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 46Diploid (2N)Typical body (somatic) cell 23Haploid (N)Gamete, Egg or Sperm cell

Chromosomes 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

Meiosis

Interphase I – All the chromosomes are duplicated and thus each consists of two identical sister chromatids. Figure 13.4 Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) 2n = 6 Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Centromere

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.

Figure Prophase I of meiosis Nonsister chromatids Bivalent Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells 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.

Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Bivalents line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Interphase and meiosis I Figure 13.8

After finishing Meiosis I, our results are two daughter cells with a haploid number of duplicated chromosomes.

Meiosis II

TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS MEIOSIS II: Separates sister chromatids Cleavage furrow Sister chromatids separate Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Two haploid cells form; chromosomes are still double Figure 13.8 Telophase I, cytokinesis, and meiosis II

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 Genetic Variation is increased by: ◦Crossing over (during prophase I) ◦(Random) Fusion of gametes ◦Independent assortment

Sexual Reproduction 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.

Independent Assortment of genes Organization/ orientation of pairs of homologous chromosomes during metaphase is random. Figure Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Two equally probable arrangements of chromosomes at metaphase I Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4

Karyotyping Non-disjunction: “not coming apart” – when chromosomes fail to separate during Meiosis 1 or 2.

Gametes contain two copies or no copies of a particular chromosome. Offspring have an extra or missing chromosome. Figure 15.12a, b Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + 1 n  1 n – 1 n + 1n –1 n n Number of chromosomes Nondisjunction of homologous chromosomes in meiosis I Nondisjunction of sister chromatids in meiosis II (a) (b)

Down’s Syndrome – Trisomy 21 ◦The person has 3 (instead of 2) 21 st 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).

Chromosomal abnormalities Trisomy 18, Trisomy 13 Turner’s Syndrome – females with only one X Klinefelter’s Syndrome – males with XXY

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%