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Chapter 6-1: Chromosomes and Cell Reproduction
An adult human body produces about 2 trillion cells every day. This is about 25 million new
cells per second!
Formation of New Cells by Cell Division
What are some of the reasons cells undergo cell division?
1. growth
2. development
3. repair
4.
asexual reproduction
5.
formation of gametes
Regardless of the type of cell division that occurs, all of the information stored in the molecule
DNA (deoxyribonucleic acid) must be present in each of the resulting cells.
Remember back to Chapter 3, what is the function of DNA?
1. DNA stores the information that tells cells which proteins to make and when to make
them.
2. This information directs a cell’s activities and determines its characteristics.
Prokaryotic Cell Reproduction
DNA:
Circular
Attached to the inner cell membrane
Reproduce by binary fission
Asexual reproduction = identical offspring
2 stages
1. DNA is copied.
2. Cell divides
Binary fission in Paramecium
o A new cell membrane is added to a point on the membrane between the two DNA
copies. The growing cell membrane pushes inward and the cell is constricted in
the middle.
o A new cell wall forms around the new membrane.
Eukaryotic Cell Reproduction
DNA is organized into units called genes.
o A gene is a segment of DNA that codes for a protein or RNA molecule.
o A single molecule of DNA has thousands of genes.
o Genes determine how a body develops and functions.
o When genes are being used, the DNA is stretched out in the form of chromatin so
that the information it contains can be used to direct the synthesis of proteins.
Cell division
o DNA replicates (it makes a copy of itself)
o DNA condenses into chromosomes by coiling around proteins, which makes them
visible.
o
The two exact copies of DNA that make up each chromosome are called sister
chromatids.
o
The sister chromatids are attached at a point called the centromere.
o
The chromatids become separated during cell division and placed into each new
cell.
*** Demonstration of DNA as 6 ft. strands; replicate strands and coil them into chromosomes
around play-doh / clay (protein) w/ 2 sister chromatids connected @ the centromere.
Fact: As many as 500 chromosomes lined up end to end would fit in a 0.2 cm space—about
the thickness of a nickel.
How Chromosome Number and Structure Affect Development
Somatic (body) cells
o 23 pairs of chromosomes (46 chromosomes)
o
Differ in size, shape, and set of genes.
o Complete set of all chromosomes is essential to survival.
Sets of Chromosomes
Each of the 23 pairs of chromosomes consists of two homologous chromosomes, or
homologues, which are similar in size, shape, and genetic content.
Each homologue in a pair of homologous chromosomes comes from one of the two
parents.
46 chromosomes = 2 sets of 23 chromosomes; one set from Mom and one set from Dad.
Comparison of Somatic Cells and Gametes
Somatic Cells in Humans
Gametes in Humans
Body cells
Sex cells = sperm and eggs
Diploid = 23 pairs of chromosomes = 46
Haploid = 23 individual chromosomes
Diploid number is represented by ―2n‖
Haploid number is represented by ―n‖
When haploid (n) gametes fuse
in a process called fertilization,
they form a diploid (2n) zygote,
which is the first cell of a new
individual.
Chromosome Numbers
The number of chromosomes in cells is constant within a species.
Although most species have different numbers of chromosomes, some species have the
same number.
Many plants have far more chromosomes (Ex: ferns w/ 500).
A few have only 1 pair of chromosomes.
Autosomes and Sex Chromosomes
23 pairs of chromosomes in humans
o 22 pairs of Autosomes
 Chromosomes that are not directly involved in determining sex or gender
o 1 pair of sex chromosomes

Determine the sex of an individual

XY = male: the genes that cause a fertilized egg to develop into a male are
located on the Y

XX = female: any individual without a Y chromosome is female

Sex of an individual is determined by the male.
Structure and number of sex chromosomes vary in different organisms.
o Some insects (grasshopper) have no Y chromosome.

XX = female

XO = male; the O indicates the absence of a chromosome
o In birds, moths & butterflies

XX = male

XO = female
Change in Chromosome Number
Karyotype – photo of the chromosomes in a dividing cell that shows the chromosomes
arranged by size with the sex chromosomes as number 23.
Normal karyotype of male (sex)
Karyotype of Down syndrome female (sex)
Humans with more than 2 copies of a chromosome (trisomy) will not develop normally.
o Down syndrome = chromosome 21 trisomy
o Incidence of Down syndrome births increases with the age of the mother

Mothers under 30 = 1 in 1,500

Mothers 37 years old = 1 in 290

Mothers over 45 = 1 in 46
o All the eggs a female will ever produce are present in her ovaries at birth

As female ages, eggs can accumulate an increasing amount of damage.
o Males produce new sperm throughout life.
Disjunction is the separation of homologous chromosomes.
Nondisjunction is the failure of 1 or more chromosomes to separate.
o One gamete ends up with both copies of a chromosome
o The other gamete receives none.
Change in Chromosome Structure
Changes in chromosome structure are called mutations.
Breakage of a chromosome can lead to 4 types of mutations
o Deletion – a piece of chromosome breaks off completely – often fatal
o Duplication – a chromosome fragment attaches to its homologous chromosome,
which will then carry 2 copies of a certain set of genes.
o Inversion – chromosome piece reattaches to the original chromosome but in a
reverse orientation.
o Translocation – a piece of chromosomes reattaches to a nonhomologous
chromosome.
Chapter 3 Intro: History of Cytology
Objectives:
Identify the scientists who discovered the cell theory.
Explain the cell theory.
History of Cytology
Many people fell that Anton van Leeuwenhoek invented the first microscope.
However, this has been disputed in recent years - Zacharias Jansen is now thought to
be the original inventor in 1595.
During the 1600’s Leeuwenhoek used his microscopes to look at drops of pond water
and other liquids.
He discovered that the water contained tiny living things, which he called “animalcules.”
At about the same time, Robert Hooke, an English physicist, used a microscope to
observe plants.
He pointed out that the woody parts (cork) of plants contained tiny rectangular
chambers, which he called cells.
In 1839, German biologist Theodor Schwann found that some animal tissues closely
resembled the circular tissues of plants.
As he observed the tissues with better and better microscopes, he concluded that
animals are composed of cells as well.
Also during this time, Robert Brown discovered an object near the center of many cells –
the nucleus.
German biologist Matthias Schleiden expanded on Brown’s work, suggesting that the
cell’s nucleus plays a role in cell reproduction.
In 1855, German physician Rudolf Virchow proposed that animal and plant cells are
produced only by cell division.
The discoveries and observations of these scientists make up what is now known as the
cell theory.
1. All living things are composed of cells.
2. Cells are the smallest working units of living things.
3. All cells come from preexisting cells by cell division.
Things all cells have in common:
1. Cell membrane
2. Cytoplasm
4. Ribosomes
5. Cytoplasm
3. DNA
Cell Types
Prokaryotes
– Bacteria
Eukaryotes
– Unicellular (Protozoa) = Protists
– Fungi
– Plants
– Animals
Prokaryotes
Eukaryotes
Unicellular organisms (bacteria)
Can be single-celled or multicullular
No nucleus
Nucleus
No membrane-bound organelles
Many membrane-bound organelles
Circular DNA
Linear DNA
Reproduces quickly (20 minutes)
Cells reproduce slowly – 24+ hours
Relatively small in size
Relatively larger in size
Peptidoglycan cell wall
Cell Wall (Fungi, Plants) or Not (Animals)
– Not made of peptidoglycans
Plasma/Cell Membrane = “Gatekeeper”
Lipid bilayer
Filters what goes in and out
– Active Transport (Sodium)
– Facilitated Transport (Glucose)
– Passive Diffusion (Water)
Communication with environment
Cell Wall = “Retaining wall”
Peptidoglycans in prokaryotes
– Targeted by antibiotics like penicillin
Also in fungi and plants
– Different chemistry (cellulose in plants)
– Provides rigid support
Nucleus = “Brain of the cell”
Consists of:
– Nuclear membrane
– Chromatin (DNA + proteins)
– Nucleolus (rRNA
ribosomes)
DNA replication and transcription occurs in the nucleus
Controls all cell activities.
Centrioles = “Construction Foremen”
Found only in animal cells (US!)
Directs construction of the spindle during cell division
Composed of 9 triplets of microtubules arranged in a circle.
Come in pairs
Mitochondria = “Powerhouse of the Cell”
ATP (energy) production occurs here
All mitochondria come from mother
Contain own enzymes and DNA
Inner and Outer membrane space with cristae in between
May have originally been a ―captured‖ bacterium put to use by cell
Ribosomes = “Factories of the Cell”
Found free in cytoplasm or attached to ER
Translates mRNA (from DNA) into protein
Two subunits
– 50S and 30S 70S in prokaryotes
– 60S and 40S 80S in eukaryotes
– Made up of proteins and rRNA
Not a true organelle (found in prokaryotes)
Endoplasmic Reticulum (ER) = “Highways of the Cell”
Rough (with ribosomes)
– Membrane protein synthesis
– Transport and vesicle formation
Smooth (no ribosomes)
– Synthesis and metabolism of lipids
– Detoxification (lots in liver cells)
Golgi Apparatus = “UPS for the Cell”
Processes, Packages, and Distributes
– Processing of proteins to final form
– Packaging into vesicles
– Distribution of vesicles to final destinations (secretion, membranes, etc.)
Lysosomes and Peroxisomes = “Garbage Disposal of the Cell”
Lysosomes
– Highly acidic
– Have pH sensitive enzymes that break down proteins and lipids
Peroxisomes
– Produce and metabolize H2O2
– May impact aging? (get/leak more as you age)
Vesicles = “Transport Bins of Cell”
Bud off of and merge with membranes
Endocytosis – forms vesicle carrying substance into the cell
Exocytosis – vesicle carries (secretes) substance out of the cell
Cytoskeleton = “Skeleton and motion of cell”
Three basic types:
– Microtubules
– Actin filaments
– Intermediate filaments
Centrosomes serve as microtubule organizing center
– In animals, the centrosome has two centrioles, which play role in cell division
forming the mitotic spindle
Cytosol (cytoplasm) =“Soup of the Cell”
Made up of water, ions, and macromolecules of the cell
Organelles float within cytosol
Many reactions and signaling cascades take place within the cytosol
Chloroplasts =“Solar Cells of Plants”
Synthesize Sugar from Sunlight (Photosynthesis)
Stacked grana and thylakoid membranes filled with chlorophyll (green pigment)
Energy stored via the Calvin (Dark) Cycle using carbon dioxide to form sugar
Vacuoles = “Water Tower in Plants”
Small in animals; used for storage
One large central vacuole in plants
Membrane surrounds water or other storage materials
Also supplies ―turgor pressure‖ against cell wall of plants to allow them to stand up
provide structural strength
If depleted, plants wilt
Plant Cells vs. Animal Cells
Plant Cells
Animal Cells
Have a cell wall
Do not have a cell wall
Have chloroplasts
Do not have chloroplasts
Large central vacuole
Many small vacuoles
No centrioles
Pair of centrioles
Chapter 6-2: The Cell Cycle
Objectives:
Identify the major events that characterize each of the five phases of the cell cycle.
Describe how the cell cycle is controlled in eukaryotic cells.
Relate the role of the cell cycle to the onset of cancer.
The Life of a Eukaryotic Cell
Eukaryotic cell division is more complex than prokaryotic cell division because
o
It involves dividing both the cytoplasm and the chromosomes inside the nucleus
o
Many internal organelles must reproduce or be manufactured and properly
rearranged before the cell can divide.
The Cell Cycle
The cell cycle is a repeating sequence of cellular growth and division during the life of
an organism.
A cell spends 90 percent of its time in the first three phases of the cycle, which are
collectively called interphase.
A cell will enter the last 2 phases only if it’s ready to divide.
The Cell Cycle
The five phases of the cell cycle are:
1. First growth (G1) phase: During the G1
phase, a cell grows rapidly and carries out
its routine functions.
2. Synthesis (S) phase: A cell’s DNA is copied
during this phase.
3. Second growth (G2) phase: In the G2 phase,
preparations are made for the nucleus
to divide. Microtubules are rearranged.
4. Mitosis: Mitosis is the process during cell
division in which the nucleus of a cell is divided into two nuclei, each with the same
number and kinds of chromosomes as the original cell.
5. Cytokinesis: The process during cell division in which the cytoplasm divides is called
cytokinesis.
Control of the Cell Cycle
The cell cycle has key checkpoints (inspection points) at which feedback signals from
the cell can trigger the next phase of the cell cycle (green light).
Other feedback signals can delay the next phase to allow for completion of the current
phase (yellow or red light).
Control occurs at three principal checkpoints:
1. Cell growth (G1) checkpoint:
This checkpoint makes the
decision of whether the cell
will divide.
2. DNA synthesis (G2)
checkpoint: DNA replication is
checked at this point by DNA
repair enzymes.
3. Mitosis checkpoint: This
checkpoint triggers the exit from
mitosis.
When Control Is Lost: Cancer
Certain genes contain the
information necessary to make the proteins that regulate cell growth and division.
If one of these genes is mutated, the protein may not function, and regulation of cell
growth and division can be disrupted.
Cancer, the uncontrolled growth of cells, may result.
Chapter 6-3: Mitosis and Cytokinesis
Objectives:
Describe the structure and function of the spindle during mitosis.
Summarize the events of the four stages of mitosis.
Differentiate cytokinesis in animal and
plant cells.
Chromatid Separation in Mitosis
During mitosis, the chromatids on
each chromosome are physically
moved to opposite sides of the
dividing cell with the help of the spindle.
Spindles are cell structures made up of both centrioles and individual microtubule fibers
that are involved in moving chromosomes during cell division.
Forming the Spindle
When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward
opposite poles of the cell.
As the centrioles move apart, the spindle begins to form.
Separation of Chromatids by Attaching Spindle Fibers
The chromatids are moved to each pole of the cell in a manner similar to bringing in a
fish with a fishing rod and reel.
When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite
poles.
As soon as the chromatids separate from each other they are called chromosomes.
Mitosis and Cytokinesis
Mitosis
Step 1 Prophase: The nuclear envelope dissolves and a spindle forms.
Step 2 Metaphase: During metaphase the chromosomes move to the center of the cell
and line up along the equator.
Step 3 Anaphase: Centromeres divide during anaphase.
Step 4 Telophase: A nuclear envelope forms around the chromosomes at each pole.
Mitosis is complete.
Mitosis: Label the following picture.
Polar fibers - spindle microtubules that extend from the two poles of a dividing cell.
Kinetochore fibers - pull chromosomes to opposite poles
Cytokinesis
As mitosis ends, cytokinesis begins.
During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane
grows to enclose each cell, forming two separate cells as a result.
The end result of mitosis and cytokinesis is two genetically identical cells where only one
cell existed before.
Label the following pictures:
Cytokinesis in animal cell
Cytokinesis in plant cell