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AS/A-level Biology - Cells

Cells

· Cells,Mitosis,Binary Fission,Virus,A-level Biology

Cells

  • All cells arise from other cells.
  • In a cell that is not undergoing division, DNA is visible as an indistinct mass called chromatin and nuclear membrane as well as nucleolus are still intact.

  • Centrioles lie closely to one another.

Do you still remember the process of mitosis in AS/A-level Biology?

Mitosis

Interphase

  • There are two growth phases where the cell makes organelles as well as enzymes to get ready for mitosis.
  • There is also an S-phase where the DNA replicates.

Prophase

  • DNA undergoes spiralization where it gets shorter and fatter.
  • Two identical chromatids are held together at the centromere.
  • Nucleolus and nuclear membrane disintegrate and centrioles move to opposite poles of the cell and form a spindle.

Metaphase

  • Chromosomes lies at the equator of the cell where the kinetochore of the centromere is attached to the spindle fibres.

Anaphase

  • Centromeres divide when spindle fibres contract pulling each sister chromatid to opposite poles of the cell.
  • Nuclear membrane reforms and the spindle disintegrates

Telophase (cytokinesis)

  • The cell divides into two daughter cells.
  • The end product being the two daughter cells have equal amount of DNA as each other as well as being compared to the parent cell.

Cell Division

  • Cell division can be lethal as uncontrolled cell division can lead to cancers and tumours.

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Binary fission

  • It involves a bacterial cell increasing in length and doubling its parts.
  • It forms a cross septum and divides into two.

  • The time involved to double a population is called the generation time.

  • Two copies are separated by the growth of cell membrane.

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Virus

  • Viruses also replicate.

  • They inject DNA and RNA into the host cell leaving the protein coat or can enter by endocytosis.

  • Retroviruses have an enzyme called reverse transcriptase which causes viral DNA to be made on a template of viral RNA/DNA depending on what the virus has injected.

  • This is called reverse transcription.

  • It acts as a gene and is passed onto daughter cells and when activated it causes the cell to burst (lysis) because of a large amount of viruses replicating quickly.

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Mitotic Index

  • n/N x 100
  • n = number of cells in mitosis
  • N = number of cells observed

That's the end of the topic!

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Drafted by Bonnie (Biology)

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