The Cell Cycle

A mechanism for cell growth and division

Why Do Cells Divide?

• Growth and repair of tissue in multi-cellular organisms/Eukaryotes.

• Method of reproduction for single celled organisms/Prokaryotes (binary fission).

• Also important in the specialization of cells into tissue and organs.

• Keeps cell size small (very important).

Cell Size

Surface Area (length x width x 6)

Volume (length x width x height)

Ratio of Surface Area to Volume

Ratio of Surface Area to Volume in Cells

Section 10-1

Why Are Cells Small?

• MOST CELLS ARE SMALL FOR TWO REASONS:

1. A SMALL CELL HAS MORE SURFACE AREA THAN A LARGE CELL FOR A GIVEN VOLUME OF CYTOPLASM and can absorb more nutrients more efficiently/quickly. 

2. THE CELL'S NUCLEUS (THE BRAIN) CAN ONLY CONTROL A CERTAIN AMOUNT OF LIVING, ACTIVE CYTOPLASM therefore large cells would require enormous amounts of DNA in order to function properly.

Overview of The Cell CycleThe Cell Cycle is divided into 4 stages:

–G1-phase –S-phase–G2-phase

- M phaseCytokinesis

The Cell cycle is an endless repetition of mitosis, cytokinesis, growth,

DNA replication

Interphase- period of time between divisions and includesG1-phase period of initial growthS-phase “synthesis” DNA is being replicatedG2-phase Second period of growth and preparation for division

M-phase =“mitosis” is the division of the nucleus Cytokinesis division of the cytoplasm and organelles

Cell Division

Interphase• The longest part of the cell life cycle• Divided into three distinct stages:

G1 or Gap 1 phase-• initial growth of newly formed cell, organelles duplicated,

protein production• Cells increases in mass and volume, organelles are

replicated• Dna exists as uncoiled Chromatin

S or Synthesis Phase- • DNA and chromosomal protein replication (chromosome

doubled)*Chromosomes are not visible, exists as chromatin (uncoiled DNA)

G2 or Gap 2 Phase- • growth after DNA replicates, cell prepares itself for division

making proteins needed to form the spindle fibers which will move the chromosomes in mitosis.

Chromosomes • DNA in Eukaryotic cells is

condensed and packaged into units called chromosomes

• Humans have 46• Each Chromosome consist

of two DNA molecules.• Copies where made in

DNA replication(S-phase)• Because these two Dna

molecules are exact, Each copy is called a Sister Chromatid

Centromere

gene

Section 10-2 Mitosis and Cytokinesis

•Mitosis is nuclear division of cells process where cells divide, sort, and package, genetic material (DNA) prior to cell division.•Divided into four distinct stages

–Prophase–Metaphase–Anaphase–Telophase

•Cell Division involves mitosis and the separation of the cytoplasm and organelles in Cytokinesis

Overview of The Cell Cycle• Interphase

– G1-phase – S-phase– G2-phase

M phase “Mitosis” (Division of the nucleus and separation of genetic

material)

Divided into: Prophase, Metaphase,Anaphase,Telophase.

Cytokinesis– Actual division of

the cell and cytoplasm into two daughter cells

G1 phase

S phase

G2 phase

Cell Division

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomes

Metaphase

Centriole

Spindle

Centriole

Chromosomes (paired chromatids)Prophase

Centromere

Spindle forming

Section 10-2

Figure 10–5 Mitosis and Cytokinesis

Prophase• Chromosomes condense/coil and

become more visible• Centrioles move towards

opposite poles• Nuclear membrane breaks down

and disappears

•Centrioles are now at opposite poles of the nucleus•Spindle fibers extend from the centrioles at the poles to the centromeres on the chromatids

Prometaphase

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomes

Metaphase

Centriole

Spindle

Centriole

Chromosomes (paired chromatids)

Prophase

Centromere

Spindle forming

Section 10-2Figure 10–5 Mitosis and

Cytokinesis

Metaphase

• Chromosomes line up at the equatorial plane of the cell

• Spindle fibers are attached to the centromere of each chromatid

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

AnaphaseIndividual

chromosomes

Metaphase

Centriole

Spindle

CentrioleChromosomes

(paired chromatids)

Prophase

Centromere

Spindle forming

Section 10-2

Figure 10–5 Mitosis and Cytokinesis

Anaphase

• Begins when the spindle fibers start pulling the sister chromatids toward the poles.

• Kineticore (centromeres) detach/break and spindles drag each sister chromatid to the poles.

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomes

Metaphase

Centriole

Spindle

CentrioleChromosomes

(paired chromatids)

Prophase

Centromere

Spindle forming

Section 10-2

Figure 10–5 Mitosis and Cytokinesis

Telophase

• Final phase of mitosis

• Chromosomes decondense

• Nuclear membrane begins to reform around each set of chromosomes

• Spindle fibers begin to break down

• Cell membrane begins pinching or cleaving

Centrioles

Chromatin

Interphase

Nuclear envelope

Cytokinesis

Nuclear envelope reforming

Telophase

Anaphase

Individual chromosomes

Metaphase

Centriole

Spindle

CentrioleChromosomes

(paired chromatids)

Prophase

Centromere

Spindle forming

Section 10-2

Figure 10–5 Mitosis and Cytokinesis

Cytokinesis• Last stage of the cell cycle, also the

shortest in terms of time• Separation of the cytoplasm and

organelles• Cytokinesis in plant and animal cells

differs-animal cells divide by pinching from the outside towards the inside forming an indentation known as a cleavage furrow

-plants divide from the inside towards the outside by forming small membranous vesicles known as a cell plate(c)

Summary of Mitosis -a form of cell division that produces two daughter cells, each having the same genetic content as the parent cell.

Parent cell 46 chromosomes 2 daughter cells 46 chromosomes

-Produces diploid cells- having 2 copies of each chromosome (2n).-Chromosomes are made up of identical chromatids (sister chromatids) -Chromatids are separated and then segregated to different daughter cells. -Produces two daughter cells (genetic clones) 

- occurs in the somatic cells (body cells) forming muscle, epithelial (skin), nervous, and connective tissues

How is the Cell Cycle Controlled?

• Genes (segments of DNA) instruct the cell to produce certain proteins and enzymes.

• Genes are like light switches, can be turned on/off

• Genes contain protein units of information (cyclins) that instruct the cell in division and act as internal regulators of cell division

• External regulators also regulate growth• Contact inhibition-cell to cell communication

between the plasma membranes of side by side cells

What if the Cell Cycle goes Out of Control?

• Mistakes in the internal regulation of the cell cycle is known as CANCER

• Uncontrolled division of cells results from an overproduction of cyclin proteins by genes

• External factors can also influence genes and ultimately cell division

• Mutagenic agents- nicotine,UV light, PCB’s