Meiosis Overview video

Purpose of Meiosis Also called a reduction division Daughter cells end up with half the number of chromosomes as the parent cell Goes from diploid (2N) to haploid (1N) Daughter cells contain one of each chromosome and one sex chromosome Produces gametes (sperm and eggs)

Sperm surround an egg prior to fertilization Oogenesis in Rana ovary Spermatogenesis Developing sperm

Homologous Chromosomes Two chromosomes (1 from each parent) that look the same Same size Same banding Carry the same genes Alleles of the genes may differ Both carry gene for eye color but one chromosome carries the blue allele and one carries the brown allele Pair up and split during meiosis

Formation of the chiasmata Occurs when two chromatids overlap in prophase 1 Bivalents form Two homologous chromosomes One from mom and one from dad Can occur between sister chromatids Creates recombinants…new combinations of genes Can occur between non-sister chromatids Forms a cross shaped structure called a chiasmata Segments of the non-sister chromatids break off and reattach on the other chromatid Bozeman video….watch it at home

The process Meiosis I Meiosis II reduction of chromosomes Separation of homologous chromosomes into separate cells Random orientation of chromosomes: no two gametes are identical Cells go from 2N to 1N Crossing over occurs during prophase 1 Meiosis II Separation of sister chromatids into individual chromosomes

Chromosomes during Meiosis I Interphase Cell growth and DNA replication Prophase I Chromosomes condense Nucleolus becomes invisible Spindle forms Synapsis Homologous chromosomes pair up side by side Now called bivalent and cross-over points are called chiasmata Nuclear membrane disappears Metaphase I Bivalents move to the equator of the cells Anaphase I Homologous pairs split up, one chromosome of each pair goes to each pole Telophase I Chromosomes arrive at the poles Spindle disappears

10.1.1: Meiosis II Prophase II Metaphase II Anaphase II Telophase II New spindle is formed at right angles to the previous spindle Metaphase II Chromosomes move to the equator Anaphase II Chromosomes separate Chromatids move to opposite poles Telophase II Chromosomes arrive at the poles Spindle disappears Nuclear membrane reappears Nucleolus becomes visible Chromosomes become chromatin

Mendel and Independent Assortment Each allele moves independently of all other alleles Exceptions to this rule exist Think hair color and eye color Alleles are often located very close to one another on the same chromosome

Non-disjunction Chromosomes can fail to separate during meiosis. Trisomy 21 Trisomy 18 Trisomy 13 Photos: Cytogenetics Dept. Waikato Hospital Chromosomes can fail to separate during meiosis. This is called non-disjunction. It results in abnormal numbers of chromosomes in the gametes. Non-disjunction can occur: if chromosomes fail to separate at anaphase I. if sister chromatids fail to separate during anaphase II Fertilization of an abnormal gamete with a normal gamete (or vice versa) results in an abnormal chromosome number. This is known as an aneuploidy, e.g. trisomies occur where there are three instead of the normal pair of a chromosome (right).

Karyotyping The process of finding the chromosomal characteristics of a cell Chromosomes are stained to show banding

Pre-natal testing Karyotyping is generally done during pre-natal testing Chronic villus Done at 11 to 12 weeks gestation Cells are taken from the chronic villus (cells that became the placenta) Risk of miscarriage is 1% Advantage is diagnosis in early pregnancy Amniocentesis Done in the 16th week of pregnancy Takes amniotic fluid that surrounds the fetus Cells are cultured and pictures of the dividing cells are taken Creates a karyotype Risk of miscarriage is 0.5%. Recommended to all women over the age of 35 due to increased risk of non-disjunction

Genetic Variation The number of different types of gametes that can be created due to crossing over during prophase I is infinite Each time the crossing over occurs the gamete produced will be different than if straight meiosis were to occur Crossing over can take place on any chromosome Every gamete is different Fertilization takes a unique gamete from dad and a unique gamete from mom to create a new individual

Hank Time