D2.1 Cell and Nuclear Division

D2.1.1 Generation of new cells in living organisms by cell division

• New cells are only produced from pre-existing cells

D2.1.2 Cytokinesis

• Cytokinesis: The splitting of cytoplasm in the parent cell between daughter cells

• Mitosis: The splitting of nuclear content or chromosomes

• Cytokinesis between animal and plant cell is different

  • Animal cell: Cleavage furrow forms in the middle of the cell

    • Constriction belt of actin and myosin

  • Plant cell: Cell plate formation

    • Many vesicles fuse together to form a new cell wall

D2.1.3 Equal and Unequal Cytokinesis

• Equal cytokinesis: Division of cytoplasm and organelles into 2 daughter cells

  • Each daughter cell must receive at least 1 mitochondrion because mitochondria can only arise from pre-existing mitochondria

• Unequal cytokinesis: Unequal distribution of organelles/cytoplasm during reproduction

  • E.g. Yeast budding

    • Nucleus is divided - mitosis

    • Small outgrowth formed on parent cell

    • Receives nucleus but a small amount of cytoplasm

    • Cell wall forms in between daughter cell and parent cell → budding off

  • E.g. Oogenesis

    • Production of ovum

    • Meiosis 1 → 1 large cell with almost all the cytoplasm + 1 polar body

    • Only large cells proceeds to 2nd round of division → Mature oocyte

D2.1.4 Roles of mitosis and meiosis in eukaryotes

• Mitosis is nuclear division resulting in the continuity of the chromosome number and genome

• Meiosis is nuclear division resulting in reduction of chromosome number and diversity in the genome

• Parent cell divides without undergoing mitosis → anucleate cell

  • limited lifespans

  • cannot synthesize proteins

D2.1.5 DNA Replication

• DNA Replication occurs before mitosis and meiosis

  • occurs in S phase of interphase

• Replicated DNA during mitosis are in the form of sister chromatids, held together by the centromere and cohesin loops

D2.1.6 Chromosomes

• DNA is immensely long → condensed into chromosomes for efficiency and avoiding knots/tangles/breaks in DNA

• Microtubules - attach to kinetochores on the centromere and move the chromosomes to opposite poles

• Kinetochores - protein structures assembled on the centromere

  • Acts as Microtubule motors → remove subunits of tubulin from microtubules → shortening/move to opposite ends

D2.1.7 Phases of mitosis

• Prophase

  • Replicated DNA is getting condensed

  • Microtubules are assembled at opposite poles of the cell called MTOC (Microtubule organizing centers)

  • Nuclear membrane breaks down

• Metaphase

  • Chromosomes are formed and aligned at the equator of the cell

• Anaphase

  • Microtubules attach to kinetochores and split sister chromatids

  • Move to opposite poles (kinetochore cuts down microtubules)

• Telophase

  • Nuclei reform

  • DNA re-condenses

  • Cytokinesis has begun

D2.1.9 Meiosis

• Homologous chromosomes: Chromosomes with the same sequence of genes as each other

• Diploid: Organism with 2 sets of chromosomes - homologous pairs - 2n

  • humans - 46

• Haploid: Organism with 1 set of chromosomes - non-homologous - n

  • humans - 23

  • e.g. Gametes

• Meiosis 1 is a reductive division → daughter cells are haploids at the end

• Divides twice (Meiosis 1 and 2)

  • First division → 2 haploid cells

  • Second division → 4 haploid cells

D2.1.10 Nondisjunction

• Nondisjunction: Uneven splitting of homologous chromosomes or sister chromatids

  • Anaphase 1 or 2

  • Results in daughter cells with either an extra chromosome or missing one

• Down syndrome

  • Extra copy of chromosome 21

  • Heart/vision disorders, limited lifespan, etc

D2.1.11 Meiosis leads to variation

• Meiosis creates genetic diversity

  • Random orientation of bivalents

  • Crossing over

• Bivalent: A pair of homologous chromosomes

• Crossing over

  • prophase 1

  • 2 non sister chromatids brought together

  • Chiasma formed from crossing over

• Number of chromosome combinations = 2^n

D2.1.12 Cell proliferation

• Cell proliferation: Rapid increase in the number of cells - rapid cell division

  • Growth

  • Cell replacement

  • Tissue repair

• In plants - Meristem (growth region)

  • tips of stems and roots

• Skin - cell replacement and repair

  • Cell division → replace dead skin cells

  • Regenerate cells in wounds

D2.1.13 Cell cycle

• Mitosis, G1, S, G2

• G1: Active growth phase, naturally functioning

• S: DNA synthesis, copies chromosomes

• G2: Prepares for division - growth, produce organelles/proteins/membranes, DNA packed as chromatin fibers

• Cells can enter G0

  • Resting phase

  • Cell is neither dividing or preparing to divide

D2.1.14 Cell growth during interphase

• Cell must double in size

• DNA is replicated/doubled

• Increase in most organelles

D2.1.15 Control of the cell with cyclins

• Cell cycle checkpoints hold cells until it is appropriate for them to proceed to the next phase

  • G1, G2, M

  • Ensure that Cyclin is present in adequate amounts

    • Cyclins are proteins that coordinate changes throughout the cell cycle

    • CDKs (Cyclin-dependent kinases) are activated by cyclins, which bind to and activate other proteins

• G1 Checkpoint:

  • Ensures there is adequate room in the environment for more cells

  • Ensures there is enough energy and materials available

• G2 Checkpoint:

  • Ensures there is enough energy and materials available

  • Ensures DNA has been completely replicated and checked for errors

  • Regulated by Maturation-promoting-factors (MPFs)

    • Made of CDK, cyclin, and phosphate

• M checkpoint:

  • Checks that all chromosomes are attached to microtubules

D2.1.16 Consequences of mutations

• Mutations in the cell cycle (at checkpoints) can lead to uncontrolled cell division → tumor

• Mutagens increase mutations and tumor formation

  • mutagenic chemicals

  • radiation

• Proto-oncogenes affect cell proliferation

  • Can mutate into oncogenes → promote cell proliferation

• Tumor-suppressor genes prevent uncontrolled cell proliferation

• Mutations to proto-oncogenes and tumor-suppressor genes can increase tumor formation

  • Uncontrolled cell division - ignoring checkpoint checks

D2.1.17 Differences in tumors

• Benign tumors

  • Primary tumor

  • Not serious, removable

• Malignant tumors

  • Secondary tumor

  • loses attachment to nearby cells

  • Metastasis: Spread to other systems/tissues