In diploid cells there are two chromosomes of

In diploid cells, there are two chromosomes of each type (2 n), called homologous chromosomes • Homologous chromosomes are not identical, but they do contain the same genes (different versions of the same gene are called alleles)

• For example: - Gene: HAIR COLOR GENE - Alleles: BLONDE, BROWN, RED, BLACK • Everyone has the same gene, but we have different versions (different alleles) of that gene.

Fig. 13 -3 b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome

In humans, the diploid number is 46

• Meiosis produces gametes that are haploid, meaning they have one of each pair (n) of homologous chromosomes.

Fig. 13 -4 Key 2 n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set)

Fig. 13 -7 -3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Meiosis is called “REDUCTIVE DIVISION” because it begins with diploid germ cells and produces haploid eiosis II Haploid (1 n) GERM CELL Diploid (2 n) Meiosis I Diploid (2 n) Gamete Haploid (1 n)

In some ways, meiosis resembles mitosis

Unlike mitosis, meiosis has two divisions - meiosis I and meiosis II • During meiosis I, homologous chromosome pairs separate - Each of the two daughter cells receives a haploid number of chromosomes - Each chromosome is still duplicated • In meiosis II, the sister chromatids separat cytokinesis follows, resulting in 4 haploid cells

Meiosis II Haploid (1 n) GERM CELL Diploid (2 n) Meiosis I Diploid (2 n) Gamete Haploid (1 n)

Stages of meiosis - Interphase: each DNA replicates to produce two genetically identical sister chromatids which are attached at the centromere • This is DNA replication remember helicase, single stranded binding proteins, DNA polymerase, leading strand, lagging strand, Okazaki fragments, and ligase

Meiosis I separates the homologous chromosome pairs • Prophase I: - Chromosomes condense - Homologous chromosome pair to form a tetrad • • • One member of the pair is from the mom, the other from the dad A tetrad is the association of four chromatids (two from each homologue) The homologous chromosomes are attached at chiasmata - Spindle microtubules form and the nuclear envelope disappears

• Metaphase I - Tetrads align at the equator of the cell - Spindle fibers attach to the centromere region of each homologous chromosome pair.

• Anaphase I: - Homologous chromosomes separate and move to the poles - Sister chromatids remain attached at their centromere and move as a single unit towards the same pole - Each pole randomly receives a mixture of maternal and paternal chromosomes

• Telophase I: chromosomes decondense, nuclear membrane may reform • Cytokinesis occurs, forming two daughter cells (remember cleavage furrow and cell plates)

Meiosis II separates the two sister chromatids of each chromosome • Prophase II: - Chromosomes recondense and spindle fibers reform - Chromosomes progress towards the equator

• Metaphase II: chromosome line up at the equator

• Anaphase II: - Centromeres of sister chromatids separate - Sister chromatids of each pair, now individual chromosomes, move towards opposite poles of the cell

• Telophase II: nucleus reforms • Cytokinesis results in four haploid daughter cells

• The diploid chromosome number is restored at fertilization when two gamete nuclei fuse to form the zygote

Determining the Phase 1. Is the cell haploid or diploid? - If diploid …meiosis 1 - If haploid … meiosis 2 2. What are the chromosomes doing? - Prophase - Metaphase - Anaphase - Telophase

Fig. 13 -9 a MITOSIS MEIOSIS Parent cell Chromosome replication Prophase Chiasma Chromosome replication Prophase I Homologous chromosome pair 2 n = 6 Replicated chromosome MEIOSIS I Metaphase I Anaphase Telophase Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2 n Daughter cells of mitosis 2 n MEIOSIS II n n Daughter cells of meiosis II

Fig. 13 -9 b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, and telophase Two, each including prophase, metaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2 n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

NONDISJUNCTION

• Amniocentesis removes a small amount of fluid from the sac that surrounds the baby in the womb (uterus). • Chorionic villi are tiny finger-shaped growths found in the placenta. The genetic material in chorionic villus cells is the same as that in the baby's cells. During CVS, a sample of the chorionic villus cells is taken for biopsy.