1. Chapter 10 Section 1 Meiosis
Ms. Diana and Ms. Suad

2. Chromosomes and Chromosome Number
Each characteristic of living things is called a trait.
Examples of traits: hair color, height, eye color
The instructions of each trait are located on chromosomes, which are found in the nucleus of cells.
The DNA found on chromosomes are arranged in parts called genes. Genes control the production of proteins.

3. Chromosomes and Chromosome Number

4. Homologous Chromosomes
Human body cells have 46 chromosomes.
Each parent passes on 23 chromosomes, resulting in 23 pairs of chromosomes.
Homologous chromosomes – chromosomes that make up a pair, one chromosome from each parent.
Homologous chromosomes have the same length and the same centromere position, and they carry genes that control the same inherited traits.

5. Homologous Chromosomes

6. Haploid and Diploid Cells
In order for the number of chromosomes to stay the same from generation to generation, an organism produces gametes.
Gametes – sex cells that have half the number of chromosomes.
In humans, each gamete contains 23 chromosomes.
n represents the number of chromosomes in a gamete
Haploid – a cell with n number of chromosomes

7. Haploid and Diploid Cells
The process by which one haploid gamete combines with another haploid gamete is called fertilization.
After fertilization, the cell now will contain a total of 2n chromosomes (n from a female, n from a male).
Diploid – a cell that contains 2n number of chromosomes.

8. Meiosis
Meiosis – process that forms gametes. It is a type of cell division that reduces the number of chromosomes.
Meiosis reduces the number of chromosomes by half by separating homologous chromosomes.
Meiosis involves two cell divisions called meiosis I and meiosis II.

9. Meiosis I Steps of Meiosis I:
Interphase – replication of DNA, synthesis of proteins, metabolic processes.
Prophase I – pairing of homologous chromosomes occurs (synapsis), nuclear envelope breaks down, crossing over occur
Crossing over – process during which chromosomal segments are exchanged between a pair of chromosomes.
Metaphase I – homologous chromosomes line up opposite one another. Chromosomes attach to spindle fibers.
Anaphase I – homologous chromosome separate and move to opposite poles of the cell.
Telophase I – the spindle breaks down, chromosomes unwind and form two nuclei, the cell divides.

10. Steps of Meiosis I

11. Meiosis II Steps of Meiosis II:
Prophase II – spindle fibers form in each cell from meiosis I.
Metaphase II – chromosomes move to the equator of the daughter cells.
Anaphase II – chromatids separate and move towards the poles of the cell.
Telophase II – nuclear membrane and nucleolus reappears in each of the four new cells. Each cell contains half of the original cell’s number of chromosomes.

12. Steps of Meiosis II

13. Importance of Meiosis
Remember mitosis consists of only one set of division phases and produces two identical diploid daughter cells.
Meiosis consists of two sets of divisions and produces four haploid daughter cells that are not identical.
Meiosis is important because it is the reason genetic variation exists.

14. Meiosis Provides Genetic Variation
During prophase I, pairs of homologous chromosomes line up at the equator.
How the chromosomes line up at the equator is a random process that results in gametes with different combinations of chromosomes.
So you can get four gametes with four different combinations of chromosomes.

15. Meiosis vs. mitosis

16. mitosis vs. meiosis

17. Asexual Reproduction vs. Sexual Reproduction
Asexual reproduction – an organism inherits all of its chromosomes from 1 single parent. The result is identical offspring.
Sexual reproduction – an organism inherits all of its chromosomes from 2 parents. Requires meiosis to produce gametes, genetic variation is present.

18. Questions