The Cell Cycle and Cell Division «The cell cycle is a regular pattern of growth, DNA replication, and cell division. The cell cycle has four main stages. «The main stages of the cell cycle are G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). 1. Gap 1 (G 1 ): cell growth and normal functions 2. DNA synthesis (S): copies DNA 3. Gap 2 (G 2 ): additional growth 4. Mitosis (M): includes division of the cell nucleus (mitosis) and division of the cell cytoplasm (cytokinesis) «Mitosis occurs only if the cell is large enough and the DNA undamaged. «Some cells are unlikely to divide (G 0 ): Resting stage 1
Interphase Interphase prepares the cell to divide. During interphase, the DNA is duplicated. Parent cell centrioles spindle fibers centrosome nucleus with DNA Cells divide during mitosis and cytokinesis. 2
Prophase During prophase, chromosomes condense and spindle fibers form. Prophase " Chromosomes condense at the start of mitosis. DNA wraps around proteins (histones) that condense it. DNA double helix DNA and histones Chromatin Supercoiled DNA 3
Chromosome Structure chromatid DNA plus proteins is called chromatin. One half of a duplicated chromosome is a chromatid. Sister chromatids are held together at the centromere. Telomeres protect DNA and do not include genes. telomere centromere telomere Condensed, duplicated chromosome Metaphase During metaphase, chromosomes line up in the middle of the cell. 4
Anaphase During anaphase, sister chromatids separate to opposite sides of the cell. Telophase During telophase, the new nuclei form and chromosomes begin to uncoil. 5
Cytokinesis differs in animal and plant cells. In animal cells, the membrane pinches closed. In plant cells, a cell plate forms. Cytokinesis " Mitosis and cytokinesis produce two genetically identical daughter cells. «Cells divide at different rates. The rate of cell division in eukaryotes varies with the need for those types of cells. Many organisms will reproduce by cell division/ mitosis (ex: potatoes, starfish etc.) 6
KEY CONCEPT DNA structure is the same in all organisms. " 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 nitrogen-containing base deoxyribose (sugar) 7
The nitrogen containing bases are the only difference in the four nucleotides. " 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 sugarphosphate backbone on the outside with bases on the inside. 8
Watson and Crick s discovery built on the work of Rosalind Franklin and Erwin Chargaff. Franklin s x-ray images suggested that DNA was a double helix of even width. Chargaff s rules stated that A=T and C=G. " 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. G A C T 9
KEY CONCEPT Gametes have half the number of chromosomes that body cells have. " 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. Gametes have DNA that can be passed to offspring. body cells sex cells (sperm) sex cells (egg) 10
" 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. Body cells are diploid. Half the chromosomes come from each parent. 11
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. 12
Meiosis makes haploid cells from diploid cells. Meiosis occurs in sex cells. Meiosis produces gametes. 13