Meiosis IB Topics 4.2 and 10.2 Chapter 13 Campbell

What is meiosis? Sexual reproduction  Offspring  Genetic diversity in a species Eukaryotic organisms  Fertilization; fusion of sperm and egg Sex cells or gametes Due to fertilization, humans have pairs of chromosomes (one from each parent)  A nucleus with 2 chromosomes of each type is diploid (2n)  A nucleus with 1 chromosome of each type is haploid (n)

Why meiosis? In meiosis, the daughter cells contain half the number of chromosomes of the parent cell gametes (sex cells) are haploid  When two haploid cells come together, they make a diploid cell So meiosis is the cell division of gametes

How does meiosis work? Meiosis involves 2 divisions  Meiosis 1 and meiosis 2  Both superficially resemble mitosis Homologous chromosomes  Chromosomes that make up a pair that have the same length, centromere position and staining position

Steps of meiosis Good animation  ions/content/meiosis.html ions/content/meiosis.html

Crossing over Occurs during prophase 1 of meiosis Also called synapsis  All of the chromatids of the two homologous chromosomes become tightly associated  Combined pair is a bivalent or a tetrad The maternal and paternal chromosomes exchange corresponding sections of DNA  Once complete, new combinations of alleles have been created  The process by which offspring possess a combination of alleles different form that of either parent is called recombination

Chiasma An X-shaped structure formed between non-sister chromatids during prophase 1 Physical manifestation of crossing over Persist through metaphase 1

Meiosis and genetic variation The random orientation of chromosomes at metaphase 1 leads to variation within offspring The 4 haploid cells produced are genetically different from each other b/c:  1. independent assortment of maternal and paternal homologous chromosomes: The way the bivalents line up at the equator in meiosis 1 is entirely random In humans, there are 23 pairs of chromosomes, so the # of possible combinations is 2^23 (8 million)  2. crossing over of segments: New combinations of genes on the chromosomes Unimaginable degree of variation

Non-disjunction Meiosis is sometimes subject to errors  One example is when homologous chromosomes fail to separate at anaphase  Termed: non-disjunction  Result: gamete has an extra chromosome or is deficient in a chromosome  Human fertilization: 45 or 47 chromosome

Non-disjunction 20Nondisjunction%20Meiosis%20II.htm 20Nondisjunction%20Meiosis%20II.htm

Down syndrome and Turner’s syndrome Down syndrome  Trisomy 21  Person has 3 chromosome 21 instead of 2  Hearing loss, heart and vision disorders, physical deformity (range), and mental/growth retardation Turner’s syndrome  Girls are missing one of their X chromosomes  Intellectually normal, but physical/sexual maturity problems

Testing for Down’s Syndrome Blood test performed on expectant mother Look for unusual levels of AFP & HCG  AFP = alpha-fetoprotein  HCG = human chorionic gonadotropin Trisomy 21  Abnormally low (25% lower) levels of AFP  Abnormally high (2x higher) levels of HCG If blood test raises concern, then parents are advised to have a karyotype produced

Karyotype An organized image of metaphase fetal chromosomes Technicians stain chromosomes How do we obtain fetal chromosomes?  Amniocentesis Needle through mother’s abdominal wall, using ultrasound to guide the needle Amniotic fluid is drawn (cells of the fetus are used)  Chorionic villus sampling (CVS) Cells are taken from the placenta, specifically the chorion, rather than the amniotic fluid Can be done earlier than amniocentesis Sampling tool entered through vagina