1. Chromosomes & Meiosis

2. Chromosome Number
genes are located on the chromosomes in the cell nucleus
each organism must inherit a single copy of every gene from each parent
when an organism produces its own gametes (egg or sperm), each set of genes must be separated from each other so that each gamete contains just one set of genes

3. Genes on a Chromosome

4. Chromosome Number
homologous chromosomes– each set of chromosomes that comes from the male parent has a corresponding (matching) chromosome from the female parent
example – a human has 46 chromosomes (23 from the female and 23 from the male)
the 23 from the male has a matching partner from the female

5. Homologous Chromosomes

6. Chromosome Number Cont…
We have two terms when it comes to chromosome numbers:
diploid cells contains both sets of chromosomes (means “two-sets”)
In humans, 2N = 46 chromosomes
the gametes of a sexually reproducing organism contain a single set of chromosomes
haploid means “one set” or 23 chromosomes.

7. Haploid vs. Diploid

8. Haploid vs. Diploid

9. Meiosis meiosis is a process of reduction.
It cuts the number of chromosomes per cell in half through the separation of homologous chromosomes.
Meiosis contains two distinct divisions called meiosis I and meiosis II
at the end of meiosis II the diploid cell that started out has become 4 daughter cells

10. Meiosis

11. Introduction to Meiosis
each chromosome is replicated prior to the start of meiosis I (interphase)
each chromosome pairs with its corresponding homologous chromosome to form a tetrad
a tetrad is a group of 4 chromosomes
chromosomes exchange a portion of their chromatids in a process called crossing-over

12. A Tetrad & Crossing Over

13. Crossing Over Crossing Over
Crossing-Over is a term that we as scientists use to explain how DNA from two parents comes together to form a new individual.

14. Prophase I
In Prophase 1, crossover occurs between the tetrad (4 chromatids).
They begin to exchange DNA between the chromosomes.
Crossing over results in the exchange of alleles between homologous chromosomes
Crossing over produces new combinations of alleles

15. Crossing Over

16. Metaphase I
Homologous chromosome pairs are spread across the metaphase plate
Microtubules (spindle fibers) from each pole are attached to the centromere of one member of each homologous pair

17. Anaphase I Centromeres do NOT split
One homologous pair of chromosomes is pulled to each pole

18. Telophase I
homologous chromosomes separate and two new cells are formed
neither daughter cell has a complete set of chromosomes
the chromosomes and alleles present in the new cells are different from each other and from the diploid cell that began the process
Cells may enter interphase for a brief period of time

19. Meiosis I Meiosis I Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.

20. Meiosis II Meiosis is the second meiotic division
chromosome replication does not take place

21. Prophase II each chromosome has two chromatids
crossing over does not take place

22. Metaphase II
chromosomes line up as singles in the center of each cell

23. Anaphase II
Chromosomes are pulled apart into two chromatids by the spindle fibers
The chromatids are now chromosomes
Chromosomes migrate to the opposite poles

24. Telophase II Each new daughter cell becomes a haploid cell
Cells contain only half the number of chromosomes
Chromosomes consist of only one chromatid
When chromosomes are copied later in interphase, only half the number is copied

25. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.

26. Gamete Formation in Males
In males, the haploid gametes are called sperm
Meiosis is called spermatogenesis
Meiosis begins in males at puberty
It takes approximately 74 hours
Occurs daily in males for the duration of their life
Results in 4 genetically different sperm cells

27. Gamete Formation in Females
in females, the haploid gametes are called eggs
Meiosis is called oogenesis
cell divisions are uneven resulting in one egg receiving most of the cytoplasm
The larger egg becomes the egg that is involved in reproduction
the other three smaller cells become polar bodies and do not participate in reproduction
Meiosis begins before birth and finishes at conception

28. Mitosis in Gametes

29. Comparing Mitosis and Meiosis
mitosis is the process by which asexual reproduction occurs
mitosis results in the production of two genetically identical diploid cells
the diploid cells give rise to two diploid daughter cells
the new sets of chromosomes are identical to each other and to the parent
mitosis allows the body to grow and replace cells

30. Comparing Mitosis and Meiosis
meiosis results in four genetically different haploid cells
the diploid cell gives rise to 4 different haploid cells
the chromosomes are not identical to each other or the original diploid cell
meiosis is the process for sexually reproducing organisms to produce gametes