Download The Cell Cycle

The Cell Cycle
Mitosis phase
Prophase
Metaphase
Anaphase
Telophase
The Chromosome
Hemoglobin gene
Hemoglobin gene
These are two chromosomes. Chromosomes are only temporary structures that serve the purpose of boxing up the DNA
in a package that prevents information getting lost during the process of cell division. What is shown above is called a
pair of homologous chromosomes. One member of each pair is inherited from each of your parents. They contain the
same genes (beads) in the same locations. What may differ is the form of that gene. The actual trait that is expressed in
you is dependent on the information on both chromosomes.
Karyotype: A picture of an organism’s chromosomes. This picture is taken right after all the DNA copies itself in preparation for cell division.
By studying a karyotype, certain problems can be detected in an organism.
Normal Male 46
Down’s Syndrome (Trisomy 21) 47
Normal Female 46
Klinefelter’s Syndrome (Male 47)
Interphase
Cell membrane
Nuclear Membrane
DNA
During interphase, the cell is performing its day to day activities. Prior to cell division, the cell grows in size, replicates
its DNA and makes more copies of all the organelles. The cell spends most of its life in this state.
Prophase
Centromere
Centriole
Chromatids
(2 identical chromosomes)
During prophase, the DNA is now visible as chromosomes. Each chromosome consists of two identical parts called
chromatids. The two chromatids are joined together at a point called the centromere. The nuclear membrane dissolves
and organelles called centrioles move to opposite sides of the cell.
Metaphase
Spindle fibers
The chromatid pairs position themselves vertically, down the middle of the cell. The centrioles, at east and west ends of
the cell are like winches on a tow truck. Cable -like structures called spindle fibers extend from each centriole and
connect with the centromere of each chromatid pair.
Anaphase
Like a cable being wound around a winch, the spindle fibers pulled to opposite sides of the cell, seperateing the pair of
chromatids.
Telophase
New nuclear membranes
One pair of homologous
chromosomes
New nuclear membranes surround each of chromosomes. Notice that this is still one cell but has two copies of DNA.
There is one important part of the cell’s life cycle to complete….
Cytokinesis
Animal Cell
Cleavage furrow
Plant Cell
Cell Plate
During cytokinesis, the cytoplasm and organelles are distributed equally between the two new cells. In animals cells, the
membrane constricts down the middle to form two cells. In plant cells, a new cell wall is formed down the middle,
separating the one megacell into two smaller ones. Now the chromosomes will unwind and the DNA will appear as a
hazy mass as it once did during interphase. Each cell will now carry on its specific functions
A Human Being
Sperm cell
23 chromosomes
haploid = N = 23
+
Egg cell
23 chromosomes
haploid = N = 23
Stem Cells
=
Zygote
46 chromosomes
diploid = 2N = 46
Mitosis
2N Cell Specialization
Muscle Cells
2N Mitosis
2N Cell Specialization
Skin Cells
2N Mitosis
2N Cell Specialization
2N Cell Specialization
Brain Cells
Gonads
Cell at this stage are PLURIOPOTENT, meaning they can become
anything at all.
MEIOSIS (the creation of sex cells)
Meiosis is very similar to mitosis. The main difference is
that the cell goes through TWO divisions instead of just
one. So the result is not two identical diploid cells, but
four cells having only one member of each chromosome
pair (haploid). Look at the pictures below that point out
the differences from mitosis.
Prophase I: Chromatid pairs form a TETRAD (a group of
four). In mitosis, the pairs were separate from one
another.
METAPHASE 1:
Through a process called “crossing over”, the chromatids
become intertwined with each other and actually
exchange pieces with one another.
ANAPHASE 1:
The tetrads line up in the cell during metaphase 1 and
are pulled apart during anaphase 1. Note that the
sections of the chromosomes are not the same as when
they came out of interphase (all red or all yellow). The
chromatids are still connected by the centromere.
During Telophase / Cytokinesis 1, the cells split, but the
cells have two copies of one chromosome pair. Because
these are sex cells, they only want one… so……
… they go through a Prophase II, Metaphase II,
Anaphase II….
… and finally Cytokinesis II. The result is four cells that
have only one member of each homologous pair, or 23
chromosomes.
Nondisjunction: the failure of chromosomes to divide during meiosis