Cell Cycle and Mitosis

Cell Division The process by which a cell divides into two new “daughter” cells. Daughter cells are the NEW cells  the product of cell division.

Cell Division Before cell division occurs, the cell replicates (copies) all of its DNA. This solves the problem of information overload Each cell gets one complete copy of genetic information

Cell Division Cell division also solves the problem of increasing size by reducing cell volume. Cell division results in an increase in the ratio of surface area to volume. This allows for the efficient exchange of materials within a cell.

Cell Division and Reproduction Asexual Reproduction – the production of genetically identical offspring from a single parent. Sexual Reproduction – fusion of two cells The offspring produced by sexual reproduction inherit some of their genetic information from each parent.

Asexual Reproduction Occurs in many single-celled organisms. Ex, bacterium Cell division Also occurs in multi-cellular organisms Budding – small bud growing off hydra plant will eventually break off and become a new plant

Advantages of Asexual Reproduction Asexual reproduction is very fast. If conditions are right, quick reproduction can be a very good thing. If conditions remain the same, genetically identical offspring is a good thing too.

Disadvantages of Asexual Reproduction A lack of genetic diversity is a disadvantage when conditions change in ways that do not fit the characteristics of an organism.

Sexual Reproduction Many animals and plants reproduce sexually Many single-celled organisms also reproduce sexually Offspring are produced by the fusion of special reproductive cells formed by each of two parents.

Advantages of Sexual Reproduction Provides genetic diversity In a changing environment, genetic diversity provides the right combination of characteristics needed to survive.

Disadvantages of Sexual Reproduction Process of finding a mate and raising offspring requires time.

Chromosomes and the Cell Cycle Threadlike structure within the nucleus that contains genetic information Bundle of DNA Replicated at the beginning of the cell cycle Passed on from parent cell to daughter cell

Prokaryotic Chromosomes Prokaryotes lack nuclei Most prokaryotes contain a single, circular DNA chromosome that contains all, or nearly all, of the cell’s genetic information.

Eukaryotic Chromosomes Eukaryotic cells usually have more DNA than prokaryotes. Eukaryotes contain multiple chromosomes. Fruit flies have 8 chromosomes per cell. Humans have 46 chromosomes per cell.

Eukaryotic Chromosomes Chromosomes make it possible to separate DNA precisely during cell division. Chromosomes form a close association with histones (type of protein). This complex of chromosomes and proteins is known as chromatin.

Chromatin

Cell Cycle During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells.

Prokaryotic Cell Cycle The process of cell division in prokaryotes is a form of asexual reproduction. This process is known as binary fission.

Eukaryote Cell Cycle Consists of 4 phases G1 phase S phase G2 phase M phase

Interphase Period of cell cycle between cell divisions. Divided into 3 phases: G1 phase S phase G2 phase

G1 Phase Cells do most of their growing during G1 phase. Cells increase in size during this phase. Cells synthesize new proteins and organelles during this phase.

S Phase S stands for synthesis (to make). During this phase, new DNA is synthesized (made) when the chromosomes are replicated (doubled). The cell at the end of this phase contains twice as much DNA as it did at the beginning.

G2 Phase During this phase, many of the organelles and molecules required for cell division are produced. Chromatid – one of two identical “sister” parts of a duplicated chromosome Centromere – region of a chromosome where the two sister chromatids attach Centriole – structure in an animal cell that helps to organize cell division

M Phase Two new daughter cells are produced during this phase. M comes from the word mitosis. This phase has two stages – Mitosis Cytokinesis In many cells, this process may overlap.

Mitosis Consists of 4 stages – Prophase Metaphase Anaphase Telophase

Prophase During prophase, the genetic material inside the nucleus condenses and the duplicated chromosomes become visible. Outside the nucleus, a spindle starts to form. Cell starts to build spindles Spindles – fanlike system of microtubules that will help to separate the duplicated chromosomes. Spindle fibers extend from tiny paired structures called centrioles

Prophase and Centrioles Centrioles are only found in animal cells. During prophase centrioles start to move toward opposite ends (poles) of the cell. As prophase ends, the chromosomes coil more tightly, the nucleolus disappears, and the nuclear envelope breaks down.

Prophase

Metaphase During metaphase, the centromeres of the duplicated chromosomes line up across the center of the cell. Spindle fibers connect the centromere of each chromosome to the two poles of the spindle.

Metaphase

Anaphase During anaphase, the chromosomes separate and move along spindle fibers. Once anaphase begins, each sister chromatid is now considered an individual chromosome. Anaphase comes to an end when this movement stops and the chromosomes are completely separated into two groups.

Anaphase

Telophase During telophase, the chromosomes, which were distinct and condensed, begin to spread out in into a tangle of chromatin. A nuclear envelope reforms around each cluster of chromosomes. The spindle begins to break apart. A nucleolus becomes visible in each daughter nucleus.

Telophase

Cytokinesis Cytokinesis completes the process of cell division – it splits one cell into two. This process is different in animal cells and in plant cells.

Cytokinesis in animal cells The cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts.

Cytokinesis in plant cells The cell membrane is not flexible enough to pinch because of the rigid cell wall. A structure known as the cell plate forms halfway between the divided nuclei. The cell plate gradually develops into cell membranes that separate the two daughter cells. A cell wall then forms in between the two new membranes.

Cytokinesis in plant cells