The Cell Cycle and Mitosis. AP Biology. Chromatin. Chromosomes. VS . 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
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
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
Chromatin Up Close
Coiling into Chromosomes
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
G0 phase: cells do not divide
Ex: Nerve cells
Programmed Cell Death
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:
Why do cells divide?
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
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
The chromosomes are lined up down the equator by the spindles
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
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
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
Cytokinesis in a plant cell
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
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.
There is a checkpoint at the end of metaphase. Kinetochores produce a delay signal until spindles attach.
Regulation by External Factors
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
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?