BIOCELL GEN Lecture 4.1 genetics

• Non-disjunction


  Alteration of chromosome number

  
• Aneuploidy


  Offspring with abnormal chromosome number
• Types of aneuploidy


  • Trisomic (2n + 1)
  • Monosomic (2n - 1)
• Most types of aneuploidy are spontaneously aborted
• Some types of trisomy have a lesser impact
• Down syndrome


  One per 700 births in US, incidence correlates with age of the mother
• XYY


  No syndrome, but generally taller than other individuals
• XXX


  Indistinguishable from normal females except by karyotype, one occurrence per 1000 females
• XXY, XXXXY


  Result in Klinefelter syndrome, once per 2000 live births, male but sterile, sometimes with feminine body characteristics
• XO, Turner syndrome


  The only viable monosomy in humans, 1 per 5000 births, female but usually sterile
• Polyploidy


  More than one complete set of chromosomes

  
• Types of polyploidy


  • Triploidy (3n)
  • Tetraploidy (4n)
• Polyploidy is common in plants, known in fishes & amphibians, one mammal Tympanoctomys barrerae is tetraploid
• Tympanoctomys barrerae is not tetraploid but is so large due to repeated genome-region expansion

  
• Translocation


  An example of alteration of chromosome structure
• Deletion


  Can be 'final', or it can lead to duplications, inversions and translocation
• Deletions, duplication, and translocation are often lethal
• Reciprocal translocation and inversions, gene balance is normal, but expression may be altered due to proximity to other genes
• Cri du chat


  Specific deletion on chromosome 5, severe mental retardation and early death
• Chronic myelogenous leukemia (CML)


  Cancer of the cells which produce white blood cells, reciprocal translocation of Chr 22 & Chr 9
• Down syndrome


  A duplication of most of Chr 21 on another chromosome

  
• Across organisms, not all alterations are deleterious, as some may ultimately add to biodiversity
• Mitochondrial DNA (mtDNA)


  Small loop of double-stranded DNA, with many genes which influence metabolic function (electron transport chain and ATP synthase)

  
• Mitochondria and plastids self-replicate, and are inherited through the ovum, and generally not through the sperm, thus follow rules for maternal inheritance (non-Mendelian)

  
• Genomic imprinting


  Certain traits in mammals depend on whether they inherited the allele from the male or female parent, happens in gamete formation where one allele is silenced
• The function of genomic imprinting is not specified
• Objectives of Genetics Section


  • To understand the basic principles of Meiosis, mitosis and inheritance
  • Mendel's influence on genetic research
  • The chromosomal basis of inheritance
  • DNA structure, replication and repair
  • RNA transcription
  • The translation of proteins

  
• The lecture(s) on Chapter 16 will illustrate: The double-helix structure of DNA, Base pair function in DNA, Enzyme and protein functions in DNA replication, DNA proofreading and repair, The replication of the ends of DNA molecules

  
• Transformation


  A change in genotype and phenotype due to the assimilation of external DNA by a cell
• Avery, McCarty & MacLeod (1944) found of all cell contents, only DNA could transform bacteria
• Bacteriophages


  Viruses that infect bacteria, mostly just DNA covered in a coat of protein
• Only the 32P cells produced radioactive T2 phages in the Hershey-Chase experiment
• Prior to mitosis, DNA exactly doubles in a cell, and is divided exactly equally in the daughter cells

  
• Normal cells have exactly twice the DNA of gametes
• E. Chargaff (1948) reported that DNA base composition varies from species to species, and within one species varies little, and also found A = T, G = C, or Chargaff's Rule

  
• Rosalind Franklin and her x-ray diffraction photo


  Deflection patterns of x-rays through an aligned DNA molecule
• Watson & Crick (1953). Nature 171: 738, Watson & Crick (1954). Proc. Roy. Soc. (A) 223: 80
• The double-helix of DNA


  Ribbons of sugar-phosphate 'backbones' with base 'rungs', while Chargaff's Rule must apply, the order along the helix is free to vary

  
• The basic concept of DNA replication


  Unwinding, separation, and complementary base pairing to form two new double-stranded DNA molecules

  
• The Meselson-Stahl experiment tested three models of DNA replication and the data is only consistent with the semiconservative model