Biology 265 EVOLUTION

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Biology 265EVOLUTION

• Lecture 3

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Overview

• What is variation and how is it inherited?
• Mendelian Genetics
• Chromosome Theory of Heredity
• DNA is genetic material
• Structure of DNA
• Causes of variation (sex, recombination, mutation)

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Johann Gregor Mendel(1822-1884)

• Experiments in Plant Hybridization (1865,1866)
• Had read Darwins Origin of Species (1859) with
  interest
• Saw no immediate connection
• Nor did anyone else for decades

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Who was that robed man?

• A monk
• Monastery of St. Thomas in Czech Republic
• 1884 I am convinced that my scientific work
  will be appreciated before long by the whole
  world.

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It took a while...

• 1866 Ignored
• 1900 Rediscovered by de Vries, Correns, and
  Tshermak
• 1930 Haldane, Fisher and Sewell-Wright gt
  evolutionary synthesis
• 1953 Watson and Crick gt modern genetics

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Mendel liked peas

• 1856 to 1861 he raised some 10,000 plants of
  varieties of the edible pea (Pisum sativum)
• established true-breeding lines (always same type
  of offspring)
• studied frequency of traits in hybrids

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Mendel - Experiment 1

• Mendel suspected that heredity depended on
  contributions from both parents
• and that specific characteristics from each
  parent were passed on
• rather than being blended together in the
  offspring.

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Mendels First Law

• Principle of segregation
• In the formation of gametes, the paired
  hereditary determinants separate (segregate) such
  that each gamete is equally likely to contain
  either one CHANCE

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Mendels Second Law

• The principle of independent assortment
• Segregation of the members of any pair of alleles
  is independent of the segregation of other pairs
  in the formation of reproductive cells
• (partly true)

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Dominance

• The spherical seed phenotype is observed with the
  genotypes "SS (homozygous) or "Ss"
  (heterozygous)
• the character "spherical seed" is dominant and
  the character "wrinkled seed recessive.

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Founder of genetics?

• never seen or heard of chromosomes
• he did not conceive of pairs of elements in the
  cell representing and determining the pairs of
  contrasted characters (Olby, 1979)
• he did apply mathematics to heredity

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THEODOR BOVERI (1862-1915)

• WALTER STANBOROUGH SUTTON (1877-1916)
• Described meiosis
• Recognized link between chromosomes and Mendels
  factors

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What is a chromosome?

• a very long DNA molecule and associated proteins,
  that carry portions of the hereditary information
  of an organism

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Experimental Evolutionists

• Aimed to recreate evolution in the lab
• Controlled experiments
• Analysis of variation
• Where does variation come from and how is it
  inherited?
• Rediscovered Mendels laws that offered
  statistical method for analyzing inheritance

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THOMAS HUNT MORGAN (1866-1945)

• chromosomal theory of heredity
• genetic linkage
• crossing over
• non-disjunction

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Morgans Fly Lab (1904)

• genes in chromosomes
• every chromosome many genes
• there are chromosomes connected with sex
• genes on one chromosome are inherited together
• genes on different chromosomes are inherited
  separately

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Morgans conclusions

• Every form of a gene (allele) is located at the
  same place (locus) in each homologous chromosome.
  
• Genes on the same chromosome are "linked" and are
  inherited together
• unless crossing-over takes place
• the frequency of crossing-over between alleles of
  two different genes is proportional to physical
  distance between them

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Recombination gt Variation

• During meiosis, genetic material is exchanged
  between chromosomes by crossing over (metaphase
  I)
• Recombination is one cause of variation

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DNA is genetic material

• 1928 Frederick Griffith
• transformation of bacteria with material from
  dead bacteria
• 1952 Hershey and Chase
• 35S labels protein and was left outside bacterial
  cells with the empty viral coats
• 32P labels DNA and entered the bacteria causing
  the production of new virus generations

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But what was its structure

• And why was that such a big deal?

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The Double Helix

• On the last day of February 1953, according to
  James Watson
• Francis Crick announced to the patrons of the
  Eagle pub in Cambridge
• "We have discovered the secret of life."

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ROSALIND E. FRANKLIN (1920-1958)

• X-ray crystallography of DNA
• Died before Nobel prize awarded

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Watson, J. D. 1981The double helix a personal
account of the discovery of the structure of DNA.
London Weidenfeld and Nicolson.
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Replication

• It has not escaped our notice that the specific
  base pairing we have postulated immediately
  suggests a copying mechanism for the genetic
  material. (Watson Crick, 1953)
• Meselson and Stahl (1958) confirmed mechanism

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Variation - Mutation

• During replication mistakes are made
• Mistakes (copying errors) are corrected
• But not always
• Uncorrected errors during meiosis are passed on
  to offspring as mutations

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How does DNA control traits?
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DNA carries the heredity factors - genes

• Genes can be as short as 1000 base pairs or as
  long as several hundred thousand base pairs.
• Humans have between 100,000 and 300,000 genes
  with at estimated 3 billion base pairs

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Gene

• a region of DNA that controls a hereditary
  characteristic.
• It usually corresponds to a sequence used in the
  production of a specific protein

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But how do genes control traits?
Its amazingly simple!!
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Genetic Code

• Har Khorana
• Nirenberg Matthaei (1961)
• Using radiolabelled amino acids and synthetic
  mRNA they decoded the genetic material

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Synthetic Theory of Evolution

• Genetics supports natural selection
• Pre-Mendel, it was thought that traits blended
  and were passed on as the new blended trait
• Mendel showed that traits dont blend
• They are inherited as discrete units
• Genetic variation is not diluted, it is preserved
  for natural selection to act upon
• Variation is produced by chance