Meiosis Self Manager Reflective Learner Effective Participator L.O: To describe how gametes are formed by meiosis and that this creates genetic variation MUST: Recall that gametes are formed when cells divide by meiosis SHOULD: Describe how meiosis halves the number of chromosomes and that fertilisation restores the normal number of chromosomes COULD: Explain how meiosis leads to genetic variation Self Manager Reflective Learner Effective Participator

Why do siblings look similar but not identical? THINK PAIR SHARE

Mitosis - recap One cell One cell division Two genetically identical cells containing the full amount of DNA

What is meiosis? Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis. The cells that are formed by meiosis have half the number of chromosomes as a body cell (they are haploid). The main features of meiosis are: - The cell divides twice, forming four gametes The gametes are genetically different from each other.

Meiosis One cell First cell division Second cell division Four genetically different cells containing half the DNA

Gametes are formed in the reproductive organs by meiosis. Male Female Reproductive Organs Testes Ovaries Gametes Sperm Egg

CLIP

Genetic causes of variation Boardworks GCSE Science: Biology Similarity and Variation Many characteristics are caused by the genes that we inherit from our parents. These are never exactly the same as either parent’s genes, but a new combination. When sex cells (gametes) are produced in the parents, genes are swapped between the chromosomes so that they are not exactly the same as the parents’. At fertilisation, any sperm could fuse with the egg, mixing the genes from both parents to form a very different combination. This means that offspring are always different to their parents, creating genetic variation. 8

What happens next? Once two gametes have joined, the cell contains a FULL set of DNA. It then begins to divide by MITOSIS. As an embryo develops, the cells DIFFERENTIATE (become different types – skin, liver, RBC etc).

MEIOSIS MITOSIS One division Two divisions Starts with one cell Two cells formed Four cells formed Cells have half the DNA Cells have the full DNA Cells are identical Cells are not identical Takes place in body cells Creates gametes Is a form of cell division MITOSIS MEIOSIS

ANSWERS MEIOSIS MITOSIS One division Two divisions Starts with one cell Two cells formed Four cells formed Cells have half the DNA Cells have the full DNA Cells are identical Cells are not identical Takes place in body cells Creates gametes Isa form of cell division MITOSIS MEIOSIS 1. One division 2. Two divisions 4. Two cells formed 5. Four cells formed 3. Starts with one cell 6. Cells have half the DNA 7. Cells have the full DNA 12. Is a form of cell division 9. Cells are not identical 8. Cells are identical 10. Takes place in body cells 11. Creates gametes

TASK Self Manager LOW/MID LEVEL: Complete Student Practical sheet HIGH LEVEL: Complete student practical sheet including Student Follow up

Exam Question

Exam questions- Peer assessment (a) any one from • chromosomes in pairs • inherited one of each pair from each parent • one of each pair in egg and one of each pair in sperm • so sex cells / gametes can have half the number 1 (b) any two from: • code • combination / sequence of amino acids • forming specific / particular proteins / examples If no other mark gained allow reference to controlling characteristics / appearance for 1 mark 2

Exam questions- Peer assessment (c) (i) C 1 (ii) 30 1 (d) (i) for growth / repair / replacement / asexual reproduction1 (ii) 44 or 22 pairs 1

Exam questions- Peer assessment (a)     (i)      meiosis allow meiosis 1 (ii)     testis / testes allow testicle 1 (b)     (i)      23 1 (ii)      fuses / joins with cell D / with egg cell or used in fertilisation allow fuse with another cell 1 prevents doubling of chromosome number / restores original no. / 46 / diploid no. / normal no. / full no./23 from each parent / from each gamete 1

Meiosis Self Manager Reflective Learner Effective Participator L.O: To describe how gametes are formed by meiosis and that this creates genetic variation MUST: Recall that gametes are formed when cells divide by meiosis SHOULD: Describe how meiosis halves the number of chromosomes and that fertilisation restores the normal number of chromosomes COULD: Explain how meiosis leads to genetic variation Self Manager Reflective Learner Effective Participator