Molecular Genetics

Chromosomes
1. Chromosome Structure
  1. Two strands twisted into a double helix
  2. DNA molecule wraps around core of histone proteins
  3. Twist tightly into a fiber
  4. Fiber coils then coils again to form hollow cylinder
  5. Duplicated chromosome has 'X' shape
2. Chromosome Number
  1. The total number of chromosomes
  2. Diploid: 46 chromosomes but 23 pairs
  3. Karyotype
3. Types of Chromosomes
  1. All except one pair of a diploid cell's chromosomes are autosomes
  2. Sex chromosomes are 'X' and 'Y'; differ between male (XY) and female (XX)
DNA Structure
1. Made up of 4 types of nucleotides
  1. Consists of five-carbon base, three phosphate groups, nitrogen-containing base
  2. Adenine(A), Cytosine(C), Guanine(G), and Thymine(T)
2. Erwin Chargaff
  1. 1950
  2. First rule: Equal amounts of Adenine and Thymine as are Cytosine and Guanine
  3. Second rule: different species differ in amount of Adenine and Guanine
3. James Watson & Francis Crick
  1. Suspected DNA molecule was a helix
  2. 1953: Built first accurate DNA model based on Franklin's work
  3. Article appeared first in a series of articles about DNA
4. Rosalind Franklin
  1. Specialized in X-ray crystallography
  2. First clear x-ray diffraction image of DNA
  3. Presentation in 1952
  4. Article appeared third in a series of articles about DNA
  5. Died in 1958; did not share in Nobel Prize in 1962 given to Watson, Crick, and Wilkins for the discovery of DNA structure
5. Double Helix
  1. A to T and C to G; supports Chargaff's first rule
  2. Bases inside helix
6. Base Pairings
  1. Order of pairs vary vastly among species; supports Chargaff's second rule
DNA Replication & Repair
1. Enzymes and other molecules open the double helix to expose the bases
2. Primers base-pair with the exposed single DNA strands
3. DNA polymerases assemble new strands of DNA from nucleotides, using the parent strands as templates
4. DNA ligase seals any gaps that remain between bases of the "new" DNA, so a continuous strand forms
5. Each parental DNA strand serves as a template for assembly of a new strand of DNA. Both double helixes serve as templates.
6. DNA polymerases copy 50 nucleotides per second in eukaryotes and 1000 nucleotides per second in bacteria.
7. Polymerases proofread and reverse synthesis if a mistake occurs
8. Repair enzymes and other proteins remove and replace damaged or mismatched bases in DNA before replication begins
9. Mutations occur when proofreading mechanism fails
10. Repair enzymes cannot recognize a mutation after the DNA has been replicated, because each new DNA strand base-pairs properly with its parent strand

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Molecular Genetics

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