Published on March 7, 2014
MEIOSIS Formation of gametes, division of the sex cell (egg and sperm) (Readapted from Slideshare): 1. Jay Swan 2. Karl Pointer 3. Mbrown
Chromosome Matching 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.
• Homologous chromosomes are matched in: • Similar length, • Centromere (attaches sister chromatids together) position • gene locations (locus). • A locus (plural, loci) is the position of a gene. • Different versions or variations (alleles) of a gene may be found at the same locus on maternal and paternal chromosomes.
MEIOSIS The process to make cells with half the number of chromosomes for sexual reproduction Usually humans and most animals and some plants have diploid (2n) body cells. Meaning that they have two sets of chromosomes (one from each parent) Meiosis occurs in our germ cells that produce gametes (Sperm & egg) • Meiosis results in four cells which are genetically different from parent cell and from each other. • The end products of Meiosis are 4 Haploid (n) cells
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.
Why do we need Meiosis? It is the fundamental basis of sexual reproduction Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.
Meiosis must reduce the chromosome number by half (n) Fertilization then restores the 2n number
Summary of the Meiotic process
MEIOSIS has two distinct stages MEIOSIS I consisting of 5 phases: Interphase I, Prophase I, Metaphase I, Anaphase I, Telophase I. MEIOSIS II consisting of 4 phases Prophase II, Metaphase II, Anaphase II, Telophase II.
MEIOSIS 1: Interphase Cell build up energy DNA Replication (to make duplicated chromosomes Cell doesn’t change structurally.
MEIOSIS 1: Prophase 1
Prophase 1 in detail 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.
PROPHASE 1 • Early prophase 1 Homologous pair. Crossing over occurs. Late Prophase 1 Chromosomes condense. Spindle forms. Nuclear envelope fragments.
Prophase 1: Crossing over • Synapsis – the pairing of homologous chromosomes • Group of 4 chromatids Homologous chromosomes (each with sister chromatids) Join to form a TETRAD
Prophase 1: Crossing over Homologous chromosomes in a tetrad cross over each other - Genes are exchanged
Metaphase I Spindle fibre attached to a kinetochore Metaphase plate Homologous pairs of chromosomes align along the equator of the cell The two chromosomes attach to one spindle fiber by means of the kinetochore of the centromere.
Anaphase 1 Spindle fibers contact. Homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres.
Telophase 1 Nuclear envelopes reappear Spindle fibres disappear. Cytokinesis (when the cytoplasm divides) divides cell into two.
Telophase 1 and Cytokinesis • 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.
Meiosis ii: prophase II • Chromosomes coil and become compact (if uncoiled after telophase I). • Nuclear envelope and nucleolus, if re-formed, dissappears again. • Centrioli move to opposite poles, forming spindle fibers between them.
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.
Anaphase 2 - Spindle fibers contract. - Duplicated chromosomes split in half (centromere dividing in 2) Sister chromatids separate and move to opposite poles.
Meiosis 2: Telophase 2 • 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.
MEOITIC DIVISION 2: SUMMARY Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis
Results of Meiosis Gametes (egg & sperm) form Four haploid cells (n) with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome
When Chromosome number is altered 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.
Trisomy 21 (Down Syndrome) produces a characteristic set of symptoms, which include: 1. mental retardation, characteristic facial features, 2. short stature, 3. heart defects, 4. susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and 5. shortened life span. The incidence increases with the age of the mother.
Bibliography • Jackson, R., & Jackson, J. M. (2011). Henderson's Dictionary of BIOLOGY (15th ed.). (E. Lawrence, Ed.) (pp. 181-183). London: Pearson Education Limited. • Livingstone, C. D., & Nobbe, M. E. (1998). The Molecules of Life. Oxford: Biochemistry Department, University of Oxford. • Reece, J., Urry, L. A., Cane, M. L., Wasserman, S. A., Minrsky, P. V., & Jackson, R. B. (2011). Campbell Biology (9th ed.). San Francisco: Pearson Benjamin Cummings. • Urry, L. A., & Cane, M. L. (2011). The Molecular Basis of Inheritance. In J. B. Reece, Campbell Biology (pp. 351370). San Francisco: Pearson Benjamin Cummings.
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