA sequence info for the genes of individuals in generation 2 and 3, you could see if one of the alleles in the propositus has a mutation not found in his parents.

beadle and tatum concluded that each gene coded for one enzyme. why is this inaccurate?

there are many genes that make different kinds of nonenzymatic proteins

Beadle and Tatum supplementation experiment 2

they performed a supplementation test to determine the order of genes in the pathway. they grew mutant genes on minimal medium and supplemented the mutant genes with different intermediates to see if they are able to grow. + means growth, - means no growth. they found out the order of genes (ARG-E, ARG-F, ARG-G, ARG-H) and concluded that each gene coded for one enzyme

why is some mutation necessary?

to create new alleles for genetic diversity and to continue evolution

why does the ames test use a liver enzyme extract?

to recreate metabolism that would normally happen in the human body

what is a common mistake in DNA replication?

trinucleotide repeats. DNA polymerase can expand (if new strand slips) or contract (if template strand slips) repeats during replication.

what is a non-poly Q disease?

triplet repeat expansion disease gene. disease alleles are nonfunctional (no protein). different triplets, 5'UTR, 3'UTR or intron (outside the coding region). does not affect sequence, affects amount of protein. fragile X syndrome

what is a poly Q disease?

triplet repeat expansion disease gene. disease alleles generate abnormal protein. always CAG. always inside Q codon. Huntington's

Beadle and Tatum supplementation experiment 1

used X-rays to make mutations in a fungus called Neurospora (bread mold). each haploid spore is grown in complete medium, then switched to minimal medium (only prototrophs). tried to find which nutrient (supplement) an auxotroph needs to grow. they isolated arginine auxotrophs- fungi with mutations that prevent them from making arginine acid (can't grow without it). they performed a complementation test and found there are 4 genes (ARG-E, ARG-F, ARG-G, ARG-H) required for arginine synthesis. the chemical intermediates were known, they proposed the 4 genes each encode an enzyme that catalyzes the reactions required for arginine synthesis.