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 eukaryotesMitosis: a form of cell division that produces two daughter cells of identical genotypes.2N2N 4NMeiosis: a form of cell division in a diploid cell that produces four haploid cells (Gametes)N2N 4NMeiosis 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 traits2N 2N| |4N 4NN N|2N4NMendel’s laws occur only in germ cells of the parents during gamete formation/meiosisYou will most often deduce what happened during meiosis by looking at phenotype in the developed progenymeiosismitosis
5 Digression: Chromosome number Species Chromosome numberin haploid cells (n)Human 23Monkey 21Mouse 20Frog 13Fruit fly 4C. Elegans 6Corn 10S. Cerevisiae 16S. Pombe 3Smallest 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” ATotally differentHaploidBHaploids have 1N (or 1n with n=2) DNA contentDiploids have 2N (or 2n with n=2) DNA contentTetraploids have 4N (or 4n with n=2) DNA contentChromosome number = Autosome + sex chromosome99.99% identicaln=2AaTotally differentDiploidbB99.99% identical
7 MitosisMitosis 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 identicalchromosome complements.
8 Mitotic cell cycle in diploids HomologousChromosomes99.99% identicalHomologous chromosomesAaAan=22NBbReplication of DNASister chromatidstelomereAaAan=24NcentromereBbEach DNA mol is a chromatidTwo chromatids attached at centromere (via proteins) are sister chromatidsSister chromatids are 100% identical to each other
9 Sister chromatids separate to opposite poles MitosisChromosomes line up at the metaphase plate.Abn=24NBaSister chromatids separate to opposite polesAn=24NAaaBBbb
10 MitosisAAan=22NaBBbbTwo Daughter cells created that are identical to original mother cellAan=22NBbCan Mitosis occur in haploid cells?
11 Mitosis in haploid and diploid Replication of DNAReplication of DNAAAn=12Nn=14NaAAAAaan=11Nn=12N
12 Chromosomes Basic terms and key features of the chromosome: Telomere: end of chromosomesCentromere: It is the constricted region where the microtubules attach and help pull the sister chromatids apart during mitosisSister chromatids: replicated chromatids in G2. The two sister chromatids are identical to one another. During prophase and metaphase they look like:AHomologue- chromosome pair in a diploid. They are similar but not identical.AaMetaphase plate: the region midway between the two spindle poles in which the chromosomes align during metaphaseHaploid (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 MeiosisMeiosis: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.N2N > 4N >Nrepli 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 theThe condensed chromosomes proceed directly into Prophase II of meiosis Prophase II/Metaphase II chromosomes align at plateAnaphase IISister chromatids segregate to the opposite polesTelophase IIFour haploid meiotic productsMeiosis 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.AaaAORbBBb
17 Random assortment (a) (b) A a a A OR B b B b AnaphaseI Sister chromatid centromeres do not separateThe two sister chromatids go to the same poleaAaABbORBb(a)(b)TelophaseI/CytokinesisCell dividesReductional 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 gametesAaaABbBbCell division without intervening replication!!Similar to mitotic metaphaseaaAAaaAABBbbBBbb25%25%25%25%
19 Meiosis In diploids with 2 chromosomes you get 4 different gametes A a BbAabBAabBAabBabBAABabAbaBABABababAbAbaBaBIn diploids with 2 chromosomes you get 4 different gametes
20 How did we get genetic diversity? AABBaabbABabAaBbAaBbABABababAbAbaBaBNonParental GametesRecombinant GametesParental Gametes
21 Gene ShufflingUnlike mitosis, the meiotic products are not genetically identical. There are two reasons for thisThe arrangement of paired homologues on the plate at Metaphase I is random. The random arrangement is the basis for Mendel's principle of independent assortment2. 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 IA DBCn=2 organism4Na dbcHomologous chromosomes pair in prophaseIAt least one crossover occurs per homologous pairA DBCa dbcAnaphaseIA-D B-CA DA dBCA-d B-CAnaphaseIIa Da dbca-D b-ca-d b-c
23 Mitotic and meiotic recombination Recombination can occur during Mitosis and MeiosisRecombination is initiated by the formation of a double strand break followed by rejoining of the DNA strandsOnly meiotic recombination serves the role of re-assorting genesMitotic 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 chromosomesOn chromosome 9 in corn there were two markers:Endosperm composition: Seed color:Wx waxy C coloredwx starchy c colorlessIn 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 longerWxCStarchy n colorlessWaxy n coloredwxcXWxCwxcWCF1wc
25 F1 heterozygous plant crossed to homozygous plant WCwcXwcwcwcWCwcParentalwcwcWcwcRecombinantwCwcThe genetic recombinants were also cytological recombinants. This strongly supported the model that recombination involves a physical exchange between homologous chromosomes