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Meiosis
Honors Biology
Introduction to Heredity
Offspring acquire genes from parents by inheriting chromosomes
Inheritance is possible because:
– Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg
Actual transmission of genes depends on the behavior of chromosomes
•Chromosomes-organizational unit of hereditary material in the nucleus of eukaryotic organisms•Contain hundreds of thousands of genes, each of
which is a specific region of the DNA molecule, or locus
Human Life Cycle
Each somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid)
Karyotype: male
Autosomes: non-sex chromosomes
Sex chromosomes:determine gender (XX; XY)
Human Life Cycle
Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y
With 23 chromosomes, they are haploid
haploid number: n = 23
diploid number: 2n = 46
Haploid sperm + haploid ova zygote (2n)fertilization
2nn n
Meiosis
Reduces chromosome number from diploid to haploid
Increases genetic variation among offspring
Steps resemble steps in mitosis
Single replication of DNA is followed by 2 consecutive cell divisions Meiosis I Meiosis II
Produces 4 different daughter cells which have half the number of chromosomes as the original cell
In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange
genetic information The homologous pairs are then separated, and two
daughter cells are produced
Interphase I
Chromosomes replicate (still as chromatin)
Duplicated chromosomes consist of 2 identical sister chromatids attached by centromere
Centriole pairs replicate
Figure 8.14, part 1
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes(withcentriolepairs)
Nuclearenvelope
Chromatin
Sites of crossing over
Spindle
Sisterchromatids
Tetrad
Microtubules attached tokinetochore
Metaphaseplate
Centromere(with kinetochore)
Sister chromatidsremain attached
Homologouschromosomes separate
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Meiosis I
This cell division separates the 2 chromosomes of each homologous pair and reduce the chromosome number by one-half
Prophase I
Chromosomes condense
Synapsis occurs (homologues pair)
Chromosomes seen as distinct structures; each chromosome has 2 chromatids, so each synapsis forms a tetrad
Prophase I
Sister chromatids held together by centromeres; non-sister chromatids held together by chiasmata where crossing-over occurs (exchange of DNA)
Late Prophase I
Centriole pairs move apart and spindle fibers formNuclear envelope disappears and nucleoli disperse
Prophase I
Metaphase I
Homologous chromosomes line up along metaphase plate
Metaphase I
Anaphase I
Homologous chromosomes separate, independently from others
Anaphase I
Telophase I and Cytokinesis
Each pole now has a haploid set of chromosomes (each with 2 sister chromatids)Usually, cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells (cleavage furrow forms in animals; cell plate forms in plants)
Telophase I
Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells
Figure 8.14, part 2
MEIOSIS II: Sister chromatids separate
TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II
Cleavagefurrow
Sister chromatidsseparate
TELOPHASE IIAND CYTOKINESIS
Haploiddaughter cellsforming
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Meiosis II
This cell division separates the 2 sister chromatids of each chromosome
Prophase II
Spindle apparatus forms and chromosomes move toward metaphase II plate
Prophase II
Metaphase II
Chromosomes align singly on the metaphase plate
Metaphase II
Anaphase II
Sister chromatids of each pair (now individual chromosomes) separate and move toward opposite poles of the cell
Anaphase II
Anaphase II
Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell
Cytokinesis occurs producing 4 haploid daughter cells (each genetically different)
Telophase II
Telophase II
Key Differences Between Mitosis and Meiosis
Meiosis is a reduction division Mitotic cells produce clones (same xsome #) Meiosis produces haploid cells
Meiosis creates genetic variation Mitosis produces 2 identical daughter cells Meiosis produces 4 genetically different
daughter cells Meiosis is 2 successive nuclear divisions Mitosis has one division
Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings
Spermatogenesis
Process of sperm production
Results in 4 viable sperm
Oogenesis
Process of egg (ova) production
Results in 1 viable egg and 3 polar bodies that will not survive
Polar bodies result from an uneven division of cytoplasm
Mechanisms of Genetic Variation
Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations
Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination
Random fertilization—random joining of two gametes
Independent Assortment
Figure 8.16
POSSIBILITY 1 POSSIBILITY 2
Two equally probable
arrangements of chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2 Combination 3 Combination 4
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Mechanisms of Genetic Variation
Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations
Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination
Random fertilization—random joining of two gametes
Figure 8.18A
TetradChaisma
Centromere
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 8.17A, B
Coat-color genes Eye-color genes
Brown Black
C E
c e
White Pink
C E
c e
C E
c e
Tetrad in parent cell(homologous pair of
duplicated chromosomes)
Chromosomes ofthe four gametes
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
How crossing over leads to genetic recombination
Figure 8.18B
Tetrad(homologous pair ofchromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II and completion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
1
2
3
4
Coat-colorgenes
Eye-colorgenes
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Crossing Over
In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids
Creates new combinations of genes not seen in either parent
Mechanisms of Genetic Variation
Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations
Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination
Random fertilization—random joining of two gametes
Random Fertilization
Random as to which gametes join and form a gamete
Importance of Genetic Variation
Essential to evolution (change over time)
Variation can cause changes that leads to different traits Some favorable Some unfavorable
Errors and Exceptions in Chromosomal Inheritance
Alterations in chromosome number or structure causes some genetic disorders Physical and chemical disturbances Errors during meiosis
To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes
and one pair of sex chromosomes
ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
Preparation of a karyotype
Figure 8.19
Blood culture
1
Centrifuge
Packed redAnd white blood cells
Fluid
2
Hypotonic solution
3
Fixative
WhiteBloodcells
Stain
4 5
Centromere
Sisterchromatids
Pair of homologouschromosomes
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Human female bands
Figure 8.19x1
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Human female karyotype
Figure 8.19x2
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Human male bands
Figure 8.19x3
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Human male karyotype
Figure 8.19x4
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Alterations of Chromosome Numbers
Nondisjunction—pair of homologues do not move apart during Meiosis I, or sister chromatids do not separate during Meiosis II Results in uneven distribution of chromosomes to
daughter cells
Alterations of Chromosome Numbers
Aneuploidy: abnormal chromosome number Trisomy: three copies of chromosomes Monosomy: one copy of a chromosome Trisomy and monosomy are usually lethal
Abnormal chromosome count is a result of nondisjunction Either
homologous pairs fail to separate during meiosis I
Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings
Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n n
Number of chromosomes
Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings
Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Figure 8.21C
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings
Trisomy 21 (Down Syndrome)*Short stature, characteristic facial features, and heart defects (varying severity)*Most common serious birth defect (1 out of 700 births)*Mothers 35+ years of age have higher chance of having a Down baby
The chance of having a Down syndrome child goes up with maternal age
Figure 8.20C
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Down syndrome karyotype
Figure 8.20Ax
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Nondisjunction with Sex Chromosomes
Table 8.22
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Klinefelter’s karyotype
Figure 8.22Ax
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
XYY karyotype
Figure 8.22x
Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: chromosomal fragment is lost during
cell division Duplication: fragment may join to the
homologous chromosome Inversion: fragment may reattach to the original
chromosome but in the reverse orientation Translocation: fragment joins a nonhomologous
chromosome
Alterations of Chromosome Structure
Chromosome Mutation: Deletion
Deleted region
BeforeDeletion
AfterDeletion
Cri du Chat Syndrome: Partial deletion 5p
Before inversion
After inversion
Inverted region
Chromosome Mutation: Inversion
Chromosome 4
Chromosome 4Chromosome 20
Chromosome 20
Regionbeingmoved
Before TranslocationAfter Translocation
Chromosome Mutation: Translocation
Chromosomal changes in a somatic cell can cause cancer
Figure 8.23C
Chromosome 9
A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia
Chromosome 22Reciprocaltranslocation
“Philadelphia chromosome”
Activated cancer-causing gene
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Philadelphia Chromosomet(9,22)
Translocation
Figure 8.23Bx
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Acknowledgements
Unless otherwise noted, illustrations are credited to Pearson Education which have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.