biology chapter 11. introduction to geneticspg 262
TRANSCRIPT
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BIOLOGY CHAPTER 11
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INTRODUCTION TO GENETICSPG 262
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11-1 THE WORK OF GREGOR MENDEL
Gregor Johann Mendel-(born 22nd July 1822, died 6th January 1884)
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Introduction:"EARLY IDEAS ABOUT HEREDITY"
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Until the 19th century people believed that
heredity was a BLENDING inheritance
and the nature of these factors was unknown…
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*The resemblance of children was explained
by the THEORY OF BLENDING
INHERITANCE.
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Indeed you do see a little of both parents in a
child...
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GENETICS- the branch of biology that studies
heredity….the scientific study of heredity.
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HEREDITY- the passing of traits from parents to their young...biological
inheritance.
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Heredity is the reason we have different
species....cats have kittens, dogs have puppies, oak trees produce acorns...
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NO THIS CANNOT HAPPEN!NO! This CANNOT Happen!
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"GREGOR MENDEL" PG 263
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Gregor Mendel , an Austrian monk born in 1822, is known as the "father of genetics".
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At the age of 21 Mendel entered a monastery in "Brno"and in 1851 was sent to the Univeristy of Vienna to study science
and math.
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He spent 2 years at the University and then
returned to the monastery and spent
the next 14 years teaching at the
monastery.
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Mendel was a teacher & was also responsible for tending the garden at the monastery. From
this responsibility came the foundation of
genetics.
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Mendel studied the garden pea plants and conducted experiments that unlocked some of the secrets of heredity.
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Teacher Tube Video Clip
• http://www.teachertube.com/v.php?viewkey=2cbe4813cae2f7715336
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Mendel found that pea plants have both male and female parts and that normally, pollen
from the male part of a pea flower fertilizes the female egg cells of the
same pea flower.
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TRUE BREEDING- when allowed to self pollinate,
the parents produce offspring identical to the
parent plant.
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SELF POLLINATION- the process in which pollen falls from the
male part of a flower to the female part of a flower of the same
plant.
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PUREBRED- belonging to a group of organisms
that can produce offspring having only one form of a trait in
each generation.
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*Mendel used pea plants that had been
allowed to self pollinate for several generations
because they were purebred.
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The purebred pea plants would produce offspring
that were identical to themselves.
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Purebred pea plants were the basis of
Mendel's experiments.
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Tall plants produced only tall offspring. Short
plants produced only short offspring. Plants that had green seeds
produced offspring with green seeds.
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Mendel’s pea plants produced seeds by self pollination. These true breeding plants were the basis for Mendel's
experiments.
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The seeds that were produced inherited all of
their characteristics from the single plant that "created" them.
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Mendel prevented self-pollination in some
plants and fertilized the eggs of a flower with the pollen from a different
plant.
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CROSS POLLINATION- the transfer of pollen from the flower of one plant to the flower of
another plant...fertilization of a
plant's eggs by the pollen of another plant.
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Cross pollination produces seeds that are
the offspring of 2 different plants.
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Through cross pollination Mendel was
able to cross plants with different characteristics.
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Mendel studied a few isolated traits...ones
that were easily observed.He studied 7 traits...See figure 11-3
page 264
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TRAIT- a specific characteristic that a
living thing can pass on to its young
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Traits Medel Studied: Seed Shape, Seed Color,
Seed Coat Color, Pod Shape, Pod Color,
Flower Position, Plant Height.
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Studying only a few traits made measuring the effects of heredity
much easier.
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"GENES AND DOMINANCE" PG 264
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Mendel crossed pea plants with different
characteristics for the same trait....tall with
short.
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Flower positions: axial and terminal...axial
along the sides...terminal at the
end.
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HYBRID- an organism that results from
crossing parents with differing traits or characteristics…
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From Mendels crosses he got HYBRIDS.
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Mendel thought that he would get half short and half tall plants when he crossed a short and tall
plant…
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to his surprise all of the offspring were tall....the short characteristic had apparently disappeared.
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Conclusions from Mendels 1st Set of
Experiments:
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1. The individual factors that do not blend with
one another, control the trait of a living thing.
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Mendel used the word merkmal to refer to
these factors..In German merkmal means
character…
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Merkmal - factors that control traits.Today the
word merkmal is replaced with the word
GENE.
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Bill Nye Gene video Clip
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GENE-chemical factors that control
traits. Each of the traits that Mendel studied was controlled by one gene that had 2 contrasting forms..tall and short;
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ALLELES- different forms of a gene.
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Allele
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(conclusions from 1st set of experiments...) 2. PRINCIPLE OF DOMINANCE- some of the alleles or factors are dominant
and some are recessive.
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The effects of a dominant allele is seen
even when the contrasting recessive
allele is present.
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The effects of a recessive allele are not seen when
the dominant allele is present.In his 1st
experiments tall and yellow alleles were
dominant while short and green alleles were
recessive…
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Dominance is seen in many traits but does not
apply to all genes.
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"SEGREGATION" PG 265
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Mendel wanted to know what happened to the
recessive characters.So, he allowed several of the hybrid plants to reproduce by self-
pollination.
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To keep track of the different groups of
seeds he gave them names.
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P GENERATION- PUREBRED PARENTAL
PLANTS
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F1 FIRST FILIAL GENERATION- the first
generation of plants produced by cross
pollination.
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The word filius is a Latin word that means
son.The next generation produced from crossing the F one plants would be referred to as the F2 generation and so on.
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"THE F1 CROSS" PG 266;When the 1st filial
generation was crossed the plants produced (F2)
showed the recessive traits. WHY?
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"EXPLAINING THE F 1 CROSS" PG 266
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Mendel assumed that the presence of the
dominant tall allele had masked the recessive short allele.In some of the offspring of the F2 generation the allele was not masked.…
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This segregation or separation puzzled
Mendel…
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He suggested that during the formation of the egg and pollen cells, the tall and short alleles
in the F1 plants were separated from each
other.
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SEGREGATION- the separation of alleles
during gamete formation.
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11-211-2
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PUNNETT SQUARE- a diagram that shows the
possible gene combinations in the offspring that result
from a cross.
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Alleles are represented by
letters that serve as symbols.The
DOMINANT allele is represented by a capital letter.
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The RECESSIVE allele is represented by a
lower case letter that corresponds to the
dominant allele symbol.
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11-2 page 267 "GENETICS AND PROBABILITY”
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Mendel applied the math concept of
"probability" to biology. PROBABILITY- the likelihood that a
particular event will occur.
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PROBABILITY = # times a particular event
occurs/ # of trials EX: flipping a coin
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In probability you only get the expected ratio for large numbers of trials...the larger the number of trials the
closer you get to expected values.
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Previous events do not affect future
outcomes.Ex: each flip of the coin is a separate
independent event;
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GAMETES- reproductive cells
(sex cells);The gametes produced by each parent are placed along the left hand side and the top of
the punnett square.
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The offspring are represented by each square.The probable
results are often expressed as ratios.
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PHENOTYPE- physical characteristics;
GENOTYPE- genetic make-up
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HOMOZYGOUS- organisms that have 2 identical alleles for a particular trait. (TT or
tt);Homozygous organisms are
PUREBRED.
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HETEROZYGOUS- organisms that have
two different alleles for a particular trait. (Tt);
Heterozygous organisms are hybrids.
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11-3
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"INDEPENDENT ASSORTMENT" (page
271)
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Mendel also questioned if the segregation of one pair of alleles affect the segregation of another pair.For example: does the gene for seed shape
have anything to do with seed color?
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To find out the answer to this Mendel crossed purebred plants that
produce round yellow seeds with purebred plants that produced wrinkled green seeds.
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TWO FACTOR CROSS- a cross that involves 2
traits. See the cross in figure 11-9 & 11-10 pages 270 & 271
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The F1 plants from the above mentioned cross will produce seeds that
are round and yellow...the dominant
traits show up in a hybrid and the recessive ones seem to disappear.
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Segregation is still not proved to be
independent...another cross is needed.The F1 generation plants need
to be crossed to produce an F2
generation.
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"THE 2 FACTOR CROSS F2”
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Mendel concluded from his results of the F1
cross that genes could segregate
independently during the formation of
gametes...genes can undergo independent
assortment.
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The only exception to independent assortment
is for genes that are located on the same chromosome...they
cannot undergo independent assortment.
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A SUMMARY OF MENDEL'S WORK:
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-Genes control heredity. In sexually reproductive
organisms genes are inherited from each
parent.
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-When 2 or more forms of one gene exist, some forms of the gene may be dominant and some
forms may be recessive.
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-Genes of different traits may assort
independently of one another.
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APPLYING MENDEL'S PRINCIPLES
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Mendel's ideas about heredity and his applications of
mathematics and statistics to Biology were ahead of their
time.
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Mendel's pioneering work in genetics
remained unappreciated in his lifetime.
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More than 20 years after his death, Mendel's experiments and conclusions were
recognized as important breakthroughs in
Biology.
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"USING THE PUNNETT SQUARE"
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ONE FACTOR CROSS...crossing one
trait
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TEST CROSS- the cross of an organism of
unknown genotype with a homozygous recessive
individual.
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"TWO FACTOR CROSS”
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SEE FIGURE 11-9 page 270
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11-4 MEIOSIS PG 275
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The sex cells or gametes that carry the genetic information are formed by a special kind of cell division known as
MEIOSIS.
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The number of chromosomes, the
structures that contain the genetic information,
is reduced by half during meiotic division.
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By fertilization the full number of
chromosomes is restored.
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These 2 processes Meiosis and fertilization- allow for infinite variety
in the selection and recombination of
genetic traits.
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From Mendel's genetics we know that an
organism inherits a single copy of each
gene from each of their parents.
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These 2 copies are segregated from one another during the
formation of gametes.
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EX: fruit fly...each body or somatic cell of a fruit
fly contains 8 chromosomes.
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If chromosomes were not separated the
offspring would have 16 chromosomes…their
offspring would have 32 and so on.
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Drosophila melanogaster
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CHROMOSOME NUMBER PG 275
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The chromosomes of the fruit fly can be
divided into 2 sets…4 chromosomes from the
male parent and 4 chromosomes from the
female parent.
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Each chromosome in the male set has a corresponding
chromosome in the female set.
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HOMOLOGOUS- a description of
chromosomes that occur in pairs; having a
corresponding structure…the
chromosomes are called HOMOLOGS.
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EX: Each of the chromosomes from the
male parent have a corresponding
chromosome from the female parent.
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A cell that contains both sets of homologous
chromosomes are said to be DIPLOID.
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A diploid cell contains 2 complete sets of
chromosomes and 2 complete sets of
genes.The diploid number is sometimes represented by 2N…so for the Drosophila 2N=
8.
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DIPLOID- "two sets"…a cell that contains
both sets of homologous
chromosomes.All of an organisms cells
(except for sex cells) contain 2 alleles for a
given trait.
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Gametes or sex cells contain only a single set of genes because alleles are separated during the process of
gamete formation.
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The GAMETES of Sexually reproducing organisms contain a
single set of chromosomes and
genes.
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HAPLOID- a cell that contains a single set of
chromosomes..represented by the symbol
N.Drosophila, N =4…
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“THE PHASES OF MEIOSIS”
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MEIOSIS- a process of reduction division in which the number of
chromosomes per cell is cut in half and homologous
chromosomes that exist in a diploid cell are
separated.
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In most organisms meiosis takes place in 2 stages..1st & 2nd meiotic
divisions.
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The phases of meiosis are very different from the phases of MITOSIS.
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SEE FIG 11-15 page 276…
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MEIOSIS I….
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Special cells in reproductive organs
undergo a round of DNA replication…this
resembles mitosis but it is not the same!
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In Prophase 1 of meiosis, each
chromosome seeks out its corresponding
homologous chromosome to form a special structure called
a tetrad.
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There are 4 chromatids in a tetrad;
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In a process called
“CROSSING OVER” homologous
chromosomes may exchange portions of their chromatids, an
exchange of genes that produces new
combinations of genes…see figure 11-16 page
276
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METAPHASE 1
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Metaphase ITetrads (rather than
individual chromosomes) line up in the center of the cell.
ANAPHASE IThe homologous
chromosomes separate and (telophase 1) two new cells
are formed.
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The 2 new cells have sets of chromosomes that are different from the parent cells and different from each
other.
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MEIOSIS II….
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The 2 cells produced by meiosis I enter
meiosis II.In the second meiotic division the cells do not under go DNA replication so
each cell’s chromosomes contains
2 chromatids.
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In METAPHASE II of meiosis, : chromosomes line up in the middle of the cell. In ANAPHASE II they separate…each of
the 4 daughter cells receives 2 chromatids .
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The daughter cells contain the HAPLOID
number = 2 chromosomes each.
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*The amount of genetic material has
been reduced and the combinations of
chromosomes in each gamete have been made at random.
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“MEIOSIS & GENETICS”
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Meiosis I results in segregation and
independent assortment.
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GAMETE FORMATION…
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In males the haploid gametes produced by
meiosis are called SPERM.Pollen grains
contain haploid sperm cells.
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The female gamete is called an egg in
animals and an OVULE in higher plants.
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In females the cell divisions are uneven, and the egg or ovule
gets most of the cytoplasm…see figure
11-17 pg 278;
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3 other cells called polar bodies are produced in
the female during meiosis..they are called
polar bodies.
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“COMPARING MITOSIS AND MEIOSIS”
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Mitosis results in the production of 2
genetically identical cells.
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A diploid cell divides and gives rise to 2
diploid daughter cells that are genetically
identical to each other and identical to the original parent cell.
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Meiosis begins with a diploid cell but
produces 4 haploid cells that are different from the original diploid cell and different from one
another.WHY?
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Homologous chromosomes are
separated during the 1st meiotic division and
crossing over occurs…giving new gene
combinations on the chromosomes.
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*MITOSIS results in the production of 2
genetically identical diploid cells.
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*MEIOSIS results in the production of 4
genetically DIFFERENT haploid cells.
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TEST CH 11