Mitosis & Meiosis

Phases of the Cell Cycle consists of two phases Mitotic (M) phase = mitosis and cytokinesis) Interphase = cell growth and copying of DNA in preparation for cell division Interphase - about 90% of the cell cycle can be divided into sub-phases G1 phase -“first gap” – preparation for S phase S phase “synthesis” – duplication of DNA G2 phase - “second gap”- preparation for mitosis http://www.wisc-online.com/objects/index.asp?objID=AP13604

Some terms to know -genome = all the DNA within the nucleus -parent cell = cell about to undergo division (either mitosis or meiosis) -daughter cell = cell that results from either mitosis or meiosis -somatic cell = any cell within the body other than an egg or sperm -a somatic cell has two complete sets of chromosomes -one set is called the haploid number of chromosomes (n) -therefore the somatic cell is said to be diploid (2n) e.g. humans n = 23 (2n = 46) -germ cell or gamete = sex cell -gamete has only one set of chromosomes and is haploid

Cellular Organization of the Genetic Material before the cell can divide – it must duplicate its DNA REMINDER: eukaryotic chromosomes consist of chromatin, a complex of DNA and histone proteins following replication, the DNA condenses into chromosomes starts just prior to Mitosis 20 m every eukaryotic species has a characteristic number of chromosomes in each cell nucleus e.g. humans – n=23 e.g. drosophila – n=2 e.g. dog – n=39

chromosome = organized structure of DNA and protein chroma = color soma = body DNA is duplicated during the S phase of the cell cycle then starts to condense just before mitosis each condensed & duplicated chromosome is made of two sister chromatids these chromatids will separate during cell division and be partitioned into daughter cells chromatids are joined by a structure called a centromere highly condensed region of the duplicated chromosome role in attaching the duplicated chromosome to the spindle 0.5 m Centromere Sister chromatids Figure 12.4 A highly condensed, duplicated human chromosome (SEM).

Mitosis is conventionally divided into five phases Prophase Metaphase Anaphase Telophase Cytokinesis begins during the latter stages of mitosis For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files.

Figure 12.7 Exploring: Mitosis in an Animal Cell

Mitosis http://www.loci.wisc.edu/outreach/bioclips/CDBio.html http://faculty.stcc.edu/BIOL102/Lectures/lesson8/mitosis.htm Prophase: 1. the replicated DNA begins to condense into sister chromatids joined at the centromere  (duplicated) chromosome 2. the centrioles (replicated at G2) move apart from each other 3. the spindle forms between the centrioles (made of microtubules) 4. the condensing, duplicated chromosomes attach to the spindle (via a kinetochore) 5. the nuclear envelope disintegrates 6. the nucleolus disappears Spindle – structure that includes the two centrioles, two asters and the spindle microtubules than span the cell Kinetochore – region of the centromere where the chromosome attaches to the spindle Prophase Fragments of nuclear envelope Nonkinetochore microtubules Kinetochore Kinetochore microtubule Centromere Kinetochore Microtubules Chromatid

Metaphase: centrioles are now at opposite ends of the cell and the spindle is complete 1. the chromosomes move and line up along the equator of the cell = metaphase plate

Anaphase: shortest of the mitotic phases 1. the sister chromatid separate into daughter chromosomes -one chromatid/chromosome moves toward one centriole of the cell, the other the opposite -pulled apart by the action of the spindle ** At the end of this phase – each end of the cell has equivalent numbers of chromosomes – same number as the parent cell

Telophase: reverse of Prophase & takes place along with the event Cytokinesis 1. nuclear envelope reforms & two daughter nuclei result 2. the nucleolus reappear 3. the spindle disappears 4. daughter chromosomes uncoil  chromatin

Cytokinesis: division of the cytoplasm into the 2 daughter cells (cytokinesis = cell motion) -mitosis can occur with cytokinesis  a single cell with 2 nuclei -so actual cell division is cytokinesis!!! -mitosis is actually the division of the nucleus = karyokinesis (a) Cleavage of an animal cell (SEM) Cleavage furrow Contractile ring of microfilaments Daughter cells 100 m -formation of a cleavage furrow that expands to split the cell into daughters

10 m Cell plate 5 Telophase Plant cell Cytokinesis: No cleavage furrow possible -vesicles containing parts of the new cell wall form and migrate to the middle of the cell -vesicles fuse together to produce a cell plate -cell plate grows and eventually splits the parent into two daughter cells (b) Cell plate formation in a plant cell (TEM) Vesicles forming cell plate Wall of parent cell Cell plate New cell wall Daughter cells 1 m 10 m Telophase 5 Cell plate Figure 12.10 Cytokinesis in animal and plant cells.

Meiosis results in four haploid daughter cells with half the number of un-replicated chromosomes as the parental cell

The Stages of Meiosis Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes Interphase Meiosis I Meiosis II 2 1 after chromosomes duplicate in interphase, two divisions follow Meiosis I: results in two haploid daughter cells containing half the number of duplicated chromosomes as the parent parent – 46 duplicated chromosomes daughter cell – 23 duplicated chromosomes Meiosis II: results in four haploid daughter cells containing single chromatid/chromosomes new “parent cell” – 23 duplicated chromosomes daughter cell – 23 chromatid/chromosomes

Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Homologous chromosomes Fragments of nuclear envelope Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Centromere (with kinetochore) Metaphase plate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Sister chromatids remain attached Homologous chromosomes separate Each pair of homologous chromosomes separates. Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. Figure 13.8 Exploring: Meiosis in an Animal Cell Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I and cytokinesis

many similarities with mitotic prophase Prophase I many similarities with mitotic prophase chromosomes begin to condense within the nucleus the centrioles migrate and the spindle begins to forms chromosomes attach to the spindle BUT a unique event happens – synapsis = pairing of 2 homologous chromosomes the paired chromosomes are called a tetrad this pairing is necessary for a special event called crossing over crossing over results in the exchange of genetic information between chromosomes http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html

What are homologous chromosomes? a set of one maternal chromosome and one paternal chromosome e.g. maternal chromosome 1 + paternal chromosome 1 homologous, duplicated chromosomes pair up with each other inside a cell during meiosis

Metaphase I Metaphase Metaphase I tetrads line up at the metaphase plate - with one chromosome facing each pole & the other chromosome facing the other pole crossing over takes place during this phase alignment is different from mitosis Metaphase I Metaphase

Crossing Over introduces genetic variability by exchanging genetic information between homologous chromosomes Anaphase II Gametes Metaphase II Anaphase I Metaphase I

Anaphase I the duplicated homologous chromosomes separate each duplicated chromosome moves toward each pole - guided by the spindle apparatus

Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Division in meiosis II also occurs in four phases Prophase II Metaphase II Anaphase II Telophase II and cytokinesis Meiosis II is identical to mitosis Figure 13.8 Exploring: Meiosis in an Animal Cell