Biology (UP) - The Cell Cycle


When do some cells divide?

All cells have the ability to divide, but some lose it

  • Intestinal epithelial lining: replaced every five days by division
  • Liver cells: Only divide to repair damage, then they stop
  • Bone marror cells: Divide repeatedly to produce red and white blood cells
  • Meristem cells: Divide for new growth
  • Cambium cells: Divide to form vascular tissue like xylem and phloem
  • More specialised cells, like neurones/nerve cells only go through the cell cycle once and never divide again. This is one reason why damage to nerves is so bad.
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What is the cell cycle?

Eurkaryotic cells have a defined cycle of growth and division (division is mitosis). It can vary in time from cell to cell, and ends with mitosis.

Each phase involves specific activities, and varies in length from one organism to another

  • Affected by availability of nutrients
  • Between each stage, it "checks" to see if it is ready to move on to the next stage
  • Enzymes "proof-read" the copied chromosomes to check for mistakes or mutations. The cell may kill itself if harmful mutations are discovered. This is called apoptosis
  • Bacterial cell cycles are completed every twenty minutes
  • Muscle cells never complete the cycle, which is known as terminal differentiation. 
  • Uncontrolled and repeated cell division by mitosis results in cancer and tumours 
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What happens in interphase?

  • Genetic material in the nucleus appears as chromatin
  • The cell grows and the organelles are replicated. Proteins are made
  • DNA is unravelled and copied. Each length of DNA forms a pair of strands.
  • ATP content increased for energy
  • These strands are identical = sister chromatids. They are joined at the centromere
  • When the DNA is copied, it is checked for errors to prevent passing on mutations
  • No apparent activity - cannot be seen under a microscope
  • Split into G1, S and G2 phase
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What is the definition of mitosis?

Process by which a nucelus divides into two, each with an identical set of chromosomes. The two nuceli formed are genetically identical. There are four phases: prophase, metaphase, anaphase and telophase, which is followed by cytokinesis, when the cell divides into two genetically identical daughter cells. 

In mitosis, a cell inherits two sets of chromosomes, meaning the parent and daughter cells are diploid. The symbol 2n represents this where n = number in set of chromosomes.

It increases cell numbers in organism. Growth and repair and asexual reproduction depend on mitosis. 

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What happens in G1, S and G2 phases during interph

  • G1 = First growth phase - the longest phase: Protein synthesis and the cell grows because the volume of the cytoplasm increases. Most organelles produced. Cell differentiation. The length depends on internal and external factors. If the cell is not going to divide again then it remains in this phase. DNA content = 23 pairs
  • S = Replication phase: DNA replicates. This must occur for mitosis to take place. This phase is only entered if cell division is about to follow. DNA content = 46 pairs. 
  • G2 = Second growth phase - short: Short gap before mitosis/cell division. Cytoskeleton of the cell begins to break down and protein microtubule components begin to reassemble into spindle fibres. This is required for cell division. DNA content = 46 pairs
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What happens in prophase?

  • The nucleolus begins fading
  • Centrioles duplicate -> move to opposite poles
  • Centrioles form spindle fibres (collectively called the spindle apparatus)
  • Chromosomes shorten and thicken -> become visible
  • Nuclear envelope starts disintergrating. Chromosomes lie free in cytoplasm.
  • Plant cells lack centrioles, but do develope a spindle apparatus, so centrioles are not as important
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What happens in metaphase?

  • Chromosomes seem to be made up of two chromatids. Each chromatid is a copy of DNA from the parent cell. They are attatched to the centromere
  • Chromosomes line up alone equator
  • One chromatid of each pair lies on either side
  • Spindle fibres attatch to centromere
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What happens in anaphase?

  • Centromere divides into two. 
  • Spindle fibres contract
  • Sister chromatids are pulled apart -> move to opposite ends of the cell
  • Now referred to as chromosomes
  • Energy for this process provided by mitochondria, which gather around spindle fibres. 
  • If cells treated with chemicals that destory spindle, the chromosomes are unable to reach the poles. 
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What happens in telophase and cytokinesis?

  • Chromatid separation = complete
  • Chromosomes become longer and thinner, and disappear leaving only widespread chromatin
  • Spindle fibres disintergrate
  • Each chromatid -> now a chromosome
  • Nuclear envelope reforms around each group of chromosomes
  • Cell divides along waist = cytokinesis
  • Both daughter cells formed are diploid
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How do prokaryotic cells replicate?

Prokaryotic cells replicate by binary fission. In binary fission, the cell replicates its genetic material before physically splitting into two daughter cells.

  • DNA tightly winds
  • DNA unwinds. Circular DNA and plasmids replicate. The main DNA loop only replicates once, but plasmids can replicate loads of times.
  • Cell elongates and individual copies move to different parts 
  • Cell membrane begins to grow . DNA loops move to opposite poles.Both circluar DNA copies attatch to the cell membrane.
  • Cleavage furrow in cell membrane pinches cell together. The cytoplasm begins dividing into two. Proteins connecting cells are broken.
  • New cell walls form between the two molecules of DNA. Divides into two identical daughter cells, each with one copy of circular DNA and a variable number of plasmid copies. 

Prokaryotic cells may divide equally or unequally. It is a stable process as bacteria have stable genomes and few mutations take place. 

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How do viruses replicate?

Viruses are non-living and acellular so cannot undergo cell division, nor do they need energy to survive and they don't regulate their temperatures. They replicateby using attatchment proteins on the surface to bind to complementary receptor proteins on the surface of host cells. They inject their nucleic acid into the host cell, giving their genetic information and providing 'intructions' for the host cell's metabolic processes to start producing viral components, nucleic acid, enzymes and structural proteins, which assemble into new viruses. 

Antibiotics don't work on viruses because they have different structures and ways of dividing to bacteria. Some patients may be given antibiotics if suffering from virus, but only if they have secondary bacterial infection. Antiviral drugs are hard to make because they may damage host cell as well as viruses, but those that do work, do so by preventing growth of viruses.

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