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MCDB 1041 Class 3 Chromosomes
and Cell division
Webpage: mcdb.colorado.edu/courses/1041/
Learning goals:
1.  Diagram the cell cycle
2.  Describe the basic principle of mitosis
(why do cells undergo mitosis?)
3.  Diagram how mitosis proceeds, including
the numbers of chromosomes present at
each phase of mitosis
4.  Distinguish between chromosome,
replicated chromosomes, and sister
chromosomes
Chromosomes, continued
then Mitosis
We inherit our genetic makeup, or genes from our parents
by inheriting a chromosome of each type (number) from
each of our parents:
a
A
Chromosome 1 (from Dad)
Chromosome 1 (from Mom)
These two chromosomes are often referred to as a pair of
chromosome 1s : homologous chromosomes
These homologous chromosomes, one from your father, and one from
your mother have
a.  The same genes, but not necessarily the same sequence
b.  The same genes with the same exact sequence
c.  Different genes
homologous chromosomes A
Chromosome 1 (from Dad)
a
Chromosome 1 (from Mom)
The A (dominant) and the a (recessive) represent two
different versions of the same gene, referred to as
alleles
True or false: The difference between allele A and allele
a could be as small as a single DNA base pair
a.  True
b.  False
An allele is a simply a variation in a DNA sequence
A B
In this picture (from a
human male), the two
chromosomes you see
for each number
(indicated A and B) are:
a.  Exact replicas of
each other
b.  Slightly different
from each other
c.  Each were inherited
from a different
parent
d.  A and C
e.  B and C
What are A and B called?
a.  Sister chromatids
b.  Homologous chromosomes
Cell division is fundamentally important to
life
In an embryo: a single cell divides into many thousands
of cells and these cells make all the different parts of
our bodies
In an adult: certain cells called stem cells keep dividing
to replenish cells that die: skin cells, blood cells, bone
CANCER: Cell division gone wrong
Normal cell division
and death (1) of an abnormal cell (2)
Cell division starting with a
cancerous cell
Cancer cells have many defects
•  Mutations
•  Allow cell to live when it should die
•  Allow cell to divide when it shouldn t
•  Chromosome problems
•  Wrong number ( too many, too few)
•  Rearrangements
When these cells divide, more problems arise…the
checks and balances are gone
SO, we need to understand mitosis!
The Cell Cycle: when cells are dividing, they go
through the following phases:
•  S phase: DNA Synthesis
•  G phase: Gap or “Growth” phase
•  M phase: Mitosis (nuclear
division): 4 stages
Stem cells keep going through the cell cycle
Other cells exit
Cancer cells behave like stem cells
Chromosomal Separa-on– Mitosis Blue: DNA
Green: Spindle
Proteins
h"p://www.cbp.pi".edu/faculty/yong_wan/images/main_cell_cycle.jpg Look at the handout and work
with your group on the activity
There are 5 stages shown in your handout.
Interphase Here Prophase
Anaphase
Telophase
are their namesMetaphase
(not in the correct
order!):
Telophase, Prophase, Interphase, Anaphase, Metaphase
Clicker questions: review the concepts
Below is a karyotype of a genetically normal female.
The karyotype was made from cells undergoing
metaphase of mitosis. The circle contains how many
chromatids?
A.  1
B.  2
C.  4
D.  6
E.  8
Consider a replicated chromosome as a unit.
How many replicated chromosomes are present
in each human cell at the end of mitosis,
after the two new cells have separated?
a.
b.
c.
d.
e.
0
2
23
46
92
At the entry to mitosis (prophase) in a
human cell:
a.  There are 46 replicated chromosomes
b.  There are 46 chromosomes
c.  There are 46 sets of homologous
chromosomes
d.  I am confused
What is the correct order of the phases of
the cell cycle?
a.  G1 S G2 M
b.  G1 G2 S M
c.  G1 G2 M S
d.  S G1 G2 M
Why does it matter?
•  The genome must be replicated before M phase (mitosis)
•  The other phases (G1 and G2) are important growth phases, and
also serve as time points at which the cell is “Checked” to make
sure contents are correct
Mitosis is critical for making more cells
as our bodies grow or repair
These cells that divide are called
somatic cells
Germ cells = gametes = sperm or egg
The germ cell precursors do not
undergo mitosis but rather MEIOSIS—
next week!