1. Genetics 1 Reduction Division - Meiosis

2. Review: Cell Cycle and Mitosis
Draw the cell cycle llcycle.swf List the steps of mitosis

3. What stages?

4. Keyword Definition basic unit of heredity; codes for a specific trait
the specific location of a gene on a chromosome
the total hereditary endowment of DNA of a cell or organism
all body cells except reproductive cells
reproductive cells (i.e. sperm & eggs)
elongate cellular structure composed of DNA and protein - they are the vehicles which carry DNA in cells
cellular condition where each chromosome type is represented by two homologous chromosomes
cellular condition where each chromosome type is represented by only one chromosome
chromosome of the same size and shape which carry the same type of genes
one of two duplicated chromosomes connected at the centromere
region of chromosome where microtubules attach during mitosis and meiosis
GENE
LOCUS
GENOME
SOMATIC
GAMETES
CHROMOSOME
DIPLOID (2n)
HAPLOID (n)
HOMOLOGOUS
CHROMATID
CENTROMERE

5. Meiosis
The process by which a diploid nucleus (2n) divides to produce four haploid daughter nuclei (n) which are not genetically identical to each other or the parent nucleus.
Produces gametes.
Results in genetic variation.

6. Stages of Meiosis Follows G2 stage of interphase
DNA replication has already occurred.
Two distinct divisions – meiosis 1 followed by meiosis 2

7. Meiosis 1 and 2
Meiosis 1
Homologous pairs of chromosomes are separated
Meiosis 2
Sister chromatids are separated

8. At end of Interphase
DNA replicated but not coiled so no chromosomes visible
Nuclear membrane present
Two centrioles
Nucleolus present

9. Chromosome Structure

10. Meiosis 1 Early Prophase 1
DNA condenses and becomes visible
Two chromatids attached at a CENTROMERE
Centrioles begin to move
Nucleolus disappears

11. Meiosis 1 – Mid Prophase 1
Homologous chromosomes pair up and lie along side each other (SYNAPSIS)
Each pair called a BIVALENT
Centrioles move further towards poles

12. Meiosis 1 – Late Prophase 1
Chromatids tangle at points called CHIASMATA (chiasma sing.)
Chromosomes may break and rejoin
CROSSING OVER occurs
Reformed chromatids called RECOMBINANTS

14. Meiosis 1 – Metaphase 1 Nuclear membrane breaks down Spindle forms
Bivalents move to the equator and attach to the spindle by the centromere

15. Meiosis 1 – Anaphase 1
Contraction of spindle fibres pulls whole chromosomes towards poles
Homologous chromosomes separate

16. Meiosis 1 – Telophase 1
Nuclear membrane reforms around each set of chromosomes
Spindle fibres disappear
Centrioles divide into two

17. Cytokinesis Division of the cytoplasm occurs.
There may be a short pause

19. Meiosis 2 – Prophase 2
Centrioles move towards opposite poles

20. Meiosis 2 – Metaphase 2 Nuclear membranes disappear
Spindles form at right angles to the first
Chromosomes move to the equator and attach to spindle fibres by centromeres

21. Meiosis 2 – Anaphase 2 Centromeres divide
Sister chromatids move towards opposite poles

22. Meiosis 2 – Telophase 2 Each chromatid now called a chromosome
Nuclear membrane reforms around each group of chromosomes
Spindle fibres disappear and centrioles may divide

23. Cytokinesis Division of the cytoplasm
Four haploid daughter cells are produced
Each cell genetically different

25. Variation Results from:
Crossing over – during prophase 1 creates new combinations of genes
Reduction – fertilisation of haploid gametes produces new combinations of genes and restores the chromosome number from generation to generation
Independent assortment –during metaphase 1 it is random which way the chromosomes line up

26. Independent Assortment

27. Possible combinations
No. of possible combinations in a gamete = 2n
(where n = haploid number of the species)
In humans n = 23
So any one of 8,388,608 combinations of maternal and paternal chromosomes may be found in ONE gamete.
Crossing over means the variety is endless

28. Comparison between Mitosis and Meiosis
Meiosis animation
Mitosis v Meiosis
List three similarities between the two types of cell division.
Complete the table on the worksheet by selecting the appropriate statements