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Molecules of Genetic Inheritance

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Basic information of Cytogenetics

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The material in living beings that is responsible for the inheritance of characteristics is called genetic material.

The molecule of genetic inheritance must possess four major characteristics:


•Storage of information

•Expression of information

•Variation by mutation

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•Pre-Mendelian ideas on heredity

1859 Charles Darwin Published The Origin of Species.

•1865 Mendelian Inheritance

•Classical genetics 1900 Hugo de Vries, Carl Correns and Erich von Tschermak

1903 Chromosomes are discovered to be hereditary units

1905 British biologist William Bateson coins the term "genetics"

1908 Hardy-Weinberg law derived.

1910 Thomas Hunt Morgan shows that genes reside on chromosomes

1913 Alfred Sturtevant makes the first genetic map of a chromosome

1913 Gene maps show chromosomes containing linear arranged genes

1927 Physical changes in genes are called mutations

1928 Frederick Griffith transformation experiment

1931 Crossing over is the cause of recombination

1941 Edward Lawrie Tatum and George Wells Beadle show that genes code for


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Mutation and adaptation:

Jean-Baptiste Lamarck (1744-1829)

• Proposed “inheritance of acquired traits” ~1801; induction by the environment; also known as transformism or transmutation.

• Ideas largely ignored or attacked during his lifetime.

• Never won the acceptance and esteem of his colleagues and died in poverty and obscurity.

• Today Lamarck is associated with a discredited theory of heredity (but Lamarckism persisted until 1930s/1940s).

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Charles Darwin (1809-1882)

• Heritable adaptive variation results from random mutation and natural selection (1859, The Origin of Species).

• Contrary to Larmarck, inheritance of adaptive traits does not result from induction by environmental influences.

• But differential survival (selection) and heritable variation (originally arising from mutation).

• Years following Darwin and rediscovery of Mendel resulted in controversy (until 1930s/1940s) about the relative importance of mutation and selection.

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Experimental test of Lamarck’s “inheritance of acquired traits”

Salvador Luria and Max Delbrück (1943)

• An E. coli population started from one cell should show different patterns of T1 resistance depending on which theory is correct.

1. Adaptive theory states that cells are induced to become resistant when T1 is added; proportion of resistant cells should be the same for all cultures with the same genetic background.

2. Mutation theory states that random events confer resistance to T1; duplicate cultures with the same genetic background should show different numbers of T1 resistant cells.

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Fig. Fluctuating populations of E. coli infected with T1 phage.Luria and Delbrück (1943)

Adaptive theory prediction: proportions or resistant cells are the same

Mutation theory prediction: proportions are function of genotypes

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Gregor Mendel

• Formulated basic laws of heredity during mid 1800’s

• Austrian Botanist and monk

• Experimented with peas

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Mendel• Studied inheritance of seven pairs

of traits• Bred and crossbred thousands of

plants• Determined that some traits were

dominant and other recessive

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Mendel• Findings were published in


• Largely ignored for 34 years

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Johan Friedrich Miescher

• Swiss Biologist• Isolated nuclei of white blood

cells in 1869• Led to identification of nucleic

acid by Walter Flemming

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Walter Sutton• Determined in 1903 that

chromosomes carried units of heredity identified by Mendel

• Named “genes” in 1909 by Wilhelm Johannsen, Danish Botanist

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Thomas Hunt Morgan• Studied genetics of fruit flies• Early 1900’s• Experimented with eye color• His work contributed to the

knowledge of X and Y chromosomes

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Thomas Hunt Morgan• Nobel Peace Prize in 1933 for

research in gene theory

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Griffith's Transformation Experimen(1928)

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Avery, MacLeod and McCarty Experiment (1944)

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Fraenkel-Conrat and Singer (1956)

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•The DNA era 1944 Oswald Theodore Avery, Colin McLeod and Maclyn McCarty prove DNA as

the genetic material

1950 Erwin Chargaff

1950 Barbara McClintock discovers transposons in maize

1952 Hershey-Chase prove the genetic material of phages to be DNA

1953 James D. Watson and Francis Crick DNA structure is a double helix

1958 Meselson-Stahl demonstrate that DNA is semiconservatively replicated

1961The genetic code is arranged in triplets

1970 Howard Temin showed using RNA viruses that Watson's central dogma is not

always true

1970 Restriction enzymes discovered

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•The genomics era

1977 Fred Sanger, Walter Gilbert, and Allan Maxam sequenced DNA

1985 Kary Banks Mullis discovers the polymerase chain reaction

1989 Francis Collins and Lap-Chee Tsui, sequence the first human gene encoding the

CFTR protein, defects in this gene cause cystic fibrosis

1995 The genome of of a free living organism Haemophilus influenzae sequenced

1996 Saccharomyces cerevisiae is the first eukaryote genome sequence to be released

1998 The first genome sequnce for a multicellular eukaryote, C. elegans is released

2001 First draft sequences of the human genome are released simultaneously by the

Human Genome Project and Celera Genomics.

2003 (14 April) Successful completion of Human Genome Project with 99% of the genome

sequenced to a 99.99% accuracy

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Erwin Chargaff (1950)

Erwin Chargaff was a biochemist who first figured out the equation for the different bases. He concluded that:

• The amount of adenine (A) will always equal the amount of thymine (T).

• The amount of guanine (G) will always equal the amount of cytosine (C).

• The sum of the purines (A+G) equals the sum of pyrimidines (C+T).

• The ratio [C+G] / [A+T] was typically less than unity (that is, [C+G] is less abundant).

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The X-Ray photograph shows the diffraction pattern of a crystallized DNA molecule. The cross pattern in the middle is characteristic of a helical molecule with regular repeats; the broad bands at top and bottom give some indication of the periodicity.

Franklin's X-Ray Crystallography Experiments (1950s)

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James Watson and Francis Crick (1953)

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There are three main kinds of ribonucleic acid, each of which has a specific job to do.




•RNA is used as the genetic material in some viruses e.g.Human• Immunodeficiency Virus (HIV)

•dsRNA is also involved in some cellular processes, such as RNA• interference

•Some RNAs are known with catalytic activity and are called ribozymes

•Certain RNAs are associated with specific proteins to form ribonucleoproteins that participate in post-transcriptional processing of other RNAs.

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Class Nucleic acid Replication Example

I Duplex DNA Semiconservative T4,adeno and herpes viruses

II Single-stranded DNA(+)

Via duplex replicative form

ΦX, minute virus of mouse(MVM)

III Duplex RNA Via (+) strand RNA intermediate


IV + strand RNA Via(-)strand RNA intermediate

MS2,polio,foot and mouse disease virus

V (-) strand RNA

Via(+)strand RNA intermediate

Measeles, flu and rabies viruses

VI + strand RNA Via DNA duplex Leukemia virus, HIV

Baltimore(1971) classification of Viruses

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The 1970 version of the Central Dogma.

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What Drives Evolution?

There are 5 forces of change.

Only natural selection makes a population better adapted (more fit) to its environment.

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Changes in allele frequency within populations drive evolution.

Microevolution considers mechanisms that cause generation-to-generation changes in allele frequency within populations.

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Gene Flow or Migration

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Populations Are the Units of Evolution

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The Genetic Basis of Evolution

For evolution to occur, genetic differences must at least partially account for phenotypic differences.

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Mutations “Just Happen”

Mutations occur at random without regard to whether they have a beneficial, neutral or harmful effect.

For this reason, mutations are a randomly acting evolutionary force.

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Endangered Species Are in the Narrow Portion of a Genetic Bottleneck and Have Reduced Genetic Variation

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A Galapagos Finch, the Subject of a Classic Study of Evolution in Action

Peter and Mary Grant and their colleagues observed how beak depth, a significant trait for feeding success, varied in populations experiencing climactic variations.