Sexual Reproduction (OCR Gateway GCSE Biology: Combined Science)

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Sexual Reproduction

  • Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
    • The gametes of animals are the sperm cells and egg cells
    • The gametes of flowering plants are the pollen cells and egg cells
  • Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
  • The formation of gametes involves meiosis

Gametes are haploid cells

Animal gametes for sexual reproduction

Advantages and Disadvantages of Sexual Reproduction Table

Advantages & Disadvantages of Sexual Reproduction table

Numbers of Chromosomes

  • A gamete is a sex cell (in animals: sperm and ovum; in plants: pollen nucleus and ovum)
  • Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells - we say they have a haploid nucleus
  • This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells
  • In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes
  • When the male and female gametes fuse, they become a zygote (fertilised egg cell)
  • This contains the full 46 chromosomes, half of which came from the father and half from the mother - we say the zygote has a diploid nucleus

Sexual reproduction in humans

An overview of sexual reproduction in humans


  • Cells in reproductive organs divide by meiosis to form gametes (sex cells) for sexual reproduction
  • 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 to form the zygote (fertilised egg)
  • This halving occurs during meiosis, the chromosome number is halved from diploid to haploid
  • Key facts about meiosis:
    • Meiosis produces four genetically different haploid daughter cells
    • There are 2 divisions involved in the process
    • It is the process used to produce gametes

Meiosis I and II

  • Before meiosis can begin, DNA replication must occur in the nucleus of a diploid cell
  • Next, chromosomes pair up and align 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 daughter cells


The importance of meiosis

  • Produces gametes e.g. sperm cells and egg cells in animals or 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 genetically different from any others

Exam Tip

Sometimes we refer to the number of chromosomes as 'n' where:

  • n = 1 set of chromosomes (a haploid cell or gamete). In humans this is 23 individual chromosomes.
  • 2n = 2 sets of chromosomes (a diploid cell). In humans this is 23 pairs of chromosomes

You are not required to know the stages of meiosis for your exam. They are only included here to help with your understanding of the role of meiosis (i.e. the production of four daughter cells, each with half the number of chromosomes, which results in genetically different haploid gametes).

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