1. Meiosis
Chapter 7 Section 1

2. Mitosis vs. Meiosis Meiosis: Mitosis:
occurs because the cell gets too big and for healing.
produces genetically identical diploid daughter cells.
Meiosis:
occurs in the production of gametes.
produces haploid daughter cells.

3. Somatic vs. Gametes Somatic Cells –
They are all the cells in the body except for sperm and eggs
Always diploid!
Produced by mitosis.
Gametes –
In males – sperm/pollen In females – eggs
Always haploid!
Produced by meiosis.

4. Gametes
Each sperm and egg cell contains half the normal number of chromosomes – essential to sexually reproducing organisms.
When a egg is fertilized by the sperm, the offspring inherits one chromosome for each homologous pair from each parent.
It gains the normal number of chromosomes.

5. Overview of Meiosis
G1 Phase
Meiosis produces gametes through two stages of cell division.
Meiosis begins with a cell that has the normal number of chromosomes.
Due to the cell having completed the S (synthesis) phase of the cell cycle, the chromosome has replicated DNA.
After S Phase
G2 Phase

6. Overview of Meiosis
Second reduction
Meiosis I: separation of homologous chromosomes – with each of the 2 new daughter cells get one chromosome from each pair.
Meiosis II: each chromosome is separated into two chromosomes, so that each of the four daughter cells get half the normal number of chromosomes
First reduction
Unreplicated chromosomes

7. Phases of meiosis
During meiosis, two cell divisions – divided into 2 stages – stages I and II.
Each stage, the cell undergoes the same phases as mitosis: prophase, metaphase, anaphase and telophase.

8. phases of Meiosis Prophase I: Nuclear envelope breaks down.
Homologous chromosomes pair up
Crossing over occurs.

9. Crossing-over
Crossing-over occurs in Prophase I after the homologous chromosomes pair up.
DNA strands from one chromosome join with the other, breaking off and piecing themselves back together.
END RESULT: each chromosome contains pieces of DNA from its homolog

10. Result of Crossing-Over
Each homolog has a slightly different set of alleles.
Crossing-over shuffles the genetic material so that each sister chromatid has a different combination of alleles.
Instead of the two original allele combinations, there are now four

11. Result of crossing-over
Crossing over makes it possible for offspring to inherit unique combinations of alleles.
This increases the genetic variation in the gene pool of a species.

12. phases of Meiosis Metaphase I:
Spindles attach to the centromeres of the homologues.
Homologous chromosomes align at the equator.

13. phases of Meiosis Anaphase I: Spindles shorten
Homologous chromosomes separate

14. phases of Meiosis Telophase I: Cytokinesis occurs.
Two cells have formed.
At the end of telophase I, one parent cell has been divided into two new daughter cells with half the number of chromosomes – haploid cell.

15. phases of Meiosis Prophase I Prophase II: Chromosomes condense
Spindle fibers form
Prophase II

16. phases of Meiosis Metaphase I Metaphase II:
Spindles attach to the centromeres of the sister chromatids.
Chromosomes align at the equator.
Metaphase II

17. phases of Meiosis Anaphase I Anaphase II: Spindle fibers shorten.
Sister chromatids separate.
Anaphase II

18. phases of Meiosis Telophase I Telophase II:
Chromosomes begin to unwind and stretch out.
Nuclear envelope reforms.
Cytokinesis occurs.
Telophase II
At the end of telophase II, two daughter cells have been divided into four new daughter that are haploid.

19. Review Time! Mitosis Meiosis I Meiosis II Number of daughter cells
Mitosis
Meiosis I
Meiosis II
Number of daughter cells
Chromosome number
Crossing-over occurs (yes/no)
Sister chromatids separate
Daughter cells genetically identical to parent cell
Daughter cells genetically identical to each other