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We’re going to go through this diagram step-by-step:

And we’re going to learn that there are two different ways for cells to divide:
mitosis (for making exact copies) and meiosis (for making sperm and egg cells)

This diagram shows us both processes at once (Mitosis is on the left)

Mitosis: middle top of the diagram (“Parent cell”)

In this cell, how many chromosomes are there in total?        4

How many blue chromosomes are there?                             2

Why are there 2 of them?                                                       There is 1 from each parent

How many red chromosomes are there?                              2

Why are there 2 of them?                                                       There is 1 from each parent

– – – –

In human cells, how many chromosomes are there in total?                        46

Why are they found in pairs?                                                  There is 1 from each parent

In human cells, how many chromosomes are from the mom?         23

In human cells, how many chromosomes are from the dad?           23

Mitosis: Prophase
Instead of four single chromosomes, what do we have now?           4 pairs of chromosomes. Original ones were duplicated

What do we call the set of the original and copy?                              Sister chromatids

What is the total number of chromosomes right now?                      4 x 2 = 8

– – – – –

In a human cell, what would the total number of chromosomes be?            46 x 2 = 92

Mitosis: Metaphase

The cell builds a spindle apparatus – a network of thin protein fibers.
They stretch from one side of the cell to the other side.
Why is this called a “spindle”? Do you remember seeing pictures like this in history books?
A spindle is a disk-shaped tool, with a hole in the center, used to spin threads into cloth.


Mountain climbers use metal anchors to dig into a rock wall, attach ropes with a carabiner, and climb up the wall.

(Carabiners are a metal loop with a spring-loaded gate. These let you quickly connect or disconnect.)

Cells do something similar.  The cell itself is the rock wall.
The ropes are the spindle apparatus.
The chromosomes are the mountain climber.

When cells divide, half of the chromosomes “climb” one way, while half of them climb the other way.

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During metaphase, we see the sister chromatids attach to the spindle apparatus.
The thin protein “ropes” connect to them. The objects attached to them are chromosomes.
skills_crevasse Carabiner Anchor rope
carabiner mountain climber graphic

Mitosis: Anaphase and Telophase (In the main diagram, the images on the lower left)

The sister chromatids have split apart. Half of them moved to the left, half moved to the right.

Then the parent cell split apart into two daughter cells.

Result? We have 2 new cells.  Each is an exact copy (“clone”) of the parent.

There may be a few random mistakes (“mutations”) which make the daughter cells slightly different from the parent.

Let’s look at this in more detail:
Meiosis I Metaphase I

The two chromosomes in each bivalent separate and migrate toward opposite poles.

Meisos B

The homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.
Meiosis C

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-LS1-4. Construct an explanation using evidence for why the cell cycle is necessary for the growth, maintenance, and repair of multicellular organisms. Model the major events of the cell cycle, including (a) cell growth and DNA replication, (b) separation of chromosomes (mitosis), and (c) separation of cell contents.
State Assessment Boundary: Specific gene control mechanisms or specific details of each event (e.g., phases of mitosis) are not expected in state assessment.

Disciplinary Core Idea Progression Matrix: “In multicellular organisms, the processes of mitosis and differentiation drive an organism’s growth and development. Each chromosome pair contains two variants of each gene. Offspring that result from sexual reproduction inherit one set of chromosomes from each parent”


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