Cell Division in Eukaryotic and Prokaryotic Cells — AQA A Level Biology 7402 Revision Notes & Topical Questions

Mitosis is part of the precisely controlled cell cycle, the regulated sequence of events between one cell division and the next. Transitions between phases are triggered by chemical signals called cyclins. Interphase has three phases:

Mitosis is nuclear division producing two daughter nuclei that are genetically identical to the parent nucleus. It underlies growth, tissue repair, replacement of cells and asexual reproduction. Remember the sequence with PMAT: prophase (Preparing), metaphase (Middle), anaphase (Away/apart), telophase (Two).

In diagrams, different chromosome colours may represent maternal and paternal origin. Read the question carefully, as not every organism has the human diploid number of 46; use the diploid number given.

You must be able to recognise each stage from electron micrographs and explain why you chose it.

Cytokinesis follows telophase and is not part of mitosis. After the nuclear envelope has reformed at each pole, the cytoplasm divides, producing two genetically identical daughter cells. Remember that cytokinesis is a process that occurs after mitosis, not a stage within it.

Growth in plants occurs in regions called meristems, so the root tip meristem can be used to study mitosis. Garlic or onion (Allium cepa) root tips are commonly used; bulbs can be encouraged to grow roots by suspending them over water for a week or two.

Sizes of cells or structures may look inconsistent between slides because cell structures are 3D and different samples are cut at different planes. Optical microscopes also cannot show every structure.

The mitotic index is the proportion of cells in a sample that are undergoing mitosis:

Mitosis is normally controlled. Cancer results from uncontrolled mitosis, forming a tumour (an abnormal mass of cells). Mutations in genes controlling the cell cycle, such as oncogenes and tumour suppressor genes, can lead to this loss of control.

Cell division in prokaryotes is binary fission.

Viruses are acellular infectious particles. Being non-living, they do not undergo cell division. They are relatively simple in structure: genetic material (DNA or RNA) enclosed in a protein coat called a capsid; some also have an outer lipid envelope. Attachment proteins on the surface are used to bind to host cells.

Viruses replicate inside a host cell:

The Unit 2 content on cell structure provides details of how HIV replication occurs by this process.

Different viruses have different attachment proteins, so they can only bind to specific host cell types that have the correct (complementary) receptor proteins. Some viruses use prokaryotic host cells (bacteriophages), whilst others use eukaryotic animal or plant cells. In addition, some viruses can infect only one specific cell type, whereas others infect many different cell types.

Because viruses use host machinery, drugs that block viral replication must be very specific to avoid harming host cells. Vaccines work by exposing the immune system to viral antigens so that memory cells can respond rapidly. Antigenic change in viruses such as influenza means new vaccines are needed regularly.