AQA GCSE Biology

Revision Notes

6.1.2 Meiosis

Meiosis

  • Cells in reproductive organs divide by meiosis to form gametes (sex cells)
  • The number of chromosomes must be halved when the gametes 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 notes

The process of cell division by meiosis to produce haploid gamete cells

Process

  • Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes
  • First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
  • Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
  • A total of four haploid daughter cells will be produced

Importance

  • Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants
  • 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

Fertilisation

  • Gametes join at fertilisation to restore the normal number of chromosomes
  • When the male and female gametes fuse, they become a zygote (fertilised egg cell)
  • This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete
  • The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo
  • Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring
  • The process of cells becoming specialised is known as cell differentiation

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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