Chapter 8 Chromosomes & Mitosis
Why do cells replicate or divide?
I. Creation of Chromosomes- occurs just before the cell replicates in the
A. Remember: DNA is grouped on what we call chromatin (DNA +
proteins (histone)) (46 strands of chromatin- in humans,
chimpanzees have 48, lettuce has 18).
B. If the cell is going to replicate, the DNA on the chromatin must
replicate first. Giving us 2 long identical chromatin strands for
each DNA or 46 pairs of chromatin. But these long strands are not
easy to work with.
C. Each chromatin coils up, or condenses forming a chromatid.
D. The identical or sister chromatids are joined by a centromere (holds
the sister chromatids together) and are now called a chromosome
(46 pairs of chromosomes or 96 strands of chromatin).
E. Types of chromosomes: sex chromosomes (2)- determine the sex of
an organism. It can either be an X or a Y chromosome. Females are
XX & males are XY. Autosomes- all of the other chromosomes (44).
II. Cell cycle: repeating set of events that makes up the life of a cell.
A. Interphase: time between cell division. Consists of 3 sub phases.
1. G1 phase: cell growth between cell division & DNA
replication. (Time, variable, usually 8-10 hrs.)
2. S phase: DNA replication occurs providing 2 identical
chromatin strands (6-8 hrs).
3. G2 phase: period of cell growth & preparation for cell
division (4-6 hrs).
a. Centrioles replicate (2-4).
B. Mitosis- Cell reproduction in organisms in which two identical cells
are made from one cell. Occurs in somatic cells or body cells (not in
reproductive cells). This occurs in varying rates depending on the
cells reproducing. Ex. The lining of your stomach replaces with
new cells every 2 days, while the cells of your brain or heart are
never replaced. (Is the DNA message changed at all during
mitosis)? 5 phases: Interphase, Prophase, Metaphase, Anaphase, &
1. Prophase (for all these, we are just concerning ourselves
with the nucleus).
a. Sister chromatin coil up into sister chromatids.
b. Each sister chromatid is joined together by a
centromere to form a chromosome (46) (92
c. Nucleolus & Nuclear membrane break down and
d. Centrioles (necessary for cell division) move to the
north and south poles of the cell.
e. Fibers form between the centrioles and begin
connecting to the centromeres of the chromosomes.
2. Metaphase- Chromosomes line up in the center of the
3. Anaphase- the fibers connecting the centrioles to the
centromeres pull the sister chromatids apart to the
4. Telophase- opposite of prophase:
a. Fibers break down and disappear.
b. The nucleolus and nuclear membranes return.
c. The chromatids unwind back to 2 sets of chromatin
(46 in each cell).
d. Cytokinesis- the cell divides into 2 identical cells or
daughter cells, both with their own nucleuses, with 46
chromatin a piece.