Chapter 8: Mitosis - Cell Division and Reproduction

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Transcript Chapter 8: Mitosis - Cell Division and Reproduction

Chapter 8:
The Cellular Basis of Reproduction and
Inheritance
Introduction
Stages of an Organism’s Life Cycle:
Development: All changes that occur from a
fertilized egg or an initial cell to an adult
organism.
Reproduction: Production of offspring that
carry genetic information in the form of
DNA, from their parents.
Two types of reproduction:
1. Sexual Reproduction
2. Asexual Reproduction
Types of reproduction:
1. Sexual Reproduction:
 Most
common type of animal reproduction.
 Male and female gametes or sex cells (sperm and egg
cell) join together to create a fertilized egg or zygote.
 The offspring has genetic information from both parents.
 Offspring are genetically different from each parents and
their siblings.
 Advantages:


Ensures genetic diversity of offspring.
• Population more likely to survive changing environment.
Disadvantages:



Cannot reproduce without a partner of opposite sex.
Considerable time, energy, and resources spent to find a suitable
mate.
Parents only pass on 1/2 (50%) of their genetic information to
each offspring.
2. Asexual Reproduction:

Production of offspring by a single parent through:




Splitting: Binary fission in bacteria.
Budding: Yeasts, plants
Fragmentation: Sea stars
Parthenogenesis: “Virgin birth”. Several insect species.
Offspring inherit DNA form one parent only.
 Offspring are genetically identical to parent and siblings,
unless mutations occur.
 Advantages:




Can reproduce without a partner of opposite sex.
Don’t spend time, energy, and resources to find a suitable mate.
Parents pass on 100% of their genetic information to each
offspring.
 Disadvantage:

No genetic diversity of offspring.
• Population less likely to survive changing environment.
Cells Only Arise from Preexisting Cells
New cells are made through cell division:

Unicellular organisms (Bacteria, protozoa):
Division of one cell into two new organisms through
binary fission or mitosis.

Multicellular organisms (Plants, animals):
If sexual reproduction:
1. Growth and development from zygote or fertilized egg.
 Original cell divides by mitosis to produce many cells,
that are genetically identical to first cell.
Cells later develop specific functions (differentiation).
2. Reproduction requires:
 Meiosis: Special type of cell division that will generate
gametes or sex cells, with 50% of individual’s genetic
material.
Bacteria (Procarytoes) Reproduce Asexually by
Binary Fission
Features of Bacterial DNA
Single, relatively small circular chromosome:
About 3-5 million nucleotide base pairs
 Contains only about 5-10,000 genes

 Binary
fission
Single circular DNA is replicated
 Bacterium grows to twice normal size
 Cell divides into two daughter cells
 Each daughter cell with an identical copy of DNA
 Rapid process, as little as 20 minutes.

Bacteria Reproduce Asexually by Binary Fission
Eucaryotic cell division is a more complex and time
consuming process than binary fission
Features of Eucaryotic DNA
1. DNA is in multiple linear chromosomes.

Unique number for each species:
• Humans have 46 chromosomes.
• Cabbage has 20, mosquito 6, and fern over 1000.
2. Large Genome: Up to 3 billion base pairs (humans)


Contains up to 50,000-150,000 genes
Human genome project is determining the sequence of entire
human DNA.
3. DNA is enclosed by nuclear membrane.
Correct distribution of multiple chromosomes in each
daughter cell requires a much more elaborate process
than binary fission.
Human Body Cells Have 46 Chromosomes
DNA: Found as Chromosomes or Chromatin
Chromosomes
Tightly packaged DNA
Found only during cell
division
DNA is not being used
for macromolecule
synthesis.
Chromatin
Unwound DNA
Found throughout cell
cycle
DNA is being used
for macromolecule
synthesis.
Eucaryotic Chromosomes Duplicate Before
Each Cell Division
Cell Cycle of Eucaryotic Cells
 Sequence
of events from the time a cell is formed,
until the cell divides once again.
 Before cell division, the cell must:
 Precisely
copy genetic material (DNA)
 Roughly double its cytoplasm
 Synthesize organelles, membranes, proteins, and other
molecules.
 Cell
cycle is divided into two main phases:
 Interphase:
Stage between cell divisions
 Mitotic Phase: Stage when cell is dividing
Eucaryotic Cell Cycle:
Interphase + Mitotic Phase
The Life Cycle of a Eucaryotic Cell:
Interphase: Time between cell divisions.




Most cells spend about 90% of their time in interphase.
Cells actively synthesize materials they need to grow.
Chromosomes are duplicated.
Interphase can be divided into three stages:
1. G1 phase: Just after cell division.
Cell grows in size, increases number of organelles, and
makes proteins needed for DNA synthesis.
2. S phase: DNA replication.
Single chromosomes are duplicated so they contain two
sister chromatids.
3. G2 phase: Just before cell division.
Protein synthesis increases in preparation for cell division.
Duplication of Chromosomes During S
stage of Interphase
DNA replication during
S stage of Interphase
Single chromosome
Two identical sister
chromatids joined by
a centromere ( )
The Life Cycle of a Eucaryotic Cell:
Mitosis: The process of eucaryotic cell division.
 Most
cells spend less than 10% of time in mitosis.
 Mitosis
is divided into four stages:
1. Prophase: Cell prepares for division.
2. Metaphase: Chromosomes line up in “middle” of cell.
3. Anaphase: Sister chromatids split and migrate to
opposite sides of the cell.
4. Telophase: DNA is equally divided into two new
daughter cells. Cytokinesis usually occurs.
 Cytokinesis:
Division of cytoplasm.
Mitotic Phase: Mitosis + Cytokinesis
Mitotic Phase: Mitosis + Cytokinesis
Mitosis: The Stages of Cell Division
1. Prophase

Chromatin condenses into chromosomes, which appear
as two sister chromatids joined by a centromere.

Nucleoli disappear.

Nuclear envelope breaks apart.

In animal cells, mitotic spindle begins to form as
mictotubules grow out of two centrosomes or
microtubule organizing centers (MTOCs).
• Each centrosome is made up of a pair of centrioles.

Microtubules attach to kinetochores on chromatids and
begin to move chromosomes towards center of cell.

Centrosomes begin migrating to opposite poles of cell.
Interphase and Prophase of Mitosis in Animal Cell
Mitosis: The Stages of Cell Division
2. Metaphase

Short period in which chromosomes line up along
equatorial plane of cell (metaphase plate).

Chromosomes are completely condensed and easy to
visualize.

Mitotic spindle is fully formed.

Kinetochores of sister chromatids face opposite sides
and are attached to spindle microtubules at opposite
ends of the cell.
Metaphase, Anaphase, and Telophase of
Mitosis in an Animal Cell
Mitosis: The Stages of Cell Division
3.Anaphase

Centromeres of sister chromatids begin to separate.

Each chromatid is now an independent daughter
chromosome.

The separate chromosomes are pulled toward opposite
ends by spindle microtubules, attached to the
kinetochores.

Cell elongates as poles move farther apart.

Anaphase ends when a complete set of chromosomes
reaches each pole.
Mitosis: The Stages of Cell Division
4. Telophase

Cell continues to elongate.

Cell returns to interphase conditions:
• A nuclear envelope forms around each set of
chromosomes.
• Chromosomes uncoil, becoming chromatin threads.
• Nucleoli reappear.
• Spindle microtubules disappear.

Cytokinesis usually occurs at the end of this stage
Mitotic Phase: Mitosis + Cytokinesis
 Cytokinesis

The division of cytoplasm to produce two daughter
cells. Usually begins during telophase.
• In animal cells: Division is accomplished by a
cleavage furrow that encircles the cell like a ring in
the equator region.
• In plant cells: Division is accomplished by the
formation of a cell plate between the daughter cells.
Each cell produces a plasma membrane and a cell
wall on its side of the plate.
Cytokinesis in Animal and Plant Cells
Animal Cell
Plant Cell
External Factors Control Mitosis
1. Anchorage
 Most
cells cannot divide unless they are attached
to a solid surface.

May prevent inappropriate growth of detached cells
2. Nutrients and growth factors
 Lack
of nutrients can limit mitosis
 Growth factors: Proteins that stimulate cell
division.
3. Cell density
 Density-dependent
inhibition: Cultured cells will
stop dividing after a single layer covers the petri
dish. Mitosis is inhibited by high cell density.

Cancer cells do not demonstrate density inhibition
Density Dependent Inhibition of Mitosis
Normal Cells Stop Dividing at High Cell Density
Cancer Cells are Not Inhibited by High Cell Density
Cell-Cycle Control System
There are three critical points at which the cell
cycle is controlled*:
1. G1 Checkpoint: Prevents cell from entering S
phase and duplicating DNA.
 Most
important checkpoint.
 Amitotic cells (muscle and nerve cells) are frozen here.
2. G2 Checkpoint: Prevents cell from entering
mitosis.
3. M Checkpoint: Prevents cell from entering
cytokinesis.
*Cells must have proper growth factors to get
through each checkpoint.
Cell Division is Controlled at Three Key Stages
Growth factors are
required to pass
each checkpoint
Cancer is a Disease of the Cell Cycle
 Cancer
kills 1 in 5 people in the United States.
 Cancer cells divide excessively and invade other
body tissues.
 Tumor: Abnormal mass of cells that originates
from uncontrolled mitosis of a single cell.
 Benign
tumor: Cancer cells remain in original site.
Can easily be removed or treated
tumor: Cancer cells have ability to “detach”
from tumor and spread to other organs or tissues
 Malignant
 Metastasis:
Spread of cancer cells form site of origin to
another organ or tissue.

Tumor cells travel through blood vessels or lymph nodes.
Metastasis: Cancer Cells Spread
Throughout Body
Functions of Mitosis in Eucaryotes:
1. Growth: All somatic cells that originate after a
new individual is created are made by mitosis.
2. Cell replacement: Cells that are damaged or
destroyed due to disease or injury are replaced
through mitosis.
3. Asexual Reproduction: Mitosis is used by
organisms that reproduce asexually to make
offspring.
Mitosis Replaces Dead Skin Cells
Chromosomes are matched in homologous pairs
Homologous Chromosomes:
 Eucaryotic
chromosomes come in pairs.
Normal humans have 46 chromosomes in 23 pairs.
 One
chromosome of each pair comes from an
individual’s mother, the other comes from the father.
 Homologous
chromosomes carry genes that control the
same characteristics.

Examples: Eye color, blood type, flower color, or height
 Locus:
Physical site on a chromosomes where a given
gene is located.
 Allele:

Different forms of the same gene.
Example: Alleles for blood types A, B, or O.
Homologous Pair of Chromosomes:
One Comes From Each Parent
Homologous Chromosomes: Code for the Same Genetic
Traits, but Have Different Alleles
There are two types of chromosomes:
1. Autosomes: Found in both males and females.


In humans there are 22 pairs of autosomes.
Autosomes are of the same size and are homologous.
2. Sex Chromosomes: Determine an individual’s gender.


One pair of chromosomes (X and Y).

The X and Y chromosomes are not homologous.

The X chromosome is much larger than the Y chromosome and
contains many genes.

The Y chromosome has a small number of genes.
In Humans and other mammals females are XX and
males are XY.
Chromosomes of Normal Human Male:
44 (22 Pairs) Autosomes + XY
Normal Genetic Complement of Humans:
Females: 44 autosomes (22 pairs) + XX
Males: 44 autosomes (22 pairs) + XY
Note: In most cases, having additional or missing
chromosomes is usually fatal or causes serious defects.
Down’s syndrome: Trisomy 21. Individual’s with an extra
chromosome 21. Most common chromosomal defect (1 in
700 births in U.S.). Mental retardation, mongoloid facial
features, heart defects, etc.
Gametes have a single set of chromosomes
Humans have two sets of chromosomes, one
inherited from each parent.
 Diploid Cells: Cells whose nuclei contain two
homologous sets of chromosomes (2n).
 Somatic
cells are diploid (almost all cells in our body).
 In humans the diploid number (2n) is 46.

Haploid Cells: Cells whose nuclei contain a single
set of chromosomes (n).
 Gametes
are haploid (egg and sperm cells).
 In humans the haploid number (n) is 23.
Fertilization: Haploid egg fuses with a haploid
sperm to form a diploid zygote (fertilized egg).
Meiosis Produces Haploid Gametes From Diploid Parents
Fertilization Produces Diploid Offspring from Haploid Gametes
Mitosis versus Meiosis
Mitosis
Meiosis
One cell division
Two successive cell divisions
Produces two (2) cells
Produces four (4) cells
Produces diploid cells
Produces haploid gametes
Daughter cells are genetically Cells are genetically different from
identical to mother cell
mother cell and each other
No crossing over
Crossing over*
Functions: Growth,
Functions: Sexual reproduction
cell replacement, and
asexual reproduction
*Crossing over: Exchange of DNA between homologous chromosomes.
Meiosis: Generates haploid gametes

Reduces the number of chromosomes by half,
producing haploid cells from diploid cells.

Also produces genetic variability, each gamete is
different, ensuring that two offspring from the
same parents are never identical.

Two divisions: Meiosis I and meiosis II.
Chromosomes are duplicated in interphase prior
to Meiosis I.
 Meiosis
I: Separates the members of each homologous
pair of chromosomes. Reductive division.
 Meiosis
II: Separates chromatids into individual
chromosomes.
STAGES OF MEIOSIS
Interphase:
Chromosomes
replicate
Meiosis I:
Reductive division.
Homologous
chromosomes separate
Meiosis II:
Sister chromatids
separate
Meiosis I: Separation of Homologous Chromosomes
1. Prophase I:

Most complex phase of meiosis (90% of time)

Chromatin condenses into chromosomes.

Nuclear membrane and nucleoli disappear.

Centrosomes move to opposite poles of cell and
microtubules attach to chromatids.

Synapsis: Homologous chromosomes pair up
and form a tetrad of 4 sister chromatids.
Crossing over: DNA is exchanged between
homologous chromosomes, resulting in genetic
recombination. Unique to meiosis.
 Chiasmata: Sites of DNA exchange.

Prophase I: Crossing Over Between
Homologous Chromosomes
Meiosis I: Separation of Homologous
Chromosomes
2. Metaphase I:

Chromosome tetrads (homologous
chromosomes) line up in the middle of the cell.

Each homologous chromosome faces opposite
poles of the cell.
Meiosis I: Homologous Chromosomes
Separate
Stages of Meiosis: Meiosis I
3. Anaphase I:

Chromosome tetrads split up.

Homologous chromosomes of each pair separate,
moving towards opposite poles.

Random assortment: One chromosome from each
homologous pair is shuffled into the two daughter
cells, randomly and independently of the other pairs.

Random assortment increases genetic diversity of
offspring. Possible combinations: 2n.

One human cell can generate 223 or over 8.3 million
different gametes by random assortment alone.
Random Assortment of Homologous Chromosomes
During Meiosis I Generates Many Possible Gametes
Meiosis I: Separation of Homologous
Chromosomes
4. Telophase I and Cytokinesis:

Chromosomes reach opposite poles of the cell.

Nucleoli reorganize, chromosomes uncoil, and
cytokinesis occurs.

New cells are haploid.
Meiosis II: Separation of Sister Chromatids
During interphase that follows meiosis I, no DNA
replication occurs.
Interphase may be very brief or absent.
Meiosis II is very similar to mitosis.
1. Prophase II:

Very brief, chromosomes reform.

No crossing over or synapsis.

Spindle forms and starts to move chromosomes
towards center of the cell.
Meiosis II: Separation of Sister Chromatids
2. Metaphase II:

Very brief, individual chromosomes line up in
the middle of the cell.

Kinetochores of chromatids face opposite poles.
3. Anaphase II:

Chromatids separate and move towards
opposite ends of the cell.
Meiosis II: Separation of Sister Chromatids
Meiosis II: Separation of Sister Chromatids
4. Telophase II:

Nuclei form at opposite ends of the cell.

Cytokinesis occurs.
Product of meiosis:
Four (4) haploid gametes, each genetically
different from the other.
Meiosis Produces Four Genetically Different Gametes
Meiosis in Males and Females
Spermatogenesis:
 Four sperm cells are made.


Starts in puberty and occurs continuously.
Males produce millions of sperm cells a month.
Oogenesis:
 Only
one large egg is produced. The other three
cells are small polar bodies.
 Oogenesis starts before birth in females, stops at
Prophase I, and resumes during puberty.
 Meiosis is completed only after fertilization.
 Females make one mature egg/month.
Mitosis versus Meiosis (Review)
Mitosis
Meiosis
One cell division
Two successive cell divisions
Produces two (2) cells
Produces four (4) cells
Produces diploid cells
Produces haploid gametes
Daughter cells are genetically Cells are genetically different from
identical to mother cell
mother cell and each other
No crossing over
Crossing over*
Functions: Growth,
Functions: Sexual reproduction
cell replacement, and
asexual reproduction
*Crossing over: Exchange of DNA between homologous chromosomes.
Crossing Over in Meiosis Increases Genetic Diversity
Sources of Genetic Variability in
Sexual Reproduction
1. Crossing Over: After crossing over and synapsis,
sister chromatids are no longer identical.
2. Independent Assortment: Each human can
produce over 8.3 million different gametes by
random shuffling of chromosomes in meiosis I.
3. Fertilization: A couple can produce over 64 trillion
(8.3 million x 8.3 million) different zygotes during
fertilization. This figure does not take into account
diversity created by crossing over.
Accidents During Meiosis Can Cause
Chromosomal Abnormalities

Nondisjunction: Chromosomes fail to separate.

Members of a pair of homologous chromosomes fail to
separate during meiosis I or:

Sister chromatids fail to separate during meiosis II.
 Nondisjunction

increases with age.
Gametes (and zygotes) will have an extra
chromosome, others will be missing a chromosome.

Trisomy: Individuals with one extra chromosome, three
instead of pair. Have 47 chromosomes in cells.

Monosomy: Missing a chromosome, one instead of pair.
Have 45 chromosomes in cells.
Nondisjunction of Chromosomes During
Meiosis Produces Abnormal Gametes
Accidents During Meiosis Can Result
in a Trisomy or Monosomy
 Most
abnormalities in numbers of autosomes are
very serious or fatal.


Down’s syndrome: Caused by a trisomy of
chromosome number 21 (1 in 700 births). Mental
retardation, mongoloid features, and heart defects.
Most abnormalities of sex chromosomes do not
affect survival.

Klinefelter Syndrome: Males with an extra sex
chromosome (XXY) (1 in 1000 male births).

Turner Syndrome: Females missing one sex
chromosome (XO) (1 in 2500 female births).
Down’s Syndrome is More Common in
Children Born to Older Mothers
Abnormal Numbers of Sex Chromosomes
Usually Do Not Affect Survival
Klinefelter Syndrome (XXY)
Incidence: 1:1000 male births
Turner Syndrome (XO)
Incidence: 1 in 2500 female births