Download Mitosis and Meiosis Compared

Exam II Lectures and Text Pages
• I. Cell Cycles
–
Mitosis (218 – 228)
–
Meiosis (238 – 249)
• II. Mendelian Genetics
• III. Chromosomal Genetics
• IV. Molecular Genetics
–
Replication
–
Transcription and Translation
• V. Microbial Models
• VI. DNA Technology
Mitosis and Meiosis Compared
• 1. Meiosis is a reduction division.
• 2. Meiosis creates genetic variation.
• 3. Meiosis has two nuclear divisions.
• Be able to identify unlabeled diagrams of various
stages in mitosis and meiosis.
A Comparison of Mitosis and Meiosis
• The processes of mitosis and meiosis are
similar in some ways, but there are some key
differences
• Meiosis can be distinguished from mitosis
– By three events in Meiosis l
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1. Synapsis and Crossing Over
• In Prophase I, homologous chromosomes
form tetrads (synapsis) and physically connect
(form chiasmata) to exchange genetic
information (crossing over)
2. Tetrads align on the Metaphase I Plate
• At metaphase I, paired homologous
chromosomes (tetrads) are positioned on the
metaphase I plate
• At metaphase, individual chromosomes are
aligned on the metaphase place
3. Separation of Homologues – The Reduction Division
• Anaphase I Separates members of pairs of
chromosomes (homologues). Centromeres do not
divide and sister chromatids stay together.
• Anaphase Separates sister chromatids of
individual chromosomes. Centromeres divide and
sister chromatids move to opposite poles. (Similar
to Anaphase II)
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A Comparison of Mitosis and Meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Metaphase
Chromosomes
positioned at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
Tetrads
positioned at the
metaphase plate
Metaphase I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
Figure 13.9
Sister chromatids separate during anaphase II
Mitosis and Meiosis Compared
• Meiosis II is virtually identical in
mechanism to mitosis
– Sister chromatids separate
– But, Meiosis II begins with a haploid cell.
Meiosis and Sexual Lifecycles Produce Genetic Variation
• Genetic variation produced in sexual life cycles
contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces much genetic variation
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Origins of Genetic Variation Among Offspring
• Meiosis and fertilization are the primary
sources of genetic variation in sexually
reproducing organisms. Genetic variation
results from:
• ----Independent assortment
• ----Crossing over during prophase I of
meiosis
• ----Random fusion of gametes during
fertilization
Independent Assortment of Chromosomes
The random distribution of maternal and paternal
homologues to the gametes
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
During prophase I:
each homologous pair
aligns on the
metaphase I plate.
Each pair consists of
one maternal and one
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
paternal chromosome.
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Independent Assortment of Chromosomes
The orientation of any homologous pair to the poles is
random.
Key
There is a 50-50
chance that any
one daughter cell
produced by
meiosis I will
receive the
maternal
homologue, and a
50-50 chance it
will receive the
paternal
homologue.
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
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Independent assortment
Each pair of chromosomes sorts its maternal and paternal
homologues into daughter cells independently of the
other pairs
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Figure 13.10
Combination 1
Combination 2
Combination 3
Combination 4
Independent Assortment of Chromosomes
• A gamete produced by meiosis contains just
one of all the possible combinations of maternal
and paternal chromosomes.
– The process produces 2n possible combinations of
maternal and paternal chromosomes in gametes
(n is the haploid number).
– Each human gamete contains one of over eight
million possible assortments of chromosomes (223)
inherited from that person's mother and father.
Independent Assortment of Chromosomes
• Genetic variation results from this reshuffling
of chromosomes. Maternal and paternal
homologues will carry different genetic information
at many of their corresponding loci.
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Crossing Over
• The exchange of genetic
material between homologues
during prophase I
–
Produces recombinant chromosomes
that carry genes derived from two
parents
–
Occurs when homologous portions of
nonsister chromatids trade places.
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
–
X-shaped chiasmata: become
visible at places where homologous
strand exchange occurs.
Metaphase II
Daughter
cells
Figure 13.11
Recombinant
chromosomes
Random Fertilization
• The fusion of gametes in humans
– Will produce a zygote with any of about 64
trillion diploid combinations
Evolutionary adaptation depends on genetic variation in a population
• Heritable variation is the basis for Darwin's theory
that natural selection is the mechanism for
evolutionary change.
• Natural selection:
– Increases the frequency of heritable variations that
favor the reproductive success of some individuals
over others
– Results in adaptation, the accumulation of heritable
variations that are favored by the environment
– In the face of environmental change, genetic variation
increases the likelihood that some individuals will have
heritable variations that help them cope with the new
conditions.
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Sources of Genetic Variation
• There are two sources of genetic variation:
– Mutations. Random and relatively rare. They
are structural changes in a gene. They are the
original source of genetic variation.
– Sexual reproduction. Independent
assortment and crossing over in meiosis I, and
random fusion of gametes during fertilization
produce new combinations of genes in every
generation.
Exam II Lectures and Text Pages
• I. Cell Cycles
–
Mitosis (218 – 228)
–
Meiosis (238 – 249)
• II. Mendelian Genetics (251 – 270)
• III. Chromosomal Genetics
• IV. Molecular Genetics
–
Replication
–
Transcription and Translation
• V. Microbial Models
• VI. DNA Technology
Mendelian Genetics
• 1. Be sure to learn the necessary
vocabulary
• 2. Be able to use a Punnett square.
• 3. Where in meiosis would segregation and
assortment occur?
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Explaining Inheritance
• From observations of ornamental plant breeding,
biologists in the 19th century realized that both
parents contribute to the characteristics of
offspring.
• Blending hypothesis of heredity: hereditary
material from each parent mixes in the offspring; once
blended, like two liquids in solution, the hereditary
material is inseparable and the offspring's traits are
some intermediate between the parental types.
• According to this hypothesis:
– 1. Individuals of a population will reach a uniform
appearance after many generations.
– 2. Once hereditary traits are blended, they can no
longer be separated out to appear again in later
generations.
Explaining Inheritance
• The blending hypothesis is inconsistent
with observations that:
– 1. Individuals in a population do not reach a
uniform appearance; heritable variation among
individuals is generally preserved.
– 2. Some heritable traits skip one generation,
only to reappear in the next.
Explaining Inheritance
• Modern genetics began in the 1860s when
Gregor Mendel, an Augustinian monk,
discovered the fundamental principles of
heredity. Mendel's great contribution to
modern genetics was to replace the
blending hypothesis of heredity with the
particulate theory of heredity.
• Particulate theory of heredity: parents
transmit to their offspring discrete inheritable
factors (genes) that remain as separate factors
from one generation to the next.
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Gregor Mendel
– Documented a particulate mechanism of
inheritance and basic principles of heredity
through carefully planned breeding
experiments with garden peas
Figure 14.1
He applied an experimental, quantitative approach
• He attended the University of Vienna from
1851-1853. He was influenced by two professors:
– Christian Doppler: a physicist, trained Mendel to
apply a quantitative experimental approach to the
study of natural phenomena.
– Franz Unger: a botanist, interested Mendel in the
causes of heritable variation in plants.
Breeding Garden Peas in An Abbey Garden
• Mendel probably chose to work with peas
because:
– They are available in many easily
distinguishable varieties
– Because he could strictly control matings to
ensure parentage
• Petals of the pea flower enclose the pistil and
stamens, which prevents cross-pollination.
• Immature stamens can be removed to prevent
self-pollination.
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Crossing Pea Plants
•
He bred pea plants by transferring pollen from one flower to another
with an artist's brush.
1
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3 Pollinated carpel
Stamens
Carpel (male)
(female)
matured into pod
4 Planted seeds
from pod
When pollen from a white flower fertilizes
TECHNIQUE
RESULTS
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
Figure 14.2
Vocabulary
• Character: a heritable feature, such as flower
color
• Trait: a variant of a character, such as purple or
white flowers
He chose characters that differed in an “either-or” manner
• He chose seven characters, each occurred in
two alternative forms:
– 1) Flower color (purple or white)
– 2) Flower position (axial or terminal)
– 3) Seed color (yellow or green)
– 4) Seed shape (round or wrinkled)
– 5) Pod shape (inflated or constricted)
– 6) Pod color (green or yellow)
– 7) Stem length (tall or dwarf)
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A Typical Mendelian Breeding Experiment
• Mendel mated two contrasting, true-breeding
varieties, which he hybridized (cross-pollinated)
in monohybrid crosses
• True breeding: Always producing offspring with
the same traits as the parents when the parents
are self-fertilized
• The first-generation, true-breeding parents
– Are called the P generation
Offspring Generations
• The hybrid offspring of the P generation
– Are called the F1 generation (first filial)
• When F1 individuals self-pollinate (monohybrid
cross)
– The F2 generation (second filial) is produced
Principles of Heredity
• Mendel observed the transmission of traits for
at least three generations and arrived at two
principles of heredity:
– the law of segregation
– the law of independent assortment
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