CELLULAR BIOLOGY

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CELLULAR BIOLOGY MENDELIAN GENETICS
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Cellular Biology
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THE CELL

• Smallest functional unit of living organisms
• Each cell maintains its own homeostasis
• Each cell contributes to the homeostasis of the
  tissue, organ and/or organism

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THE CELL

• All living organisms consist of cells
• Humans have trillions of cells (Yeast - one
  cell)
• Cells are of many different types (blood, skin,
  nerve), but all arose from a single cell (the
  fertilized egg)
• Each cell contains a complete copy of the genome
  (the program for making the organism), encoded in
  DNA

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SOME DEFINITIONS

• Chromosomes made up of a protein core and
  strands of DNA in the nucleus of a cell (46
  chromosomes make up 1 human cell)
• DNA (deoxyribonucleic acid) Molecule that
  carries the genetic code, ladder with rungs made
  of base pairs
• (letters A,C, T, G)
• Codon Sequence of 3 bases. Each 3-letter word
  stands for an amino acid. (word GCA
  Alanine)
• Genes Portions of the DNA molecule that code
  for specific proteins (sentence)
• Protein chain of amino acids (see p. 54 for
  protein synthesis)
• Replication Copying of genetic code during cell
  division

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The Question of Inheritance

• Darwin was very concerned with the question
• How are traits inherited?
• Variation within populations was the raw material
  that powered the evolutionary process
• Could not explain, however, the source of this
  variation or how it was distributed across
  generations
• Towards the end of his life, Darwin admitted that
  the greatest challenge to his theory was its
  inability to explain the mechanism of inheritance

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Theories of Inheritance During Darwins Time

• Blending Inheritance
• Belief of how inheritance operated offspring
  received a combination of all characteristics of
  each parent through the mixture of their bloods
• If true, unique traits would be diluted, and
• Individuals would become more uniform

If not, how are individual adaptive traits
maintained and selected for within
populations? Czech monk, Gregor Mendel, working
in obscurity, came up with the answer
particulate inheritance
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Gregor Johann Mendel (1822 - 1884)

• 1865 Mendel published the results of his
  experiments with garden peas in which he crossed
  parents and examined offspring, became the
  father of genetics.
• Concluded that each parent contributes
  particles or genetic units to their offspring
• The particulate nature of inheritance and the
  laws of inheritance final pieces in the puzzle
  that fulfilled Darwins vision of evolution by
  natural selection
• Mendels contribution DID NOT become known until
  early in the 20th century

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Some Definitions

• Allele Variants of a gene. Every gene has 2
  alleles with different instructions for
  developing a certain phenotype
• Monogenic Trait coded for by a single gene (ex.
  Taster trait for PTC)
• Polygenic Trait coded for by more than one gene
  (ex. Skin color)

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Characteristics of Good Model Organisms
Mendels work with garden peas was successful
because they had

• a well-known genetic history
• short generation time
• large number of progeny
• small size and easy to handle
• wide availability of mutants - genetic variation
  within garden peas

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Mendel had true-breeding varieties of peas for 7
different traits including
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One of Mendels Monohybrid Crosses
Gametes Cells of sexual reproduction (sperm
egg), contain only ½ the chromosomes of normal
cell Genotype alleles of an organism 1/4 SS
1/2 Ss 1/4 ss 1 SS 2 Ss 1 ss Phenotype
physical expression of genetic code 3/4 Smooth
seeds 1/4 wrinkled seeds
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Mendels Model

• Genes do not blend together
• Pea genes have two versions or alleles
• Each gamete gets one allele
• Males females contribute equally
• Some alleles are dominant, while others are
  recessive

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What Accounts for Species Variation?

• Independent Assortment Alleles segregate into
  gametes independently (Mendels Law)
• Crossing Over Sections of chromosomes switch
  between chromosome pairs during meiosis
• Recombination Exchange of genetic material
  between pairs of chromosomes during meiosis