CELL DIVISIONWhy do Cells Divide?

Science Starter

◻ Why do cells divide?

All Cells Come From Pre-Existing Cells

Up until the late 17th century it was believed that life could arise spontaneously; like maggots appearing on rotting meat.

In an uncovered jar, maggots and flies appear ‘spontaneously’.

If the jar is covered in mesh, the flies still appear, but not inside the jar!

All Cells Come From Pre-Existing Cells

◻ DNA controls everything our cells do (by controlling which proteins will be made)

◻ DNA must be copied and divided equally into the two cells, or one of the resulting cells will die

◻ All cells in your body have the exact same DNA

What is a Chromosome?

◻ A chromosome is made of two supercoiled DNA strands that are exact copies of one another, and joined in the middle.

Chromosome Numbers

◻ Humans have 46◻ A potato has 48◻ A goldfish has 94◻ A pea has 14◻ Dogs have 72◻ A Shrimp has 254!◻ A fruit fly has 4

The Cell Cycle

Mitosis: Interphase

Normal cell growth occurs; cell roughly doubles its size

Exact copy of DNA is made

Cell Prepares for division

Mitosis: Prophase

1

2Chromosome 1

Chromosome 2

DNA coils and condenses into chromosomes Centrosomes move to poles of cell, spindle fibres appear Nuclear membrane (the outside of the nucleus) disappears

Mitosis: Metaphase

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2

Metaphase Plate

Chromosomes align at the metaphase plate (equator of cell) Spindle fibres attach to the centre of each chromosome

Mitosis: Anaphase

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2

Spindle fibres separate the chromosomes by pulling them towards the centrioles at each pole Remember that each side is still getting one full copy of each chromosome, because they were doubled in interphase!

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1

Mitosis: Telophase

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2

One full set of daughter chromosomes is now at each pole Spindles and centrioles are disassembled and re-used Nuclear membrane forms around each set of chromosomes

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1

Mitosis: Cytokinesis

12

Cytoplasm and all other cellular organelles are divided evenly among the two new cells (cytokinesis).

Chromosomes uncoil into chromatin.

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1

ChromatidChromatin

Science Starter

◻ What is the end goal of mitosis?

◻ What important steps must be taken to get to this goal?

◻ When does mitosis occur in cells?

◻ Where does mitosis occur in living things?

Science Starter

◻ With a partner, draw the stages of mitosis for a cell with 4 chromosomes.

What Phase of Mitosis?

What Phase of Mitosis?

What Phase of Mitosis?

What Phase of Mitosis?

Phases of Mitosis