Unit 1: Cell

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Unit 1 Cell Molecular Biology

• Cell Growth
• Cell Cycle

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Learning Intentions

• Interphase G1, S and G2 phases
• Mitosis the M phase (4 stages)
• Cytokinesis
• Mitotic Index
• Control of the cell cycle.
• Abnormal cell division cancer cells.

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Four Phases of Cell Division
Cell growth and synthesis of organelles
Cell growth and synthesis of organelles
Phase G1, S and G2 are collectively known as
interphase M phase Mitosis (cell division)
DNA replication
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Cell Division And The Cell Cycle
Cytokinesis is controlled by actin fibres which
split the cytoplasm in two - stage 6
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Stages of Mitosis
Stage
Description
Prophase
No distinct chromosome. Nuclear envelope intact
Prometaphase
Chromosome become visible. Nucleus breaks down
Metaphase
Chromosomes line up across the centre of cell
(metaphase plate)
Anaphase
Chromosomes divide into chromatids which are
pulled to opposite poles by spindle fibres. These
are made of microtubules and radiate from the
centrosome.
Telophase
Daughter chromosomes ( chromatids) reach opposite
poles and begin to de-condense
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Interphase
Follows the M phase and involves cell growth
and DNA replication. Made up of G1, S and G2
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Mitosis Prophase

• The replicated chromosomes each consisting of two
  closely associated sister chromatids condense
• Outside the nucleus the mitotic spindle assembles
  between the two centrosomes which have replicated
  and moved apart.

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Mitosis Prometaphase

• The nuclear envelope breaks down
• Chromosomes attach to the spindle microtubules
  via structures called kinetochores and move
  towards the equator of the cell.

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Mitosis Metaphase

• The chromosomes are moved to the equator by the
  spindle fibres
• The kinetochores of all chromosomes align on the
  equator, midway between the poles at a structure
  known as the metaphase plate

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Mitosis - Anaphase

• The paired chromatids from each chromosome
  separate to form two sister chromatids.
• Daughter chromosomes are pulled to opposite poles
  by the simultaneous shortening and lengthening of
  microtubules

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Mitosis - Telophase

• The two sets of daughter chromosomes arrive at
  the poles
• A new nuclear envelope reassembles around each
  set forming two separate daughter nuclei and
  marking the end of Mitosis.

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• Nuclear division is controlled by microtubules
  (tubulin protein) which form the spindle fibres
  and move chromosomes to opposite poles of the
  cell during mitosis.

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Cytokinesis

• Once the cell completes M phase (mitosis) the
  division of the cytoplasm produces 2 daughter
  cells this is known as CYTOKINESIS.

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• In animal cells the cytoplasm is divided into two
  by a contractile ring of actin and myosin protein
  which pinches in the cell to create two daughter
  cells.
• In plant cells a new cell membrane and cell wall
  are built between the two daughter nuclei and
  cuts the cytoplasm in half

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Mitotic Index

• Mitosis duration and frequency varies greatly
  between cell types.
• The Mitotic index calculates the percentage of
  cells undergoing division in a given sample.
• Only cells goinf through M phase are counted (not
  Interphase)
• This can be the first indication of cancerous
  cells and a developing tumour.

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Mitotic Index Number of mitotic cells X
100 Total number of cells