AP Biology: Cell Division

Interphase

• 90% of time

• Some cells then go to G0 (ex, nervous/brain cells that don’t divide)

  • Most other cells go through cell division

• G1: normal cell activities, growth of the cell

• S: DNA replication

  • sister chromatids

• G2: prepares for mitosis, copies organelles

Cell divison in Eukaryotic cells: Mitosis

• for repair and replacement —> division of nucleus and chromosomes

• Prophase: chromatin condenses

  • centrioles move to poles

  • spindle forms

• Prometaphase

  • nuclear envelope breaks down

  • nucleous dissapears

  • spindle starts attaching to chromosomes

    • at the centromere using kinetochromes

• Metaphase

  • chomosomes line up in the middle (metaphase plate)

• Anaphase

  • chromosomes pulled apart at centromeres

  • or driven up using motor proteins

• Telophase

  • nucleolus + nucleus returns

  • spindle breaks down

  • one centriole per cell

  • chromosomes decondense

Cytokinesis in Mitosis

• begins during anaphase

• division of the cytoplasm

• two separate identical cells

• animals —> cleavage furrow

• Plants —> cell wall —> cell plate

Meiosis

• no prometaphase

• starts as a diploid cell —> one set from each parent

• Prophase 1: chromosomes cross over

  • homologous chromosomes

  • must be touching (synapses —> tetrads) and exchange genes

  • Chiasmata are where crossing over occurs

  • recombinant chromosomes

• Metaphase/anaphase: independent assortment

  • random lining up/division of homologous pairs

  • 2^n (haploid number)

  • genes further apart sort independently —> closer together sort together

• Anaphase: Homologous chromosomes are separated not sister chromatids

• Telophase —> haploid

• cytokinesis

• no interphase

• repeat process

• creates 4 genetically different haploid cells

Cytokinesis: meiosis

• every male creates 4 sperm each round of meiosis

  • dont havent many other components than DNA

• Females —> 1 egg and 3 polar bodies

  • polar bodies are destroyed

  • all nutrients + organelles go to one egg

Ways to create genetic diversity

• crossing over

• independent assortment

• random fertilization —> sperm + egg combinations

Binary Fission

• Prokaryotes (bacteria and archaea) reproduce by binary fission 

• In bacteria, the single chromosome replicates, beginning at the origin of replication

• The two daughter chromosomes actively move apart while the cell elongates

  • Without using centrioles/spindles 

• The plasma membrane pinches inward, dividing the cell into two

  • Two completely identical cells 

Cell controls

• Both internal and external controls regulate the cell cycle control system 

  • External: growth factors

    • Proteins released by certain cells that stimulate other cells to divide 

      • Ex: platelet-derived growth factor → stimulates the division of the human fibroblast cells  

  • Internal: kinases and cyclins

    • sets of regulatory proteins and protein complexes

Checkpoints

• The cell has a specific checkpoint where the cell cycle stops until a go-ahead signal is received

  • Checkpoints → processes that check if things are ok 

    • G1: ensures no damage to DNA, the cell is a good size, and if the cell has nutrients/ prepped and ready to go

    • G2: DNA properly replicated 

    • M: ensures the spindle fibers are correctly connected to the centromere 

      • If not → will not divide

        • Errors could cause Down Syndrome and other disabilities

Density dependent inhibition + anchorage dependence

• density-dependent inhibition 

• Crowded cells stop dividing 

  • Most cells also exhibit anchorage dependence in which they must attach to something to divide (ex, inside of a culture flask)

Cancer cells

• Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence 

• Do not respond to signals that normally regulate the cell cycle 

• Do not need growth factors to grow and divide

  • Make their own growth factor/convey a growth factor’s signal without the presence of the growth factor 

  • They may have an abnormal cell cycle control system 

    • Any lack of function in a necessary chemical or protein

      • Ex: cyclins have issues

    • Mutations or errors in checkpoints
      

• Cells in culture that can divide indefinitely are said to have undergone a process called transformation 

• Cancer cells that are not eliminated by the immune system form tumors, masses, or abnormal cells within otherwise normal tissue 

• If abnormal cells remain only at the original site → the lump is called a benign tumor

• Malignant tumors invade surrounding tissues and undergo metastasis, exporting cancer cells to other parts of the body where they may form additional tumors

Treatments

• Radiation → pinpoint location 

  • High level of damage to normal cells → only really done twice

    • Also damages DNA

• Immmunetherapy 

  • Targets certain cancers using the patient's own immune cells

    • Using viruses 

• Chemotherapy 

  • Targets fast-dividing cells anywhere in the body

    • Causes hair loss, nausea, and a compromised immune system

      • Loose hair cells and the stomach lining, as they are fast-dividing cells