Chromatin is DNA and associated proteins.
When the cell is not dividing, individual pieces of chromatin called chromosomes may be condensed into 10 nm, 30 nm or 300 nm fibers. During cell division, DNA becomes further compacted and chromosomes such as those in the photograph below become visible. Click here for more information about chromosome structure.
Below: Human chromosomes (female)
Diploid cells (2N) have two complete sets of chromosomes. The body cells of animals are diploid.
Haploid cells have one complete set of chromosomes. In animals, gametes (sperm and eggs) are haploid.
chromosomes are two chromosomes that are the same. This happens because
diploid organisms have two of each chromosome. Each of the pairs is a
homologous pair. One of the homologous chromosomes was inherited from
the individual's mother and the other one was inherited from the
individual's father. For example, the two chromosomes #1 are
homologous. However, a chromosome #1 and a chromosome #2 are not
homologous because they are different chromosomes.
small segment of DNA that contains the information necessary to
construct a protein or part of a protein (polypeptide) is called a
gene. Genes are the unit of inheritance.
cell divides by pinching into two. Each of two daughter cells produced
contains genetic material inherited from the original (parent) cell.
Single-celled organisms divide to reproduce.
division in multicellular organisms enables the organism to grow larger
while the cells remain small. A large surface:volume ratio is due
to small cell size.
Organisms with many cells can have cells
which are specialized for different functions and tasks. For
example, red blood cells are specialized for carrying oxygen but
neurons (nervous tissue) are specialized for conducting signals from
one cell to another.
Some cells of multicellular organisms must divide to produce sex cells (gametes).
produces two daughter cells that are identical to the parent
cell. If the parent cell is haploid (N), then the daughter cells
will be haploid. If the parent cell is diploid, the daughter
cells will also be diploid.
N --> N
2N --> 2N
This type of cell division allows multicellular organisms to grow and repair damaged tissue.
Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell.
2N --> N
Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.
is necessary in sexually-reproducing organisms because the fusion of
two gametes (fertilization) doubles the number of chromosomes.
Meiosis involves two divisions producing a total of four daughter cells.
Click here to go to the chapter on meiosis.
A chromatid is a single DNA molecule.
chromosomes have two chromatids; normally, each one is identical to the
other. The point where the two chromatids are attached is called the centromere.
Splitting chromosomes into two will double their number because each chromatid is identical.
DNA replication occurs when a single-stranded chromosome produces a second chromatid.
Click here to review DNA synthesis (replication).
Overview of the Cell Cycle
| ||Interphase (G1 and G2)|
Chromosomes are not easily visible because they are uncoiled.
| ||Prophase |
The chromosomes begin to coil.
The spindle apparatus begins to form as centrosomes move apart.
The nuclear membrane disintegrates.
Kinetochores form on the chromosomes.
Kinetochore microtubules attach to the chromosomes.
The chromosomes become aligned on a plane.
The chromatids separate (The number of chromosomes doubles).
The nuclear membrane reappears.
The chromosomes uncoil.
The spindle apparatus breaks down.
The cell divides into two.
|G1 Interphase |
The chromosomes have one chromatid.
|G2 Interphase |
The chromosomes are replicated. Each one has two sister chromatids.
The link below is an animation that shows chromosome movement during mitosis in a hypothetical species with 2N = 4.
Click here to begin the animation. After the screen opens, press Ctrl-F to view the animation in full screen mode.
The cell cycle alternates between interphase and mitosis as diagrammed below.
Mitosis has these four phases: prophase, metaphase, anaphase, and telophase.
prophase, chromosomes begin condensing as DNA becomes coiled. The genes
cannot function (produce mRNA and therefore protein) when the DNA is
coiled. Coiling facilitates movement.
The nucleolus disappears.
A system of microtubules needed to move the chromosomes begins to form during prophase. The microtubules, also called spindle fibers, form from an area of the cell called the centrosome.
During interphase, the cell has one centrosome but just before
prophase, the centrosome duplicates, producing a second
centrosome. During prophase, microtubules radiate from each
centrosome. Some of the microtubules extend from one centrosome toward
The entire complex of centrosomes and spindle fibers is called the spindle apparatus.
Each centrosome of an animal cell contains two centrioles. Plant cells do not have centrioles but they do form spindle fibers.
The nuclear membrane becomes fragmented.
Protein plates called kinetochores form on the centromeres of each chromosome.
microtubules attach to the kinetochores.
Click on the image above to enlarge it.
metaphase, the chromosomes move to the center of the cell
(diagram below, photograph above). This line of chromosomes is referred
to as the metaphase plate.
The structures in the diagram below are referred to as the spindle apparatus.
Kinetochore microtubules are attached to the chromosomes. Polar
microtubules are not attached to chromosomes but overlap each other. Asters are short microtubules that radiate from the centrosomes. The spindle
apparatus can be seen on the drawing of a cell in metaphase below.
Metaphase ends when chromosomes split, thus doubling the number of chromosomes.
the chromosomes split at the end of metaphase, the chromosome number is
doubled. For example, the number of chromosomes and chromatids during
each phase in a human cell is:
|Phase||# Chromosomes||# Chromatids|
Microtubules lengthen and shorten by the addition or removal of tubulin dimers. Click here for details in the chapter on cells.
Kinetochore microtubules shorten in the region of the kinetochore, pulling the chromosomes apart.
Polar microtubules push against each other and thus, push the two centrosomes apart. This, in turn, also pulls the chromosomes apart.
The chromosomes move toward poles of cell.
Cytokinesis (division of the cell) begins in anaphase. A cleavage furrow forms as
actin filaments underneath the plasma membrane constrict in a band
called the contractile ring. Two cells will be produced as this process
Telophase begins when chromosomes reach the poles of the daughter cells.
Many of the events in telophase are the reverse of prophase. The chromosomes uncoil, the nuclear membranes around daughter nuclei appear, the spindle apparatus breaks down, and the nucleolus reappears.
Cytokinesis is completed as telophase ends.
This is the non-dividing phase of the cell cycle.
During interphase, the nucleus is visible and the chromosomes are uncoiled and invisible.
Interphase includes G1, S and G2.
Each chromosome has one chromatid.
The cell grows in size.
Synthesis of organelles occurs.
This is when DNA synthesis occurs.
Each chromosome has two chromatids.
The synthesis of enzymes and other proteins in preparation for mitosis occurs during this period.
Cells that permanently leave the cycle
Some cells remain permanently in G1. Examples: skeletal muscle, nerve cells
Some cells remain permanently in G2. Example: cardiac muscle
Below: Whitefish blastula X 400
Plants form a spindle apparatus as animals do but plants lack centrioles.
Instead of furrowing, vesicles derived from the Golgi apparatus fuse at the equator to form a cell plate. The vesicles contain materials necessary to construct a cell wall between the cells.
Mitosis in Allium (Onion)
Click here for the answers to the questions below.
How many chromosomes are there in each of the three diagrams below? How many chromatids?
If a parent cell had 6 chromosomes, how many during each phase listed below?
If a cell had 4 chromosomes that were single-stranded, how many chromosomes and chromatids during each phase listed below?
each phase of mitosis (prophase, metaphase, anaphase, telophase) in a
cell that has 2N = 4 chromosomes. Show the following in your drawings:
chromosomes, kinetochore microtubules, and nuclear membrane.
Control of the cell cycle
Mammalian cells typically divide only about 50 times.
This limit is set by the presence of repeated sequences of DNA at the tips of the chromosomes called telomeres.
young cells, the sequence TTAGGG is repeated hundreds or thousands of
times but each time the cell divides, it loses 50 to 200 of these
repeats. Cells that have divided many times have fewer of these repeats
When the telomere is reduced to a certain size, the cell will no longer divide.
Telomeres are restored to their original length by an enzyme called telomerase. This enzyme contains a single strand of RNA that is used to synthesize the telomeres.
Telomerase is usually found in cells involved in the production of gametes. It is not normally found in somatic cells.
Some cells stop dividing in G1; others stop in G2.
Kinases are enzymes that activate proteins by transferring a phosphate group
from ATP to the protein being activated. An activated protein is needed
for the cell cycle to proceed from G1 to S. Similarly, another activated protein is needed to move the cycle from G2 to mitosis.
Kinases activate these proteins and thus stimulate the cell cycle to continue.
are normally inactive and must be activated before they can activate
other proteins. Cyclin-dependent kinases become activated by combining
with a protein called cyclin.
The activated complex is involved in stimulating the cell cycle to resume.
level of cyclin fluctuates (cycles). At low levels, kinases are not
activated and the cell cycle is halted. At high levels, activation
occurs and the cycle resumes.
Growth factors are molecules that stimulate nearby cells to divide by promoting the binding of cyclin to kinase.
normal conditions, cyclin combines with kinase only when growth factors
are present. For example, damaged tissue releases growth factors to
stimulate cell division needed to repair the tissue.
S-Kinase combines with S-cyclin and the resulting active complex stimulates DNA replication.
The "S" in S-kinase and S-cyclin refers to DNA synthesis.
Enzymes triggered by the active kinase-cyclin complex then destroy the S-cyclin.
M-Kinase combines with M-cyclin and the active complex initiates several mitotic events:
- chromosome condensation (coiling)
- nuclear membrane disintegration
- the synthesis of the spindle apparatus
The active kinase-cyclin complex also activates enzymes that destroy the M-cyclin.
cells do not undergo mitosis. When the cell divides, the circular
chromosome replicates itself (DNA synthesis) and the cell pinches into
This process is called binary fission.