Chapter 2 Mitosis & Meiosis

Cell Types Prokaryotes (bacteria) Eukaryotes Single cell No organelles, no nucleus (nucleoid – circular DNA) Eukaryotes Multicellular (generally) Organelles, organized nucleus

Bacteria Cheek cells

Cell Organization Nucleus Rough/Smooth ER Ribosomes Cell membrane/cell wall Cell coat (animal cells) Cytoplasm, cytoskeleton Mitochondria, chloroplasts, etc.

Nucleus Chromatin/chromosomes Nucleolus (RNA Synthesis) Nucleolus Organization Region (NOR) Sections of DNA that code for rRNA

DNA in the nucleus Chromatin – loosely coiled DNA. DNA exists in this form until ready to divide.

Chromsomes Chromatin condenses before mitosis Chromosome – tightly packed DNA, 2 “sister chromatids” P ARM Q ARM

Karyotypes Karyotype: “chart” of all an organism’s chromosomes

Homologous Chromosomes Most organisms have 2 copies of each chromosome (homologous) Diploid (2n) – somatic cells Haploid (n) – gametes. Haploid cells contain only 1 copy of each chromosome

Homologous Chromosomes 2 copies of each chromosome, 2 copies/version of each gene (ALLELE) Genes are located at identical sites on sister chromatids (LOCI)

Cell Cycle A sequence of cell growth and division Numerous factors control when cells divide

Chrom duplicate during INTERPHASE (90% of cell’s life) G1 phase - cells grow and synthesize biological molecules S phase - DNA replication G2 phase - gap of time between S phase and mitosis (preparation for division) G0 phase

Mitosis Why do cells need to divide? Zygote How much time did you spend as a single egg?

Mitosis Purpose is to ensure the orderly distribution of chromosomes Four Stages: Prophase Metaphase Anaphase Telophase

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Interphase Chromosomes are extended & uncoiled, forming chromatin Centrioles duplicate

Prophase (early) Chromosomes condense, centrioles divide and move apart

Prophase (middle) Also called “prometaphase” Chromosomes doubled, clearly visible Centrioles at poles, spindle fibers form Nucleolus dissapears Nuclear membrane dissolves

Metaphase Chromosomes line up in the middle of the cell Spindle fibers extend from the poles, attached to the chromosomes

Anaphase Centromeres split and sister chromatids (now referred to as chromosomes) migrate to the poles

Telophase Two separate nuclei form Cell returns to conditions similar to interphase Nuclear envelope reforms; nucleoli reappear Cytokinesis occurs

Cytokinesis In animals cells, a furrow develops caused by filaments that encircle the equatorial region In plant cells, a cell plate forms originating from the Golgi complex

Meiosis Why must cells undergo meiosis? Reduce chromosome number Production of gametes (gametogenesis) Sexual reproduction involves the union of gametes to form a zygote Sexual reproduction results in greater variation amongst offspring – offspring are not clones of their parents

How many combinations? Number of combinations possible 2n, where n is the haploid number of the organism If n = 3, there are 8 possible combos Humans: n = 23, there are 223 ~ 8 million possible combinations of chromosomes

A few notes about chromosomes…. Autosomes: non-sex chromosomes How many do humans have? Sex Chromosomes Females: XX Males: XY Only small parts of these have the same genes, most of their genes have no counterpart on the other chromosome

Meiosis – an overview Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice The first division, meiosis I, separates homologous chromosomes The second, meiosis II, separates sister chromatids

Chromosomes separate Chromatids separate

Meiosis contains 2 stages, Meiosis I and Meiosis II Meiosis I and II each include prophase, metaphase, anaphase, and telophase

Meiosis I - Prophase I Homologous chromosomes pair and undergo synapsis (special proteins) Synapsis is the association of four chromatids (two copies of each homologous chromosome) The resulting complex is called a bivalent or tetrad In humans, there are 23 tetrads and 92 chromatids in this phase

Prophase I At several sites, the chromatids of tetrads are crossed (chiasmata) and segments of chromosomes are traded (crossing over)

Synaptonemal complex forms between members of the tetrad & genetic material is exchanged by crossing over Crossing over ensures greater genetic variation

Crossing Over

Meiosis I - Metaphase I Tetrads line up at the equator of the cell

Meiosis I - Anaphase I The homologous chromosome separate and move to the poles Each pole receives a mixture of maternal and paternal chromosomes

Meiosis I - Telophase I Chromosomes decondense The nuclear membrane may reform Cytokinesis usually occurs

Meiosis II - Prophase II Brief Recondensation of the chromosomes Very similar to conditions in prophase of mitosis

Metaphase II & Anaphase II Metaphase II- Chromosomes line up at the equator Anaphase II- the chromatids separate and are now called chromosomes

Telophase II There is one copy of each homologous chromosome at each pole Nuclei form around chromatids Cytokinesis separates the cytoplasm At the end of meiosis, there are (typically) 4 haploid daughter cells

Mitosis vs. Meiosis In mitosis a single division results in two genetically identical daughter cells and there is no crossing over In meiosis, two sets of divisions occur resulting in four genetically different cells. A great deal of genetic diversity occurs caused by synapsis and independent assortment

Meiosis & Genetic Variation The events of meiosis & fertilization are responsible for the variation in each new generation Independent assortment Crossing over Random fertilization

Independent Assortment Contributes to genetic variability due to random orientation of tetrads at metaphase plate 50-50 chance that a particular daughter cell of meiosis I will get the maternal chromosome, 50-50 chance that it will receive paternal chromosome

Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Begins very early in prophase I - homologous chromosomes pair up gene by gene Homologous portions of two nonsister chromatids trade places. Humans - occurs 2 – 3x/chromosome pair

Crossing Over One sister chromatid may undergo different patterns of crossing over than its “mate” Once these undergo independent assortment in meiosis II, variation of gametes increases even more!

Fertilization An ovum is one of ~ 8 million possible chromosome combinations Successful sperm represents one of 8 million different possibilities Resulting zygote - 1 in 70 trillion (223 x 223) possible combinations Crossing over adds even more variation

Taken together… All three mechanisms reshuffle the various genes carried by individual members of a population Mutations, still to be discussed, are what ultimately create a population’s diversity of genes