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Information about Meiosis

Published on March 9, 2014

Author: MASIKANE93





MEIOSIS  The form of cell division by which gametes, with half the number of chromosomes also known as haploid, are produced.  Diploid (2n) (n) → haploid  Meiosis is sexual reproduction.

MEIOSIS cont..  Sex cells divide to produce gametes (sperm or egg).  Gametes have half the # of chromosomes.  Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis  Meiosis is similar to mitosis with some chromosomal differences.

Spermatogenesis n=23 human sex cell n=23 sperm n=23 2n=46 haploid (n) diploid (2n) n=23 n=23 n=23 Meiosis I Meiosis II

Interphase I  Similar to mitosis Interphase.  Chromosomes replicate (S phase).  Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. centromeres  Centriole pairs also replicate, INSIDE THE CENTROSOME

Interphase I  Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Meiosis I (four phases)  Cell division that reduces the chromosome number by one-half. a. b. c. d. Four phases: phases Prophase I Metaphase I Anaphase I Telophase I

PROPHASE I  Longest and most complex phase (90%).  Chromosomes condense.  Synapsis occurs: homologous chromosomes come together to form a BIVALENT TETRAD. TETRAD  Tetrad= TWO chromosomes of THREE OR Tetrad FOUR chromatids (sister and nonsister chromatids).  IN THIS PHASE THE PROCESS CROSSING OVER TAKES PLACE- EXCHANGE OF GENETIC MATERIAL.

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids

HOMOLOGOUS CHROMOSOMES  PaIr Of chrOMOSOMES (MatErnal and PatErnal) that arE SIMIlar In ShaPE and SIzE.  hOMOlOgOuS PaIrS (tEtradS) carry gEnES cOntrOllIng thE SaME InhErItEd traItS.  Each lOcuS (POSItIOn Of a gEnE) IS In thE SaME POSItIOn On hOMOlOguES.  huManS havE 23 PaIrS Of hOMOlOgOuS chrOMOSOMES. a. 22 PaIrS Of autOSOMES b. 01 PaIr Of SEx chrOMOSOMES

Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal

Crossing Over ???????????? Crossing over (variation) may occur between nonsister chromatids at the chiasmata. chiasmata Crossing over: segments of nonsister chromatids break and over reattach to the other chromatid. chromatid Chiasmata (chiasma) are the sites of crossing over. over

Crossing Over nonsister chromatids chiasmata: site of crossing over vARIATION Tetrad variation

Sex Chromosomes XX chromosome - female XY chromosome - male

Prophase I spindle fiber aster fibers centrioles

Metaphase I  Shortest phase  Tetrads align on the metaphase plate. plate  INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

Metaphase I OR metaphase plate metaphase plate

Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres. centromeres

Anaphase I

Telophase I  Each pole now has haploid set of chromosomes. chromosomes  Cytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II  No Interphase II (or very short - no more DNA replication) replication  Remember: Meiosis II is similar to Mitosis

Prophase II  Same as prophase in mitosis

Metaphase II  Same as Metaphase in Mitosis metaphase plate metaphase plate

Anaphase II  Same as Anaphase in Mitosis  Sister chromatids separate

Telophase II Same as Telophase in Mitosis. Mitosis Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg

Telophase II

Meiosis sex cell n=2 sperm n=2 n=2 2n=4 haploid (n) diploid (2n) n=2 n=2 n=2 meiosis I meiosis II


Variation  Important to population as the raw material for natural selection. Question: What are the three sexual sources Of genetic

Answer: 1. Crossing over (Prophase I) 2. Independent assortment (Metaphase I) 3. Random fertilization Remember: VaRiAtIoN iS gOoD!

Fertilization  The fusion of a sperm and egg to form a zygote. zygote A zygote is a fertilized egg


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.


Centromere Sister chromatids Pair of homologous chromosomes Sex chromosomes

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

 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, 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.


D. New species can arise from errors in cell division  Errors in mitosis or meiosis may produce polyploidy 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

PRESENTED and EDITED BY KHANYISILE MASIKANE ACKNOWLEDGEMENTS TO Hasimah Azit AND Janice Williamson Images taken from: Campbell, N. Reece, J.2007. BIOLOGY 8th edition. Pearson Education.

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