Published on March 9, 2014
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
SUMMERY OF THE MEIOSIS PROCESS 3
MEIOSIS HAS 2 STAGES: MeiosIs I Meiosis II PROPHASE II INTERPHASE METAPHASE II PROPHASE I ANAPHASE II METAPHASE I TELOPHASE II ANAPHASE I TELOPHASE I © 2012 Pearson Education, Inc.
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.
SCIENTIST OBSERVING A HUMAN KARYOTYPE
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 n1 n1 n1 Abnormal gametes n1
MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n1 n1 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 • http://www.slideshare.net/freesmart/meiosispowepo?qid=fe02f4ec-0e9e-4a97-a2581795f193ff7e&v=qf1&b=&from_search=1 • http://www.slideshare.net/CDA-PamelaOrtiz/meiosis-ppt14835989 • Mrs J.Williamsons 2014 sildes 3A FET MEIOSIS • http://www.slideshare.net/RawanShahien/mitosis-andmeiosis-6360090
1.UNIT 3: MEIOSIS REDUCTION DIVISIONCampbell & Reece, 2010 Chapter 13 by Dudrah Moyo 2. 1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS In humans, somatic ...