Meiosis 3.3 & 10.1

Essential Idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes. 3.3 Meiosis Understandings: One diploid nucleus divides by meiosis to produce four haploid nuclei   The halving of the chromosome number allows a sexual life cycle with fusion of gametes DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation Orientation of pairs of homologous chromosomes prior to separation is random Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number Crossing over and random orientation promotes genetic variation Fusion of gametes from different parents promotes genetic variation Applications: Non-disjunction can cause Down syndrome and other chromosome abnormalities Studies showing age of parents influences changes of non-disjunction Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks Skill: Draw diagrams to show the stages of meiosis resulting in the formation of four haploid cells

Chromosomes replicate in interphase before meiosis. Essential idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells 10.1 Meiosis Understandings: Chromosomes replicate in interphase before meiosis. Crossing over is the exchange of DNA material between non-sister homologous chromatids. Crossing over produces new combinations of alleles on the chromosomes of haploid cells. Chiasmata formation between non-sister chromatids in a bivalent can result in an exchange of alleles. Homologous chromosomes separate in meiosis I. Sister chromatids separate in meiosis II. Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I. Skills: Drawing diagrams to show chiasmata formed by crossing over,

Production of gametes (egg and sperm cells) A. Need to have half as many chromosomes as somatic cells so that when they fuse chromosome number is maintained B. Called a reduction division

Remember, DNA has been duplicated during S phase before meiosis begins A. Same number of chromosomes, but now sister chromatids

Increasing genetic diversity A. Crossing over – exchange of genetic material between homologous chromosomes 1. Chromatids overlap, forming chiasmata, and swap segments

2. Allows DNA from a person’s maternal chromosomes to mix with DNA from paternal chromosomes a. Chromosomes that end up in gametes are a mosaic (increases variety)

Random Orientation 1. The way maternal and paternal chromosomes line up on metaphase plate is entirely by chance 2. Makes chances of creating the same gamete twice very slim

Sexual Reproduction 1. Chances of making the same gamete twice (NOT including crossing over) can be calculated by 2n a. Ex. Humans 223 = 1 in 8,388,608 2. Now compound that by adding a second parent 3. Compound it again with crossing over

Phases of Meiosis Prophase I 1. Chromosomes condense 2. Homologues pair up forming bivalents and crossing over occurs 3. Spindle fibers form

Metaphase I Nuclear membrane disintegrates Homologous pairs line up together on metaphase plate in random orientation

Anaphase I 1. Spindle fibers separate homologous pairs and pull them to opposite poles of cell

Telophase I Spindle fibers disintegrate Usually chromosome de-condense Often cytokinesis occurs

Prophase II 1. Chromosomes condense again 2. New spindle fibers form

Metaphase II 1. Chromosomes line up single file on the metaphase plate (random orientation) 2. Spindle fibers attach to sister chromatids

Anaphase II 1. Sister chromatids pulled to opposite poles 2. Cleavage furrow starts to form in animal cells

Telophase II 1. Chromosomes de-condense 2. Nuclear envelopes form in each of the four haploid nuclei 3. Cytokinesis results in 4 cells

Make a chart as a table: Brainstorm similarities and differences between mitosis and meiosis.

Non-disjunction A. Sometimes homologous pairs or sister chromatids are not properly separated

Can result in monosomy (only one copy of a chromosome) or trisomy (3 copies of a chromosome) 1. Down’s Syndrome – 3 copies of chromosome 21

Most common in sex chromosomes a. Turner Syndrome (XO) b. Klinefelter Syndrome (XXY) c. XYY Syndrome d. Triple X Syndrome Other examples a. Trisomy 13 – Patau Syndrome b. Trisomy 18 – Edward Syndrome

Non-disjunction and maternal age

Obtaining cells for karyotyping Amniocentesis – use needle to remove amniotic fluid from around the fetus Chorionic villi – tissue from placental tissue in uterine wall Amnio miscarriage risk about 1%, villi about 2%