Cell Bio Lecture 6

• Mitosis is used for asexual cell division and separates chromosomes into identical sets.
• DNA, as the "master molecule" of life, has to:

  • Reliably carry heritable information (genotype) that underpins an organism's phenotype
  • Be able to replicate itself for cell division and species reproduction
  • Translate DNA sequences into amino acid sequences (genetic code)
  • Generate and preserve genetic variation through mutation and DNA repair mechanisms
• Scientists were initially unsure whether DNA or proteins encoded genetic information
• Properties of DNA that made it ideal for storing and transmitting genetic information:

  • Ability to reliably carry heritable information
  • Capacity for self-replication
  • Mechanism for translating DNA sequences into amino acid sequences
  • Generation and preservation of genetic variation through mutation and DNA repair
• Properties of proteins that make them less than ideal for storing and transmitting genetic information:

  • Lack of self-replication ability
  • Inability to carry heritable information as reliably as DNA
  • Limited capacity to generate and preserve genetic variation
• Properties that could have led scientists to think proteins were the genetic material:

  • Initial uncertainty about whether DNA or proteins encoded genetic information
  • Lack of understanding of DNA's structure and functions
• Gregor Mendel established the concept of a gene in the mid-19th century through his study of inheritance in pea plants
• Terms introduced by Gregor Mendel:

  • Gene: a heritable characteristic
  • Alleles: variations of a gene
  • Dominant trait: trait that masks expression of a recessive allele
  • Recessive trait: trait that is "masked" by the dominant allele
• Frederick Griffith's experiment in 1928 provided clues to the molecular identity of genetic material
• Griffith's experiment showed that heat-killed infectious bacteria could transform harmless live bacteria into pathogens through a process called transformation
• Fractionation experiments were conducted to narrow down the molecules responsible for transformation
• Only the DNA fraction from fractionation experiments could transform the harmless bacteria into pathogens
• Viruses, identified in the early 20th century, were found to be very simple and composed of only protein and DNA
• Hershey and Chase's experiments with bacteriophages confirmed that DNA was the genetic material, not protein
• DNA was isolated and studied chemically, revealing it as a polymer of nucleotides, specifically deoxyribonucleotides
• Erwin Chargaff's rules established that within an organism, the amount of A = T and G = C, providing crucial insights into DNA composition
• Maurice Wilkins and Rosalind Franklin used X-ray crystallography to study the molecular structure of DNA
• Rosalind Franklin's X-ray diffraction image ("Photo 51") revealed the helical structure of DNA
• Watson and Crick used Franklin's image to deduce the geometry of DNA and proposed the double helix structure
• Watson and Crick deduced the correct base pairing in DNA: A-T pairs have 2 hydrogen bonds, G-C pairs have 3 hydrogen bonds
• DNA structure consists of two outer sugar-phosphate backbones with nitrogenous bases paired in the molecule's interior, forming an antiparallel double helix
• In DNA, one polynucleotide runs 5’ → 3’ while the other runs 3’ → 5’
• The spacing of base pairs in DNA is 0.34 nm
• DNA forms an antiparallel double helix structure
• The two DNA strands wind around each other to form a right-handed helix with 10 bases per turn
• The coiling of the DNA helix creates two grooves along the molecule:

  • Major groove: nitrogenous bases are more accessible
  • Minor groove: nitrogenous bases are more occluded by the backbone
• Isolated DNA usually conforms to the B-DNA form, which is a right-handed helix with a major groove much larger than the minor groove
• When bound to protein, DNA can conform to the A-DNA form, which is similar to B-DNA but wider and with less difference in the size of grooves
• Under exotic conditions, DNA can recoil into a left-handed Z-DNA form
• In eukaryotes, a single length of DNA found in the nucleus is called a chromosome and can be over 100 million base pairs long
• Humans have 23 pairs of chromosomes (46 total), with one set inherited from the mother and the other from the father
• A diploid cell (2n) has two sets of chromosomes, with humans having a diploid number of 46 (2n = 46)
• Karyotyping is often performed as a prenatal test after chorionic villus sampling (CVS) or amniocentesis to diagnose diseases based on abnormal number or structure of chromosomes
• The number of chromosomes is not as important as the total information stored within them, as similar species may differ in the number and structure of chromosomes but possess similar genes
• Bacteria have single, multiple, linear, or circular chromosomes, with a single circular chromosome being most common
• Bacterial DNA is negatively supercoiled and folded into loops, localized to a region of the bacterial cell called the nucleoid
• Chromatin changes in packaging during the cell cycle, with dense packing (heterochromatin) silencing genes
• Chromatin packaging occurs at different levels: 2-nm fiber, 10-nm fiber, 30-nm fiber, and 700-nm fiber
• Each nucleosome consists of 147 bp DNA wrapped around histone proteins, with 50 bp linkers between nucleosomes
• Supercoiling condenses DNA in bacteria, but this is not sufficient in eukaryotes