Double the side of a cube and its surface area grows four times while its volume grows eight times. Cells are the same: if one grows too big, its surface cannot keep up with its volume for taking in what it needs and getting rid of wastes, so at a certain size it divides in two.
In mitosis the cell first copies its DNA during interphase, then passes through prophase, metaphase, anaphase and telophase to make two daughter cells with the same number of chromosomes as the parent.
Stepping through the diagram, X-shaped chromosomes line up across the middle of the cell, split into two strands that are pulled to opposite ends, and the cell divides into two.
The extra cells this produces are what let the body grow and wounds heal.
Why Do Cells Divide?
🔬 Bigger Is Worse for a Cell
①As a cell grows, its volume increases with the cube of its length but its surface only with the square, so volume grows much faster
②When surface area is small compared with volume, exchanging materials becomes hard
③So once a cell reaches a certain size, it divides instead of growing larger
④A many-celled organism grows because the number of its cells increases
Stages of Mitosis
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How chromosomes move during interphase, prophase, metaphase, anaphase and telophase
Interphase
📖 What Happens at Each Stage
①Interphase: DNA is copied; this takes up most of the cell cycle
②Prophase: the loose, thread-like chromatin coils into X-shaped chromosomes, the nuclear membrane breaks down and spindle fibers form
③Metaphase: the chromosomes line up across the middle of the cell (the equatorial plate)
④Anaphase: spindle fibers pull the two chromatids of each chromosome to opposite ends
⑤Telophase: nuclear membranes re-form and the cytoplasm divides, giving two daughter cells
Because meiosis halves the chromosome number, offspring still have 2n chromosomes after a sperm and an egg join.
Unit Test
Which statement about cell division is correct?
①In mitosis the chromosome number is halved
②Meiosis divides only once
③In metaphase of mitosis the chromosomes line up at the equator
④Nothing happens during interphase
⑤In anaphase the chromosomes gather at the equator
▸ ③ In metaphase of mitosis the chromosomes line up at the equator
1
In mitosis the chromosome number stays the same (① wrong, meiosis halves it), and meiosis divides twice in a row (② wrong).
2
DNA is copied during interphase (④ wrong), and in anaphase the sister chromatids move to the poles while alignment at the equator is metaphase (⑤ wrong); in metaphase of mitosis the chromosomes line up at the equator (③ correct).