Biology
Chapter 8
Cell Reproduction
Mitosis and Meiosis
1
Objectives
• Explain the nature of cell division
• Discuss the significance of mitosis
• Identify and describe the phases of mitosis
• Distinguish between sexual and asexual
reproduction
• Describe the forms of asexual reproduction
• Define meiosis and its importance to sexual
reproduction
• Describe the major differences between mitosis and
meiosis
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Pennsylvania State Standards
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S11.A The Nature of Science
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S11.A.1 Reasoning and Analysis
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Assessment Anchor
S11.A.1.3 Describe and interpret patterns of change in natural and human-made systems.
Eligible Content
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S11.A.2.1.3 Use date to make inferences and predictions, or to draw conclusions, demonstrating
understanding of experimental limits.
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S11.A.3.2.1 Compare the accuracy of predictions represented in a model to actual observations and
behavior.
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S11.B Biological Sciences
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S11.B.1 Structure and Function of Organisms
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•
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Assessment Anchor
S11.B.1.1 Explain structure and function at multiple levels of organization
Eligible Content
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S11.B.1.1.3 Compare and contrast cellular processes (e.g., photosynthesis and respirations, meiosis and
mitosis, protein synthesis and DNA replication).
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S11.B.2.2.1 Describe how genetic information is expressed(i.e. DNA, genes, chromosomes, transcription,
translation, and replication)
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S11.B.2.2.2 Compare and contrast the functions of mitosis and meiosis in passing on genetic information.
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Chapter 8 Cell Reproduction
• 8-1 Chromosomes
• 8-2 Cell Division
• 8-3 Meiosis
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Focus Concept
Cell reproduction perpetuates life,
It allows for the growth and reproduction
of organisms, and passes genetic
information to future generations.
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8-1 Chromosomes:
• Chromosome Structure
• Chromosome Numbers
• Diploid and Haploid Cells
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The Chromosome
• Threadlike structure within a cell which
contains the genetic information that is
passed on from one generation of cells to the
next.
• The are located in the nucleus of a cell.
• Human somatic cells contain 46
chromosomes (or 23 pairs)
• Somatic cells – normal body cells
• Gametes – reproductive cells (sperm and
egg)
• Filling the activity of specific regions of the
DNA
7
Chromosome Structure
• Chromatin – the genetic material that
makes up chromosomes.
– Chromatin is composed of DNA and
proteins
• The DNA is super-coiled into a very
compact structure during cell division.
• Histones: proteins that help maintain
the shape of the chromosome and aids
in the tight packing of DNA
• Nonhistone: do not participate in the
packing of DNA. Involved in contro
8
Chromosome Formation
• During cellular division, chromatin condenses or
coils to form the rod-like chromosomes.
– The DNA double helix begins to coil and wrap tightly around
proteins(histones) to form a nuclesome.
– Nucleosome – DNA which is tightly wrapped around
histones.
– The nuclesome then begins to twist up to form coils.
– The coils then begin to twist to form larger coils called
super coils.
– Super coils – large coils of nuclesome coils which make up
chromosomes.
– Each chromosome consists of two halves, each is referred
to as a chromatid.
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Chromatids
• Each identical half of a chromosome
• Form as the DNA makes a copy of itself
before cell division
• Each new cell receives one chromatid
from each chromosome.
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Centromere
• Constricted area of each chromatid
• Helps hold the two chromatids together
• Aids in movement of chromosomes
during cell division.
15
Chromatin
• Less tightly coiled DNA-protein
complex.
• Regions of DNA uncoil between cell
divisions so information can be read
and used to direct the activities of the
cell.
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17
Prokaryote DNA
• Bacterial DNA is simpler than
eukaryotes.
• Usually only one chromosome which is
attached to the inside of the cell
membrane
• Consists of a circular DNA molecule
and associated proteins.
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Chromosome Numbers of Various Species
Adder’s
tongue fern
Carrot
1,262
Fruit fly
8
18
Garden pea
20
Cat
32
Gorilla
48
Chimpanzee
48
Horse
64
Dog
78
Human
46 or 23 pairs
Orangutan
48
Lettuce
18
Earthworm
36
Sand dollar
52
19
Chromosome Numbers
• Each species has a characteristic
number of chromosomes in each cell.
• Chromosome number does not indicate
species complexity.
• The human chromosome number is 46
or 23 pairs.
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Sex Chromosomes
• Chromosomes that determine the sex
of an organism.
• May also carry genes for other
characteristics.
• Either X or Y.
– Normal females XX
– Normal males XY
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Autosomes
• All of the other chromosomes in an
organism
• Humans: 2 sex chromosomes + 44
autosomes = 46 total chromosomes
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Homologous Chromosomes
• Occurs in Meiosis diploid cells
• Also called Homologues or
homologous pairs
• Every cell of an organism produced by
sexual reproduction has 2 copies of
each autosome.
• One copy is received from each parent
• Same size, shape and carry genes for
the same traits.
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Sister Chromatids
• Each duplicated chromosome is made up of
two halves. Each half is referred to as a
sister chromatid.
• Sister chromatids – (occur during mitosis)
and the DNA they contain are exact copies of
each other.
• Formed when DNA is copied during
interphase.
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Karyotype
• A photomicrograph of the
chromosomes in a dividing cell.
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Chromosomal Abnormalities
• Deletion: a portion of a chromosome is
lost
• Duplication: the deletion becomes
incorporated into its homologue so that
the segment appears twice on the same
chromosome.
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• Inversion
• Translocation
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Chromosomal Abnormalities
• Nondisjunction: the failure of
chromosomes to separate properly
during meiosis. Results in too many or
too few chromosomes.
• Trisomy: an abnormality in which a cell
has an extra chromosome or section of
a chromosome
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Nondisjunction
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Turner Syndrome
• Female with only
one X chromosome
• XO
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Klinefelter Syndrome
• Male with two X
chromosomes
• XXY
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Other Sex Chromosome
Aneuploidies
• XYY genotype - taller than average;
after about age 35, extra Y often
degenerates and is not passed on to
offspring.
• XXX genotype - some developmental
deficiencies; some instances of mental
retardation
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Down Syndrome
• Trisomy 21
Link to website with other chromosomal abnormalities
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Diploid Cells (2n)
• Cells having 2 sets of chromosomes.
Have both chromosomes from each
homologous pair.
• All normal human cells (body cells)
except reproductive cells are 2n
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Hapliod (1n)
• Contain only one set of chromosomes.
• Have only half the number of
chromosomes that are present in
diploid cells.
• Sperm and egg cells (gametes) are 1n
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Fertilization
• When a sperm cell (1n) and an egg cell
(1n) combine to create the first cell of a
new organisms it becomes 2n.
• If the reproductive cells were diploid,
the new cell would have too many
chromosomes and would not be
functional.
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8-2 Cell Division
• Cell Division in Prokaryotes
• Cell Division in Eukaryotes
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Cell Division in Prokaryotes
• Binary Fission: the division of a
prokaryotic cell into two different
offspring cells.
• Consists of three stages.
(Fig.8-4 p148)
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Cell Division in Eukaryotes
• Mitosis and Cytokinesis: cellular
division that results in new cells with
genetic material identical to the original
cell.
• Occurs in:
– the reproduction of unicellular organisms
– the addition of cells to a tissue or organ in
a multicellular organism (growth, repair)
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The Cell Cycle
• The repeating set of events that make
up the life of a cell from mitosis to
mitosis..
• Cell division is one phase of the cell
cycle. (includes both mitosis and
cytokinesis)
• Interphase: the time between cell
divisions. The cell spends most of its
life in interphase.
• IPMATC
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Cell cycle animation
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Interphase
• G1 phase: offspring cells grow to mature
size. The time “gap” following cell division
and preceding DNA replication
• S phase: the cell’s DNA is copied,
“synthesized”
• G2 phase: the time “gap” following DNA
synthesis and preceding cell division.
Growth and preparation for division.
• G0 phase: cells can enter this state when
fully developed. Do not copy DNA and do not
divide. Ex. Central nervous system cells.
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Mitosis / M phase
• The equal division of the nucleus
• Continuous process that can be
divided into 4 phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
– PMAT
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Prophase:
• 1st phase of mitosis
• Shortening and tight coiling of DNA
into rod-shaped chromosomes which
are visible with a light microscope
• The 2 copies of each chromosome stay
connected to one another by the
centromere
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• The nucleolus and nuclear membrane
break down and disappear
• Centrosomes appear next to the
disappearing nucleus.
– In animal cells, each centrosome contains a pair
of small cylindrical bodies called centrioles. Not
present in plants.
– The centrosomes move toward opposite poles of
a cell
– Spindle fibers (made of microtubules) radiate
from the centrosomes. Called mitotic spindles
and help to equally divide the chromatids between
the two offspring cells.
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Metaphase
• 2nd phase of mitosis
• Spindle fibers move the chromosomes
to the center of the dividing cell
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Anaphase
• 3rd phase of mitosis
• The chromatids of each chromosome
separate at the centromere and slowly
move, centromere first, toward the
opposite poles of the dividing cell
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Telophase
• 4th phase of mitosis
• Spindle fibers disassemble and the
chromosomes return to a less tightly
coiled chromatin state.
• Nuclear envelope reforms around each
set of chromosomes and a nucleolus
forms in each of the newly forming
cells
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Cytokinesis
• Occurs at the end of telophase
• The division of the cytoplasm
– Animals: begins with a pinching inward of
the cell membrane midway between the
dividing cell’s 2 poles. Cleavage furrow
uses the action of microfilaments.
– Plant cells: vesicles formed by the Golgi
apparatus fuse at the midline of the
dividing cell forming a membrane-bound
cell wall called the cell plate.
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Cytokinesis
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• Link to Steps of Cell Cycle Animation
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Link to web site
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8-3 Meiosis
• Stages of Meiosis I and Meiosis II
• Formation of Gametes
• Asexual and Sexual Reproduction
71
Meiosis
• A process of nuclear division that
reduces the number of chromosomes
in new cells to half the number in the
original cell
• The halving of the chromosome
number counteracts a fusion of cells
later in the life cycle of the organism.
• In humans, meiosis produces haploid
reproductive cells called gametes
Link to meiosis movie
(sperm, egg)
72
Stages of Meiosis
• Cells undergo:
– G1, S, and G2 phases of interphase
– Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I)
– Meiosis II(Prophase II, Metaphase II, Anaphase II,Telophase II)
• One cell goes through two divisions to
produce 4 gametes.
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Prophase I
• DNA coils tightly into chromosomes, spindle fibers
appear, nucleus and nucleolus disassemble.
• Synapsis: chromosomes line up next to its
homologue. Each pair of homologous chromosomes
is called a tetrad.
• Crossing over occurs: portions of a chromatid twist
around one another, break off, and attach to adjacent
chromatids on homologous chromosomes. Permits
the exchange of genetic material between maternal
and paternal chromosomes and results in genetic
recombination by producing a new mixture of
genetic material.
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Link to crossing over animation
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Metaphase I
• Tetrads line up randomly along the
midline of the dividing cell.
• Spindle fibers from each pole attach to
the centromere of one homologous
chromosome in each tetrad.
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Anaphase I
• Each homologous chromosome of a
tetrad moves to an opposite pole
• Random separation of homologous
chromosomes is called independent
assortment and results in the random
separation of the maternal and paternal
chromosomes which results in genetic
recombination.
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Telophase I
• Chromosomes reach the opposite ends
of the cell and cytokinesis begins.
• The new cells contain a haploid number
of chromosomes of the original cell but
each new cell contains two copies of
the chromosome because the original
cell copied its DNA before meiosis I.
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Link to meiosis II animation showing independent assortment
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Meiosis II
• Occurs in each cell formed during
meiosis I and is not preceded by the
copying of DNA
• Prophase II: spindle fibers form and
begin to move the chromosomes
toward the midline.
• Metaphase II: chromosomes line up at
the midline
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Meiosis II
• Anaphase II: chromatids separate and
move toward the opposite poles of the
cell
84
Meiosis II
• Telophase II: nuclear membrane forms
around the chromosomes in each of
the 4 new cells.
• Cytokinesis II: occurs during telophase
II, resulting in 4 new cells, each of
which contains half the original cell’s
number of chromosomes. (1n)
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• Link to animation of meiosis
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Formation of Gametes
• Gamete: haploid reproductive cells
produced during meiosis
• Meiosis occurs only within
reproductive organs in humans
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Testes
• Involved in the production of sperm
cells or spermatozoa.
• Meiosis produces 4 haploid spermatids
that develop into mature sperm cells
during spermatogenesis
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Ovaries
• Involved in the production of mature
egg cells or ova.
• During oogenesis, a diploid
reproductive cell divides meiotically to
produce 1 mature egg cell (ovum) and 3
polar bodies.
• The one egg cell receives most of the
cytoplasm and the 3 polar bodies
degenerate.
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Asexual Reproduction
• The production of offspring from one
parent.
• Does not usually involve meiosis or the
union of gametes.
93
Asexual Reproduction
• In unicellular organisms, new
organisms are created either by binary
fission or mitosis
• In multicellular organisms, new
organisms form by budding off
portions of their bodies or by some
forms of regeneration. Offspring are
genetically identical to the parent.
94
Sexual Reproduction
• The production of offspring through meiosis and the
union of a sperm and an egg.
• Offspring are genetically different from the parents
because genes combined in new ways during
meiosis.
• Evolutionary advantage: it enables species to adapt
rapidly to new conditions.
– Example: if a disease strikes a grain crop, a few plants may
have genetic variations that make them resistant to the
disease. Many individuals die, but a few resistant plants
survive and produce offspring.
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Review
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• Cell Division Tutorials
• Another Mitosis Animation
• On-line Onion Root Tip Activity
• Cell Cycle and Mitosis Tutorial
• Karyotype Activity
• Meiosis Tutorial
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THE END
QUESTIONS?
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Objectives
• Explain the nature of cell division
• Discuss the significance of mitosis
• Identify and describe the phases of mitosis
• Distinguish between sexual and a sexual
reproduction
• Describe the forms of asexual reproduction
• Define meiosis and its importance to sexual
reproduction
• Describe the major differences between mitosis and
meiosis
112
REVIEW FOR
CHAPTER TEST
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