Levels of Organization in DNA Double helix Chromatin

Levels of Organization in DNA • Double helix • Chromatin with nucleosome packaging via histone proteins • Chromosome

Outcome of Mitosis • Result: two DIPLOID cells that are identical to one another • Diploid: describes a cell or nucleus that contains two complete sets of paired chromosomes, one from each parent • Haploid: describes a cell or nucleus that contains only one set of unpaired chromosomes • Animation for Review: https: //highered. mheducation. com/sites/0072495855/student_view 0/chapter 2/animation__mitosis_and_cyto kinesis. html

Centromere • Homologous chromosomes: pair of chromosomes with the same general structure, and sequence of genes ; one chromosome comes from mom, the other from dad • Sister chromatids: either of the two identical copies of a replicated chromosome ; one half of the duplicated chromosome

• Locus/loci: the specific location or position of a gene on a chromosome (Ex: gene for eye color vs. enzyme A vs. cytochrome C)

How do chromosomes differ? Alleles • Allele: specific form or variation of a gene ; you inherit one from mom, and one from dad ; source of variation in traits • Alleles differ from other alleles by one to few bases (nucleotides) • Example 1: alleles for blue eyes vs. brown eyes for the gene of eye color • Example 2: alleles for A blood-type vs alleles for B blood type for gene of blood type

Meiosis • Meiosis: process where a single diploid cell divides to produce four cells containing half the original amount of DNA (haploid) • Produces gametes (eggs and sperm) – also called “sex cells” • Only occurs in sex organs (testes or ovaries)

• • How Meiosis Works: http: //highered. mheducation. com/sites/0072495855/student_view 0/chapter 3/animation__how_meiosis_works. html Stages of Meiosis: http: //highered. mheducation. com/sites/0072495855/student_view 0/chapter 28/animation__stages_of_meiosis. html

How does meiosis contribute to the variation in traits? • Crossing Over during Prophase I: – Parts of non-sister chromatids cross over and exchange genetic material – Produces chromatids with a new combination of alleles (recombinants) – Can occur along any part or length of the chromosome • Independent (or Random) Assortment during Metaphase I: – Chromosomes line up randomly – The inheritance of one gene/trait is independent of the inheritance of another trait (this means that an allele of one chromosome has an equal chance of being paired with any allel on another chromosome) **ONLY FOR NON-LINKED TRAITS – 2^23 combinations in humans

What is synapsis? • Synapsis: Pairing of homologous chromosomes

What is a tetrad? • Tetrad: two pairs of homologous chromosomes next to each other • “Homologous chromosomes in a tetrad cross over each other, resulting in sister chromatids with a different set of alleles. ”

Variation due to Crossing Over • Chiasma/Chiasmata: specific location where alleles cross over • Recombinant (HL Only): allele combinations that were not present in the original parents ; result from crossing over

Recombinant Chromosomes After Crossing Over

Variation Due to Independent Assortment

Outcome of Meiosis • Gametes (sperm or egg cells) that are unique in their genetic composition, and haploid nuclei

Why is meiosis considered a “reduction division”? • Meiosis consists of two divisions. DNA is replicated before the first division, resulting in two diploid cells. DNA is not replicated before the second division, which causes resulting daughter cells to have haploid nuclei.

Meiosis in Testes vs. Ovaries Spermatogenesis Oogenesis

Formation of Gametes in Testes vs. Ovaries

Stages of Gamete Development

Gamete Formation in Ovaries (Oogenesis)

Gamete Formation in Testes (Spermatogenesis)

Overview of Spermatogenesis

Overview of Oogenesis

11. 4. 2 Outline the processes involved in spermatogenesis within the testes, including mitosis, cell growth, the two divisions of meiosis and cell differentiation • Spermatogenesis describes the production of spermatozoa (sperm) in the seminiferous tubules of the testes • The first stage of sperm production requires the division of germline epithelium by mitosis • These cells (spermatogonia) then undergo a period of growth • This is followed by two meiotic divisions that result in four haploid daughter cells • These haploid cells then differentiate to form sperm cells • The developing sperm cells are nourished throughout by the Sertoli cells

11. 4. 5 Outline the processes involved in oogenesis within the ovary, including mitosis, cell growth, the two divisions of meiosis, the unequal division of cytoplasm and the degeneration of polar body • Oogenesis describes the production of female gametes (ova) within the ovary • The process begins during foetal development, when a large number of cells (oogonia) are formed by mitosis before undergoing a period of growth • These cells begin meiosis but are arrested in prophase I until puberty • At puberty, some follicles continue to develop each month is response to FSH secretion • These follicles complete the first meiotic division to form two cells of unequal size • The cell with less cytoplasm is a polar body (which degenerates), while the larger cell forms a secondary oocyte • The secondary oocyte begins the second meiotic division but is arrested in prophase II (until fertilisation) • It is released from the ovary (ruptured follicle develops into corpus luteum) and, if fertilisation occurs, will complete meiosis • The second meiotic division will produce an ovum and a second polar body