You are performing mitosis. Where is this occurring? Describe what is happening.

Mitosis Nuclear division plus cytokinesis – produces two identical daughter cells – Part of cell cycle

Cell Cycle Set of events in cell growth and division into two daughter cells. Some cells cease to divide and are not considered to be part of the cell cycle. – Examples are: Cells from the heart, eye, and N.S. Cell cycle duration – 8 minutes in some embryos – as long as one year in liver – Most mammalian cells about 24 hours.

Cell Cycle

Stages of Cell Cycle G 1 – Gap 1 S – Synthesis – when DNA is replicated G 2 – Gap 2 M – Mitosis – when chromosomes separate and cytokinesis occurs

G1G1 Cells grow in size – Produces RNA and synthesize protein Cells may become quiescence - no longer proliferating – Cells maintain themselves An important cell cycle control mechanism activates (G1 Checkpoint) – It ensures that everything is ready for DNA synthesis.

S phase - synthesis Cell replicates genetic material A family of enzymes make an exact replica of the DNA. In human cells, just 6-hours are needed to replicate 46 chromosomes – totalling six billion nucleotides, kept in exact sequence as parental DNA – A mistake = mutation

G 2 Phase Cell continues to grow and produce new proteins. At the end of this gap is a control checkpoint (G2 Checkpoint) to determine if the cell can enter M (mitosis) and divide.

M Phase - Mitosis Cell growth and protein production All of the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis lasts one to two hours. There is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division.

Regulation of the cell cycle How cell division (and thus tissue growth) is controlled is very complex. – Errors can lead to uncontrolled cell growth – which is how tumors are formed The passage of a cell through the cell cycle is controlled by proteins in the cytoplasm.

Control of cell cycle

Main proteins in animal cells Cyclins – Levels rise and fall with the stages of cell cycle. – G1 cyclin (cyclin D) – S-phase cyclins (cyclins E and A) – mitotic cyclins (cyclins B and A)

cyclin dependent kinase Cdks – G 1 Cdk (Cdk4) – S-phase Cdk (Cdk2) – M-phase Cdk (Cdk1) Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. They add phosphate to protein They are the major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M.

MPF – Maturation Promoting Factor – triggers progression through the cell cycle

p53 – Protein blocks the cell cycle if the DNA is damaged – If the damage is severe this protein can cause apoptosis (cell death).

p27 a protein that binds to cyclin and cdk blocking entry into S phase.

Steps in Mitosis Interphase, prophase, prometaphase, metaphase, anaphase, and telophase.

Interphase Stages G 1, S, and G 2 of the cell cycle. The cell preparing for mitosis Chromosomes are not clearly visible

Interphase At start of S (synthesis) stage, each chromosome is composed of one coiled DNA double helix molecule, which is called a chromatid. At the end of this stage, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids (joined at the centromere). During S phase, the centrosome is also duplicated.

Chromosome – two chromatids

Prophase Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.

Prophase

Prometaphase The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.

Prometaphase

Metaphase Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate – helps to ensure each new nucleus will receive one copy of each chromosome.

Metaphase Metaphase plate

Anaphase Paired chromosomes separate at the kinetochores Chromatids move to opposite sides of the cell.

Anaphase

Telophase Chromatids arrive at opposite poles of cell. New membranes form around the daughter nuclei. The chromosomes disperse. The spindle fibers disperse. Cytokinesis begins.

Telophase