MEIOSIS

Meiosis cell divisiongametes, halfchromosomes,The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n)Diploid (2n)  haploid (n) Meiosissexual reproduction.Meiosis is sexual reproduction. Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II).

Fertilization spermeggzygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

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

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

Oogenesis *** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

Interphase I mitosisSimilar to mitosis interphase. Chromosomes(S phase).Chromosomes replicate (S phase). chromosome chromatids centromeresEach duplicated chromosome consist of two identical sister chromatids attached at their centromeres. CentrioleCentriole pairs also replicate.

Interphase I NucleusnucleolusNucleus and nucleolus visible. nuclear membrane nucleolus cell membrane chromatin

Meiosis I (four phases) Cell division chromosome one-half.Cell division that reduces the chromosome number by one-half. four phasesfour phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

Prophase I Longest and most complex phase (90%).Longest and most complex phase (90%). ChromosomesChromosomes condense. Synapsis homologous chromosomes tetradSynapsis occurs: homologous chromosomes come together to form a tetrad. Tetradchromosomes chromatidsTetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Homologous Chromosomes chromosomesmaternalpaternalPair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads)Homologous pairs (tetrads) carry genes controlling the same inherited traits. locus(position of a gene)Each locus (position of a gene) is in the same position on homologues. homologous chromosomes.Humans have 23 pairs of homologous chromosomes. autosomes a.22 pairs of autosomes sex chromosomes b.01 pair of sex chromosomes

Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.A method of organizing the chromosomes of a cell in relation to number, size, and type.

Homologous Chromosomes PaternalMaternal eye color locus eye color locus hair color locus hair color locus

Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues. Homologue

Autosomes (The Autosomes code for most of the offspring’s traits) In Humans the “Autosomes” are sets

21 trisomy – Downs Syndrome Can you see the extra 21 st chromosome? Is this person male or female?

Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. XX chromosome - femaleXY chromosome - male In Humans the “Sex Chromosomes” are the 23 rd set

Boy or Girl? The Y Chromosome “Decides” X chromosome Y chromosome

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

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

Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Sex Chromosomes XX chromosome - femaleXY chromosome - male

Prophase I centrioles spindle fiber aster fibers

Metaphase I Shortest phaseShortest phase Tetradsmetaphase plateTetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS: Formula: 2 n Example:2n = 4 then n = 2 thus 2 2 = 4 combinations thus 2 2 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

Question: In terms of Independent Assortment - how many different combinations of sperm could a human male produce?In terms of Independent Assortment - how many different combinations of sperm could a human male produce?

Answer Formula: 2 nFormula: 2 n Human chromosomes:2n = 46Human chromosomes:2n = 46 n = 23 n = = ~8 million combinations2 23 = ~8 million combinations

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

Anaphase I

Telophase I haploid chromosomesEach pole now has haploid set of chromosomes. CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase I

Meiosis II No interphase IINo interphase II DNA replication (or very short - no more DNA replication) Remember: Meiosis II mitosisRemember: Meiosis II is similar to mitosis

Prophase II prophasemitosissame as prophase in mitosis

Metaphase II metaphasemitosissame as metaphase in mitosis metaphase plate

Anaphase II anaphasemitosissame as anaphase in mitosis sister chromatids separatesister chromatids separate

Telophase II telophasemitosisSame as telophase in mitosis. Nuclei form. CytokinesisCytokinesis occurs. Remember:four haploid daughter cells produced.Remember:four haploid daughter cells produced. gametes = sperm or egg

Telophase II

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

Non-disjunction Non-disjunction is the failure of homologous chromosomes, or sister chromatids, to separate during meiosis. Non-disjunction results with the production of zygotes with abnormal chromosome numbers…… remember…. An abnormal chromosome number (abnormal amount of DNA) is damaging to the offspring. Non-disjunction is one of the Two major occurrences of Meiosis (The other is Crossing Over)

Non-disjunctions usually occur in one of two fashions. The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18 th set, Trisomy 21…. Three chromosomes in the 21 st set. If an organism has Monosomy 23 it has only one chromosome in the 23 rd set.

Common Non-disjunction Disorders Down’s Syndrome – Trisomy 21 Turner’s Syndrome – Monosomy 23 (X) Kleinfelter’s Syndrome – Trisomy 23 (XXY) Edward’s Syndrome – Trisomy 18

Karyotype (picture of an individual’s chromosomes) One of the ways to analyze the amniocentesis is to make a Karyotype What genetic disorder does this karyotype show? Trisomy 21….Down’s Syndrome

Question: 20 chromosomes(diploid) chromosomesA cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes (haploid)10 chromosomes (haploid)

Fertilization spermeggzygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

Question: 40 chromatids chromosomesA cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

Answer: 10 chromosomes10 chromosomes