Occurs in reproductive organs during gamete formation Meiosis Meiosis is a reduction division of a diploid nucleus to form haploid nuclei. Occurs in reproductive organs during gamete formation

Homologous chromosomes homologous chromosomes = chromosomes with the same gene loci in the same sequence which are capable of pairing up to form bivalents during the first prophase of meiosis.

1.pairing of homologous chromosomes 2.crossing over 3.two divisions 4.results in four haploid cells

Stages of meiosis

Meiosis Prophase 1 chromosomes condense nucleolus disappears spindle forms synapsis of bivalent: pairing of homologous chromosomes crossing over occurs between non-sister chromatids nuclear membrane disappears

Crossing over Homologous chromosomes pair forming bivalents Portions of non-sister chromatids overlap, break at chiasmata, reattach to new chromatid in a system of reciprocal exchange Chromosomes with new combinations are known as recombinants

chromosomes continue to condense Meiosis Metaphase 1 chromosomes continue to condense spindle microtubules attach to centromeres bivalents line up at equator

separation of homologous pairs; chromosomes moved to poles by spindle Meiosis Anaphase 1 separation of homologous pairs; chromosomes moved to poles by spindle

chromosomes at poles spindle disappears chromosomes partially uncoil Meiosis Telophase 1 chromosomes at poles spindle disappears chromosomes partially uncoil

Meiosis prophase 2 chromosomes condense again Newspindle forms (at right angles to previous spindle)

Meiosis metaphase 2 spindle microtubules attach to centromeres chromosomes move to the equator

Meiosis Anaphase 2 sister chromatids separate spindle microtubules move chromatids to opposite poles

Meiosis Telophase 2 chromosomes (=chromatids) arrive at poles spindle disappears nuclear membrane reappears nucleolus reappears chromosomes decondense into chromatin

Genetic variety in gametes Crossing over in prophase 1 Random orientation in metaphase 1 maternal and paternal chromosomes assort to daughter cells randomly possible arrangements of chromosomes in haploid daughter cells = (2)nth, where n = number of homologous pairs in humans, n = 23, and possible arrangements = (2)23 = about 8 million Chance fusion of gametes

https://highered. mcgraw-hill https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html

https://highered. mcgraw-hill https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

Here is the link to the McGraw Hill Animation http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__random_orientation_of_chromosomes_during_meiosis.html

Mendel’s law of independent assortment and meiosis. Independent assortment occurs as a result of the alignment of homologs during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell each pair of alleles separates independently of every other pair of unlinked alleles

non-disjunction can lead to changes in chromosome number Down syndrome: in either oogenesis or spermatogenesis, chromosome 21moves to pole as a pair instead of singly; egg or sperm contain an extra copy of chromosome 21, and fertilized egg has 3 copies of chromosome 21 Down syndrome is also known as trisomy 21

non-disjunction can lead to changes in chromosome number. which is why Down syndrome is also known as trisomy 21

Here is the link to this video http://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::550::400::/sites/dl/free/0078695104/383925/Chapter11_NGS_VisualizingNondisjunction_10_10_06.swf::Visualizing%20Nondisjunction

Trisomy A trisomy occurs when there are three copies of a chromosome in a cell. This is the result of non-disjunction during meiosis , Followed by fertilization with another gamete

Down Syndrome Trisomy 21 Physical Characteristics Low muscle tone (babies appear "floppy") Flat facial features, with a small nose Upward slant to the eyes Small skin folds on the inner corner of the eyes Small, abnormally shaped ears Single deep crease across the center of the palm Hyperflexibility (excessive ability to extend joints) Fifth finger has only one flexion furrow instead of two Extra space between the big toe and the second toe Enlarged tongue that tends to stick out

Health Problems associated with Down Syndrome Heart Defects Visual Problems crossed eyes nearsightedness farsightedness cataracts 15-20 times greater risk of developing leukemia hormonal problems 50% of adults with Down syndrome have thyroid disease Approximately 10%-12% of babies born with Down syndrome also have abnormalities in the gastrointestinal tract that require surgery for correction. Life expectancy for people with Down syndrome has increased dramatically in recent decades - from 25 in 1983 to 60 today The incidence of births of children with Down syndrome increases with the age of the mother. But due to higher fertility rates in younger women, 80% of children with Down syndrome are born to women under 35 years of age. - See more at: http://www.ndss.org/Down-Syndrome/Down-Syndrome-Facts/#sthash.LnK6hJfY.dpuf

http://www.medindia.net/animation/amniocentesis.asp

http://www.youtube.com/watch?v=sxEf_ddmpZk

What’s the difference? Chorionic Villus Sampling Amniocentesis Takes a small sample of cells from the developing placenta Draws a small sample of amniotic fluid from the uterus (amniotic fluid) May take place during 1st trimester (Earlier diagnosis) Completed during 2nd trimester CVS thought to be more risky for miscarriage but a 20 year study at the University of California, San Francisco concluded that the miscarriage rate for either test averages less than 2 percent

Karyotyping Chromosomes are arranged in pairs according to their size, structure, banding pattern. Karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities. Created during metaphase of MITOSIS? Why does this make sense?

Karyotyping Video http://www.youtube.com/watch?v=BjX90u1pVnM