Cell Cycle

The Cell Cycle

• During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells. These two daughter cells will begin the cycle again.

• Consists of 4 phases:G1 phaseS phaseG2 phaseM phase

The Cell Cycle

M phase

Events of the Cell Cycle• G1 phase: The cell increases in size and synthesize

new proteins and organelles.• S phase: DNA replication.• G2 phase: Various organelles and molecules

required for cell division are created at this time .

Note: G 1 phase, S phase, G 2 phase all make upinterphase which is the longest event of thecell cycle.

• M phase: Mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm) take place during this phase.

Interphase

• Longest event in a cell’s life.• Period of great metabolic activity.• When a cell can no longer maintain

homeostasis it must divide.

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Helicase

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DNA Replication

• An enzyme called DNA Helicase causes the base pairs of DNA to split down the middle forming 2 single strands.

• The nucleus contains free floating nucleotides from the food you eat.

• These nucleotides are attached to each single strand of DNA by another enzyme DNA Polymerase to form 2 identical double stranded DNA molecules.

Interphase

Questions

• Read pages 240 to 245.

• Answer questions 1 to 4 on page 243.

What is Mitosis?

• Mitosis is the division of the cell nucleus.

• There are 4 main events or phases make up

mitosis:

Prophase

Metaphase

Anaphase

Telophase

Prophase• Replicated chromatin from interphase begins to shorten ,

thicken , and coil up to form Chromatid Pairs .• Nucleolus & nuclear envelope disappears.• Centrioles split and move to opposite sides of the cell (moves

to the poles). Protein fibers begin to radiate from each. These structures are called Asters .

• When asters reach opposite sides of cell they form a Spindle .

Prophase

• Note: Plant Cells do not have centrioles or asters but somehow these spindles still form?

Metaphase

• Shortest stage of mitosis.

• Chromatid pairs align at the equator of the spindle.

Anaphase• Centromere splits

and chromatid pairs separate.

• Spindle fibers begin to contract and drag the Chromosomes to opposite sides of the cell.

Telophase

Telophase

• Final stage of mitosis , chromosomes reach the poles and begin to uncoil to form chromatin.

• Spindle fibers disappear , nuclear envelope & nucleolus begin to re-appear.

• In early telophase the cell membrane begins to indent at the equator to form a Cleavage Furrow (start of actual cytoplasm division called Cytokinesis ).

• In late telophase cleavage furrows have indented further.

• MITOSIS IS NOW OVER !!

What is Cytokinesis

• Refers to the division of the cytoplasm to produce 2 genetically identical daughter cells.

• Animal & Plant cell’s achieve cytokinesis differently.

Animal Cell

• Animal cell divides from the outside in by forming these cleavage furrows. The cleavage furrows eventually pinches off the middle.

Plant Cell Cytokinesis

• Plant cells divide from inside out through the formation of a Cell Plate .