Chap 5

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CHAPTER 5

  • Life depends on the stable and compact storage of genetic information
  • Genetic information is carried by very long DNA molecules and encoded in the linear sequence of nucleotides A, T, C, G
  • Each molecule of DNA is a double helix composed of a pair of complementary strands of nucelotides, held together by hydrogen bonds between C-G and A-T base pairs.
  • A strand of DNA has a chemical polarity due to the linkage of alternating sugars and phosphates in its backbone. The two strands of the double helix run anti-parallel — that is, in opposite orientations.
  • The genetic material of a eukaryotic cell is contained in a set of chromosomes, each formed from a single, enormously long DNA molecule that contains many genes.
  • When a protein-coding gene is expressed, part of its nucleotide sequence is copied into RNA, which then directs the synthesis of a specific protein.
  • The DNA that forms each eukaryotic chromosome contains, in addition to genes, many replicated origins, one centromere, and two telomeres. These sequences ensure that the chromosome can be replicated efficiently and passed on to daughter cells.
  • Chromosomes in eukaryotic cells consist of DNA tightly bound to a mass of specialized proteins. These proteins fold the DNA into a compact form. The complex of DNA and protein in chromosomes is called chromatin.
  • The most abundant chromosomal proteins are the histones, which pack DNA into a repeating array of DNA-protein particles called nucleosomes.
  • Nucleosomes pack together, with the aid of histone H1 molecules, to form a 30-nm fiber. The fiber is generally coiled and folded, producing more compact chromatin structures.
  • Chromatin structure is dynamic: by temporarily decondensing its structure — using chromatin remodeling complexes and enzymes that covalently bond histone tails — the cell can ensure that proteins involved in gene expression, replication, and repair have rapid, localized access to the necessary DNA sequences.
  • Some forms of chromatin have a pattern of histone tail modification that causes the DNA to become so highly compacted that the packaged genes cannot be expressed to produce RNA and protein.
  • Chromatin structure can be transmitted from one cell generation to the next, producing a form of epigenetic inheritance that helps a cell to remember the state of gene expression in its parent cell.

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