DNA Mutations

Question 1

How do Mutations Arise?

Answer 1

• Environmental factors (UV, X-rays, mutagens)
• Spontaneous
• Errors during replication (that are not corrected…)

Question 2

Mutation in Viruses

Answer 2

• Much higher than multicellular organisms
• RNA viruses have the highest rate due to delicate RNA backbone

Question 3

Somatic Mutations

Answer 3

Mutation only at specific cells in specific tissues.

• Occur in non-germline cells
• Cannot be inherited
• Cells are in the G0 phase (no longer dividing) – mutation negligable

Question 4

Germline Mutations

Answer 4

All cells inherit mutation in offspring

• Occur in germline cells (sperm, egg)
• Can be inherited (passed to progeny)

Question 5

Lederberg Experiment

Answer 5

Part 1 Setup:

• Bacteria grown in petri dish w/ nonselective agar (agar plate1)
• Replica plating: agar plate1 stamped with cloth, and stamped onto SELECTIVE agar plate2 (containing penicilin antibiotic)
• Agar plate2 only grows bacteria resistant to pencillin

Observations:

• Only a few of the cells from plate1 could grow on plate2 (position of cell growth in plate2 tells where mutation is on plate1)

Part 2 setup:

• Mutant colony on nonselective medium was extracted and culture diluted
• Cells spread on selective medium

Observations:

• Pure culture of antibiotic resistant bacteria

Conclusion: Mutations exist in populations prior to penicillin exposure

Question 6

Physiological Consequences of Mutations in Genes

Answer 6

Cell death, cancer, aging, disease

Question 7

Mutagens

Answer 7

Agents that can increase probablility of mutations at specific regions along DNA.

• Radiation
• Chemicals

Question 8

Importance of DNA Ligase in Repair

Answer 8

• Repairs breaks in the DNA backbone that cause mutations

Question 9

Mismatch Repair

Answer 9

• DNA polymerase proofreads DNA by scanning for potential mismatches
• Mismatch creates a kink in DNA molecule that is recognized
• DNA backbone cleaved by nuclease in region including mutation
• Another enzyme removes successive bases from cut DNA
• DNA polymerase and DNA ligase close gap

Result: Intact DNA strand that matches w/ accurate complementarity to template DNA.

Significance: Cancers have problems in mismatch repair (colorectal cancer)

Question 10

Base Excision Repair

BER

Answer 10

• Uracil (only in RNA) incorporation acts as a signal for repair
• Detected by DNA Uracil Glycosylase and cleaved from backbone
• Lack of nitrogenous base detected by AP endonuclease which cleaves the backbone and removes the sugar
• Completely new nucleotide then needed
• DNA synthesis facilitated by DNA polymerase and ligase

Result: Intact DNA strand that matches w/ accurate complementarity to template DNA.

Question 11

Nucleotide Excision Repair

NER

Answer 11

• One or more damaged bases signals the repair process
• Enzyme cleaves DNA backbone at sites flanking damage
• Region with damaged bases removed
• Gap filled by new DNA synthesis, using the ungapped strand as the template

Result: Result: Intact DNA strand that matches w/ accurate complementarity to template DNA.

Question 12

Point Mutation

Answer 12

Single nucleotide pair substitution in a DNA sequence.

• Can arise during DNA replication
• Mutation escapes proofreading, becoming a permanent change in genome
• AKA: Single nucleotide polymorphisms (SNPs)
• Can have serious consequences if mutation is in an important protein

Question 13

Sickle-Cell Amenia

Answer 13

• Variant form of hemoglobin causes distortion of RBC
• Caused by a single-nucleotide substitution causing missence/nonsynonymous mutation in Beta globin protein subunits in hemoglobin
• Glutamate –> Valine
• Leads to decreased ability for hemoglobin to bind to oxygen

Question 14

Nonsynonymous Mutation

Answer 14

• Nucleotide substitution that also changes amino acid
• Missense

Question 15

Synonymous Mutation

Answer 15

• Nucleotide substitution that does NOT change amino acid
• Silent
• Due to the redundant nature of the genetic code

Question 16

Nonsense Mutations

Answer 16

• A nucleotide substitution that creates a STOP codon
• Creates a premature ppt chain –> nonfunctional proteins

Question 17

Insertion Mutations

Answer 17

• One or more extra nucleotides are inserted into replicated DNA

Question 18

Deletion Mutations

Answer 18

• Skipping or removal of one or more nucleotides during DNA replication

Question 19

Effect of Insertion and Deletion Mutations

Answer 19

• Insertion/Deletion of three (or multiples) nucleotides will add/delete one (or more) amino acids from protein
• Not in multiples of three? –> Frameshift mutation

Question 20

Mutant CFTR

Answer 20

• Three nucleotide deletion in CFTR transporter protein
• Causes Cystic Fibrosis (cannot move Cl- ions across cells, leading to sticky mucus and inflamed airways)
• Largely degraded prior to embedding in cell membrane

Question 21

Frameshift Mutation

Answer 21

• Caused when no. of nucleotides inserted/deleted are NOT in multiples of three
• Improper grouping of nucleotide downstream of deletion
• Changes reading frame of translation
• Can end in a nonsense premature mutation
• Proteins are often nonfunctional

Question 22

Chomosomal duplications

Answer 22

• Causes little harm to diploids due to presence of normal homologous chromosome
• Duplication & Divergence: Extra gene copies can even be an advantage, leading to a new gene being formed

Question 23

Chromosomal deletion

Answer 23

• Chromosomal fragment lost –> Entire genes are lost
• Centromers lost –> Entire chromosome can be lost within few cell divisions (not able to be allocated properly during mitosis)
• Deletion in one homologous chromosome may persist depending on whether other homologue can provide code for enough of gene product required for survival (only for diploids)
• Deletions in embryos either cause death or abnormalities

Question 24

Chromosomal Inversion

Answer 24

• Normal order of a sequence of genes is reversed
• Chromosome fragment breaks off and then reattaches to the same chromosome, but in the reverse order
• Do not lead to serious consequences as all the genes are still present
• Problems w/ gamete formation…. breaks occur within a gene
• Small inversions explain why order of genes vary within populations

Question 25

Chromosomal Reciprocal Translocations

Answer 25

* Portion of one chromosome breaks off and attaches to a non-homologous chromosome * Exchange is reciprocal (two way!) * Often occurs in noncoding regions of DNA, hence may not interrupt gene function * Homologous chromosomes may not pair properly during meiosis, affecting segregation * Gametes may end up missing or having extra genes → genetic abnormalities

Question 26

Beta Globin Gene Family

Answer 26

* Codes for one subunit of hemoglobin * Arose due to multiple rounds of gene duplication and sequence divergence * Single duplication event of beta globin gene leads to evolution of prenatal and adult globin genes

Question 27

Nonreciprocal Translations

Answer 27

* Portion of one chromosome breaks off and attaches to a non-homologous chromosome * No exchange occurs → only one chromosome donates DNA * Mistakes in the process in reciprocal translocation * Genetic info is missing --> impact on protein

Question 28

Other Conditions Caused By Nondisjunctions

Answer 28

* Polyploidy: Having more than two full sets of chromosomes * Trisomy: Subcategory of aneuploidy; When there is an extra chromosomes * Aneuploidy: Overarching term, situation where there are specific chromosomes where there are too many or too few Nondisjunction is the failure of the separation of chromosomes in meiosis