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There are two different ways for cells to divide:

mitosis – makes exact copies.
process by which almost all cells reproduce.
used by plants, animals, fungus, protists, bacteria.

meiosis – makes deliberately changed copies,
shuffles the genes so that they are different every time.
only sperm and egg cells are made this way.

Here we focus on mitosis.

Basic idea:

1 parent cell produces 2 identical daughter cells.

Each daughter cell is initially smaller than the parent, but then they grow back to full size.

daughter cells are – plus or minus a few genetic mistakes (mutations) – clones of the parent cell.


The diagram shows mitosis on left, & meiosis on right.

Let’s follow the left-hand sequence.

These are not human cells. These are simple models, with fewer chromosomes.


Start at the top/middle. This is the parent cell.

Here, how many chromosomes are there?        4

How many are blue (from mom)                             2

How many are red (from dad)                                  2

Why are there 2 of each? 1 chromosome from each parent

– – – –

In human cells, how many chromosomes are there?                        46

Why are they in pairs?                                                  1 from each parent

In humans, how many chromosomes are from the mom?         23

In humans, how many chromosomes are from the dad?           23

Prophase: The next step in the diagram

Instead of 4 single chromosomes, what do we have now?           4 pairs.

What must have happened?              Chromosomes were duplicated

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

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

– – – – –

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

Metaphase: The next step in the diagram

A spindle apparatus is built.

Each chromatid attaches to the spindle

They line up in the center of the cell

Anaphase and Telophase

Sister chromatids split apart. Half move left, half move right.

Parent cell splits into 2 daughter cells.

Each is a copy (“clone”) of the parent.

How do chromosomes move ?

Cells build a spindle apparatus – a beautiful network of thin protein fibers.

The chromosomes then climb along the spindle like a mountain climber goes up ropes.

mitosis spindle From Sara Moutinho Pereira CID Lab

Drosophila S2 cell undergoing mitosis. The mitotic spindle is in yellow and chromosomes are in blue. (Credits: Sara Moutinho Pereira/CID Lab)

Why is this called a spindle?
Recall from history class – a spindle is a disk-shaped tool, with a hole in the center, used to spin threads into cloth.
When biologists first discovered this, it reminded them of this.


How do the chromosomes climb along this spindle?
Like how mountaineers climb up a rope.

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

carabiner mountain climber graphic

The metal clip is a carabiner. This image from iafstore.com, “Discensore Gigi + moschettoni Oval e Argon screw”

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

Cells have carabiners, too! Here’s the analogy:

cell                     =  mountain
spindle              = ropes
chromosomes = mountain climber
Kinetochore    = carabiner

Centromere vs Kinetochore microtubule

Chromosome (mountaineer) use a kinetochore (carabiner) to attach to the spindle apparatus (rope) to move up the cell (mountain.)

skills_crevasse Carabiner Anchor rope

from pbskids.org/nova/denali/crevasse.html

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.

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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