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What are sex cells? Sex cells in animals and plants are called gametes. In animals, the gametes are eggs (ova) and sperm. In mammals, egg cells are produced in the ovaries, and sperm cells are produced in the testes. How are these cells specialized for their roles in reproduction? food source in cytoplasm for embryo lots of mitochondria for providing energy enzymes to digest egg cell membrane flexible tail for swimming protective outer cell layers

Meisosis and Fertisation Learning Objective: Explain the role of meiosis in the production of sex cells. Learning Outcomes C : Describe the process of meiosis with diagrams. B: Compare and contrast meiosis and mitosis. A: Apply understanding of both meiosis and mitosis to exam style questions.

What is Meiosis? Meiosis is a special type of cell division used to make sex cells (gametes) Gametes need to have half the ordinary number of chromosomes (haploid number ) What are the sex cells in humans called?

How many chromosomes in gametes? If gametes had the same number of chromosomes as body cells, what problem would this cause at fertilization? The embryo would have double the number of chromosomes – 92 instead of 46 in humans. Why does this not happen? Only one chromosome from each homologous pair in the parent cell is copied to the gametes during cell division. This means that human gametes only have 23 chromosomes. Gametes are said to be haploid cells.

Meiosis Gametes are ‘sex cells’. They're called ova in females and sperm in males. During sexual reproduction two gametes combine to form a new cell which will grow to become a ne organism. Gametes are haploid (1 copy of each chromosome) Two gametes combine resulting in a cell (zygote) that has the right number of chromosomes.

Formation of gametes How many chromosomes here?

This means that when they fuse together during fertilisation the zygote has the same number of chromosomes as the parents’ cells Why do you think this is important?

Why is meiosis important? The new cells are all different because the chromosomes get shuffled – each egg or sperm only has half the parents chromosomes and which half is random So if there are 23 pairs of chromosomes there are 2 23 possible combinations (in other words a huge number) The important things to remember about meiosis are that it promotes variation within a species and maintains the correct chromosome number

What happens in meiosis? 1. The chromosomes become visible and get thicker

What happens in meiosis? 2. The chromosomes are replicated and can now be seen as x-shapes

What happens in meiosis? 3. The nuclear membrane disappears. The chromosomes line up in homologous pairs in the middle of the cell along the spindle

What happens in meiosis? 4. The pairs of chromosomes separate and are pulled to opposite ends of the cell along the spindle

What happens in meiosis? 5. The cell splits into two, each cell gets 1 chromosome from each pair

What happens in meiosis? 6. Chromatids separate. Each cell splits into two and nuclear membrane reappears

What happens in meiosis? 7. Each new cell is DIFFERENT to the original and has HALF (n) the original number of chromosomes

Meiosis produces genetically-unique daughter cells, but how does this happen? In the early stages of meiosis, homologous chromosomes exchange DNA, which creates genetic variation and new combinations of characteristics. This is called crossing-over. homologous pair before crossing-over homologous pair after crossing-over

Put the sentences into the correct order to describe the process of Meiosis

Chromosomes are copied by a cell in the reproductive organs. The cell divides into two new cells The cells divide to produce 4 haploid daughter cells (gametes) 4 gametes are genetically different. There is variation. 4 gametes are genetically different. There is variation.

MitosisMeiosis Purpose Takes place.. Produces how many cells? What happens to number of chromosomes? How do parent and daughter cells differ genetically? Variation between daughter cells? Mitosis and Meiosis Compared

MitosisMeiosis Purpose To make daughter cells identical to the parent cells - eg during growth and repair To produce sex cells (gametes)gametes Takes place.. In all cells apart from gametes In the reproductive organs (ovaries and testes) Produces how many cells? Two daughter cellsFour gametes What happens to number of chromosomes? Same number as in parent cell Diploid = 46 (in pairs) Half as many as in parent cell (The original number of chromosomes is restored when two gametes fuse to form a zygote.) zygote Haploid = 23 (single) How do parent and daughter cells differ genetically? Not at all - genetic material is copied exactly (replicated) Contain a mixture of chromosomes from two parent gametes - so cannot be identical Variation between daughter cells? No - they are clones of each other Yes - they are genetically different from each other because chromosomes get shuffled up during division Mitosis and Meiosis Compared

Exam Question 1. Mitosis and meiosis are types of cell division. Compare these two types of cell division. (6 Marks)

A comparison between mitosis and meiosis including Mitosis ·(genetically) identical cells produced ·two daughter cells ·one division ·diploid daughter cells ·identical set of chromosomes ·occurs in the formation of body cells ·for growth and repair (of body tissues) Meiosis ·(genetically) non-identical cells ·four daughter cells ·2 divisions ·haploid daughter cells ·half the number of chromosomes ·occurs in the formation of gametes ·for sexual reproduction ·results in genetic variation