2.2 All cells arise from other cells

Name the three phases of the cell cycle in eukaryotic cells

1. interphase

2. mitosis

3. cytokinesis

Describe what happens in interphase

• (S phase) DNA replicates semi-conservatively

  • Leading to 2 chromatids (identical copies) joined at a centromere

• (G1/G2) number of organelles & volume of cytoplasm increases, protein synthesis

Describe what happens in mitosis

• Nucleus divides

• to produce 2 nuclei with identical copies of DNA produced by a parent cell

Describe what happens in cytokinesis

• cytoplasm and cell membrane divide

• to form 2 new genetically identical daughter cells

Describe the behaviour of chromosomes & role of spindle fibres in prophase

• chromosomes condense, becoming shorter / thicker

  • appears as 2 sister chromatids joined by a centromere

• nuclear envelope breaks down

• centrioles move to opposite poles forming spindle network

Describe the behaviour of chromosomes & role of spindle fibres in metaphase

• spindle fibres attach to chromosomes by their centromeres

• chromosomes align along equator

Describe the behaviour of chromosomes & role of spindle fibres in anaphase

• spindle fibres shorten / contract

• centromere divides

• pulling chromatids from each pair to opposite poles of the cell

Describe the behaviour of chromosomes & role of spindle fibres in telophase

• chromosomes uncoil, becoming longer / thinner

• nuclear envelopes reform = 2 nuclei

• spindle fibres / centrioles break down

Why do some eukaryotic cells not undergo the cell cycle?

• Within multicellular organisms, not all cells retain the ability to divide (eg. neurons)

• Only cells that do retain this ability go through a cell cycle

Explain the importance of mitosis in the life of an organism

• Growth of multicellular organisms by increasing cell number

• Replacing cells to repair damaged tissues

• Asexual reproduction

Describe how tumours and cancers form

• Mutations in DNA / genes controlling mitosis can lead to uncontrolled cell division

• Tumour formed if this results in mass of abnormal cells

  • malignant tumour = cancerous, can spread (metastasis)

  • benign tumour = non-cancerous

Suggest how cancer treatments control the rate of cell division

some disrupt spindle fibre activity / formation

• so chromasomes can’t attach to spindle by their centromere

• so chromatids can’t be separated to opposite poles (no anaphase)

• so prevent / slows mitosis

some prevent DNA replication during interphase

• so can’t make 2 copies of each chromosome (chromatids)

• so prevents / slows mitosis

Describe how prokaryotic cells replicate

Binary fission:

1. replication of circular DNA

2. replication of plasmids

3. division of cytoplasm to produce 2 daughter cells

• single copy of circular DNA

• variable number of copies of plasmids

Describe how viruses replicate

1. attachment proteins attach to complementary receptors on host cell

2. inject nucleic acid (RNA/DNA) into host cell

3. infected host cell replicates virus particles

   1. nucleic acid replicated

   2. cell produces viral protein / capsid / enzymes

   3. virus assembled then released