Organisms have tens of thousands of genes - Genes determine traits - Genes are lined up on chromosomes - 1 chromosome can contain a thousand or more genes

Each species of organisms contains a different number of chromosomes The chromosome numbers of a species is not related to the complexity of the organism

The Chromosomes in most plants and animals occur in pairs 1 chromosome comes from the male parent 1 chromosome comes from the female parent

Diploid – A cell with two of each kind of chromosome (1 from each parent) We say the cell is a diploid cell or 2n * Supports Mendel’s conclusion that organisms have two factors (alleles) for each trait Haploid – A cell with one of each kind of chromosome (gametes) We say the cell is a haploid or n *Supports Mendel’s conclusion that parents give 1 allele for each trait to their offspring

Homologous Chromosomes – Paired chromosomes Each pair of homologous chromosomes has genes for the same traits Ex. Pod shape On homologous chromosomes, the genes are arranged in the same order Because there are different possible alleles for the same gene, the two chromosomes in the homologous pairs are not always identical to each other.

Meiosis – cell division. -produces haploid gametes Meiosis occurs in the body cells that produce gametes (Male and Female reproductive organs) The process of meiosis produces: male gametes (sperm) - n, haploid female gametes (eggs) – n, haploid

When the sperm fertilizes the egg, the resulting cell is a Zygote – 2n diploid The zygote then can develop by mitosis into a multicellular organism This process is called Sexual Reproduction Two parents produce an offspring that is not identical to either parent What are the benefits to sexual reproduction?

Gamete Production 9 Meiosis Square Dance Write down the phases and what is taking place as you watch the video.

Meiosis Similar to mitosis Similar to mitosis However, several differences: However, several differences: Meiosis occurs in 2 phases (2 cell divisions: Meiosis I & Meiosis II Meiosis occurs in 2 phases (2 cell divisions: Meiosis I & Meiosis II Results in 4 cells Results in 4 cells Vocabulary: Vocabulary: Diploid (2N) – Diploid (2N) – Haploid (N) - Haploid (N) - 10 Normal amount of genetic material 1/2 the genetic material.

Meiosis Meiosis forms haploid cells. Meiosis forms haploid cells. Example In Humans: Ova (egg) and sperm. Example In Humans: Ova (egg) and sperm. Ova produced in ovaries (females) Ova produced in ovaries (females) Process is called oogenesis Process is called oogenesis Sperm produced in testes (males) Sperm produced in testes (males) Process is called spermatogenesis Process is called spermatogenesis 11

Spermatogenesis & Oogenesis 12 Sperm formation Egg formation

Overview of Meiosis 13

Meiosis I Prior to division (S phase), amount of DNA doubles Prior to division (S phase), amount of DNA doubles 14

Metaphase I Metaphase I homologous chromosomes EQUATOR of the cell During Metaphase I homologous chromosomes line- up along the EQUATOR of the cell connect CHIASMATA Areas of homologous chromosomes connect at areas called CHIASMATA Genes are exchanged at these connections Genes are exchanged at these connections 15

Crossing Over Segments of homologous chromosomes break and reattach at similar locations. Segments of homologous chromosomes break and reattach at similar locations. Results in new genetic combinations of offspring. Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction This is the main advantage of sexual reproduction 16

Chiasmata 17

Anaphase I During Anaphase I, each HOMOLOGOUS CHROMOSOME is pulled to opposite sides of the cell. During Anaphase I, each HOMOLOGOUS CHROMOSOME is pulled to opposite sides of the cell. Unlike mitosis, the CENTROMERES DO NOT BREAK. Unlike mitosis, the CENTROMERES DO NOT BREAK. Nuclei MAY OR MAY NOT reform following division. Nuclei MAY OR MAY NOT reform following division. CYTOKENESIS may or may not occur. CYTOKENESIS may or may not occur. 18

Meiosis II DNA DOES NOT double DNA DOES NOT double Chromosomes randomly line-up along metaphase plate like regular mitosis. Chromosomes randomly line-up along metaphase plate like regular mitosis. During Anaphase II, CENTROMERES BREAK and each SISTER CHROMATID is pulled to opposite sides of the cell. During Anaphase II, CENTROMERES BREAK and each SISTER CHROMATID is pulled to opposite sides of the cell. Nuclei reform and cytokinesis usually occurs (although it is often unequal). Nuclei reform and cytokinesis usually occurs (although it is often unequal). Sister chromatids 19

Overview of Meiosis 20

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