Cell division in prokaryotic and eukaryotic cells

3 cell cycle phases

• interphase

• nuclear division (mitosis)

• cell division (cytokinesis)

what do cyclins do

• The transition from one phase to another is triggered by chemical signals called cyclins

interphase

• During interphase the cell increases in size and carries out normal cellular functions

• Interphase consists of three phases:

  • G₁:

    • G stands for gap

    • Cells make enzymes and other proteins required for growth

    • At some point during G₁ a signal is received, telling the cell to divide again; at this point the cell will progress into S phase

  • S phase:

    • S stands for synthesis (of DNA)

    • The DNA in the nucleus replicates, after which each chromosome consists of two identical sister chromatids

    • S phase is relatively short

  • G₂ phase

    • The cell continues to grow and the new DNA is checked so that any errors can be repaired

    • Other preparations for cell division are made, e.g. the production of tubulin protein, which is used to make microtubules for the mitotic spindle

importance of mitosis

• Mitosis is the process of nuclear division producing two genetically identical daughter nuclei

• Each nucleus is genetically identical to the parent nucleus

• Mitosis is fundamental to many biological processes

mitosis - growth of multicellular organisms

• The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell

• This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms

• Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

mitosis - replacement and repair of cells

• Damaged tissues are repaired via mitosis, followed by cytokinesis

• Continuous cell loss (e.g. skin, gut lining) requires constant cell replacement

• Some animals show regeneration of body parts (e.g. zebrafish fins, axolotl limbs)

mitosis - asexual reproduction

• Involves one parent, producing genetically identical offspring

• In unicellular organisms (e.g. Amoeba), mitosis results in reproduction

• In multicellular organisms, offspring may detach from the parent after growth

  • e.g. runners in strawberries and budding in Hydra and yeast

stages of mitosis

• Prophase

• Metaphase

• Anaphase

• Telophase

mitosis - prophase

• Chromosomes condense (visible when stained)

• Each chromosome = 2 sister chromatids joined at a centromere

• Centrosomes move to opposite poles

• Spindle fibres (microtubules) form from centrosomes

• Nuclear envelope breaks down

mitosis - metaphase

• Centrosomes are located at opposite poles of the cell

• Spindle fibres are fully formed and are attached to the centromeres of the chromosomes

• Chromosomes align on the metaphase plate (equator of the cell)

mitosis - anaphase

• Spindle fibres shorten, pulling chromatids apart

• Centromeres divide, separating sister chromatids

• Chromatids (now chromosomes) move to opposite poles

mitosis - telophase

• Chromosomes arrive at opposite poles and begin to decondense

• Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes

• The spindle fibres break down

recognising prophase

• Chromosomes are visible

• The nuclear envelope is breaking down

recognising metaphase and anaphase

• Metaphase: Chromosomes are lined up along the middle of the cell

• Anaphase: Chromosomes are moving away from the middle of the cell, towards opposite poles

cytokinesis

• Cytokinesis occurs after telophase and so is not a part of mitosis

• It follows reformation of the nuclear envelope at each pole and involves the division of the cytoplasm

• This produces two genetically identical daughter cells

cytokinesis - differences between animal and plant cells

• Animal cells:

  • A cleavage furrow forms

  • The membrane pinches inwards to separate cells

• Plant cells:

  • A cell plate forms at the metaphase plate

  • New cell walls are built from the cell plate to separate cells

mitotic index

• The mitotic index is the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis

• The mitotic index can be calculated using the formula below:

mitotic index = number of cells with visible chromosomes ÷ total number of cells

uncontrolled cell division and cancer

• Mitosis is normally a controlled process

• Cancer results from uncontrolled mitosis, forming a tumour (an abnormal mass of cells)

• Mutations in genes controlling the cell cycle (e.g. oncogenes, tumour suppressor genes) can lead to this loss of control

  • These mutations may

    • cause continuous cell division

    • prevent programmed cell death (apoptosis)

cancer treatments

• Most cancer treatments aim to slow or stop mitosis in rapidly dividing cells

• Some examples include

  • Methotrexate: a drug which inhibits DNA nucleotide synthesis

  • Taxol: a drug which prevents disassembly of spindle fibres (freezing mitosis)

binary fission of prokaryotic cells

• Cell division in prokaryotes is called binary fission

• It is simpler than mitosis as cells have no nucleus, chromosomes, spindle fibres, or membrane-bound organelles

• Prokaryotes only need to replicate

  • a single, circular DNA molecule

  • plasmids: small, circular DNA molecules

process of binary fission

1. The circular DNA molecule is replicated

2. Plasmids replicate

3. The cytoplasm divides (roughly) equally between daughter cells

4. Each daughter cell is genetically identical and receives

   • one copy of circular DNA

   • a variable number of plasmids

division of viral particles

• Viruses are acellular infectious particles

• Being non-living, viruses do not undergo cell division

• Viruses are relatively simple in structure with:

  • A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)

  • A protein coat known as a ‘capsid’

  • Some viruses have an outer layer known as an ‘envelope’ formed usually from the membrane-phospholipids of a cell they were made in

viral replication

• Viruses are non-living and parasitic - they can only replicate inside host cells

  Replication process:

  1. Attachment proteins bind to complementary receptors on host cell surface

  2. Viral DNA or RNA is injected into the host cell

  3. Host cell uses its own enzymes and ribosomes to synthesise viral proteins and nucleic acids

  4. New viral particles are assembled

  5. Viruses are released by

     • cell lysis (bursting)

     • budding (takes host membrane – forms viral envelope)