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9
Chapter 9
Introduction to Genetics:
One Cell Becomes Two:
Mitosis and Cytokinesis
Why do cells divide?
• Growth
• Repair/regeneration
• Reproduction
– asexual
Chromatin
• Invisible most of the time - Only
visible during cell division (mitosis
or meiosis)
• During S-phase of the cell cycle – the
DNA replicates (makes an exact copy
of itself)
• This means the cell has twice as much
DNA in it after replication
• Once a chromosome has replicated, it
shortens and thickens and can now be
seen in our microscopes.
• See animation
Each descendant of a cell, in addition to
requiring nutrients, cell membrane, and
organelles, must have this information of DNA
to survive.
1. Simple division would mean the new cells had
half of what the old cell had.
2. Duplication of both cytoplasmic and nuclear
contents preceeds division so that new cells
have a complete set of everything.
3. Replication—duplication of DNA: Figure 9.5
• When cells divide by mitosis, each daughter
cell receives the same number of
chromosomes as its mother cell has – 2n.
46
46
46
• In order to do this, the chromosomes must
be copied first, then one of each copy is
placed in the new cells.
46
46
92
46
II. DNA Is Packaged in Chromosomes
(Section 9.3)
A. Organization of these long pieces of DNA
1. DNA is divided into long strands wrapped
around protein (chromatin).
2. Each strand is packaged and condensed into
a single chromosome: Figure 9.6a. Why?
Analogy of moving and packing boxes. You
only need your stuff out when you are
using it. When you move, you won’t be using
it and you want it to take up less space:
Figure 9.8
One Chromosome
• Sister
Chromatids
Each strand is an identical
copy of the other one
Centromere
Where the two chromatids
Are attached to each other –
This is different for each
chromosome
DNA
B. DNA is the key to reproduction, development,
and maintenance.
1. Genome = complete collection of an
organism’s genetic information as linked
genes in a long strand of DNA.
3. Information is found in letters A, C, G, and T in
the double helix: Figure 9.2
4. Path from DNA to protein
5. Humans have about 100,000 genes that have
all the information to make all the proteins
(especially enzymes) a cell needs.
3.
Replication takes one chromosome and
makes two identical copies, called sister
chromatids: Figure 9.6b (Interactive Activity 3)
Chromosome Number
• Each species has the same number of chromosomes in
all their cells that are made by mitosis. This is the
diploid number (2n). In humans this number is 46. So
cells of your skin and muscle and liver each have 46
chromosomes in them. Look how many chromosomes
are in the cells of these creatures:
2n = 42
2n = 78
2n = 38
2n = 94
Sex Chromosomes
• Homologous in females: XX
• Not homologous in males: XY
A male karyotype: 22 pairs of homologous
chromosomes; one pair of sex chromosomes
The cell cycle keeps record of progress of a
cell over time, like a clock: See animation
1. The cell cycle is made up of a repeating
pattern of growth, genetic duplication, and
division.
2. Typical animal cell cycle lasts about 24 hours.
3. Two main phases: interphase and mitotic phase
(about 30 minutes).
4. Interphase = G1 (gap 1 for growth, 12 hours) + S
phase (synthesis, for replication of DNA, 6
hours) + G2 (gap 2, 6 hours): -animation of figure
9.9 available under the resources for this
chapter
Mitosis
• Nuclear division resulting in nuclei
identical to parent cell – asexual
reproduction for some organisms.
• Begins after interphase
• Ends before cytokinesis
• Four phases: Prophase
Metaphase
Anaphase
Telophase
1. Prophase (P for “plain to see”)—Chromosomes
condense, nuclear envelope breaks down,
formation of spindle fibers (microtubules)
from the centrosomes.
2. Metaphase (M for “middle”)—Chromosomes
are aligned on the equator by pushing along
spindle with each sister chromatid facing
opposite poles.
3. Anaphase (A for “apart”)—Sister chromatids
separate; each new chromosome moves to the
opposite pole.
4. Telophase (T for “two nuclei”)—Chromosomes
de-condense, spindle breaks down, nuclear
envelope forms around the two separate
complements of chromosomes.
Mother cell
Prophase
Prophase
• Chromosomes become visible
• Spindle forms from protein
microtubules
• Nuclear envelope disintegrates
• Nucleolus disintegrates
• In animal cells, centrioles migrate to
opposite ends of the cell (poles) and
spindle fibers attach to them
Metaphase
• Chromosomes line up single file
at the equator of the cell
Anaphase
• Sister chromatids are pulled
apart toward opposite poles
• In animal cell, cleavage furrow
begins to form
Telophase
• Nuclear membrane forms around each
group of chromosomes
• Nucleolus reappears in each nucleus
• Spindle fibers disappear
• Chromosomes become invisible again as
chromatin
• Cytokinesis begins in plant cell by
formation of cell plate; cleavage furrow
in animal cell completely separates the
two nuclei into two different cells.
Cytokinesis
Occurs after nucleus has been duplicated
Begins in anaphase in animal cells
by the formation of a cleavage furrow
Begins in telophase in plant cells
by the formation of a
cell plate.
V. Variations in Cell Division
(Section 9.5)
A. Plant cells—Everything is similar
except for cytokinesis because plant
cells have to break down and reform
the cell wall: Figure 9.12, animation
available under the resources for this
chapter
1. Vesicles fuse near the metaphase
plate for form a cell plate that
grows outward to form a cell wall.
Find the different stages of mitosis in these onion
cells:
Meiosis
• Cell division producing cells that
have half the number of
chromosomes of the mother cell
• Produces gametes – eggs and sperm
• Occurs so that fertilization
doesn’t increase the number of
chromosomes in each generation.
Gametes = sex cells
• Eggs or sperm
• Have half the normal number of
chromosomes = haploid (n) = 23 in humans
• Sexual reproduction needs these to
combine DNA from two different
parents, producing offspring that is
different from each parent
Meiosis = Reduction Division
• Two complete cell divisions
• First cell division – separates
homologous chromosomes (reduction
of chromosome number)
• Second cell division – separates sister
chromatids (like mitosis) - Division
• Produces 4 haploid cells
M
M
E
E
I
I
O
O
S
S
I
I
S
S
I
Crossing over can occur
Homologous pairs are separated
Sister chromatids are separated
II
Gametogenesis
• Oogenesis
Production of an
egg
• Spermatogenesis
Production of
sperm
One mother cell
produces one
egg cell and
three polar
bodies that die
One mother cell
produces 4
equally sized
sperm cells
Fertilization
The sperm unites with the egg forming a
zygote (fertilized egg).
The zygote then divides by mitosis to
produce the trillions of cells that make up a
multicellular body like yours.
B. Prokaryotes (no nucleus) binary fission: Figure
9.13, animation available under the resources for
this chapter
III. When Cell Division Runs Amok:
Cancer (Sidebar)
A. Unrestrained cell division—cancer
1. Mechanisms that induce cell division can
become hyperactive. (carcinogens)
2. Mechanisms that suppress cell division can
fail. (carcinogens)
B. Genes
1. Oncogenes: stuck accelerator
2. Tumor suppressor genes: failed brakes
3. Cyclin-dependent kinases, act in linked
chain of protein activity
Skin cancer
The End