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UNIT 3: MEIOSIS REDUCTION DIVISION Campbell & Reece, 2010 Chapter 13 by Dudrah Moyo

1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS  In humans, somatic cells (body cells) have: • 23 pairs of homologous chromosomes and • one member of each pair from each parent.  The human sex chromosomes (Gonosomes) X and Y differ in size and genetic composition.  The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. © 2012 Pearson Education, Inc.

 Homologous chromosomes are matched in: • length, • centromere position, and Centromere • gene locations (locus).  A locus (plural, loci) is the position of a gene.  Different versions (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes. © 2012 Pearson Education, Inc.

 Homologous chromosome pair Centromere © 2012 Pearson Education, Inc.

2. GAMETES HAVE A SINGLE SET OF CHROMOSOMES  Humans and most animals and plants have diploid body cells.  That means they have two sets of chromosomes (homologous chromosome pair) one from each parent.  Diploid is written 2n.  It refers to the total number of chromosomes a cell can have. © 2012 Pearson Education, Inc.

 Meiosis is a process that converts diploid nuclei to haploid nuclei. • Diploid cells have 2 sets of chromosomes. • Haploid cells have 1 set of chromosomes. • Meiosis occurs in the sex organs, producing gametes—sperm and eggs.  Fertilization is the fusion of a sperm and egg cell.  The zygote has a diploid chromosome number, one set from each parent. © 2012 Pearson Education, Inc.

Haploid gametes (n  23) A life cycle n Egg cell n Sperm cell Meiosis Ovary Fertilization Testis Diploid zygote (2n  46) 2n Key Multicellular diploid adults (2n  46) Mitosis Haploid stage (n) Diploid stage (2n)


 All sexual life cycles include an alternation between • a diploid stage and • a haploid stage.  Why is meiosis so important? It produces haploid gametes which prevents the chromosome number from doubling in every generation. Produce gametes for fertilization. © 2012 Pearson Education, Inc.

3. MEIOSIS  Meiosis is a type of cell division that produces haploid gametes from diploid cells.  Two haploid gametes combine in fertilization to restore the diploid state in the zygote. 3



MEIOSIS I : INTERPHASE  Cell build up energy  DNA Replication (to make duplicated chromosomes  Cell doesn’t change structurally. © 2012 Pearson Education, Inc.

MEIOSIS I : PROPHASE I Events occurring in the nucleus: • Chromosomes coil and become individual chromosomes, nucleolus and nuclear envelope disappear. • Homologous chromosomes come together as pairs by synapsis forming a tetrad (Each pair, with four chromatids) • Non-sister chromatids exchange genetic material through the process of crossing over to ensure genetic variation. • Centrioli move to opposite poles with spindle fibers between them.

MEIOSIS I : PROPHASE I © 2012 Pearson Education, Inc.

CROSSING OVER  Genetic recombination is the production of new combinations of genes due to crossing over.  Crossing over is an exchange of genes between separate (non-sister) chromatids on homologous chromosomes. • Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment. • Genetic material are exchanged between maternal and paternal (non-sister) chromatids. © 2012 Pearson Education, Inc.

CROSSING OVER © 2012 Pearson Education, Inc.

MEIOSIS I: METAPHASE I  Centrioli has reached the poles.  Homologous pairs align at the cell equator.  The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere. . © 2012 Pearson Education, Inc.

MEIOSIS I: ANAPHASE I  Spindle fibers contract.  Duplicated chromosomes move to opposite poles. . © 2012 Pearson Education, Inc.

MEIOSIS I: TELOPHASE I • Duplicated chromosomes have reached the poles. • A nuclear envelope and nucleolus re-forms around chromosomes. • Each nucleus now has the haploid number of chromosomes. • Cell invaginates forming a cleavage furrow, which extends to for 2 separate haploid cells. © 2012 Pearson Education, Inc.

MEIOSIS II  Follows meiosis I without chromosome duplication.  Each of the two haploid products enters meiosis II. © 2012 Pearson Education, Inc.

MEIOSIS II: PROPHASE II • Chromosomes coil and become compact (if uncoiled after telophase I). • Nuclear envelope and nucleolus, if reformed, dissappears again. • Centrioli move to opposite poles, forming spindle fibers between them. © 2012 Pearson Education, Inc.

MEIOSIS II: METAPHASE II • Individual duplicated chromosomes align on the equator. • One chromosome per spindle fiber attached by means of kinetochore of centromere. • Centrioli has reached the poles. © 2012 Pearson Education, Inc.

MEIOSIS II: ANAPHASE II • Spindle fibers contract. • Duplicated chromosomes split in half (centromere dividing in 2) • Daughter chromosomes move to opposite poles. © 2012 Pearson Education, Inc.

MEIOSIS II: TELOPHASE II • Daughter chromosomes has reached the poles. • Two cells invaginate and form 4 daughter haploid cells (gametes) • They uncoil and form chromatin. • Nuclear envelope and nucleolus for around chromatin again. • Centrioli for centrosome.

SUMMERY OF MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Haploid daughter cells forming

4. SIMILARITIES AND DIFFERENCES BETWEEN MITOSIS AND MEIOSIS  Mitosis and meiosis both • begin with diploid parent cells that • have chromosomes duplicated during the previous interphase.  However the end products differ. • Mitosis produces two genetically identical diploid somatic daughter cells. • Meiosis produces four genetically unique haploid gametes.

5. GENETIC VARIATION IN GAMETES RESULTS FROM: • Independent orientation at metaphase I • Random fertilization. • Crossing over of genes during prophase I © 2012 Pearson Education, Inc.

6. KARYOTYPE • A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. • Karyotypes allow for the observation of :  homologous chromosome pairs,  chromosome number, and  chromosome structure. © 2012 Pearson Education, Inc.


KARYOTYPE Sex chromoso

7. ALTERATION IN CHROMOSOME NUMBER  An extra copy of chromosome 21 causes Down syndrome or also known as TRISOMY 21.  A. Trisomy 21 • involves the inheritance of three copies of chromosome 21 and • is the most common human chromosome abnormality. © 2012 Pearson Education, Inc.

Down syndrome Karyotype

 Trisomy 21 produces a characteristic set of symptoms, which include: • • • • • mental retardation, characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and • shortened life span.  The incidence increases with the age of the mother.

B. ACCIDENTS DURING MEIOSIS CAN ALTER CHROMOSOME NUMBER  Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during: • meiosis I, if both members of a homologous pair go to one pole or • meiosis II if both sister chromatids go to one pole.  Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Number of chromosomes n1 n1 n1 Abnormal gametes n1

MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n1 n1 Abnormal gametes n n Normal gametes

C. ABNORMAL NUMBERS OF SEX CHROMOSOMES  Sex chromosome abnormalities tend to be less severe, perhaps because of • the small size of the Y chromosome or • X-chromosome inactivation. © 2012 Pearson Education, Inc.

 In general, • a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and • the absence of a Y chromosome yields “femaleness.” © 2012 Pearson Education, Inc.

 The following table lists the most common human sex chromosome abnormalities. © 2012 Pearson Education, Inc.

D. NEW SPECIES CAN ARISE FROM ERRORS IN CELL DIVISION  Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets. . © 2012 Pearson Education, Inc.

8. ALTERATIONS OF CHROMOSOME STRUCTURE  Chromosome breakage can lead to rearrangements that can produce: • genetic disorders or, • if changes occur in somatic cells, cancer. © 2012 Pearson Education, Inc.

THESE REARRANGEMENTS MAY INCLUDE: • a deletion, the loss of a chromosome segment, • a duplication, the repeat of a chromosome segment, • an inversion, the reversal of a chromosome segment, or • a translocation, the attachment of a segment to a non-homologous chromosome that can be reciprocal.

THESE REARRANGEMENTS MAY INCLUDE: Deletion Inversion Duplication Reciprocal translocation Homologous chromosomes © 2012 Pearson Education, Inc. Nonhomologous chromosomes

LIST OF SOURCES USED • • • Mrs J.Williamsons 2014 sildes 3A FET MEIOSIS •

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1.UNIT 3: MEIOSIS REDUCTION DIVISIONCampbell & Reece, 2010 Chapter 13 by Dudrah Moyo 2. 1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS In humans, somatic ...
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