Meiosis and its stages

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Information about Meiosis and its stages
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Published on March 7, 2014

Author: XolaniMfazwe

Source: slideshare.net

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Meiosis and the stages involved in them. The powerpoint was readapted from various powerpoint presentations.

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

Crossing over

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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