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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 4. Ribosomes

2. Cytoplasm 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