THE CELL CYCLE AND MITOSIS UNIT 3 ORGANIZATION AND DEVELOPMENT

2 PHASES OF THE CELL CYCLE The cell cycle consists of Interphase – normal cell activity Cell Division – mitosis and cytokinesis INTERPHASE Growth G 1 (DNA synthesis) Growth G 2 Cell Divsion

3 DNA Genetic information – makes up an organism’s genome Packaged into chromosomes 50 µm Figure 12.3

4 MITOSIS AND CELL DIVISION Mitosis makes new cells for repair; to replace old, damaged, or dead cells. Mitosis makes new cells for growth. Somatic (non sex cells) undergo mitosis. In every mitotic division, 2 cells are made. These cells are genetically identical to the parent cell.

5 CHROMOSOMES Chromosomes in Humans: 23 pairs, 46 total Autosomes: non-sex chromosomes, chromosomes 1 – 22 Sex chromosomes: determine sex, X and Y, chromosome 23 Sets of chromosomes Diploid: two sets of chromosomes, 2n (46 in humans) Haploid: one set of chromosomes, n (23 in humans)

6 STRUCTURE OF CHROMOSOMES DNA is an extremely long double-stranded molecule. One strand of DNA makes up a chromosome. Before mitosis, the DNA exists in long, uncondensed strands called - chromatin Each unduplicated chromosome contains one DNA molecule. Duplicated chromosomes are called sister chromatids.

7 STRUCTURE OF CHROMOSOMES DNA wraps around a group of histone proteins to form a nucleosome. Higher order coiling and supercoiling also help condense and package the chromatin inside the nucleus:

8 Because of duplication, each condensed chromosome consists of 2 identical sister chromatids joined by a centromere. Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid: Chromosome Duplication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two unduplicated chromosomes Centromere Sister chromatids Sister chromatids Duplication Non-sister chromatids Two duplicated chromosomes

9 STRUCTURE OF CHROMOSOMES The centromere is where sister chromatid are connected. The kinetochore serve as points of attachment for microtubules that move the chromosomes during cell division. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metaphase chromosome Kinetochore microtubules Centromere region of chromosome Sister Chromatids

10 PHASES OF THE CELL CYCLE Interphase G 1 - primary growth S – DNA synthesis (replication) G 2 - secondary growth M - mitosis C - cytokinesis

11 INTERPHASE G 1 – Growth 1 - Cells undergo majority of growth S – Synthesis - Each chromosome replicates (Synthesizes) to produce sister chromatids Attached at centromere Contains attachment site (kinetochore) G 2 – Growth 2 - Chromosomes condense - Assemble machinery for division such as centrioles G 0 – cells exit the cell cycle from G 1 (they are not preparing for division. ** During interphase, the cells are also carrying out their function(s).

12 G 2 of Interphase A nuclear envelope bounds the nucleus. Centrosome duplicated (one for each end of the cell). Chromatin is not fully are condensed, but DNA has been replicated during S phase. Chromosomes are not yet visible. G 2 OF INTERPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Nucleolus Nuclear envelope Plasma membrane

13 Prophase Chromatin has condensed - chromosome are visible. Each duplicated chromosome appears as two identical sister chromatids joined together at the centromere. The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The centrosomes move to opposite ends of the cell and microtubules become longer. PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids

14 Metaphase The centrosomes are now at opposite ends of the cell. The chromosomes line up on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The centromeres lie on the metaphase plate. Kinetochores are attached to microtubules coming from opposite poles. The entire apparatus of microtubules is called the spindle because of its shape. METAPHASE Spindle Metaphase plate Centrosome at one spindle pole

15 Anaphase Sister separate. Each chromatid thus becomes a chromosome. Chromosomes begin moving toward opposite ends of the cell (their kinetochore microtubules shorten). The chromosomes move centromere first. The cell elongates as the other microtubules lengthen. By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes. ANAPHASE Daughter chromosomes

16 Telophase Nuclei begin to form in the “new” cells. Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope. The chromosomes become less condensed – chromatin. Mitosis is completed. TELOPHASE AND CYTOKINESIS Nucleolus forming Cleavage furrow Nuclear envelope forming

17 CYTOKINESIS Cleavage of cell into two halves – Animal cells  Constriction of actin filaments  Cleavage furrow – Plant cells  Cell plate

18 MITOSIS IN AN ANIMAL CELL G 2 OF INTERPHASEPROPHASE PROMETAPHASE Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore microtubule Nonkinetochore microtubules

19 METAPHASEANAPHASETELOPHASE AND CYTOKINESIS Spindle Metaphase plate Nucleolus forming Cleavage furrow Nuclear envelope forming Centrosome at one spindle pole Daughter chromosomes MITOSIS IN AN ANIMAL CELL

20 MITOSIS IN A PLANT CELL 1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell Nucleus Nucleolus Chromosome Chromatin condensing

21 CYTOKINESIS IN ANIMAL AND PLANT CELLS Daughter cells Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm 1 µm Vesicles forming cell plate Wall of patent cell Cell plate New cell wall (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)

22 CELL DIVISION IN PROKARYOTES Prokaryotes lack nuclei and membrane-bound organelle DNA is a circular chromosome Binary Fission: DNA is copied  two circular chromosomes New membrane begins to develop Cells grows to double its size Pinches off into two identical cells