1. Meiosis
IB Topic 4.2 – Page 93

2. Meiosis A form of cell division which results in gametes (sex cells)
Sexual reproduction
Leads to genetic variation
Fertilization = fusion of sperm and egg
Superficially resembles mitosis, but there are some fundamental differences

3. Unique Characteristics
Each new cell has only half the number of chromosomes that a typical cell in that organism has.
For example, humans have 46 chromosomes in their cells, but in the sperm and egg cells, there are only 23 chromosomes
Cells which contain half the chromosome number are called haploid cells
Cells with the full chromosome number are called diploid cells.

4. Are you listening … Identify a cell in your body that is diploid.
Identify a cell in your body that is haploid.
What is the difference between the two?

5. What do we get at the end of meiosis?
The cells formed are called daughter cells
Meiosis generates:
4 haploid daughter cells that are genetically different from one another
Unique mix of half of the genetic information of the parent cells (half from mom and half from dad)
Quick review: how does this differ from mitosis?

6. Meiosis is also called …
Reduction division
Why?
Because the # of chromosomes has been reduced
This reduction is necessary because during sexual reproduction, each parent contributes 50% of the genetic material
23 chromosome egg (n) + 23 chromosome sperm (n) = 46 chromosome (2n) fertilized egg
What do you think might happen if human cells do not have 46 chromosomes?

7. Homologous Chromosomes
In a diploid human cell, the 46 chromosomes can be grouped into 23 pairs of chromosomes called homologous chromosomes
Homologous means similar in shape and size and centromere position
They also carry the same genes
One from mom and one from dad

8. Homologous Chromosomes
Although the pair carry the same genes, they are not identical because the alleles for the genes from each parent could be different

9. The phases of meiosis
Step-by-step process by which a diploid parent cell produces 4 haploid daughter cells
Before the steps begin, DNA replication allows the cells to make a complete copy of it genetic information during interphase
Sound familiar?

10. The phases of meiosis
In order to produce four cells, the parent cell must divide two times
1st mitotic division = 2 cells
2nd mitotic division = 4 cells

11. Crossing Over – Another Fundamental Difference
Occurs during prophase 1 of meiosis
Also called synapsis
All of the chromatids of the two homologous chromosomes become tightly associated
Combined pair is a bivalent or a tetrad
The maternal and paternal chromosomes exchange corresponding sections of DNA
Once complete, new combinations of alleles have been created
The process by which offspring possess a combination of alleles different form that of either parent is called recombination

12. Why crossing over?
Crossing over allows DNA from a person’s maternal chromosomes to mix with the DNA from the paternal chromosomes
Recombinant chromatids that end up in the sperm or egg cells are a mosaic of the parent’s original chromatids.

13. Chiasma
An X-shaped structure formed between non-sister chromatids during prophase 1
Physical manifestation of crossing over
Persist through metaphase 1

14. Steps of meiosis Good animation

15. Meiosis and genetic variation
The random orientation of chromosomes at metaphase 1 leads to variation within offspring
The 4 haploid cells produced are genetically different from each other b/c:
1. independent assortment of maternal and paternal homologous chromosomes:
The way the bivalents line up at the equator in meiosis 1 is entirely random
In humans, there are 23 pairs of chromosomes, so the # of possible combinations is 2^23 (8 million)
2. crossing over of segments:
New combinations of genes on the chromosomes
Unimaginable degree of variation