KEY CONCEPT DNA structure is the same in all organisms.

DNA is short for deoxyribonucleic acid 2

DNA is composed of four types of nucleotides. DNA is made up of a long chain of nucleotides. Each nucleotide has three parts. a phosphate group a deoxyribose sugar a nitrogen-containing base phosphate group deoxyribose (sugar) nitrogen-containing base

2 categories of nitrogenous bases: The nitrogen containing bases are the only difference in the four nucleotides. 2 categories of nitrogenous bases: Pyrimidines: single ring, ex. thymine and cytosine Purines: double ring, ex. adenine and guanine 4

Chargaff’s rule states that the amount of thymine = adenine and that the amount of cytosine = guanine in DNA. 5

Franklin’s x-ray images suggested that DNA was a double helix of even width. Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

Watson and Crick determined the three-dimensional structure of DNA by building models. They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

Nucleotides always pair in the same way. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. C G T A

Bonding in DNA Phosphate group ↓ Deoxyribose Sugar → hydrogen bond covalent bond Deoxyribose Sugar → The DNA backbone: alternating deoxyribose (sugar) and phosphate group is connected by strong covalent bonds. The nitrogen bases are connected by hydrogen bonds.

hydrogen bond covalent bond T G A C T A | | | | | | A C T G A T 10

All the DNA in your body is identical…. Two haploid cells meet (fertilization) then mitosis takes over

You have body cells and gametes. Body cells are also called somatic cells. Germ cells develop into gametes. Germ cells are located in the ovaries and testes. Gametes are sex cells: egg and sperm. body cells sex cells (sperm) sex cells (egg)

Your cells have autosomes and sex chromosomes. Your body cells have 23 pairs of chromosomes. Homologous pairs of chromosomes have the same structure. For each homologous pair, one chromosome comes from each parent. Chromosome pairs 1-22 are autosomes. Sex chromosomes, X and Y, determine gender in mammals.

Body cells are diploid; gametes are haploid. Fertilization between egg and sperm occurs in sexual reproduction. Diploid (2n) cells have two copies of every chromosome. (you have 2 sets of chromosomes) Body cells are diploid. One set comes from each parent.

Haploid (n) cells have one copy of every chromosome. Gametes are haploid. Gametes have 22 autosomes and 1 sex chromosome.

Chromosome number must be maintained in animals. Mitosis and meiosis are types of nuclear division that make different types of cells. Mitosis makes more diploid cells.

Meiosis makes haploid cells from diploid cells. Meiosis occurs in sex cells. Meiosis produces gametes.

Sister chromatids:divide in meiosis II; copies of the same chromosome. Meiosis I and meiosis II each have four phases, similar to those in mitosis. Homologous chromosomes: Pairs of chromosomes that separate in meiosis I; Similar but not identical genes Sister chromatids:divide in meiosis II; copies of the same chromosome. homologous chromosomes Tetrad: 4 chromatids Crossing over: exchange of similar DNA sister chromatids sister chromatids

Meiosis I occurs after DNA has been replicated. Meiosis I divides homologous chromosomes in four phases.

Prophase I Homologous chromosomes form tetrads Crossing over (genetic recombination)

Crossing over during meiosis increases genetic diversity. Crossing over is the exchange of chromosome segments between homologous chromosomes. occurs during prophase I of meiosis I when homologous chromosomes form tetrads. results in new combinations of genes

Metaphase I Homologous chromosomes line up in middle

Anaphase I chromosome pairs separate

Telophase I and Cytokinesis separation into two haploid cells

Prophase II Each cell prepares for division

Metaphase II chromosomes line up in the middle

Anaphase II sister chromatids separate

Telophase II and Cytokinesis produces 4 unique haploid cells

Haploid cells develop into mature gametes. Gametogenesis differs between females and males. Spermatogenesis Sperm become motile. Sperm primarily contribute DNA to an embryo. Oogenesis Eggs contribute DNA, cytoplasm, and organelles to an embryo. During meiosis, the egg gets most of the contents; the other 3 cells form polar bodies.

Meiosis differs from mitosis in significant ways. Meiosis has two cell divisions while mitosis has one. In mitosis, homologous chromosomes never pair up. Meiosis results in haploid cells; mitosis results in diploid cells.