CIE IGCSE Biology

Revision Notes

17.1.4 Cell Division

Mitosis: Basics

  • Most body cells have two copies of each chromosome
  • We describe these cells as diploid
  • When cells divide their chromosomes double beforehand
  • This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)
  • This type of cell division is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction and is known as mitosis
  • Mitosis is defined as nuclear division giving rise to genetically identical cells
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Mitosis

Mitosis, IGCSE & GCSE Biology revision notesThe process of cell division by mitosis

 

Process:

  • Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x – shaped chromosomes)
  • Chromosomes line up along the centre of the cell where cell fibers pull them apart
  • The cell divides into two; each new cell has a copy of each of the chromosomes

Importance:

  • All cells in the body (excluding gametes) are produced by mitosis of the zygote
  • Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)

Occurs in:

  • Growth: mitosis produces new cells
  • Repair: to replace damaged or dead cells
  • Asexual reproduction: mitosis produces offspring that are genetically identical to the parent
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Stem Cells

  • Many tissues in the human body contain a small number of unspecialised cells
  • These are called stem cells and their function is to divide by mitosis and produce new daughter cells that can become specialised within the tissue and be used for different functions
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Meiosis: Basics

  • Meiosis is a type of nuclear division that gives rise to cells that are genetically different
  • It is used to produce the gametes (sex cells)
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Meiosis

  • The number of chromosomes must be halved when the gametes (sex cells) are formed
  • Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
  • This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
  • It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
  • After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete
  • We describe gametes as being haploid – having half the normal number of chromosomes
  • Because of this double division, meiosis produces four haploid cells

 

Meiosis, IGCSE & GCSE Biology revision notesThe process of cell division by meiosis to produce haploid gamete cells

 

Process:

  • Each chromosome makes identical copies of itself (forming X-shaped chromosomes)
  • First division: chromosomes pair up along the centre of the cell, recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair
  • Second division: chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)
  • A total of four haploid daughter cells will be produced

Importance:

  • Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum
  • Increases genetic variation of offspring

 

  • Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others
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Differences between Mitosis & Meiosis

Differences between mitosis and meiosis table, IGCSE & GCSE Biology revision notes

Exam Tip

Questions on cell division often ask for differences between mitosis and meiosis. Learn two or three and remember to BE SPECIFIC when giving your answer.

You should also know the reasons for a specific type of cell division taking place and the types of cells where each happen.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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