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Ch 13 NOTES – Meiosis
For a species to survive,
it must REPRODUCE!
Genetics Terminology:
Autosomes Sex chromosomes
Somatic cell Diploid
Gamete Haploid
Karyotype Zygote
Homologous chromosomes Synapsis
Meiosis Crossing over
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1) ASEXUAL REPRODUCTION:
● each new organism gets a set of chromosomes identical to parents
● DNA from generation to generation remains the same
● therefore, no differences or variations occur in the offspring (difficult to adapt!)
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2) SEXUAL REPRODUCTION: ● fusion of nuclei from 2 special cells: GAMETES
(female gamete = egg; male gamete = sperm)
● with 2 parents, offspring receive half of their chromosomes from the female parent and half from the male parent.
● therefore, the offspring will show new combinations of characteristics
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**Sexual reproduction leads to VARIATION!(this ensures the survival of a species)
● SOMATIC CELLS = all cells EXCEPT the gametes
● each somatic cell of an organism contains a specific # of chromosomes (humans = 46)
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Sets of Chromosomes in Human Cells
● Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs
● A KARYOTYPE is an ordered display of the pairs of chromosomes from a cell
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● the chromosomes occur in “matching pairs” (1 member of each pair came from the egg and 1 member came from the sperm)
● these pairs of chromosomes are called HOMOLOGOUS PAIRS
● Both chromosomes in a pair carry genes controlling the same inherited characteristics
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5 µmPair of homologouschromosomes
Sisterchromatids
Centromere
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● The sex chromosomes are called X and Y
● Human females have a homologous pair of X chromosomes (XX)
● Human males have one X and one Y chromosome
● The 22 pairs of chromosomes that do not determine sex are called AUTOSOMES
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Human Gamete
~cells with half the #
of chromosomes are
HAPLOID (n)
Human Somatic Cell
~cells with full # of
chromosomes are
DIPLOID (2n)
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Key
Maternal set ofchromosomes (n = 3)
2n = 6
Paternal set ofchromosomes (n = 3)
Two sister chromatidsof one replicatedchromosomes
Two nonsister chromatids in a homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
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● Gametes are haploid cells, containing only one set of chromosomes
● For humans, the haploid number is 23 (n = 23)
● Each set of 23 consists of 22 autosomes and a single sex chromosome
● In an unfertilized egg (ovum), the sex chromosome is X
● In a sperm cell, the sex chromosome may be either X or Y
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● the union of 2 haploid gametes restores the diploid # of chromosomes (Fertilization!)
● MEIOSIS: cell division in which daughter cells receive only ½ of the chromosomes of the parent cell – RESULT: formation of gametes!
● occurs only in reproductive tissues
male = testes female = ovaries
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Behavior of Chromosome Sets in the Human Life Cycle - OVERVIEW
● At sexual maturity, the ovaries and testes produce haploid gametes
● Gametes are the only types of human cells produced by meiosis, rather than mitosis
● Meiosis results in one set of chromosomes in each gamete
● Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote
● The diploid zygote develops into an adult
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Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Spermcell (n)
TestisOvary
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
FERTILIZATIONMEIOSIS
Diploidzygote(2n = 46)
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The Variety of Sexual Life Cycles
● The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
● The three main types of sexual life cycles differ in the timing of meiosis and fertilization
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● In animals, meiosis produces gametes, which undergo no further cell division before fertilization
● Gametes are the only haploid cells in animals
● Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism
ANIMALS
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Key
HaploidDiploid
Gametesn
Diploidmulticellularorganism(sporophyte)
MitosisDiploidmulticellular
organism
FERTILIZATIONMEIOSIS
Zygote
n
n
2n 2n
Animals Plants and some algae Most fungi and some protists
n n
n
n n
nn
n
n
nFERTILIZATION
FERTILIZATION
MEIOSIS
MEIOSIS
Gametes Gametes
Zygote
ZygoteMitosis
Mitosis Mitosis Mitosis Mitosis
2n2n
2n
Spores
Haploid multicellular organism (gametophyte)
Haploid multicellular organism
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● Plants and some algae exhibit an alternation of generations
● This life cycle includes two multicellular generations or stages: one diploid and one haploid
● The diploid organism, the SPOROPHYTE, makes haploid spores by meiosis
● Each spore grows by mitosis into a haploid organism called a GAMETOPHYTE
● A gametophyte makes haploid gametes by mitosis
PLANTS
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Key
HaploidDiploid
Diploidmulticellularorganism(sporophyte)
Plants and some algae
n n
n
n n
FERTILIZATIONMEIOSIS
Gametes
Zygote
Mitosis
Mitosis Mitosis
2n2n
Spores
Haploid multicellular organism (gametophyte)
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● In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
● The zygote produces haploid cells by meiosis
● Each haploid cell grows by mitosis into a haploid multicellular organism
● The haploid adult produces gametes by mitosis
FUNGI and PROTISTS
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Key
HaploidDiploid
Most fungi and some protists
nn
n
n
n
FERTILIZATIONMEIOSIS
Gametes
Zygote
Mitosis Mitosis
2n
Haploid multicellular organism
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Meiosis takes place in 2 stages:
1) Meiosis I
2) Meiosis II
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1) Meiosis I
● preceded by Interphase: DNA is copied● Prophase I: each chromosome pairs up with
its homologous chromosome to form a TETRAD (4 chromatids)
● Metaphase I: tetrads line up in the center of the cell
● Anaphase I: homologous chromosomes are pulled apart from one another
● Telophase I
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2) Meiosis II
● the 2 cells produced in meiosis I
undergo a second division (PMAT - II)
● the result: 4 haploid daughter cells!
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A Comparison of Mitosis and Meiosis:
● Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
● Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
● The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis
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Three events are unique to meiosis, and all three occur in meiosis l:
1) Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
2) At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
3) At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poles of the cell
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Propase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
2n = 6
Parent cell(before chromosome replication)
Chromosomereplication
MITOSIS MEIOSIS
Chiasma (site ofcrossing over) MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Tetradspositioned at themetaphase plate
Metaphase IChromosomes positioned at themetaphase plate
Metaphase
AnaphaseTelophase
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Sister chromatidsseparate duringanaphase
Daughtercells of
meiosis I
Haploidn = 3
Anaphase ITelophase I
MEIOSIS II
Daughter cellsof mitosis
2n2n
n
Sister chromatids separate during anaphase II
n n n
Daughter cells of meiosis II
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Property Mitosis Meiosis
DNA replication
During interphase
During interphase
Divisions One Two
Synapsis and crossing over
Do not occur Form tetrads in prophase I
Daughter cells, genetic composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal body
Produces cells for growth and tissue repair
Produces gametes
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Genetic variation produced in sexual life cycles contributes to evolution
● Mutations (changes in an organism’s DNA) are the original source of genetic diversity
● Mutations create different versions of genes
● Reshuffling of different versions of genes during sexual reproduction produces genetic variation
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Origins of Genetic Variation Among Offspring
● The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
● Three mechanisms contribute to genetic variation:
1) Independent assortment of chromosomes
2) Crossing over
3) Random fertilization
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1) Independent Assortment of Chromosomes
● Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
● In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
● The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
● For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes!
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Key
Maternal set ofchromosomes
Paternal set ofchromosomes
Possibility 1 Possibility 2
Combination 2Combination 1 Combination 3 Combination 4
Daughtercells
Metaphase II
Two equally probablearrangements ofchromosomes at
metaphase I
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2) Crossing Over● Crossing over produces recombinant
chromosomes, which combine genes inherited from each parent
● Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
● In crossing over, homologous portions of two nonsister chromatids trade places
● Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
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Prophase Iof meiosis
Tetrad
Nonsisterchromatids
Chiasma,site of crossingover
Recombinantchromosomes
Metaphase I
Metaphase II
Daughtercells
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3) Random Fertilization● Random fertilization adds to genetic
variation because any sperm can fuse with any ovum (unfertilized egg)
● The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
● Crossing over adds even more variation● Each zygote has a unique genetic identity
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Evolutionary Significance of Genetic Variation Within Populations● Natural selection results in accumulation of genetic variations favored by the environment● Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations
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