Cell division in sexual reproduction Learning objectives: understand that cells undergo meiosis to produce gametes and describe the process describe how the behviour of the chromosomes in meiosis results in gametes with a single set of genetic information fertilisation of gametes during sexual reproduction results in variation in a species

Cell division in sexual reproduction How many chromosomes are there in new human body cells produced by mitosis? 46 chromosomes (23 matching pairs) How many chromosomes are there in human sex cells? 23 unpaired chromosomes 23 unpaired chromosomes

Cell division in sexual reproduction Human sex cells have a set of 23 unpaired chromosomes - that’s half the usual number. Sex cells in animals and plants follow the same rules. They have half the usual number of chromosomes. They have a complete set of single chromosomes. Why is this important for fertilisation and reproduction?

Cell division in sexual reproduction Sex cells have half the usual number of chromosomes. Sex cells have to be produced by a special type of cell division. Sex cells are produced by meiosis. In which organs does meiosis take place?

Cell division in sexual reproduction Production of male gametes Did you know… Around 400 million sperm are produced every 24hrs! Only one sperm fertilises an egg but it must travel 100 000 times its own body length to reach it!

Cell division in sexual reproduction Meiosis in cat ovaries Production of female gametes Meiosis in plant ovary (lilies)

Cell division in sexual reproduction Meiosis begins with a single cell. original cell First the cell makes a copy of each chromosome. first division Now, there are two stages of cell division… second division How many chromosomes do the new cells have compared to the original? 4 new cells

Cell division in sexual reproduction meiosis video ...and another one just for fun! (start at 1 min 20 sec) Meiosis produces four new sex cells with half the original number of chromosomes. In animals, meiosis produces ova in females and sperm cells in males. In plants the gametes produced by meiosis are pollen grains (male) and ovules (female). Meiosis is also called reduction division. What does this mean?

Cell division in sexual reproduction Comparing mitosis and meiosis: Complete the table to show the differences: Mitosis Meiosis

Cell division in sexual reproduction Comparing mitosis and meiosis: Complete the table to show the differences: Mitosis Meiosis daughter cells genetically identical daughter cells genetically different two cells produced four cells produced one cell division two divisions cells have full chromosome number cells have half the chromosome number body cells for growth, repair, replacement gametes for sexual reproduction

Cell division in sexual reproduction The importance of meiosis: Introduces variation by the production of gametes that have half the number of chromosomes than body cells. Allows mixing of genetic information during fertilisation.

û ü ü û û ü û û ü ü True or false? All cells in the body have 2 sets of chromosomes Meiosis happens in the reproductive organs only DNA is copied before cell division starts In meiosis there is only one cell division Mitosis introduces genetic variation to a species that reproduces sexually Variation occurs when gametes from two parents are combined during fertilisation Gametes have the full number of chromosomes Mitosis occurs in the testes of animals to produce sperm cells The male gamete of a sexually reproducing plant is a pollen grain Meiosis in human females produces ova (eggs) with 23 chromosomes inside û ü ü û û ü û û ü ü

Cell division in sexual reproduction Learning objectives – review: understand that cells undergo meiosis to produce gametes and describe the process describe how the behviour of the chromosomes in meiosis results in gametes with a single set of genetic information fertilisation of gametes during sexual reproduction results in variation in a species