1. Meiosis
Chapter 11.4

2. Chromosome Review Let’s review… What is a chromosome?
Condensed chromatin (DNA + proteins)
only visible during cell division!!
How many chromosomes do you have in each of your cells?
46 (23 pairs)
But wait..do any of your cells NOT have 46?
SEX CELLS!

3. Chromosome Number What does it mean to have 23 pairs of chromosomes?
23 chromosomes: one copy of each from mom & one from dad!
Cells that have a chromosome from each parent are DIPLOID (2N)!
Somatic cells (body cells)
“two sets”

4. contain one set of genes!
Chromosome Number
Some cells don’t have a pair of each chromosome!
Gametes (sex cells) of sexually reproducing organisms
These cells are HAPLOID (N)!
“one set”
Single set of genes!
Why do they only have half the
number of chromosomes?
Each gamete can only
contain one set of genes!
They COMBINE to
Become DIPLOID!

5. Chromosome Structure Review
Chromatid
One strand of a duplicated chromosome
Joined by a centromere to its sister chromatid
Sister chromatids
Two chromatids joined by a common centromere
Each carries identical genetic information
Together called a DYAD

6. Meiosis
The genetic information that we (and other sexually reproducing organisms) inherit comes from two cells: sperm and egg
Meiosis – cell division in which the chromosome number is cut in half
Gametes (sperm and egg) divide this way!
Germ cells in the testis and ovary
undergo meiosis and produce
gametes.

7. Meiosis: Reduces the number of chromosomes/cell by half
Occurs in two steps:
Meiosis I
Meiosis II
Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages
2N = 4
N = 2

8. Meiosis
2 dyads
DNA replication occurs during interphase before the beginning of meiosis I but not before meiosis II
Chromosomes will associate with the other member of its pair during Meiosis
Homologous chromosomes (also called a TETRAD)
1 tetrad

9. Meiosis
Meiosis I is a reductional division: the number of centromeres is reduced by half after this division
Meiosis II is an equational division: the number of centromeres remains equal after this division

10. Meiosis 1: Prophase 1 Prophase I Chromosomes become visible
Homologous chromosomes pair up to form tetrads
consumes 90% of the time for meiosis
Crossing Over occurs
Results in Genetic Variation
New allele combinations!

11. Two major sources of genetic variation in Meiosis I
Crossing Over
Creates new combinations of mom and dad’s alleles
Think about chromosomal mutations!
Independent Assortment

12. Metaphase 1 As prophase I ends, tetrads attach to spindle fibers
Tetrads line up at center of cell

13. Anaphase 1 Dyads pulled toward opposite poles by spindle fibers
Disjunction: separation of chromosomes
Nondisjunction leads to polyploidy! (extra chromosomes)
Note the exchange of information between paternal and maternal chromosomes

14. Telophase 1 Separated chromosomes cluster at opposite ends of cell
Nuclear membrane forms around each cluster
Cytokinesis follows and forms 2 new cells

16. This was a reductional division: number of centromeres reduced per cell.
Prophase I: 4 centromeres, therefore 4 chromosomes
Prophase II: 2 centromeres, therefore 2 chromosomes/cell

17. Results of Meiosis 1 Two daughter cells
Neither with two complete sets of chromosomes (haploid)
Sets have been shuffled and independently assorted
Chromosomes differ between each other and the original cell

18. Meiosis II Two cells enter second meiotic division
Neither cell goes through DNA replication prior to this division!

19. Prophase II Chromosomes (dyads) become visible
Do not form tetrads because they are already separated from homologous pair!

20. Metaphase II & Anaphase II
Chromosomes (dyads) attach to spindle fibers and line up in center of cell
Remember…they aren’t paired with another chromosome!
Anaphase II – chromatids (monads) separate from each other at the centromere
move to opposite poles!

21. Telophase II & Cytokinesis
Four genetically different haploid cells produced (N)
Each monad may be an entirely new combination of maternal and paternal genetic information

22. Meiosis II

23. Meiosis Review Meiosis II Meiosis I
Prophase II: Dyads reappear, no tetrads!
Metaphase II: Dyads line up in middle
Anaphase II: Monads pulled apart (separated at centromere)
Telophase II: four new genetically different haploid daughter cells with monads
Meiosis I
Prophase I: tetrads form (homologous chromosomes pair), crossing over occurs
Metaphase I: tetrads line up in middle
Anaphase I: Dyads pulled apart
Telophase I: two new genetically different haploid daughter cells with dyads

24. Meiosis: Formation of Gametes
Meiosis results in two kinds of haploid, sexual gametes
Males produce sperm
Females produce eggs (usually only one of the four egg cells is used!)
Sperm fertilizes
egg to produce
2N zygote!
Goes through mitosis
& cell specialization
to form a new
organism!

25. Mitosis vs. Meiosis: a Comparison
Both preceded by DNA replication
Both are methods of cell division
Both include Prophase, Metaphase, Anaphase, and Telophase
Both are followed by cytokinesis
Mitosis vs. Meiosis

26. Contrasting Meiosis & Mitosis
Each daughter cell receives a complete set of chromosomes
Less genetic diversity
Doesn’t change the chromosome number of the original cell
Single cell division
Two genetically identical diploid daughter cells
Asexual reproduction
Makes somatic cells
Meiosis
Two alleles for each gene segregated and end up in different cells
Greater variety of possible gene combinations
Reduces the chromosome number by half
Two rounds of cell division
Four genetically different haploid daughter cells
Sexual reproduction
Makes gametes

28. Gene Linkage Genes on different chromosomes assort independently
What about genes on the same chromosome?
Tend to be linked!
Chromosomes assort independently, but typically genes on the same
chromosome are inherited together
Especially when close together!
Crossing over causes some genes on
the same chromosome to assort
independently

29. Gene Maps
Frequency of crossing-over between genes during meiosis is used to determine genes’ locations
Farther apart, more likely that crossing over occurs between them
Close together, crossovers rare
Use frequency of crossing over to determine distances from each other and map genes’ locations on chromosomes!