Cell Division

What is cell division important for?

• growth - foetal development, meristem tissue

• tissue repair - wound healing

• cell replacement - skin cells from stem cells in epithelium

What is a nucleosome

8 histones (+ve) loosely wrapped in DNA (-ve)

How are visible chromosomes formed?

Chromatin condenses by supercoiling

What is the metabolically active stage of cell division?

Interphase

Stages of Interphase?

• G₁ Phase

• S Phase

• G₂ Phase

With checkpoints in-between, checking the internal environment of cells.

What occurs in G₁ Phase?

• Cell grows in size

• RNA, enzymes and proteins needed for growth are synthesised

• Mitochondria and Chloroplasts replicate by binary fission

What occurs in S Phase?

DNA is replicated and sister chromatids are formed

What occurs in G₂ Phase?

Cell continues to grow. Energy stores are created.

• Tubulin is synthesised

• Centrioles are replicated

Stages of Mitosis?

• Prophase

• Metaphase

• Anaphase

• Telophase

(Then cytokinesis)

What takes place in Prophase?

• The chromosomes are condensed and appear like jelly worms

• Sister chromatids are joined by centromeres

• Centrioles migrate to opposite poles, spindle fibres are formed from microtubules

• The nucleolus disappears and the nuclear envelope dissolves into vesicles.

What takes place in Metaphase?

• Centrioles arrive at opposite poles, spindle fibres move chromosomes to equator

• Spindle fibres from opposite poles attach to sister chromatids

• Sister chromatids are lined up along the metaphase plate

What takes place in Anaphase?

• Microtubules shorten and contract pulling sister chromatids to opposite poles

• Centromere split

Sister chromatids are still considered chromosomes.

What takes place in Telophase?

• Sister chromatids arrive at opposite poles

• Spindle fibres break down, cell elongates

• Nuclear envelope reforms

• Two nucleioli formed

What regulates these stages of mitosis?

Cyclins - rise and fall in cycles, regulating the start of each stage. They can be affected by internal and external cell conditions.

How do cyclins work?

There are four - for triggering each stage of mitosis. They bind to cyclin dependent kinases.

Kinases then phosphorylate proteins that perform specific functions required for each stage.