17.0 Introduction

1. Genetic Message Is Altered in Two Ways
1. Mutation
1. Changes content of genetic message
2. Alter identity of a nucleotide
3. Nucleotide removed from or added to a gene

2. Recombination
1. Changes position of a portion of the genetic message
2. Move gene to different chromosome
3. Alter location of a part of a gene

3. Focus on Cancer as an Example of Mutation fig 17.1

1. Mutations Are Rare but Important
1. All DNA in Cells Results from Multitudes of Replications
1. Mechanisms evolved to avoid errors during replication preserve DNA from damage
2. Some mechanisms "proofread" replicated DNA strands for mistakes
1. Proofreading not perfect
2. Causes variation in nucleotide sequences of genes

2. Mistakes Happen
1. Replication errors still occur, but are rare fig 17.2
2. If errors were common instructions encoded in DNA would soon become meaningless
3. Steady trickle of change drives evolution

3. The Importance of Genetic Change
1. Evolution begins with changes in the genetic message
1. Mutation creates new alleles
2. Gene transfer and transposition alter gene location
3. Reciprocal recombination shuffles and sorts these changes
4. Chromosomal rearrangement alters organization of entire chromosomes

2. Changes in germ-line tissue
1. May result in production of more offspring
1. Such changes are preserved
2. Provides genetic endowment for future generations

2. Other changes reduce ability to produce offspring
1. Such changes tend to be lost
2. Organisms carrying such information contribute fewer offspring to future

3. Evolution viewed as selection of certain combinations of alleles from existing pool
4. Rate of evolution limited by rage of generating alternatives

3. Genetic changes in somatic tissue not passed to offspring
1. Have less evolutionary consequence
2. Immediate impact if associated with development, regulation of cell growth

2. Kinds of Mutation
1. Mutations Occur Randomly Along DNA
1. Most are detrimental
2. Consequence of damage related to function of altered gene

2. Mutations in Germ-Line Tissues
1. Effect of mutation dependent on cell in which it occurs
2. At certain point in development cells destined to be gametes separated from other cells
1. Germ-line cells versus somatic cells
2. Decision occurs late in plants and fungi
1. Mutation in any cell can pass on to progeny
2. Any cell can potentially develop into adult organism

3. Decision made early in development in animals
1. Passed on to subsequent generations
2. Part of hereditary endowment of gametes derived from that cell

3. Mutations in Somatic Tissues
1. Mutations in germ-line tissue are raw material for natural selection and evolution
2. Change occurs only if new allele combinations are produced
1. Mutation produces new alleles
2. Recombination puts alleles together in different combinations

3. Somatic mutations not passed on to next generation
1. Effects only progeny of damaged cell
2. Somatic mutations in lung tissue is leading cause of lung cancer

4. Point Mutations
1. Alterations in sequence of nucleotides
2. Summary of sources and types of mutations tbl 17.1
3. Point mutations change one to a few nucleotides
1. Due to spontaneous pairing errors during DNA replication
2. Result from radiation or chemical damage to DNA by mutagens
3. Chemical mutagens released into environment in industrial societies

5. Changes in Gene Position
1. Genes move from place to place on chromosome
1. Occurs via transposition
2. May alter expression of it or of neighboring genes

2. Chromosomal rearrangement
1. Occurs in eukaryotes
2. Large segments change location or undergo duplication

3. Point Mutations
1. Physical Damage to DNA
1. Ionizing radiation
1. High energy ejects electrons from outer shell
2. Resultant molecule is a free radical
1. Most atoms in cell are water
2. Most free radicals produced from water
3. Most damage to DNA is indirect

3. Double-strand break
1. Free radical breaks both DNA phosphodiester bonds
2. Bacterial repair enzymes cannot fix this damage

4. Eukaryotes pair damaged chromosome to homologue in synaptonemal complex
1. Possible reason for evolution of meiosis

2. Ultraviolet radiation
1. Lower energy, electrons not ejected, free radicals not formed
2. Radiation absorbed only by some organic ring compounds
1. Pyrimidine bases cytosine and thymine
2. Double bond formed between adjacent pyrimidines
3. Called pyrimidine dimer fig 17.3

3. Repair mechanisms fig 17.4
1. Cleave bond linking dimers
2. Excise dimer, repair using other strand as template
3. Filling-in process error prone
4. May create mutational changes in base sequence of gap region
5. Blocks DNA replication if not repaired

4. Causes mutations in skin cells
1. Rare hereditary disorder called xeroderma pigmentosum
2. Homozygous condition results in extensive skin tumors
3. Skin cells lack mechanism to repair even mild UV damage
4. Disease results from mutations in as many as eight different genes

2. Chemical Modifications of DNA
1. Direct modification of bases by various chemicals
2. Three classes of chemicals that act on DNA
1. Some resemble DNA nucleotides, but pair incorrectly fig 17.5
1. Include new AIDS chemotherapies
2. Block transcription and slow viral growth

2. Some remove amino group from adenine and cytosine, cause mispairing
3. Others add hydrocarbon groups to bases, cause mispairing
1. Includes potent mutagens used in laboratories
2. Released into environment, like mustard gas

3. Spontaneous Mutations
1. Not caused by radiation or chemicals
2. Nucleotides change to other conformations, or isomers
1. Form different kinds of hydrogen bonds
2. Polymerase chooses wrong base to pair with isomer

3. Slipped mispairing during chromosome pairing
1. Sequences misalign and a portion of one strand loops out
2. Generally transitory, self-correcting problem fig 17.6
1. Repair enzymes may excise unreverted loop
2. Results in deletion of hundreds of nucleotides
3. Creates frameshift mutation

4. Changes in Gene Position
1. Most Genes Are Relatively Stable Over Time
1. Chromosome location is important factor determining transcription
1. Gene not transcribed if next to tightly coiled region of DNA
2. Same gene transcribed if in any other location

2. Transcription of chromosomal regions regulated in this manner
1. Binding of certain proteins regulates degree of local coiling
2. Determines accessibility of RNA polymerase to genes within regions

2. Chromosomal Rearrangements
1. Physical alterations to chromosomes affect locations of genes
1. Translocations
2. Inversions

2. Translocations
1. Segment of one chromosome become part of another
2. Have important effects on gene expression

3. Inversions
1. Orientation of a portion of a chromosome is reversed
2. Do not usually alter gene expression
3. Effect recombination leading to serious problems in meiosis
1. Problem if inversion on one homologue only
2. After cross over event, none of gametes have complete set of genes fig 17.7

4. Particular genes or segments of chromosomes lost or gained
1. Deletions harmful since they halve the number of gene copies
2. Duplications cause imbalance and are usually harmful

5. Aneuploidy: Whole chromosome lost or gained
6. Polyploidy: Sets of chromosomes added

3. Insertional Inactivation
1. Small segments of DNA randomly move about chromosomes
1. Called transposons, encode enzyme that promotes cut-and-paste behavior
2. Destination random since enzyme doesn't recognize any particular sequence
3. Transposon inserted into gene usually causes gene inactivation
4. Called insertional inactivation
5. Example: Drosophila white-eyed mutant studied by Morgan

2. Many human gene disorders caused by transposition
1. Human Alu transposon causes X-linked hemophilia
1. Alu inserts into clotting factor IX
2. Inserts premature stop codon

2. Alu also causes inherited high levels of cholesterol
1. Called hypercholesterolemia
2. Inserts into gene coding the low density lipoprotein (LDL) receptor

3. Drosophila transposon called Mariner
1. Causes rare neurological disorder, Charcot-Marie-Tooth disease
2. Muscles and nerves in legs and feet wither away
3. Mariner inserts into gene CMT on chromosome 17, chromosome breaks

1. What Is Cancer?
1. Defined as a Disorder Causing Uncontrollable Cell Growth
1. Growing cluster of cells called a tumor fig 17.8
2. Tissue may leave main mass and spread through body fig 17.9
1. Called metastases
2. Cause more tumors at distant sites

3. Pernicious disease, affects multitudes
1. Of children born in 1985, one-third will contract cancer sometime in life
2. One-fourth of male, one-third of female children will die of it

4. Much research being done
1. Substantial progress using molecular biological techniques
2. Cancer is mutation in somatic tissue
3. Damaged genes unable to properly control cell proliferation
4. Causes mutation in genes producing proteins that regulate cell division cycle

5. Causes of cancer
1. Chemicals that mutate DNA
2. Viruses that circumvent cell's normal proliferation controls

6. Characterized by unrestrained growth and division
7. Cancer cells are virtually immortal, until host dies

2. Kinds of Cancer
1. Cancer Can Occur in Any Tissue
1. Tumors can occur in nearly any kind of tissue
1. Sarcoma if connective tissue
2. Carcinoma if epithelial tissue

2. Many cancers are deadly tbl 17.2
3. Many cancers may be preventable
1. Lung cancer linked to smoking
2. Colo-rectal cancer linked to high meat diets

4. Hereditary susceptibility associated with breast cancer fig 17.10
1. Isolation of two genes BRCA1 and BRCA2
2. Located on chromosomes 17 and 23

2. Association with Environmental Factors fig 17.11
1. Include ionizing radiation (x-rays) and chemicals
2. Cancer-causing agents called carcinogens
1. Many are also potent mutagens
2. Chemical carcinogenesis theory

3. Some Tumors Are Caused by Chemicals
1. Early Ideas
1. First presented by Hill in 1761
1. Observed tumors in heavy snuff users
2. Suggested tobacco produced cancers

2. Similar observation in 1775 by Pott
1. Chimney sweeps had frequent cancer of the scrotum
2. Suggested soot and tars responsible
3. When more attention paid to cleanliness, cancer rate dropped

2. Demonstrating that Chemicals Can Cause Cancer
1. Hypothesis directly tested in 1915 by Yamagiwa
1. Applied extracts of coal tar to 137 rabbits' skin
2. After one year tumors appeared at site of application in seven cases

2. Winder and Doll in 1949 independently linked smoking cigarettes to lung cancer
1. Smoking introduces tars into lungs
2. Cancer rates 40 times higher in smokers than nonsmokers

3. Suggestion of relationship resisted by tobacco industry

3. Carcinogens Are Common
1. Hundreds of synthetic chemicals capable of causing cancer in laboratory animals
1. Include trichloroethylene, asbestos, benzene, vinyl chloride, arsenic, arylamide
2. Also include complex petroleum products tbl 17.3

2. Carcinogens are all mutagens, capable of inducing changes in DNA

4. Other Tumors Result from Viral Infections
1. Some Tumors Result from Viral Infections
1. Viruses isolated from certain tumors
2. 15% of human cancers associated with viruses

2. A Virus that Causes Cancer
1. Report of virus associated with cancer in 1911 by Rous
1. Named Rous avian sarcoma virus (RSV)
2. Associated with chicken sarcomas
3. RSV could infect and initiate cancer in fibroblast cells in culture
4. More viruses isolated from cancerous cells

2. RSV is an RNA virus or retrovirus
1. Infects cell, make DNA copy of their RNA genome
2. Insert that copy into host genome

3. How RSV Causes Cancer
1. RSV compared to closely related virus RAV-O
1. RAV-O unable to transform cells into cancer
2. Viruses identical except for one gene, the src (sarcoma) gene

2. Isolated temperature-sensitive mutants of RSV
1. Mutants transformed cells at 35° C, but not 41° C
2. Temperature sensitivity characteristically associated with proteins

3. Likely that src gene was transcribed, not recognition site for regulatory protein
1. Suggested isolation of protein gene product
2. Possible study ensued

4. Cancer-causing gene called an oncogene
5. src protein isolated in 1977
1. Protein was an enzyme that phosphorylates tyrosine
2. Called tyrosine kinases
3. Not common in animal cells

6. Another tyrosine kinase is a plasma membrane receptor of epidermal growth factor
1. Epidermal growth factor signals initiation of cell division
2. RSV causes cancer by introducing growth-promoting enzyme into cell

4. Origin of the src Gene
1. Prepare radioactive version of gene
1. Allow it to bind to complementary segment of chicken genome
2. Examine location of radioactivity

2. Radioactive src DNA binds to site where RSV genome is inserted
3. Also binds to second site where no RSV present
1. Indicates src is not exclusively a viral gene
2. Identified as a growth-promoting gene evolved in and present in chickens

4. RSV ancestor picked up in some past infection
5. As RSV, gene transcribed under viral, not chicken regulation fig 17.12

5. Cancer and the Cell Cycle
1. Transfection Used to Study Tumors
1. Nuclear DNA isolated from tumor cells
1. Cleaved into random fragments
2. Fragments tested for ability to induce cancer

2. Most human cancers result from mutations to genes associated with cell cycle
1. Mutation in a single gene required to induce most cancers
2. Sometimes associated with cancer-causing virus
3. Compare to normal, non-mutated counterparts tbl 17.4

2. Point Mutations Can Lead to Cancer
1. May be as little as a single-point mutation
2. Example: ras-induced human bladder cancer
1. Base change from guanine to thymine
2. Convert glycine into valine

3. Telomerase and Cancer
1. Telomerases are short sequences of nucleotides repeated at ends of chromosomes
1. Covered by cap of proteins that prevent lengthening of sequence
2. Enzyme called telomerase
3. DNA polymerase can't copy chromosomes all the way to end
4. Telometric segments lost each time a cell divides
5. Sequence shortens, cannot prevent action of telomerase
6. Telomerase then lengthens sequence, preventing further action of telomerase

2. Alternation of growing and shrinking sequence called the telomere cycle
3. Telomerase found in human ovarian tumor cells
1. Contain mutations to inactivate cell control blocking telomerase gene transcription
2. Incorrectly assumed cancer cells prevented telomere shortening
3. Incorrectly presented as cause for cancer cell immortality

4. Mutations in Proto-Oncogenes: Accelerating the Cell Cycle
1. Most cancers are direct result of mutations in growth-regulating genes
1. Mutations of proto-oncogenes
2. Mutations of tumor-suppressor genes

2. Proto-oncogenes encode proteins that stimulate cell division
1. Cause cancer by wrongly activating these genes
2. Cells containing genes proliferate

3. Mutated proto-oncogenes become cancer-causing oncogenes fig 17.13
1. Induction involves change in receptor activities
2. Occurs at surface of plasma membrane
3. Normal receptors
1. Control activation of intracellular signalling pathways
2. Trigger passage of G1 check point of cell proliferation fig 11.20

4. All mutations are genetically dominant,
5. Include myc and ras
6. myc stimulates production of cyclins and Cdks
7. ras involved with epidermal growth factor (EGF)
1. Intercellular signal that triggers cell proliferation
2. Cancer-causing mutations reduce amount of EGF needed to do this

5. Mutations in Tumor-Suppressor Genes Inactivate the Cell's Inhibitors of Proliferation
1. Cell division normally blocked by proteins that prevent binding of cyclins to Cdks
1. Tumor-suppressor genes encode these proteins
2. Growth-enhancing mutant alleles are genetically recessive

2. Tumor-suppressor genes interfere with cyclin-Cdk activity
1. Rb ties up transcription factor E2F fig 17.14
2. p16 and p21 reinforce tumor-suppressing role of Rb
1. Prevent phosphorylation of Rb
2. Bind to Cdk/cyclin complex, inhibit its kinase activity

3. p53 is activated if DNA is damaged fig 17.15
1. Induces transcription of p21, binds to cyclins and Cdk
2. Smoking induces p53 mutations, lead to lung cancer

6. A Family of Cancer-Causing Genes
1. Clinical form of cancer dependent on tissue where oncogene is found
2. Only a few dozen different genes can be mutated to cause cancer

7. Cancer-Causing Mutations Accumulate Over Time
1. Proliferation controlled at several check points
2. All controls inactivated to initiate cancer
3. Induction of most cancers usually involve four genes fig 17.16
1. Most cancers occur after age of 40 fig 17.17
2. Time needed for many mutations to occur in same cells

6. Smoking and Cancer
1. Cancer Prevention
1. Most obvious strategy minimizes production of mutations
2. Decrease exposure to mutagens

2. The Association Between Smoking and Cancer
1. One-third of all cancers in U.S. attributed to smoking
2. Definite cause and effect of smoking and lung cancer fig 17.18
1. High correlation of number of cigarettes smoked and lung cancer fig 17.19
2. Risk 40 times greater if two or more packs smoked per day

3. Clear relationship between smoking and reduced life expectancy fig 17.20
1. Estimates made by life insurance companies
2. One cigarette lowers life expectancy by 10.7 minutes
3. One pack of 20 decreases life by 3.5 hours

3. Smoking Introduces Mutagens to the Lungs
1. Over one-half million people in U.S died of cancer in 1995, 29% died of lung cancer
2. In 1980s 140,000 diagnosed with lung cancer
1. 90% died within tree years of diagnosis
2. 96% were cigarette smokers

3. Smoke from cigarettes contains 3000 chemical components
1. Include potent mutagens vinyl chloride, benzo(a) pyrenes, nitroso-nor-nicotine
2. Smoking places mutagens in direct contact with lungs

4. Mutagens in the Lung Causes Cancer
1. Causes damage to genes of epithelial cells in lungs
1. Some genes have cell proliferation function
2. Damage in these genes results in lung cancer

2. Clear connection to benzo (a) pyrene (BP)
1. Lung epithelial cells absorb BP, convert it to benzo (a) pryene diol epoxide (BPDE)
2. BPDE binds to tumor-suppressor gene p53, mutates and inactivates it
1. p53 protein oversees G1 cell cycle checkpoint
2. Key mechanism for preventing uncontrolled cell proliferation

3. Destruction of p53 in lungs hastens onset of lung cancer
4. p53 mutated in over 70% of lung cancers
1. p53 mutations occur in one of three "hot spots"
2. p53 mutations caused by BPDR occur in same three spots

5. The Incidence of Cancer Reflects Smoking
1. Cigarette manufacturers deny causal connection
2. Data presented for examination fig 17.21
1. Upper graph compared smoking and lung cancer in American men
2. Lower graph represents American women
1. Smoking unpopular before World War II
2. Incidence of lung cancer increases with chances in social conventions
3. Women smoke at same rate as men, lung cancer rates approach that of men

3. Current annual rate of death is 2 per 1000 smokers each year

7. Curing Cancer
1. New Molecular Therapies
1. Receiving the signal to divide
1. Signal is a small protein growth factor released from neighboring cell
1. Growth factor received by receptor on cell surface specific for that factor
2. Mutation that increase number of receptors amplify signal, cause cancer

2. Therapies use immune system to attack cancer cells
1. Therapeutic agents are monoclonal antibodies
2. Monoclonal antibodies seek out and destroy growth factor
3. Example: Genentech's "anti-HER2" against breast cancer

2. The relay switch
1. Second step in decision process is passage of signal into cytoplasm
1. Normal cells have Ras protein that acts as relay switch
2. Growth factor binds to receptor, Ras changes into chemically active shape
3. Initiates chain of events that pass "divide" signal to nucleus
4. Mutated form of Ras is stuck in "On" position

2. Therapies utilize fact that normal Ras are inactive, must be activated
1. Anticancer drugs block farnesyl transferase activating enzyme
2. Inhibitors induce tumor regression, prevent new ones from forming

3. Amplifying the signal
1. Third step is amplification of signal within cytoplasm
1. Protein kinase Ras acts as enzyme to activate other protein kinases
2. Produces cascade of amplifying events
3. Mutations cause further increased amplification

2. Therapies initially used protein kinase inhibitors
1. Generalized inhibitors prevent protein kinases from doing other needed tasks
2. Lead to undesirable side effects

3. New therapies use anti-sense RNA directed at specific kinase mutations
1. Complementary copy RNA sticks to original, ties it up
2. Inhibit growth of cancer cells

4. Fourth step removes "brake" cells use to restrain cell growth
1. Tumor suppressor Rb blocks activity of E2F
2. E2F directs cell to copy its DNA
3. Normal cell division inhibits Rb, allows E2F to act
4. Rb destroying mutations allow cell to proliferate uncontrollably

5. Therapies just being developed
1. Focus on drugs that inhibit E2F
2. Should halt tumors resulting from inactive Rb

4. Checking that everything is ready
1. Final step ensures that DNA is healthy and ready to divide
1. Carried out in healthy cells by p53 tumor suppressor protein
2. p53 inspects DNA, stops cell division if DNA is damaged or foreign
3. Activates cell's DNA repair mechanisms
4. Triggers cell death if not repaired in timely fashion

2. Cancer causing mutations repaired or cell dies
3. Therapy involves disabled viral E1B protein
1. E1B-negative adenovirus does not grow in healthy skin cells
2. Does grow in tumor cells
3. When immune system is disabled process causes tumors to disappear

4. Therapy may not be effective if immune system is healthy
1. Patients exposed to adenovirus develop antibodies to virus
2. Antibodies may attack adenovirus therapy
3. Developing therapies involving other viruses

1. An Overview of Recombination
1. Genetic Recombination Changes the Location of a Gene
1. Mutation changes content of genetic message
2. Recombination changes the reading of the genetic message

2. Gene Transfer
1. Genetic recombination is a change in the position of a gene or gene fragment tbl 17.5
2. In gene transfer one chromosome or genome is donated to new chromosome or genome
1. Example: Acquisition of AIDS virus
2. Occurs in prokaryotes and eukaryotes
3. Most primitive process

3. Reciprocal Recombination
1. Two chromosomes trade segments
1. Occurs only in eukaryotes
2. Example: Crossing over of meiosis

2. Chromosome assortment is another form of reciprocal recombination
1. Mendelian independent assortment during meiosis
2. Also occurs only in eukaryotes

2. Gene Transfer
1. Gene Position on Chromosomes Not Fixed
1. Move to other locations on chromosomes
2. Plasmids are small, circular auxiliary genomes
1. Can enter and leave main genome at specific places
2. Found primarily in bacteria
3. Contain about 5% of bacterial genome
4. Discovered by Lederberg and Tatum, 1947

3. Transposons are small fragments of the genome
1. Migrate to other positions at random
2. Occur in prokaryotes and eukaryotes
3. Discovered by McClintock, 1950 fig 17.23

4. Both discoveries led to Nobel Prizes, in 1958 and 1983

2. Plasmid Creation
1. Formation of plasmid from circular DNA fig 17.24
1. Hypothetical DNA region, two copies of same gene
2. Loop formed at this spot, transient double duplex
3. Recombination enzymes recognize site, exchange strands
4. Called reciprocal exchange, loop freed from circle

2. DNA will replicate new plasmid if it contains replication origin
1. Replication may occur without controls present in original genome
2. Plasmids may be present in multiple copies

 

3. Integration
1. Plasmid can reenter main DNA fig 17.24
1. Plasmid recognition site aligns with matching sequence
2. Plasmid integrates into genome

2. Recombination event can occur elsewhere
1. May integrate at any site with shared sequences
2. Plasmid transfers genes to new position

4. Gene Transfer by Conjugation
1. Lederberg and Tatum: Discovery of F (fertility) plasmid
1. Only cells containing F acted as plasmid donors
2. Cells called Hfr cells (high frequency recombination)
3. Contains recognition site and transfer promoting genes
4. Cause formation of hollow tube called pilus

2. Transfer of free F plasmid
1. Contact of pilus of F⁺ cell to F^- cell lacking pili
2. Conjugation bridge forms between two cells
3. F plasmid mobilized for transfer
4. Binds to site on F⁺ just beneath pilus
5. Rolling-circle replication: DNA replication occurs at binding point
6. Replicated DNA sent to connected cell fig 17.25
7. Process called conjugation

3. Transfer of integrated F plasmid in Hfr cell
1. Similar process where entire genome copied and transferred
2. Process used to locate gene positions on chromosome fig 17.26

3. Gene Transfer by Transposition
1. Transposons Also Move from One Genomic Position to Another
1. Transposons randomly and abruptly move about chromosomes fig 17.27
1. Transposons encode transposase enzyme
2. Selects random site and inserts transposon fig 17.28
3. Destination random since enzyme doesn't recognize any particular sequence

2. Transposition relatively rare, has enormous evolutionary impact
3. Causes mutation by insertional inactivation
1. Insertion of mobile element destroys gene's function
2. May be the cause of spontaneous mutations

4. Facilitates gene mobilization
1. Genes located elsewhere brought to one location
2. Generates composite plasmid with similar genes
3. Example: Resistance transfer factors
1. Patients treated with many antibiotics at once
2. Bacteria contain antibiotic resistance genes
3. Surviving bacteria have many genes on one plasmid
4. Plasmid readily passed to other bacteria
5. Antibiotics no longer effective

4. Reciprocal Recombination
1. Crossing Over
1. Occurs during Prophase I of meiosis
1. Homologous chromosomes pair side-by-side
2. Exchange of strands at one or more locations fig 12.12
3. Produce chromosomes differing in mutation combination
4. Form gametes with new combination of alleles

2. Example: Giraffe
1. Neck length gene and leg length gene on same chromosome
2. Mutations to form long-neck allele and long-leg allele
3. Unlikely event to get both alleles in same individual
4. Recombination could readily cause cross-over of alleles

2. Unequal Crossing Over
1. Pairing mistake due to same sequences at many locations
2. Homologues line up, sequence matches with a duplicate
3. Results in unequal crossing over fig 17.29
1. Exchange segments of unequal length
2. One chromosome gains copies while its homologue looses them
3. Results in generation of hundreds of copies of a gene

3. Gene Conversion
1. Homologues not identical thus nucleotides not complementary
2. Called mismatch pairs
3. Error corrected by proofreading enzymes
1. Excise strand, fill gap complementary to other strand
2. Produces two chromosomes with same sequence
3. One mismatch pair lost, called gene conversion

5. Trinucleotide Repeats
1. New Form of Genetic Change Discovered
1. Did not involve change in nucleotide identity (mutation)
2. Did not involve change in gene position (recombination)
3. Involved increase in number of copies of repeated trinucleotide sequences
1. Root cause of a number of inherited human disorders
2. Discovered in individuals with fragile X syndrome and spinal muscular atrophy
3. Genes contain runs of repeat nucleotide triplets (CGG or CAG)
4. Repeats in afflicted individuals are in thousands compared to normal dozens

2. Most Trinucleotide Repeats are CG Rich
1. Ten additional human genes found to have repeats fig 17.31
1. Few alleles are not harmful
2. Also include Huntington's disease, myotinic dystrophy, neurological ataxias

2. Trinucleotide expansion transmits as dominant trait
1. Repeats may be within exons
2. May be outside coding sequence (fragile X)
3. Repeat number may vary among siblings
4. Repeat number increases in subsequent generations

3. Severity of disease correlated with frequency of repeats fig 17.30
4. Repeats are common, function is unknown
5. Mechanism to expand number of repeats is unknown
1. May involve unequal crossing over
2. May involve stutter in DNA polymerase when encountering triplets
3. Di- and tetranucleotide repeats not yet found

17.4 Genomes are continually evolving

1. Classes of Eukaryotic DNA
1. Comparing Bacterial and Eukaryote DNA Sequences
1. Prokaryotic genome compact with little wasted material
1. Unequal genetic exchange deletes material fig 17.32a
2. Minimum genome size maintained
3. Example: Organization of lac genes

2. Eukaryotic genome contains much duplicated material
1. Unequal genetic exchange promotes duplication fig 17.32b
2. Genome in constant state of flux
1. Production of multiple copies of single gene
2. Divergence of genes to form new genes

3. Six classes of eukaryotic DNA sequences tbl 17.6

2. Transposons
1. Multiple copies scattered throughout genome
1. Drosophila have more than 30 transposons, present at 20 to 40 sites
2. Mammalian cells have fewer transposons, but may exist in thousands of copies

2. Are transcribed but appear to have no functional role
3. Important with respect to insertional inactivation

3. Tandem Clusters
1. Repeated many times, one after another (in tandem)
2. Encode cell products required in large amounts
1. Numerous copies transcribed simultaneously
2. Example: rRNA genes
1. Visible as nucleolar organizer regions
2. Disappears in division when transcription stops
3. Reappears after division when synthesis begins

3. Sequences similar but not precisely identical
1. May differ by one to a few nucleotides
2. Separated from one another by spacer sequences
3. Spacers not transcribed, dissimilar in sequence and length

4. Multigene Families
1. Most genes found in groups of different but related genes
1. Far fewer genes than in tandem clusters
2. Genes more distinctly different than tandem clusters

2. Related in sequence
1. Derived from a single ancestral gene
2. Result from a series of unequal crossing-over events

3. Example: Evolution of hemoglobin multigene family fig 17.33
1. Formation of a and b forms by the evolution of fishes
2. Moved apart on genome between evolution of amphibians and reptiles
3. Array of 11 globin genes found in humans
1. Three genes are silent
2. Three expressed during embryonic ( ¾ and e) or fetal (g) development

4. Only four utilized in adults (d, b, a1 and a2)

5. Satellite DNA
1. Short sequences repeated several million times
2. Clustered around centromere or near ends fig 17.34
1. Remain condensed and untranscribed through cell cycle
2. Probable structural function

3. Composes 4% of eukaryotic DNA

6. Dispersed Pseudogenes
1. Pseudogenes: Silent copies of a gene inactivated by mutation
1. Affect gene's promoters
2. Shift reading frame or produce small deletions

2. Dispersed widely from original position within multigene family

7. Single-Copy Genes fig 17.35
1. Source of new genes during evolution
2. Result from duplication, conversion to pseudogenes
1. Accumulation of mutations may encode new protein
2. Initially produce only one copy that will eventually duplicate