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Lect2: Chromosomes and Genetics

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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 a
B 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 plant
W 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

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