Molecular and Cellular Biology

• Mendel based his laws on studies of garden peas. He was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible alternative traits
• Traits of Pea Plants

  • Plant Height: Tall, Short
  • Pod colour
  • Seed Shape
  • Pod Shape
  • Seed Colour: Green, Yellow
  • Round, Wrinkled
  • Smooth, Pinched
• Chromosomes appear to carry heredity
• Structure of DNA provides a mechanism for heredity
• Chargaff: '“ … a double-stranded DNA molecule globally has percentage base pair equality: %A = %T and %G = %C. …”'
• DNA is organised into genes
• The full set of genes making up an organism is its GENOME
• The genome – a functional definition: … is a biological information storage and retrieval system that defines the biological properties of a cell
• Human genome: Haploid amount (n) = 3 billion bp = 3,000 million bp = 3,000 Mb = 3 million kb = 3 × 10^9 bp (nt)
• DNA in Prokaryotes vs Eukaryotes

  Prokaryotes: DNA normally in one circular chromosome located in the nucleoid region. Additional, small pieces of DNA can be present (plasmids). Eukaryotes: DNA normally in special compartments: Most DNA is in the nucleus, organised into linear chromosomes. Some DNA also in mitochondria, organised into circular chromosomes. Plasmids can be maintained in many eukaryotes
• Human chromosome 1 = 247,199,719 base pairs of DNA
• Chromatin is the complex of DNA and proteins
• Eukaryotic DNA associates with histone via ionic interactions. Histones are complexes of smaller protein subunits. A complete histone complex consists of two copies each of four different subunits, i.e., it has eight subunits in total. This structure is termed a NUCLEOSOME
• Wrapping DNA around nucleosome shortens its length, but not enough
• Heterochromatinisation: DNA wraps around histone to form nucleosomes
• A complete histone complex consists of two copies each of four different subunits, totaling eight subunits in total
• Nucleosome


  The structure formed by histones and DNA
• Histones

  Positively charged proteins
• Linker histone (H1)

  A type of histone
• Wrapping DNA around a nucleosome shortens its length, but not enough
• Heterochromatinisation

  1. DNA wraps around histone to form nucleosomes

  2. Nucleosomes assemble into the chromatin fibre (also called solenoid structure)

  3. Chromatin fibres form bundles

  4. Loops are coiled/bundled around 'scaffold' proteins
• Regulation of chromatin structure

  Modifications like acetylation, methylation, phosphorylation influence binding of proteins that regulate more open or compacted chromatin, making DNA information accessible or repressed
• Some elements of the 'histone code' appear universal in eukaryotes from yeast to humans
• Chromatin formats in the nuclei vary
• Euchromatin contains genes that need to be actively expressed and is packaged less densely. Heterochromatin does not need to be actively expressed and is packaged densely. Heterochromatin can further be divided into constitutive and facultative heterochromatin
• The human genome consists of 22 pairs of 'autosomal' chromosomes, plus XX (females) or XY (males) 'sex' chromosomes. 'Karyotyping' is the identification and cataloguing of the chromosomes of an individual
• More than 50% of the human genome is repetitive, including exons and regulatory sequences
• Transposons (jumping genes) play a role in genome evolution
• Viruses can also integrate into host DNA
• The complete set of information in an organism's DNA is called its Genome
• What type of macromolecule helps package DNA in eukaryotic chromosomes?

  Proteins
• The N-terminal tail of histone H3 can be extensively modified, and depending on the number, location, and combination of these modifications, these changes may promote the formation of heterochromatin. What is the result of heterochromatin formation?

  Gene silencing
• Which of the following statements about the genetic code is correct?

  The genetic code is redundant.
• The human genome is divided into linear segments and packaged into structures called chromosomes. What is the total number of chromosomes found in each of the somatic cells in your body?

  46
• Which of the following statements is true regarding the structure of DNA?

  DNA is a double helix composed of two antiparallel strands.
• What is the role of the promoter region in a gene?

  Regulates gene expression by binding transcription factors
• In which part of a eukaryotic cell would you find most of the DNA?

  Nucleus
• Which of the following is not a component of a nucleotide in DNA?


  Ribose sugar
• How many chromosomes do humans typically have in their somatic cells?

  23 pairs (46 individual chromosomes)
• How many replication forks are formed when an origin of replication is opened?

  2