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13 Genetics. parents. children. Who are the parents of these children?. Heredity = Continuity of biological traits from one generation to the next. Variation = Inherited differences among individuals of the same species. Genetics = The scientific study of heredity and hereditary variation.

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13 genetics
13 Genetics



Who are the parents of these children?

Heredity= Continuity of biological traits from one generation to the next

Variation= Inherited differences among individuals of the same species

Genetics= The scientific study of heredity and hereditary variation

13 chromosome


DNA double helix of nucleotides




13 Chromosome
  • Offspring acquire genes fromparents by inheriting chromosomes

DNA = Type of nucleic acid that is a polymer of four different kinds of nucleotides.

chromosome chromatine

wound on histon peptide

Chromosomes = Organizational unit of heredity material in the nucleus of eukaryoticorganisms

Gene = Unit of hereditary information that is made of DNA and is located onchromosomes

Locus = Specific location on a chromosome that contains a gene

13 asexual life cycle
13 Asexual life cycle
  • Asexual reproduction
  • A type of reproduction involving only one parent that produces genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts
12 mitosis l
12 Mitosis l
  • Mitotic cell cycle:

In a dividing cell, the mitotic (M) phase alternates with interphase, a growth period. The first part of interphase, called G1, is followed by the S phase, when the chromosomes replicate; the last part of interphase is called G2. In the M phase, mitosis divides the nucleus and distributes its chromosomes to the daughter nuclei, and cytokinesis divides the cytoplasm, producing two daughter cells.

13 sexual life cycle
13 Sexual life cycle
  • Meiosisand fertilization result in alternation between the haploid and diploid condition

Diploid = Condition in which cells contain two sets (2n) of chromosomes

Haploid = Condition in which cells contain one set (1n) of chromosomes

Gamete = A haploid reproductive cell (sperm cells and ova)

The diploid number is restored when two haploid gametes unite

Fertilization = The union of two gametes to form a zygote

Zygote = A diploid cell that results from the union of two gametes

13 variety of life cycles
13 Variety of life cycles
  • Three basic patterns of sexual life cycles

a. Animal

In animalsgametes are the only haploid cells

b. Fungi and some protists

In many fungi and some protists, theonly diploid stage is the zygote

c. Plants and some algae

Plants and some species of algaealternate between multicellularhaploid and diploid generations

13 a sexual reproduction
13 (A)sexual reproduction
  • Like begets like, more or less: a comparison of asexual versus sexual reproduction
13 meiosis overview
13 Meiosis – overview

What happens during meiosis?

13 origins of genetic variation l
13 Origins of genetic variation l

1. Independent assortment of chromosomes

Independent assortment = the random distribution of maternal and paternalhomologues to the gametes

Theprocess produces 2npossible combinations of maternal and paternalchromosomes in gametes

13 origins of genetic variation ll
13 Origins of genetic variation ll
  • 2. Crossing over
  • Crossing over = The exchangeof genetic material between homologues; occursduring prophase of meiosis I.
  • In humans, there is an average of two or three crossovers per chromosomepair
13 origins of genetic variation lll
13 Origins of genetic variation lll
  • 3. Random fertilization
  • Random fertilization is another source of genetic variation in offspring
  • Theprocess produces 2nx 2n possible combinations of maternal and paternalchromosomes in the zygote
  • 4. Mutation
  • The ultimate source of variation: random and relatively rare structural changes made during DNA replication in a gene as a result of mistakes
13 summary
13 Summary
  • Offspring acquire genes from parents by inheriting chromosomes
  • Like begets like, more or less
  • Fertilization and meiosis alternate in sexual life cycles
  • Meiosis reduces chromosome number from diploid to haploid
  • Sexual life cycles produce genetic variation among offspring
  • Evolutionary adaptation depends on a population’s genetic variation
13 key terms
13 Key terms
  • heredity karyotype
  • zygote meiosis I and II
  • variation homologous
  • diploid cells synapsis
  • genetics chromosomes
  • meiosis tetrad
  • gene sex chromosomes
  • alternation of generations chiasma (chiasmata)
  • asexual reproduction autosome
  • clone gamete
  • sporophyte crossing over
  • sexual reproduction haploid cell
  • spores life cycle
  • fertilization gametophyte
  • somatic cell syngamy
14 heredity
14 Heredity

Theories of heredity

Blending theory of heredity = Pre-Mendelian theory of heredity proposing thathereditary material from each parent mixes in the offspring; once blended the hereditary material is inseparable and the offspring's traits aresome intermediate between the parental types

Particulate theory of heredity = Gregor Mendel's theory that parents transmit to theiroffspring discrete inheritable factors (now called genes) that remain as separate factorsfrom one generation to the next

14 mendel s experiments
14 Mendel’s experiments
  • Terms
  • Character = Detectable inheritable feature of an organism gene
  • Trait = Variant of an inheritable character allele
  • True breeding = Always producing offspring with the same traits as the parents whenthe parents are self-fertilized
  • P = true-breeding parental plants of a cross
  • F1= hybrid offspring of the P-generation
  • F2 =generation of self-pollinated F1-plants
14 monohybrid cross
14 Monohybrid cross
  • Monohybrid cross
  • Hypothesis: If the inheritable factor for white flowers hadbeen lost, then a cross between F1 plants should produce only purple-flowered plants.
  • Experiment: Mendel allowed the F1 plants to self-pollinate.
  • Results: There were 705 purple-flowered and 224 white-flowered plants in the F2generation—a ratio of 3:1.
  • Conclusion: The inheritable factor for white flowers was not lost, so thehypothesis was rejected
14 testing hypotheses using the 2 statistic
14 Testing hypotheses using the 2-statistic
  • There were 705 purple-flowered and 224 white-flowered plants in the F2generation. Hypothesis: Is this a ratio of 3:1?
  • Calculate expected values based on the result total (929):3 : 1 = 696.75 : 232.25
  • Calculate the differences between observed (O) and expected (E)
  • Standardise the differences: |O – E |2 2 =  E
  • Calculate degrees of freedom (DF) = number of differences – 1
  • Lookup in 2 table at DF
  • If p<0.05 then reject hypothesis

O E |O–E|

705 696.75 8.25

224 232.25 8.25

929 929

(8.25-0.5)2 (8.25-0.5)2

2 =  + 

696.75 232.25

The correction for continuity of

0.5 is only applied when DF=1

2 = 0.086 + 0.259 = 0.345

At DF=1 0.345 lies between 0.016

and 0.455 with 0.900>p>0.500,

so accept hypothesis

14 law of segregation
14 Law of segregation
  • By the law of segregation, the two alleles for a character are packaged intoseparate gametes
  • 1. Alternative forms of genes are responsible for variations in inheritedcharacters.
  • 2. For each character, an organism inherits two alleles, one from each parent.
  • 3. If the two alleles differ, one is fully expressed (dominant alleleP); the other iscompletely masked (recessive allelep).
  • 4. The two alleles for each character segregate during gamete production.
14 genotype and phenotype
14 Genotype and phenotype
  • Homozygous = Having two identical alleles for a given trait (e.g., PP or pp).
  • Heterozygous = Having two different alleles for a trait (e.g., Pp).
  • Phenotype = An organism's expressed traits (e.g., purple or white flowers).
  • Genotype = An organism's genetic makeup (e.g., PP, Pp, or pp).
14 testcross
14 Testcross
  • Testcross = The breeding of an organism of unknown genotype with a homozygousrecessive
14 dihybrid cross
14 Dihybrid cross
  • Mendel's law of independent assortment = Each allele pair segregates independently ofother gene pairs during gamete formation
14 laws of probability
14 Laws of Probability
  • 1. Rule of multiplication
    • Rule of multiplication = The probability that independent events will occursimultaneously is the product of their individual probabilities

Question: In a Mendelian cross between pea plants that are heterozygous forflower color (Pp), what is the probability that the offspring will be homozygousrecessive?

  • 2. Rule of addition
    • Rule of addition = The probability of an event that can occur in two or moreindependent ways is the sum of the separate probabilities of the different ways

Question: In a Mendelian cross between pea plants that are heterozygous forflower color (Pp), what is the probability of the offspring being a heterozygote?

14 extended mendelian genetics l
14 Extended Mendelian genetics l
  • Incomplete dominance = dominant phenotypeis not fully expressed in the heterozygote, resulting in a intermediatephenotype

Complete dominance = an allele is fully expressedin the phenotype of a heterozygote and masks the phenotypic expression ofthe recessive allele; the phenotypes of the heterozygote anddominant homozygote are indistinguishable

Codominance = full expression of both alleles in theheterozygote

14 extended mendelian genetics ll
14 Extended Mendelian genetics ll
  • Important points about dominance/recessiveness relationships:
  • 1. They range from complete dominance, through various degrees of incomplete dominance, to codominance
  • 2. They reflect the mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA
  • 3. They do not determine or correlate with the relative abundance of alleles in a population
14 extended mendelian genetics lll
14 Extended Mendelian genetics lll
  • Multiple alleles = Some genes may have more than just two alternative formsof a gene
14 extended mendelian genetics l v
14 Extended Mendelian genetics lV
  • Pleiotropy = The ability of a single gene to have multiple phenotypic effects

Epistasis = Interaction between two nonallelic genes in which one modifies the phenotypic expression of the other (9:3:4)

14 extended mendelian genetics v
14 Extended Mendelian genetics V
  • Polygenic inheritance = Mode of inheritance in which the additive effect of two ormore genes determines a single phenotypic character
14 nature versus nurture
14 Nature versus nurture
  • Norm of reaction= Range of phenotypic variability produced by a single genotypeunder various environmental conditions
  • The expression of most polygenic traits, such as skin color, is multifactorial; thatis, it depends upon many factors - a variety of possible genotypes, as well as avariety of environmental influences
14 human genetics
14 Human genetics
  • Humans are difficult to investigate
  • long generation time
  • few offspring
  • experiments unacceptable
  • Pedigree = A family tree that diagrams the relationships among parents and childrenacross generations and that shows the inheritance pattern of a particular phenotypiccharacter
14 human genetic disorders l
14 Human genetic disorders l
  • Recessive disorders:
  • show up in homozygous individuals
  • heterozygote are called carriers
  • frequencies differ worldwide due to local selective forces
  • Cystic fibrosisTay-Sachs diseaseSickle cell anemia
14 human genetic disorders ll
14 Human genetic disorders ll
  • Phenylketonuria
  • Dominant disorders
  • Achondroplasia (dwarfism) 1:10,000 people
  • Lethal dominant alleles are not passed to next generation
  • Late-acting alleles escape elimination: Huntington’s disease
  • Multifactoral disorders (heart disease, diabetes, cancer,
  • schizophrenia)
14 genetic testing
14 Genetic testing
  • Carrier recognition
  • Fetal testing
  • Newborn screening (PKU)
14 summary
14 Summary
  • Mendel brought an experimental and quantitative approach to genetics
  • By the law of segregation, the two alleles for a character are packaged into separate gametes
  • By the law of independent assortment, each pair of alleles segregates into gametes independently
  • Mendelian inheritance reflects rules of probability
  • Mendel discovered the particulate behavior of genes
  • The relationship between genotype and phenotype is rarely simple
  • Pedigree analysis reveals Mendelian patterns in human inheritance
  • Many human disorders follow Mendelian patterns of inheritance
  • Technology is providing news tools for genetic testing and counseling
14 key terms
14 Key terms
  • character dominant allele
  • polygenic inheritancetrait
  • law of segregation true-breeding
  • incomplete dominance multifactorial
  • complete dominance homozygous
  • monohybrid cross heterozygous
  • cystic fibrosisP generation
  • multiple alleles F1 generation
  • genotype pleiotropy
  • F2 generation testcross
  • alleles dihybrid cross
  • recessive allelehybridization
  • norm of reactioncarriers
  • codominanceepistasis
  • phenotypesickle-cell disease
  • quantitative characterlaw of independent assortment
15 chromosomal basis of inheritance
15 Chromosomal basis of inheritance
  • Chromosome theory of inheritance:
  • Mendelian genes have specific loci on chromosomes, which undergo segregation and independant assortment
  • Recombinants have new combinations of traits
  • RY and ry are parental phenotypes
  • rY and Ry are recombinant phenotypes
15 morgan s experiments with fruit flies l
15 Morgan’s experiments with fruit flies l
  • Morgan used Drosophila melanogaster, a fruit fly species
  • Short generation time
  • Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males)
  • The normal character phenotype is the wild type
  • Alternative traits are mutant phenotypes
15 morgan s experiments with fruit flies ll
15 Morgan’s experiments with fruit flies ll
  • Cross white-eyed male with a red-eyed female: F1 had red eyes
  • Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring
  • The white-eyed trait appeared only in males
  • All the females and half the males had red eyes
  • Morgan concluded that a fly’s eye color is a sex-linked gene
15 linked genes
15 Linked genes
  • Dihybrid test cross did not produce 1:1:1:1 ratio 
  • genes must be on the same chromosome: linked genes
  • Complete linkage gives ratio 1:1:0:0
  • 17% recombinants must be the resultof crossing over
15 crossing over
15 Crossing over

Recombination is result of crossing over and independant assortment

15 genetic maps
15 Genetic maps
  • The recombination frequency between cn and b is 9%
  • The recombination frequency between cn and vg is 9.5%
  • The recombination frequency between b and vg is 17%
  • How are the loci arranged?b – vg – cn orvg – b - cn
  • Geneticists can use recombination data to map a chromosome’s genetic loci:linkage map

The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombination frequency

Map distance: 1 unit is 1% recombination

15 genetic maps1
15 Genetic maps
  • Why is 9,0 (b-cn)+ 9,5 (cn-vg) > 17% (b-vg)?

This results from multiple crossing over:the further loci are apart, the greater the change for multiple crossing over events

Some genes on a chromosome are so far apart that a crossover between them is virtually certain: independent inheritance, no linkage

  • Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles
  • Morgan traced a gene to a specific chromosome
  • Linked genes tend to be inherited together because they are located on the same chromosome
  • Independent assortment of chromosomes and crossing over produce genetic recombinants
  • Geneticists use recombination data to map a chromosome’s genetic loci
  • The chromosomal basis of sex varies with the organism
  • Sex-linked genes have unique patterns of inheritance
  • Alterations of chromosome number or structure cause some genetic disorders
  • The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting)
  • Extranuclear genes exhibit a non-Mendelian pattern of inheritance
15 key terms
15 Key terms
  • chromosome theory of inheritance linkage map
  • polyploidycytological map
  • deletionwild type
  • duplicationmutant phenotype
  • hemophilia inversion
  • sex-linked genes Barr body
  • translocationlinked genes
  • nondisjunction Down syndrome
  • genetic recombination aneuploidy
  • fragile X syndromeparental type
  • trisomicrecombinants
  • monosomic
18 genetics of viruses and bacteria
18 Genetics of viruses and bacteria
  • Bacteria are prokaryotic organisms:
  • - small- no compartments
  • - single, circular chromosome
  • Viruses are smaller and simpler still, lacking the structure and most meta-bolic machinery in cells
  • - aggregates of nucleic acids and protein - genes in a protein coat
  • - reproduce in host-cells
  • - limited hoste-range
  • Ideal genetic models for study
18 discovery of viruses
18 Discovery of viruses


  • The discovery of Tobacco Mosaic Virus TMV
  • Ultimate pathogen test: Postulates of Koch
  • 1886 Mayer: very small, invisible, non-culturable bacterium?
  • 1892 Ivanowsky: pathogen passed porcelain filter – small bacterium or toxin?
  • 1898 Beijerinck: infectious agent reproduces in living host, is soluble, not killed in ethanol – named micro-organism virus
  • 1935 Stanley: crystallized the pathogen
  • 1939 Kaushe et al: first electron micrograph, particle is only 20 nm in diameter


TMV particle consists of nucleic acid enclosed by a protein coat

18 viral structure
18 Viral structure
  • 1 Viral genome
  • ds DNA, ss DNA, ds RNA or ss RNA
  • Single nucleic acid molecules that are linear or circular
  • May have four to hundreds of genes
  • 2Capsids
  • Capsid = Protein coat that encloses the viral genome, composed of many capsomeres
  • Nucleocapsid = integrated structure of nucleic acid and capsid-proteins
18 viral structure1
18 Viral structure
  • 3 Envelopes or membranes
  • Envelope = Membrane that cloaks some viral capsids
  • Helps viruses infect their host by fusing with cell-membrane
  • Derived from host cell or nuclear membrane which is usually virus-modified
  • They also have some viral proteins and glycoproteins
18 bacteriophages
18 Bacteriophages
  • The most complex capsids are found among bacteriophages
  • Of the first phages studied, seven infected E. coli. These werenamed types 1 – 7 (T1, T2, T3, ... T7).
  • The T-even phages – T2, T4, and T6—are structurally verysimilar
  • The icosohedral head encloses the genetic material
  • The protein tailpiece with tail fibers attaches the phage toits bacterial host and injects its DNA into the bacterium
18 infection cycle
18 Infection cycle
  • Viral infection begins when virus genome enters the host cell
  • Once inside, the viral genome commandeers its host, reprogramming the cell to copy viral nucleic acid and manufacture proteins from the viral genome
  • Nucleic acid molecules and capsomeres self-assemble into viral particles that exit the cell
18 phage lytic cycle
18 Phage lytic cycle
  • In the lytic cycle the phage kills the host. Virus is called virulent.
18 defenses against phages
18 Defenses against phages
  • Natural selection favors bacterial mutants with receptors sites that are no longer recognized by a particular type of phage
  • Bacteria produce restriction nucleases that recognize and cut up foreign DNA, including certain phage DNA
      • Modifications to the bacteria’s own DNA prevent its destruction by restriction nucleases
  • But, natural selection favors resistant phage mutants.
18 phage lysogenic cycle
18 Phage lysogenic cycle
  • In the lysogenic cycle phage DNA incorporates in bacterial DNA:
  • prophage. Virus is called temperate.
18 replication strategies in plant and animal viruses
18 Replication strategies in plant and animal viruses
  • –RNA and retroviruses have viral enzymes in nucleocapside
  • RNA-viruses have high mutation rates (no proof-reading during replication)
18 retrovirus
18 Retrovirus
  • HIV causes AIDS (Acquired Immuno Deficiency Syndrome)
  • HIV is a retrovirus
18 viruses and symptoms
18 Viruses and symptoms
  • The link between viral infection and the symptoms it produces is often obscure
    • Virus triggers release of hydrolytic enzymes from lysosomes
    • Viruses causes the infected cell to produce toxins that lead to disease symptoms
    • Virus has toxic molecular components (envelope proteins)
  • In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio)
  • Cure by immune system, some medicines:
    • AZT interferes with reverse transcriptase of HIV
    • Acyclovir inhibits herpes virus DNA synthesis
    • antibiotics are useless
  • Prevention by vaccination
  • RNA-viruses are hard to combat, because they mutate fast
  • Disease
  • Cause major losses
18 plant viruses l
The Current Classification of Plant Virus Genera*18 Plant viruses l
  • Plant viruses are classified
  • according to ICTV
  • guidelines:
  • Type of nucleid acid
  • Form
  • Host


18 plant viruses ll
18 Plant viruses ll
  • Plant viral diseases are spread by two major routes:
  • Horizontal transmission: infection by an external source
    • Plants are more susceptible if their protective epidermis is damaged, perhaps by wind, chilling, injury, or insects.
    • Insects are often carriers of viruses, transmitting disease from plant to plant
      • non-persistent: insect is infectious for short time
      • persistent: insect remains infectious for long period or forever (propagates in insect)
  • Vertical transmission: a plant inherits a viral infection from a parent
    • This may occur by asexual propagation or in sexual reproduction via infected seeds
18 plant viruses lll
18 Plant viruses lll
  • Once it starts reproducing inside a plant cell, virus particles can spread throughout the plant by passing through plasmodesmata
  • Agricultural scientists have focused their efforts largely on reducing the incidence and transmission of viral disease and in breeding resistant plant varieties
18 where did viruses come from
18 Where did viruses come from?
  • Candidates for the original sources of viral genomes include plasmids and transposons
    • Plasmids are small, circular DNA molecules that are separate from chromosomes
    • Plasmids, found in bacteria and in the eukaryote yeast, can replicate independently of the rest of the cell and are occasionally be transferred between cells
    • Transposons are DNA segments that can move from one location to another within a cell’s genome
  • Both plasmids and transposons are mobile genetic elements
18 viroids
Potato spindle tuber viroid18 Viroids
  • Viroids, smaller and simpler than even viruses, consist of tiny molecules of naked circular RNA that infect plants
  • Their several hundred nucleotides do not encode for proteins but can be replicated by the host’s cellular enzymes
  • These RNA molecules can disrupt plant metabolism and stunt plant growth, perhaps by causing errors in the regulatory systems that control plant growth
18 prions
normal disease-causing 18 Prions
  • Prions are infectious proteins that spread a disease
    • They appear to cause several degenerative brain diseases including scrapie in sheep, “mad cow disease”, and Creutzfeldt-Jacob disease in humans
  • According to the leading hypothesis, a prion is a misfolded form of a normal brain protein
  • It can then convert a normal protein into the prion version, creating a chain reaction that increases their numbers
18 bacteria l
18 Bacteria l
  • The short generation span of bacteria helps them adapt to changing environments
  • Genetic recombination produces new bacterial strains
  • The control of gene expression enables individual bacteria to adjust their metabolism to environmental change
  • Bacteria are haploid: mutations are immediately expressed
18 bacteria ll
18 Bacteria ll
  • The major component of the bacterial genome is one double-stranded, circular DNA molecule
    • For E.coli, the chromosomal DNA consists of about 4.6 million nucleotide pairs with about 4,300 genes.
    • This is 100 times more DNA than in a typical virus and 1,000 times less than in a typical eukaryote cell
    • Tight coiling of the DNA results in a dense region of DNA, called the nucleoid, not bounded by a membrane
  • In addition, many bacteria have plasmids, much smaller circles of DNA
    • Each plasmid has only a small number of genes, from just a few to several dozen
18 replication
18 Replication
  • Bacterial cells divide by binary fission
  • This is preceded by replication of the bacterial chromosome from a single origin of replication: rolling circle model
18 replication1
18 Replication
  • Bacteria proliferate very rapidly in a favorable natural or laboratory environment
    • E.coli can divide every 20 minutes, producing a colony of 107 to 108 bacteria in as little as 12 hours
    • In the human colon, E. coli reproduces rapidly enough to replace the 2 x 1010 bacteria lost each day in feces
  • Through binary fission, most of the bacteria in a colony are genetically identical to the parent cell
    • However, the spontaneous mutation rate of E. coli is 1 x 10-7 mutations per gene per cell division
    • This will produce about 2,000 bacteria in the human colon that have a mutation in that gene per day
    • New mutations, though individually rare, can have a significant impact on genetic diversity
    • Individual bacteria that are genetically well equipped for the local environment outgrow less fit individuals
18 recombination
18 Recombination
  • Recombination is the combining of DNA from two individuals into a single genome  generates diversity
  • Recombination occurs through three processes:
    • Transformation
    • Transduction
    • Conjugation
18 transformation l
Discovered by Griffith in 1928. 18 Transformation l
  • Transformation is the alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment
18 transformation ll
18 Transformation ll
  • Many bacterial species have surface proteins that are specialized for the uptake of naked DNA
    • These proteins recognize and transport only DNA from closely related bacterial species
    • While E.coli lacks this specialized mechanism, it can be induced to take up small pieces of DNA if cultured in a medium with a relatively high concentration of calcium ions
    • In biotechnology, this technique has been used to introduce foreign DNA into E. coli
18 generalised transduction
18 Generalised transduction
  • Transduction occurs when a phage carries bacterial genes from one host cell to another
  • In generalised transduction, a small piece of the host cell’s degraded DNA is packaged within a capsid, rather than the phage genome
    • These proteins recognize and transport only DNA from closely related bacterial species
    • When this pages attaches to another bacterium, it will inject this foreign DNA into its new host
    • Some of this DNA can subsequently replace the homologous region of the second cell
    • This type of transduction transfers bacterial genes at random
18 specialised transduction
18 Specialised transduction
  • Specialized transduction occurs via a temperate phage
    • When the prophage viral genome is excised from the chromosome, it sometimes takes with it a small region of adjacent bacterial DNA
    • These bacterial genes are injected along with the phage’s genome into the next host cell
    • Specialized transduction only transfers those genes near the prophage site on the bacterial chromosome
  • Both generalized and specialized transduction use phage as a vector to transfer genes between bacteria
18 conjugation
18 Conjugation
  • Conjugation transfers genetic material between two bacterial cells that are temporarily joined
  • One cell (“male”) donates DNA and its “mate” (“female”) receives the genes
  • A sex pilus from the male initially joins the two cells and creates a cytoplasmic bridge between cells
  • “Maleness,” the ability to form a sex pilus and donate DNA, results from an F factor as a section of the bacterial chromosome or as a plasmid
18 plasmids
18 Plasmids
  • Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules
  • Episomes, like the F plasmid, can undergo reversible incorporation into the cell’s chromosome
    • Temperate viruses also qualify as episomes.
  • Plasmids generally benefit the bacterial cell
  • They usually have only a few genes that are not required for normal survival and reproduction
    • Plasmid genes are advantageous in stressful conditions
    • The F plasmid facilitates genetic recombination when environmental conditions no longer favor existing strains
    • Many plasmids carry antibiotic resistance
18 conjugation ll
18 Conjugation ll
  • The F factor or its F plasmid consists of about 25 genes, most required for the production of sex pili
    • Cells with either the F factor or the F plasmid are called F+ and they pass this condition to their offspring
    • Cells lacking either form of the F factor, are called F-, and they function as DNA recipients
  • When an F+ and F- cell meet, the F+ cell passes a copy of the F plasmid to the F- cell, converting it into an F+ cell
18 conjugation lll
18 Conjugation lll
  • The plasmid form of the F factor can become integrated into the bacterial chromosome
  • The resulting Hfr cell (high frequency of recombination) functions as a male during conjugation
18 conjugation l v
18 Conjugation lV
  • The Hfr cell initiates DNA replication at a point on the F factor DNA and begins to transfer the DNA copy from that point to its F- partner
  • In the partially diploid cell, the newly acquired DNA aligns with the homologous region of the F- chromosome
  • Recombination exchanges segments of DNA
18 r plasmids
18 R Plasmids
  • In the 1950s, Japanese physicians began to notice that some bacterial strains had evolved antibiotic resistance
    • The genes conferring resistance are carried by plasmids, specifically the R plasmid (R for resistance)
    • Some of these genes code for enzymes that specifically destroy certain antibiotics, like tetracycline or ampicillin
  • When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population
  • Because R plasmids also have genes that encode for sex pili, they can be transferred from one cell to another by conjugation
18 key terms
18 Key terms
  • capsid provirus
  • transformation temperate virus
  • prophage prion
  • viral envelope retrovirus
  • transduction plasmodesma(ta)
  • bacteriophage (phage) reverse transcriptase
  • conjugation nucleoid
  • host range HIV
  • F factor virion
  • lytic cycle AIDS
  • episome R plasmid
  • virulent virus
  • vaccine F plasmid
  • lysogenic cycle
  • Water potential=R.T.i.M.10–3 Mpa
  • total= pressure + solutes
koch s postulates
Koch’s postulates
  • 1. The microorganism should be shown to be present in all cases of animals suffering from a specific disease but shold not be found in healthy animals
  • 2. The specific microorganism should be isolated from the diseased animal and grown in pure culture on artificial laboratory media
  • 3. This freshly isolated microorganism, when inoculated into a healthy laboratory animal, should cause the same disease seen in the original animal
  • 4. The microorganism should be reisolated in pure culture from the experimental infection