LO10

• Chromosome

  Contains thousands of genes
• Gene


  A segment of DNA that codes for a protein (protein-coding gene) or an RNA molecule (non-coding gene)
• Non-coding genes


  • Often regulate the expression of protein-coding genes
• Eukaryotes


  • Have multiple, linear chromosomes
  • Usually have much more DNA in their genome than prokaryotes
• Eukaryotes


  • Humans have 46 chromosomes in all cells except gametes, and in total those chromosomes contain ~6 billion base pairs of DNA
• Prokaryotes


  • Have a single, circular chromosome that is supercoiled in the nucleoid region
• Chromatin


  DNA double helix wound around histone proteins
• Nucleosome


  • Includes 146 bp of DNA wrapped around 8 histones
  • Nucleosomes are packed together to form compacted chromatin
• Chromosomes


  • Are further folded and compacted from chromatin
  • Additional compaction occurs prior to cell division
• Cell cycle


  1. Interphase

  2. M phase (mitosis + cytokinesis)
• Interphase


  • The longest part of the cell cycle, when no division is occurring
  • Cell is growing and carrying out normal activities, and preparing to divide
• M phase


  1. Mitosis (5 stages, results in two genetically identical daughter nuclei)

  2. Cytokinesis (division of the cytoplasm, results in two identical daughter cells)
• Interphase


  • G1 (cell grows and prepares for S phase, some cells enter G0 permanently)
  • S (DNA replication, histone synthesis, centriole duplication)
  • G2 (cell makes final preparations for M phase)
• Prophase


  1. Chromatin condenses to form mitotic chromosomes

  2. Each chromosome consists of two identical sister chromatids joined at the centromere

  3. Mitotic spindle begins to form

  4. Nuclear envelope begins to fragment, cytoskeleton disassembled
• Prometaphase


  1. Spindle microtubules attach to sister chromatids and move them towards the equator

  2. Sister chromatids become attached to spindle microtubules from opposite poles
• Metaphase


  Chromosomes are aligned along the metaphase plate in the middle of the cell
• Anaphase


  1. Sister chromatids start to separate

  2. Sister chromatids are pulled to opposite poles of the cell by shortening microtubules as the cohesin proteins that holding the chromatids together dissociate
• Telophase


  1. Nuclear envelope re-forms around each new set of chromosomes

  2. Chromosomes (which are now unduplicated) decondense

  3. Spindle microtubules are disassembled
• Cytokinesis


  1. Animal and fungal cells form a contractile ring of microfilaments and myosin to physically separate the daughter cells

  2. Plant cells form a cell plate along their equator from membranous vesicles that fuse to form plasma membrane between the daughter cells

  3. Cells have multiple copies of most organelles that are divided among daughter cells
• Binary fission


  A method of asexual reproduction that produces daughter cells genetically identical to the parent cell
• Binary fission


  1. Cell's circular chromosome replicates

  2. Duplicated chromosomes move to opposite ends of the cell

  3. Cell divides as the plasma membrane grows inward

  4. New cell wall forms between the daughter cells
• Cell-cycle checkpoints


  • Ensure that the events of a particular stage are properly completed before the next stage begins
  • G1-S checkpoint ensures cell has needed nutrients and enzymes to synthesize DNA
  • G2-M checkpoint ensures DNA replication is finished before cell begins mitosis
  • Metaphase-anaphase checkpoint ensures kinetochores are properly attached to spindle fibers along the metaphase plate
• In trisomy-21, individuals have 3 copies of chromosome 21 because chromosomes were not properly separated (during anaphase of meiosis)
• Cyclin-dependent kinases (Cdk's)


  Kinases that bind with cyclin proteins to form cyclin-Cdk complexes that regulate the cell cycle by phosphorylating needed enzymes
• Cyclin-Cdk complex


  1. After a cell passes the G1-S checkpoint, a Cdk will bind with the G1-S cyclin protein to activate (by phosphorylation) enzymes needed for DNA replication during S phase

  2. After the cyclin-Cdk complex has done its job of phosphorylation, the cyclin portion is degraded

  3. The kinase portion is an enzyme that is not degraded, but is recycled as the cell goes through another cycle of division
• Somatic cells


  Non-reproductive cells
• Somatic cells


  • Diploid (2n)
  • Contain two copies of each chromosome
• Homologous pairs


  One maternal homolog (inherited from the mother) and one paternal homolog (inherited from the father)
• Homologs contain the same genes in the same order, but they may have different variants of those genes
• Reproductive cells (gametes)


  • Typically haploid (n)
  • Contain only one copy of each chromosome
• Sexual reproduction


  1. Reduction division in the reproductive cells prior to fertilization

  2. Fertilization is the union of two haploid gametes to form a diploid zygote
• Meiosis


  1. Diploid cell undergoes two divisions (meiosis I and II) which halve its chromosome number

  2. One DNA replication event

  3. Four haploid cells are produced at the end
• Meiotic cell cycle


  Interphase → meiosis I → cytokinesis → interkinesis → meiosis II → cytokinesis
• Interkinesis


  Abbreviated interphase without an S phase
• Interphase of meiosis


  1. G 1 → S → G 2

  2. Chromosomes are duplicated during S phase, producing sister chromatids
• Prophase I


  1. Homologous chromosomes undergo synapsis

  2. Homologs join to form tetrads of four chromatids

  3. Homologs are held together by the synaptonemal complex, which facilitates crossing-over

  4. Crossing-over allows for genetic recombination

  5. Synaptonemal complex is disassembled

  6. Homologs are only held together at chiasmata where crossing-over occurred

  7. Chromatin condenses

  8. Meiotic spindle forms and centrioles move to opposite poles

  9. Nucleus fragments
• Metaphase I


  1. Tetrads are lined up along the equator

  2. Tetrads are held together only at chiasmata

  3. Sister chromatid kinetochores are attached to microtubules from the same pole, but homologs are attached to microtubules from opposite poles
• Anaphase I


  1. Homologs are pulled to opposite poles

  2. Sister chromatids remain attached at their centromeres

  3. Each pole receives a random combination of maternal and paternal homologs: independent assortment
• Telophase I


  1. Nuclei reform and chromosomes decondense

  2. Each nucleus is haploid because they only have one copy of each chromosome due to homolog separation during anaphase

  3. But each chromosome is still duplicated (i.e. with sister chromatids)
• Cytokinesis


  1. Produces two genetically different haploid daughter cells

  2. Genetically different due to crossing-over and independent assortment