MEIOSIS 3.3 & 10.1 Meiosis: A reduction division of a diploid nucleus to form four haploid nuclei. This allows for a sexual life cycle in living organisms. Number of Chromosomes Description of condition Cell Type 46 Diploid (2N) Typical body (somatic) cell 23 Haploid (N) Gamete, Egg or Sperm cell Homologous chromosomes: in a diploid cell, 46 chromosomes are grouped into 23 pairs of chromosomes. Homologous: similar shape and size, and carry the same genes Chromosomes Meiosis Interphase I – All the chromosomes are duplicated and thus each consists of two identical sister chromatids. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Figure 13.4 Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) In Meiosis I: ◦ Prophase I – Each chromosome pairs with its corresponding homologous chromosome to form a bivalent (a.k.a. tetrad) Crossing Over occurs during prophase I, then the chromosomes condense. Crossing Over During crossing over there is exchange of DNA material between non-sister homologous chromatids. This produces new combinations of alleles on the chromosomes of the haploid cells. This leads to genetic variation. Nonsister chromatids Prophase I of meiosis Bivalent Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Figure 13.11 Recombinant chromosomes Chiasmata A chiasma is an X-shaped knot-like structure that forms where crossing over has occurred. ◦ It holds a bivalent together for a while after the chromosomes condense by supercoiling. Interphase and meiosis I MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Sister chromatids Nuclear envelope Chromatin METAPHASE I Tetrad Chiasmata Metaphase plate Spindle Microtubule attached to kinetochore Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange Figure 13.8 segments; 2n = 6 in this example Homologous chromosomes separate Bivalents line up Pairs of homologous chromosomes split up After finishing Meiosis I, our results are two daughter cells with a haploid number of duplicated chromosomes. Meiosis II Telophase I, cytokinesis, and meiosis II MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS Figure PROPHASE II Cleavage furrow Two haploid cells form; chromosomes 13.8are still double METAPHASE II ANAPHASE II Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Meiosis I ◦ Homologous chromosomes separate ◦ Reduces the number of chromosomes from diploid to haploid Meiosis II ◦ Sister chromatids separate ◦ Produces four haploid daughter cells Genetic Variation is increased by: ◦ Crossing over (during prophase I) ◦ (Random) Fusion of gametes ◦ Independent assortment Genetic Variation Fusion of gametes from different parents promotes genetic variation. ◦ This allows alleles from two different individuals to be combined into one new individual. ◦ The combination of alleles is unlikely ever to have existed before genetic variation. ◦ Genetic variation is essential for evolution of a species. Sexual Reproduction Independent Assortment of genes Organization/ orientation of pairs of homologous chromosomes during metaphase is random. Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Figure 13.10 Combination 1 Combination 2 Combination 3 Combination 4 Non-disjunction: “not coming apart” – when chromosomes fail to separate during Meiosis 1 or 2. Karyotyping Gametes contain two copies or no copies of a particular chromosome. Offspring have an extra or missing chromosome. Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+1 Figure 15.12a, b n+1 n1 n+1 n –1 n–1 Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I n n (b) Nondisjunction of sister chromatids in meiosis II Down’s Syndrome – Trisomy 21 ◦ The person has 3 (instead of 2) 21st chromosomes Age of parents vs. Down Syndrome Do the DBQ on pg. 167 – 168: “Parental age and non-dsjunction” Karyotype: a property of a cell – the number and type of chromosomes present in the nucleus. Karyogram: picture of chromosomes arranged in pairs, according to their size and structure (banding patterns). Trisomy 18, Trisomy 13 Turner’s Syndrome – females with only one X Klinefelter’s Syndrome – males with XXY Chromosomal abnormalities Karyotyping is used for pre-natal (before birth) diagnosis of chromosome abnormalities. Where do we get the cells for doing a karyotype? 1) amniocentesis Extract amniotic fluid, Inside are some of the baby’s cells Risks: ◦ Miscarriage 1 in 200 to 1 in 400 ◦ Accuracy: 99.4% 2) chorionic villus sampling Tissue sample from the placenta’s projections into the uterus wall Risks? ◦ Slightly higher chance of miscarriage than amniocentesis because it is done earlier in pregnancy. ◦ Accuracy: 98%