Cell Division Lecture 10: Summary Sheet Guide

• A 107 test and an A4 summary page is a resource for students
• A link is provided which re-directs to the the university course page

"Restricted lite" A4 Summary Page

• Notes can be prepared using a preferred method
• A personal A4 summary page should be developed

Summary Sheet

• Students can use one A4 paper
• Both sides of the paper can be used
• Only handwritten or typed (10-14pt)/printed notes are allowed
• The work needs to be your own and will be checked
• It can include printed mind maps/diagrams/lecture slide images
• Handwritten notes are strongly recommended

Student Thoughts on Summary Sheets

• Preparing notes is beneficial allowing content to be ingrained
• Summary sheets are helpful since it can contain all the information in a summarised and condensed form
• Summary sheets can be used to reduce anxiety, in test situations
• Most students do not have time to use it more than a few times in the test

Lecture 10: Cell Division

• Phases and purpose of the eukaryotic cell cycle is described
• Phases and purpose of mitosis is described
• Phases and purpose of meiosis is described
• Differences between mitosis and meiosis is described

Cell Diversity and Destiny

• Cells may have different destinies
• A cell has three possible destinies: Live and function without dividing, grow and divide, and die
• Various signals signal a cell which path to take
• Homeostasis is maintained when there is balance between cell proliferation and cell death

The Eukaryotic Cell Cycle

• The cell theory states that all organisms are made of cells, and all cells come from pre-existing cells

Somatic Cell Division

• Somatic cell division involves mitosis
• Mitosis is diploid (2n) to diploid (2n)

Reproductive Cell Division

• Reproductive cell division involves meiosis
• Meiosis is diploid (2n) to haploid (1n)
• Somatic cells divide for growth and development, and for tissue renewal
• The process results in two daughter cells that are genetically and functionally identical to the parent cell
• Some somatic cells divide
• Somatic cells are mainly involved in their functions during interphase

Interphase

• Most cellular activities occur in the G1: Growth or Gap Phase 1
• Duration is variable, depending on cell type

Synthesis of DNA

• DNA replication occurs
• Strands separate at the hydrogen bonds holding the nucleotides together
• A new strand of DNA is synthesized opposite each of the old strands

Growth or Gap Phase 2

• Checks for correct DNA synthesis
• Prepares for the Mitotic Phase
• Synthesis of the proteins and enzymes required involves gathering of reactants
• Replication of centrosomes is completed

The Eukaryotic Cell Cycle–The Mitotic Phase

• The Mitotic phase is mitosis plus cytokinesis
• The phases of the cell cycle includes: Prophase, Metaphase, Anaphase, and Telophase and cytokinesis
• Some textbooks use term prometaphase

Sister Chromatid

• Human diploid cells have 46 chromosomes, with 23 from each parent
• Replicated DNA during interphase forms two identical sister chromatids per chromosome
• During prophase, DNA condenses
• During metaphase, chromosomes "line up”
• During anaphase, sister chromatids separate before the nuclear envelope reforms in telophase
• Daughter cells are "identical" to the parent cell

Completion of the Cell Cycle

• The cycle completes with somatic cell division via mitosis (2n>>>2n) resulting in two genetically identical daughter cells

Mitotic Cell Cycle Checkpoints

• G1 checkpoint
• M checkpoint
• G2 Checkpoint

G1 Checkpoints

• Multiple signals are required to pass G1 checkpoints
• Signals include: DNA is undamaged, cell size and nutrition are adequate, and appropriate signals are present
• If signals are not correct, the cell may exit to G0

M Checkpoints

• Multiple signals are required to pass M checkpoints
• A checkpoint is whether all chromosomes are attached to spindles, before proceedin

Reproductive Cell Division: Meiosis

• Occurs in the gonads (ovaries and testes)
• Gametes produced are haploid which contain a single set of 23 chromosomes
• Fertilisation then restores the diploid number of chromosomes (2n)
• Produces cells that are genetically different from the parent cell

Meiosis

• There are two stages of Meiosis: Meiosis I, and Meiosis II

Meiosis I

• Stages in meiosis I:
  • Prophase I (synapsis and crossing over, tetrads form)
  • Metaphase I (pairs of homologous chromosomes)
  • Anaphase I (sister chromatids remain attached)
  • Telophase I

Meiosis II

• Stages in meiosis II:
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II

Meiosis I - Separates Homologous Chromosomes

• Synapsis involves two sister chromatids of each pair of homologous chromosomes pairing up
• The 4 chromatids create tetrad
• Non-sister chromatids cross over, this is called recombination

Meiosis II - Separates Sister Chromatids

• Very similar to mitosis
• Is not preceded by DNA replication

Sources of Variation

• Sources of genetic variation:
  • Independent assortment at metaphase I (223 >8 million possible combinations)
  • Crossing over at prophase 1 (~1-3 crossover events per pair)
  • Fusion between two gametes (> 223 times 223 combinations!!!)