Development Including Mitosis and Meiosis Biology 155 Krilowicz Spring 2010

I. The Cell Cycle Regular sequence of events in which a cell grows and carries out regular activities, prepares to divide and then divides to produce two new cells G = gap M = mitosis or meiosis, DNA is divided between two new cells Division of cytoplasm and organelles between new cells Fig. 8.5

II. Comparison of Mitosis and Meiosis (Fig. 8.15) Mitosis A. Results in 2 new cells with the same # of chromosomes as the original cell Meiosis A. Results in 4 new cells each with ½ the # of chromosomes as the original cell 2n = diploid cell 2 X 2n cells 2n cell 4 X 1n = haploid cells

II. Comparison of Mitosis and Meiosis - continued Mitosis B. Cytokinesis is usually approximately equal C. Used for – growth repair development (embryogenesis) asexual reproduction Meiosis B. Cytokinesis can be unequal (oogenesis) C. Used for – gametogenesis

Definitions: Chromosome = DNA and its associated proteins, 46 in humans Homologous chromosomes = chromosomes that contain information to control the same genetic trait, one member of each pair came from mom and one came from dad Fig. 20B

Definitions – continued: Copies of the same chromosome that result from S phase Site where sister chromatids are held together, microtubules of spindle apparatus attach here during cell division Fig. 8.4B

Definitions – continued: Equator = center plane of the cell Spindle apparatus = portion of the cytoskeleton that orients and separates sister chromatids or homologous chromosomes during cell division; contains poles and microtubules

Steps in Mitosis Interphase nucleus with 2 homologous pairs of chromosomes S phase Interphase nucleus with 2 homologous sets of chromosomes, each replicated so they contain sister chromatids attached at centromere mitosis Spindle apparatus forms and microtubules from each pole attach to centromeres of individual chromosomes lined up on equator of the cell Spindle “contracts” and separates sister chromatids, moving them to the poles of the cell Cytokinesis 2 new cells with same genetic information as the original cell 2n 2 X 2n Fig. 8.6

Steps in Meiosis Nucleus at the end of interphase with 4 replicated chromosomes Meiosis division 1 Synapsis = physical joining of homologous chromosomes as they move to the equator of the cell, microtubules from one pole of the spindle apparatus attach to the centromere of one member of each homologous pair Spindle “contracts” and separates homologous chromosomes cytokinesis 2n2 X 1n, but with sister chromatids 4 X 1n division 2cytokinesis Individual chromosomes move to equator Spindle “contracts” and separates sister chromatids 4 new cells with ½ the genetic information of the original cell Fig. 8.14

Steps in Development A.Gametogenesis = production of gametes through the process of meiosis 1.Spermatogenesis = sperm cell production 2n 1n 2 meiotic divisions 2n (diploid) testis cell 4 X 1n (haploid) sperm cells Fig. 27.5A

A. Gametogenesis - continued B. Oogenesis = egg cell production 2n 1n 2 meiotic divisions 2n (diploid) follicle cell1 haploid (1n) egg + 3 haploid polar bodies Gets all the cytoplasm and organelles at cytokinesis essentially 3 haploid nuclei that are discarded Fig. 27.5B

B. Fertilization The union of an egg cell and a sperm cell that produces a zygote (fertile egg) and re- establishes the diploid condition 1n 2n fusion egg sperm zygote Fig. 27.9A

C. Embryogenesis embryo formation – consists of cleavage + gastrulation + organ formation All three steps will make use of mitosis to produce new cells

1. Cleavage = division of the single celled zygote into a many celled structure called a blastula (hollow ball of cells); uses mitosis so that all cells in the developing animal will have a complete set of genes Requires multiple mitotic divisions 2n Fluid-filled X.S. Fig

2. Gastrulation = transformation using complex cell movements of a blastula into a multi-layered animal (gastrula); mitotic cell division continues as developing animal continues to increase in size All cells are 2n l.s. x.s. becomes mouth or anus Breaks through to form mouth or anus gut Fig

3. Organ Formation Differentiation and specialization of embryonic tissues (endoderm, ectoderm and mesoderm) into adult tissues (epithelial, connective, muscle and nervous tissues) and organs

Final Steps in Development D. Growth = increase in size; differentiation usually continues E. Maturation = attainment of adult body form capable of reproduction metamorphosis as an example of a dramatic maturation process