1. Lect2: Chromosomes and Genetics
6th Ed: ch 4-1, 4-3, 4-4

2. Mitosis and Meiosis Traits (phenotypes) are controlled by genes
Each individual has thousands of genes and each gene has two copies in an individual.
What are the physical entities that carry the genes during growth of cells, during human development and how do these entities behave as cells grow and divide?

3. Genes reside on Chromosomes
Genes reside on chromosomes, understanding the behavior and inheritance patterns of individual genes requires an understanding of the behavior of inheritance patterns of chromosomes.
The processes of mitosis and meiosis describe the two basic patterns of chromosome behavior in eukaryotes
Mitosis: a form of cell division that produces two daughter cells of identical genotypes. 2N
2N 4N
Meiosis: a form of cell division in a diploid cell that produces four haploid cells (Gametes) N
2N 4N
Meiosis only occurs in a small specialized set of cells leading to formation of the germ cells.

4. Development Mitosis 2N ---->4N ----> 2N+2N (somatic cells)
Meiosis 2N ---->4N ---->N+N+N+N (germ cells)
The segregation and assortment of chromosomes in germ cells is important in the transmission of traits
2N 2N
| |
4N 4N
N N | 2N 4N
Mendel’s laws occur only in germ cells of the parents during gamete formation/meiosis
You will most often deduce what happened during meiosis by looking at phenotype in the developed progeny
meiosis
mitosis

5. Digression: Chromosome number
Species Chromosome number
in haploid cells (n)
Human 23
Monkey 21
Mouse 20
Frog 13
Fruit fly 4
C. Elegans 6
Corn 10
S. Cerevisiae 16
S. Pombe 3
Smallest number: The female of the ant, Myrmecia pilosula, has one pair of chromosomes per cell. Its male has only one chromosome in each cell.
Largest number: The fern Ophioglossum reticulatum has about 630 pairs of chromosomes, or 1260 chromosomes per diploid cell.

6. Chromosome number – “n”A
Totally different
Haploid B
Haploids have 1N (or 1n with n=2) DNA content
Diploids have 2N (or 2n with n=2) DNA content
Tetraploids have 4N (or 4n with n=2) DNA content
Chromosome number = Autosome + sex chromosome
99.99% identical
n=2 A a
Totally different
Diploid b B
99.99% identical

7. Mitosis
Mitosis is the period in which the chromosomes condense align along the metaphase plate and migrate to opposite poles.
In part because this is the most visibly dramatic stage in the cell cycle much research has focused on these mitotic events.
Net result: The creation of two daughter cells with identical
chromosome complements.

8. Mitotic cell cycle in diploids
Homologous
Chromosomes
99.99% identical
Homologous chromosomes A a A a
n=2 2N B b
Replication of DNA
Sister chromatids
telomere A a A a
n=2 4N
centromere B b
Each DNA mol is a chromatid
Two chromatids attached at centromere (via proteins) are sister chromatids
Sister chromatids are 100% identical to each other

9. Sister chromatids separate to opposite poles
Mitosis
Chromosomes line up at the metaphase plate. A b
n=2 4N B a
Sister chromatids separate to opposite poles A
n=2 4N A a a B B b b

10. Mitosis A A a
n=2 2N a B B b b
Two Daughter cells created that are identical to original mother cell A a
n=2 2N B b
Can Mitosis occur in haploid cells?

11. Mitosis in haploid and diploid
Replication of DNA
Replication of DNA A A
n=1 2N
n=1 4N a A A A A a a
n=1 1N
n=1 2N

12. Chromosomes Basic terms and key features of the chromosome:
Telomere: end of chromosomes
Centromere: It is the constricted region where the microtubules attach and help pull the sister chromatids apart during mitosis
Sister chromatids: replicated chromatids in G2. The two sister chromatids are identical to one another. During prophase and metaphase they look like: A
Homologue- chromosome pair in a diploid. They are similar but not identical. A a
Metaphase plate: the region midway between the two spindle poles in which the chromosomes align during metaphase
Haploid (N)- the condition in which each chromosome is present in one copy (found in gametes)
Diploid (2N): the condition in which each chromosome is present twice as members of a homologous pair

13. Meiosis
Meiosis:
While the mitotic cycle is designed to produce two cells with the identical genotype, the meiotic cycle is designed to produce four cells each with half of the chromosome complement AND non-identical genotype.
Meiosis allows the organism to maintain constant ploidy (following mating) and at the same time to shuffle the genetic deck (in the progeny)
In meiosis:
Diploid cells undergo one round of chromosome replication followed by two divisions thereby reducing ploidy and producing four haploid cells. The two divisions are referred to as Meiosis I and Meiosis II. N
2N > 4N >N
repli 2xdivi N

14. Meiosis-I Meiosis is divided into two parts- Meiosis I and Meiosis II
Interphase I: chromosomes replicate 
Prophase I: chromosomes condense members of a chromosome pair (homologues) physically associate with one another and lie side by side near the metaphase plate. This process is known as synapsis. The paired chromosome physically overlap forming structures known as chiasma. 
Metaphase I: the paired homologous chromosomes, known as bivalents, move to the center of the cell and line up along the metaphase plate. 
Anaphase I: in a process known as disjunction, the members of a homologous pair migrate to opposite poles. This effectively reduces the total number of chromosomes by half and is therefore called a reductional division. 

15. Meiosis-II
(Telophase I/Cytokinesis): if this stage were equivalent to telophase of mitosis, the nuclear envelope would reform and chromosomes would decondense but chromosomes do not decondense and the
The condensed chromosomes proceed directly into Prophase II of meiosis 
Prophase II/Metaphase II chromosomes align at plate
Anaphase II
Sister chromatids segregate to the opposite poles
Telophase II
Four haploid meiotic products
Meiosis II is analogous to mitosis; -chromosomes align along the metaphase plate and the sister chromatids separate

16. MeiosisI in diploid n=2 A a N=2 B b Chromosomes replicate a A N=4 b B
Homologous Chromosomes pair, form chiasma, and then line up on the metaphase plate at random. A a a A OR b B B b

17. Random assortment (a) (b) A a a A OR B b B b AnaphaseI
Sister chromatid centromeres do not separate
The two sister chromatids go to the same pole a A a A B b OR B b
(a)
(b)
TelophaseI/Cytokinesis
Cell divides
Reductional division

18. MetaphaseII MetaphaseIIa MetaphaseIIb
The reduced number of chromosomes in the two cells align on the metaphase plate (no pairing occurs), divide to produce four gametes A a a A B b B b
Cell division without intervening replication!!
Similar to mitotic metaphase a a A A a a A A B B b b B B b b
25%
25%
25%
25%

19. Meiosis In diploids with 2 chromosomes you get 4 different gametes A aB b A a b B A a b B A a b B a b B A A B a b A b a B A B A B a b a b A b A b a B a B
In diploids with 2 chromosomes you get 4 different gametes

20. How did we get genetic diversity?
AABB
aabb A B a b A a B b
AaBb A B A B a b a b A b A b a B a B
NonParental Gametes
Recombinant Gametes
Parental Gametes

21. Gene Shuffling
Unlike mitosis, the meiotic products are not genetically identical. There are two reasons for this
The arrangement of paired homologues on the plate at Metaphase I is random. The random arrangement is the basis for Mendel's principle of independent assortment
2. The paired homologues physically recombine (or crossover with one another) during prophaseI.

22. Crossing over There are two ways of generating genetic variation:
Random assortment of chromosomes (shuffling of chromosomes)
Pairing and Recombination between homologous (maternal and paternal ) chromosomes (crossing-over) in prophase I
A D B C
n=2 organism 4N
a d b c
Homologous chromosomes pair in prophaseI
At least one crossover occurs per homologous pair
A D B C
a d b c
AnaphaseI
A-D B-C
A D
A d BC
A-d B-C
AnaphaseII
a D
a d b c
a-D b-c
a-d b-c

23. Mitotic and meiotic recombination
Recombination can occur during Mitosis and Meiosis
Recombination is initiated by the formation of a double strand break followed by rejoining of the DNA strands
Only meiotic recombination serves the role of re-assorting genes
Mitotic recombination is utilized for repair of double strand breaks in one of a pair of sister chromatids

24. Crossing over is the result of a physical exchange between homologous chromosomes
Cytological studies in maize by Creighton and McClintock (1931) were the first to demonstrate that recombination is the result of a physical exchange between homologous chromosomes
On chromosome 9 in corn there were two markers:
Endosperm composition: Seed color:
Wx waxy C colored
wx starchy c colorless
In addition, the chromosomes were morphologically distinct. Some had a cytologically visible structure known as a knob at the telomere and others had an interchange such that it is longer Wx C
Starchy n colorless
Waxy n colored wx c X Wx C wx c W C F1 w c

25. F1 heterozygous plant crossed to homozygous plantW C w c X w c w c w c W C w c
Parental w c w c W c w c
Recombinant w C w c
The genetic recombinants were also cytological recombinants. This strongly supported the model that recombination involves a physical exchange between homologous chromosomes

26. A a A a
Mitosis Vs meiosis A a A a A a A a A a A a A a