AH Biology: Unit 1

The Cell Cycle

Cell theory

• Cells are the basic unit of all organisms (Schleiden and Schwann, 1839).

• All cells are produced by the division of pre-existing cells (Virchow, 1858).

The cell cycle

• All organisms are the result of a repeated sequence of cell growth and division.

• The sequence of events a cell goes through in order to duplicate its contents and divide is known as the cell cycle.

• The products of the cell cycle are two genetically identical daughter cells.

• What are the purposes of cell division?

• Why is it important that the daughter cells produced are genetically identical?

• Are there any circumstances in which the daughter cells produced should not be genetically identical?

Cell division in a unicellular organism

• Cell division is the organism’s mode of reproduction.

• Each round of cell division produces new individual organisms, eg yeast budding, binary fission of bacterial cells.

The cell cycle of a prokaryote

Cells can cycle every 20 minutes in optimal conditions

Replication of single circular DNA molecule

Ingrowth of plasma membrane and cell wall

Cell growth

Cell division(fission)

Cell division in a multicellular organism

• A functioning fully grown organism is the result of a series of tightly controlled cell divisions.

• Cell division occurs in the adult body in order to replace cells that have died.

Lifespan of human cells • Mucus-secreting cells and absorptive cells of the small intestine: 5–6 days.

• Skin cells ~1 month.

• Olfactory neurons ~1 month.

• Red blood cells: 120 days.

• Auditory hair cells and photoreceptors: a lifetime (and not replaced).

Stopping your cells’ cycles of duplication and division would be lethal!

Why would you expect the cell cycle of a eukaryotic cell to be more complex than that of a

prokaryotic cell?

• DNA is packaged into multiple chromosomes as chromatin and stored within the nuclear envelope.

• Cell division must be preceded by the segregation of replicated chromosomes.

The cell cycle of a eukaryotic cell is more complex

• The progress of a cell through the cell cycle is regulated by a control system, which can respond to intracellular and extracellular signals.

Why do multicellular eukaryotes require the ability to control the rate of the cell cycle?

Rate of cell production > rate of cell death = net gain of cells

Cells that proliferate rapidly and uncontrollably can form tumours.

Rate of cell production < rate of cell death= net loss of cells

Neurons are generally terminally differentiated and no longer participate in the cell cycle.

The death of large populations of neurons is associated with neurodegenerative disorders.

Healthy brain

Effects of Alzheimer’s disease

Two major phases in the cell cycle of a eukaryotic cell

1.Interphase: period of cell growth and DNA replication.

2.M phase: segregation of replicated chromosomes and division of cytoplasm to form two genetically identical daughter cells.

The major phases of the eukaryotic cell cycle

Interphase

M phase

1. Interphase

The interphase is divided into three parts:

1.G1: The initial growth phase. Protein synthesis occurs and new organelles are formed.

2.Synthesis (S): Replication of nuclear DNA.

3.G2: Second phase of growth prior to mitosis. By the end of this phase the centrosome has been duplicated.

1. Interphase

• At the end of the interphase the parent cell has usually doubled in size.

• The interphase comprises approximately 95% of the length of the cell cycle.

How long would the interphase last in cultured human cells that spend 1 hour in the M phase?

The cell cycle

G1

G2

S

Interphase

M M

2. M phase

Two processes occur during the M phase:

1.Mitosis: The accurate separation and distribution (segregation) of replicated chromosomes and the formation of daughter nuclei.

2.Cytokinesis: The division of the cytoplasm into two separate daughter cells.

Mitosis

Each stage can be recognised by the appearance and location of the cell’s chromosomes and the nucleus.

Interphase chromosomes

Prophase in an animal cell

• Replicated chromosomes condense (become shorter and fatter) and appear as pairs of identical sister chromatids. They are held together at their centromeres and along their length by protein ‘glue’.

• The centrosomes move apart and microtubules radiate out from them to form spindle fibres.

• The nuclear envelope is broken down.

A prophase chromosome

Prophase in an animal cell

Metaphase in an animal cell

• Multiple spindle fibres become attached to each sister chromatid.

• The sister chromatids in each pair are ‘captured’ by spindle fibres radiating from opposite poles of the spindle.

• The replicated chromosomes are aligned at the equator of the spindle fibres that are attached to them. This position is called the metaphase plate.

A metaphase chromosome

Metaphase in an animal cell

Anaphase in an animal cell

• The proteins holding the sister chromatids together are degraded.

• The sister chromatids separate and are now known as daughter chromosomes.

• The spindle fibres attached to each of the daughter chromosomes shorten to pull them to opposite poles of the cell.

Anaphase in an animal cell

Telophase in an animal cell

• The daughter chromosomes arrive at the cell poles and decondense (unravel).

• A new nuclear envelope is assembled around each group of daughter chromosomes to form two daughter nuclei.

Telophase in an animal cell

The underlying principle of mitosis

Conservation of the cell’s chromosome complement is achieved by:

­DNA replication prior to the M phase

­the arrangement of replicated chromosomes on the spindle fibres at the metaphase plate.

Reviewing mitosis

• Mitosis animation 1

• Mitosis animation 2

• Mitosis in a cultured lung cell

Cytokinesis

• The cell cycle ends with the division of the cytoplasm into two daughter cells. The onset of cytokinesis overlaps with the final two stages of mitosis.

• Cytokinesis is accompanied by the insertion of new sections of plasma membrane to compensate for the increased surface area of the daughter cells.

• Cytokinesis is regulated to ensure that it occurs in the correct location at the correct stage of the M phase.

Cytokinesis in animal cells

Cleavage furrow

Contractile ring

Cytokinesis in animal cells

• Cytokinesis is achieved by the action of a contractile ring of structural and regulatory proteins.

• The location of the ring within the dividing cell is specified by the spindle fibres.

• Cell division is usually symmetrical.

Why would you expect cytokinesis in plant cells to be different to cytokinesis in animal cells?

Cytokinesis in plant cells

Animation of cytokinesis in animal and plant cells.

Telophase Cytokinesis G1

Cell plate

Cytokinesis in plant cells

• The cytoplasm is sectioned off by the formation of a new cell wall (the cell plate).

• The formation of the cell plate is guided by the remains of the spindle fibres.

• Division may be asymmetrical or symmetrical.

The cell cycle: summary

Remember: I Picked My Apple To Cook

G1

G2

S

Interphase

M

Cytokinesis Telophase Anaphase

Prophase Metaphase

Mitosis

Animated overviews of the cell cycle

Cells Alive animation

McGraw-Hill Higher Education animation

Virtual Cell animation

Cell cycle tutorial activity

• Onion root tip cell analysis website

What’s a cell’s favourite sport?

Cycling!