How is your DNA organized?
Your DNA is organized into chromosomes, the pink structures pictured above. Your DNA
doesn't always look so pretty, though. It only winds tightly into chromosomes when the cell is
getting ready to divide. If your DNA wasn't organized into chromosomes, your DNA would look
like a mass of strings and would be difficult to divide up!
Mitosis and Chromosomes
The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei
(plural for nucleus) must form, so that one nucleus can be in each of the new cells after the cell
divides. In order to create two genetically identical nuclei, DNA inside of the nucleus must be
copied or replicated. This occurs during the S phase of the cell cycle. During mitosis, the copied
DNA is divided into two complete sets, so that after cytokinesis, each cell has a complete set of
genetic instructions.
Chromosomes
To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each
organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23
pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister
chromatids, forming an "X" shaped molecule (Figure below). The two chromatids are held
together on the chromosome by the centromere. The centromere is also where spindle fiber
microtubules attach during mitosis. The spindles separate sister chromatids from each other.
The DNA double helix wraps around proteins (2) and tightly coils a number of times to form a
chromosome (5). This figure shows the complexity of the coiling process. The red dot shows the
location of the centromere, which holds the sister chromatids together and is where the spindle
microtubules attach during mitosis and meiosis.
Four Phases of Mitosis
During mitosis, the two sister chromatids must be divided. This is a precise process that has four
individual phases to it. After the sister chromatids separate, each separate chromatid is now
known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid.
So: each chromosome after this separation is made of "1/2 of the X." Through this process, each
daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase,
metaphase, anaphase and telophase (Figure below).
1. Prophase: The chromosomes "condense," or become so tightly wound that you can see
them under a microscope. The membrane around the nucleus, called the nuclear
envelope, disappears. Spindles also form and attach to chromosomes to help them move.
2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The
chromosomes line up in a row, one on top of the next.
3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull
the chromatids apart, resulting in two sets of identical chromosomes.
4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes
in both cells.
An overview of the cell cycle and mitosis: during prophase the chromosomes condense, during
metaphase the chromosomes line up, during anaphase the sister chromatids are pulled to opposite
sides of the cell, and during telophase the nuclear envelope forms.
After telophase, each new nucleus contains the exact same number and type of chromosomes as
the original cell. The cell is now ready for cytokinesis, which literally means "cell movement."
During cytokinesis, the cytoplasm divides and the parent cell separates, producing two
genetically identical cells, each with its own nucleus. A new cell membrane forms and in plant
cells, a cell wall forms as well. Below is a representation of dividing plant cells (Figure below).
This is a representation of dividing plant cells. Cell division in plant cells differs slightly from
animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find
examples of the different stages of mitosis?
Vocabulary
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anaphase: Third phase of mitosis in which sister chromatids separate and move to
opposite sides of the cell.
centromere: Region of a chromosome where sister chromatids are joined together.
chromosome: Structure made of DNA and proteins that contains the genetic material of a
cell.
cytokinesis: Division of the cytoplasm.
metaphase: Second phase of mitosis in which the chromosomes are aligned in the center
of the cell.
mitosis: Division of the nucleus.
nucleus: Eukaryotic cell structure that contains the genetic material, DNA.
prophase: Initial phase of mitosis in which chromosomes condense, the nuclear envelope
dissolves and the spindle begins to form.
sister chromatids: Two identical copies of a chromosome.
spindle: Structure that helps separate the sister chromatids during mitosis; also separates
homologous chromosomes during meiosis.
telophase: Final phase of mitosis in which a nuclear envelop forms around each of the
two sets of chromosomes.
Summary
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The DNA in the nucleus wraps around proteins to form chromosomes.
During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei.
Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase.
Practice
Use the resource below to answer the questions that follow.
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Mitosis by NDSU VCell Productions at http://www.youtube.com/watch?v=C6hn3sA0ip0
(6:11)
1. When does the "classic" chromosome structure of DNA appear during mitosis? What
form is the DNA in prior to this change?
2. What problems do you think might arise if the chromosomes did not align during
metaphase?
3. When do the nuclear envelopes reform? What problems might arise if a cell started
forming the nuclear envelopes earlier?
4. In what stage do cells spend most of their "life"?
5. How long does mitosis take in the typical eukaryotic cell?
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Identify mitosis phases at http://www.neok12.com/quiz/CELDIV06
Review
1. What are chromosomes?
2. In what phase of mitosis are chromosomes moving toward opposite sides of the cell?