The Nucleus
The largest most prominent organelle
found only in eukaryotic cells.
It is surrounded by a membrane called
the nuclear envelope (or nuclear
membrane) and contains the genetic
information necessary for control of cell
structure and function.
Structurally, the nucleus is composed of three main parts:
Nuclear Envelope:
 Double membrane structure
 Contains many holes called, pores, that allow RNA
and other chemicals to pass in an out of the nucleus
but not DNA.
Nucleolus:
 Where ribosomes are constructed
 Contains RNA, DNA and proteins
 Has some of the ribosomes that synthesis proteins
(others are in the ER)
Chromatin:
 Contains DNA and proteins formed into packets of
code called chromosomes.
 When the cell divides, the chromosomes fold up on
themselves, getting wider.
Understanding the cell cycle
 Without cell division, living organisms cannot grow and
develop.
 The process that ensures each new cell has a nucleus with a
complete set of instructions (DNA) is called mitosis.
 When the cell divides, the nuclear material splits in half so
that each daughter cell gets genetics that is the same as
that of the parent cell.
 The terms mitosis and cell division are often referred
to as describing the same process.
 HOWEVER!!
 Mitosis refers to the dividing of nuclear material
 Cell Division is the COMPLETE process of copying
and dividing the WHOLE CELL!
Getting ready for mitosis
1. The nucleus must make a copy of its
chromatin. All of our body cells (somatic
cells) contain 46 chromosomes we call
them diploid.
After cell division the body cells must still
have 46 chromosomes. This is why it is
important for the nucleus to make a
copy of its genetic information.
2. Replicated chromatin coils
up- Once the replicated
chromatin coils up it forms
double stranded
chromosomes which are
joined by a centromere
Tightly
coiled
chromatin
centromere
Phase 1- Prophase
 Mitosis begins.
 During prophase, the nucleus prepares for cell division.
The genetic material shortens and thickens. The
chromosome copies are held together at their centers
(centromere), so they form a sort of ‘X’.
 The centrioles (or poles) appear and begin to move to the
opposite ends of the cell
 The nuclear membrane and nucleolus are no longer visible.
Phases of Mitosis
•Densely packed coiled
chromosome pairs become visible
with a light microscope.
•The nucleolus and the nuclear
membrane disappear.
•“Spindle fibers” begin to form from
centrioles at either end of the cell
and stretch out to connect to the
centromeres on each chromosome
pair.
Phase 2 - Metaphase
 During metaphase,
The spindle fibres
(protein structures)
attach themselves
to the centromeres
of the chromosomes
and align the chromosomes
at the equatorial plate (center of the cell).
Phases of Mitosis
•Spindle fibers pull on the
centromeres.
•Chromosome pairs line up across
the center of the cell.
Phase 3 - Anaphase
 During anaphase, the copies separate. One complete
set of chromosomes is pulled to one side of the cell.
The other complete set is pulled to the other side of
the cell.
Phases of Mitosis
•Spindle fibers contract and
shorten
•Centromeres are pulled apart
•One of each replicated
strand move to opposite ends
of the cell
Phase 4 - Telophase
 In this stage, the chromosomes reach the poles of their
respective spindles. Nuclear membrane forms before
the chromosomes uncoil. The spindle fibres
disintegrate. Mitosis Ends!
Phases of Mitosis
•One complete set of chromosomes is at each
end of the cell
•Spindle fibers begin to disappear
•Nuclear membrane forms around each set of
chromosomes
•A nucleolus appears within each new
nucleus.
•Single stranded chromosomes start to uncoil
into thin strands of chromatin.
•Two nuclei in one cell and the cell is ready to
divide.
Cytokinesis
 Cytokinesis is the final stage of cell division. During
this stage, the cytoplasm pinches in at the center of the
cell, dividing the original cell in half. When cell
division is complete, two new daughter cells are
formed.
The daughter cells are
identical to the parents cell.
Cell Cycle
The cell cycle can be broken down into three
parts.
•mitosis- copying the nucleus
•cell division- separating the cytoplasm
•interphase
Interphase - The cell is in this stage of
the cell cycle for the longest time. This is
where the cell grows and performs its
functions. This stage gets the cell ready
for cell division.
Interphase
 Interphase is the stage before cell division starts. As
a cell prepares to divide, DNA has replicated, but
has not formed the condensed structure of
chromosome. They remain as loosely coiled
chromatin.
A typical cell cycle
Rapid
growth
(4h)
Growth and
DNA replication
(1oh)
Growth and
preparation
for division
(4h)
 Work on questions 1,2,3,5 on page 23. If not completed
in class, they are for homework.
 Complete worksheets- to be handed out next day.