Chapter 8
Karyotype – complete picture of an individual’s
chromosomes
Chromosome Numbers
 Each species has a characteristic number of
chromosomes; humans have 46.
 These are in homologous pairs which have the same
size, shape, and carry genes for the same traits.
 One chromosome from a homologous pair comes from
the father, the other from the mother.
Sex Chromosomes
 Sex chromosomes determine the sex of an organism.
 In humans, sex chromosomes are either X or Y.
 XX = female; XY = male
 The other chromosomes are called autosomes.
 Diploid cells have two autosomes for each homologous
pair.
 Haploid cells have only one set for each pair.
 Gametes (sperm and egg cells) are haploid; all other
human cells are diploid.
Exploring The Mitotic Division of an Animal Cell
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Interphase
• Nuclear envelope intact.
• The nucleus contains a
nucleolus.
Prophase
• The chromosomes condense &
can be seen.
• The nucleolus disappears
•Chromosomes,
duplicated during S
phase, cannot be seen
individually because
they have not yet
condensed.
• Each duplicated chromosome
appears as two identical sister
chromatids.
•Looks like “spaghetti &
meatball”
•Centrioles move to opposite poles
• The mitotic spindle fibers begin
to form.
•The nuclear envelope fragments
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
Metaphase
• Metaphase is the longest
stage of mitosis, lasting about
20 minutes.
Anaphase
• Shortest stage of mitosis,
lasting only a few minutes.
• The centrioles are at opposite
ends of the cell.
• Sister chromatids separate
and move to opposite ends of
cell.
•The chromosomes line up on
the metaphase plate, an
imaginary plane down the
center of the cell.
•Two ends of the cell will have
equivalent—and complete—
collections of chromosomes.
Telophase
• Two daughter nuclei begin
to form in the cell.
• Nuclear envelopes re-form.
The chromosomes become
less condensed.
Cytokinesis
• “Cytoplasm Splitting” –
the cell divides into two.
• In plant cells, a new cell
wall will form between
the two cells.
• Cell membrane begins to
“pinch” forming a peanutshaped cell.
Mitosis in a plant cell
Chromatin
Nucleus
Nucleolus condensing
1 Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
Chromosome
Metaphase. The
2 Prometaphase.
3
4
spindle is complete,
We now see discrete
and the chromosomes,
chromosomes; each
attached to microtubules
consists of two
at their kinetochores,
identical sister
are all at the metaphase
chromatids. Later
plate.
in prometaphase, the
nuclear envelop will
fragment.
5
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends
of cell as their
kinetochore
microtubles shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will
divided the cytoplasm
in two, is growing
toward the perimeter
of the parent cell.
PMAT
Prophase
Metaphase
Anaphase
Telophase
Mitosis
Control of Cell Division
 What tells a cell when (and if) to proceed to the
next stage?
 Certain proteins serve as “traffic signals” ,
regulating the progress at certain checkpoints.
 The enzyme responsible for controlling the cell
cycle overall is called cyclin.
 If cells don’t respond to the regulatory proteins,
the result can be uncontrolled growth, causing
cancer.