The Cell Cycle and Mitosis

AP Biology

Chromatin

VS.

Chromosomes

Chromatin 2 m of DNA must fit in a 1x10-5 m nucleus.

DNA wrapped around histone proteins to organize it and allow it fit into the nucleus

Remember – it is condensed 200,000 x to fit in the nucleus

It is still loosely coiled enough that enzymes can get into the DNA to copy it and make mRNA for protein synthesis

It is the normal form of DNA during all phases of the cell cycle except mitosis

Chromosomes

DNA compacted 12,000 times from chromatin

Cannot read or copy the DNA in chromosomes – it is too tightly wound

Formed solely during mitosis in order to divide the doubled DNA in ½

Also, in chromosome form, the DNA is protected from destructive enzymes since they can’t get into the tightly coiled structure

Formation of Chromatin and Chromosomes

Chromatin Up Close

DNA Released from a single chromosome

Coiling into Chromosomes

Structure of the Mitotic Chromosome Showing Sister Chromatids, Centromeres, Kinetochores, and Spindle Fiber Attachment

Chromatid – ½ of a chromosome

Sister chromatid – each half of the same chromosome

Centromere – complex of proteins attached to DNA holding the sister chromatids together

Kinetochore – complex of proteins attached to the outside surface of the chromosome at the centromeric region – where spindle fibers attach

The Cell Cycle

Apoptosis

G0 phase: cells do not divide

Ex: Nerve cells

Apoptosis Programmed Cell Death

• Natural part of the cell cycle Nucleases and proteases are specifically activated chop

up the DNA and organelles Different from necrosis (premature death of cells that

occurs when the cell doesn’t have access to blood supply) Can be time activated:

• Development of nervous system• Immune system• Hand and feet• Leaf termination• Cell death after irreparable damage

Cell Cycle – “The Hourly Life of a Cell”What happens when and how

Why do cells divide?• To make a new organism• Growth• Repair• Replacement of normal cell loss• Development

Stages of Mitosis

Interphase

S Stage

Interphase Interphase is not part of mitosis

– it is the time between cell divisions

Interphase includes G1, S, and G2

During interphase the cell is doing its normal metabolic activities like protein synthesis

The cells are performing their duty as part of a tissue

The DNA duplicates to get ready for mitosis

The DNA is in chromatin form

Animal Cell

Plant Cell

Prophase

Prophase The chromatin begins to

condense into chromosomes and become visible in the nucleus

The nuclear membrane begins to break down

Centrosomes duplicate, form spindles, & move to the poles

Proteins attach to chromosomes forming kinetochores

Spindle fibers attach to the kinetochores and chromosomes begin moving

Animal Cell

Plant Cell

Metaphase

Metaphase

The chromosomes are lined up down the equator by the spindles

Animal Cell Plant Cell

Anaphase

Anaphase

The sister chromatids separate at the centromeres

Each chromatid (now called a chromosome) heads to the pole of the cell

The movement is due to kinetochore movement along the spindle fiber microtubules

Animal Cell

Plant Cell

Telophase

http://www.bioweb.uncc.edu/biol1110/Stages.htm

Telophase The chromosomes are

completely to the opposite poles

New membranes start to form around the DNA

The chromosomes begin to decondense back to chromatin

Cytoplasm begins to pinch in animal cells and a cell wall begins to form in plant cells – This is cytokinesis

Animal Cell

Plant Cell

Interphase

After telophase is complete, the cells reenter interphase and go about their normal business

The DNA is totally decondensed, new nuclei reformed, and there are totally 2 new cells

Differences Plant vs. Animal Cell Mitosis

Plant cells do not have centrioles in their centrosomes but animal cells do ?????

Plant cells cannot pinch in due to the cell wall – a new cell wall forms down the middle from the endoplasmic reticulum

Plant cells divide slower due to having to reform the cell wall

Cytokinesis in an animal cell

Cytokinesis in a plant cell

Mitosis Quiz – Animal Cells

MetaphaseInterphase

Interphase

Anaphase

TelophaseProphase

Mitosis Quiz – Plant Cells

http://biology.nebrwesleyan.edu/benham/mitosis/

MetaphaseTelophase

Anaphase

Interphase

ProphaseInterphase

Control of The Cell Cycle

Regulation by Internal Signals

There are checkpoints at the end of G1 and end of G2. Signal molecules cause the cycle to go on or stop. • Protein kinases (static levels) + cyclins

(concentration fluctuates) = active kinases Example:

• MPF is an activated kinase that promotes G2→M

There is a checkpoint at the end of metaphase. Kinetochores produce a delay signal until spindles attach.• After attachment another protein breaks down

the proteins holding the sister chromatids together.

Control of the Cell Cycle

Regulation by External Factors

Growth factors• GF’s bind to cell receptors activating

the cell cycle• Example: Platelet derived growth factor PDGF – in response to a wound, platelets release the GF which cause fibroblasts to proliferate.

Attachment proteins relay a message via cytoskeleton to halt cell cycle

Immortality Why do cells cease to divide?

When cells cease to divide, why do they deteriorate and die?

Does this happen in-vivo and Can something change this?