5.1.4 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

Animal gametes for sexual reproduction

Advantages and Disadvantages of Sexual Reproduction Table

• 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

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

• 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