Cell Division Topics covered: Why is DNA replication important? Why is cell division important? How do mitosis and meiosis compare? How do asexual and sexual reproduction compare?

DNA Replication and Cell Division DNA must replicate so that during cell division, the new cells formed each receive a complete set of genetic information

Cells must divide for: Reproduction (e.g. unicellular organisms)

Growth (e.g. 1 fertilized egg --> human of ~100 trillion cells)

Healing and tissue repair (e.g to replace dead or damaged cells)

Mitosis & the Cell Cycle Mitosis occurs when a parent cell divides to produce two identical daughter cells Mitosis refers to the process of dividing the nuclear material Cytokinesis refers to the process of separating the cytoplasm and its contents into equal parts The cell cycle consists of mitosis, cytokinesis and interphase The Cell Cycle

Interphase G1 phase: cell grows S phase: DNA is replicated G2 phase: cell prepares for mitosis DNA is visible in the nucleus as strands called chromatin

Phase 1 of Mitosis: Prophase Centrioles move to opposite poles of the cell Chromatin condenses and shortens into chromosomes Spindle fibres form between the centrioles Nuclear membranes starts to disappear

Phase 2 of Mitosis: Metaphase Spindle fibres attached to centromeres pull chromosomes into place Chromosomes line up across the equator of the cell Centrioles duplicate

Phase 3 of Mitosis: Anaphase Chromatids separate at the centromere Single-stranded chomosomes are pulled to opposite poles by spindle fibres contracting

Phase 4 of Mitosis: Telophase Two nuclear envelopes form Single-stranded chromosomes uncoil to become chromatin Cytokinesis occurs after telophase: Organelles are distributed between the two daughter cells and the cell membrane pinches inward

Mitosis in Onion Root Cells

Meiosis vs Mitosis The purpose of mitosis is to maintain genetic continuity (the number of chromosomes in each daughter cell stays the same) The purpose of meiosis is to produce gametes (sex cells) which unite during sexual reproduction (the number of chromosomes in each sex cell is half of parent cell)

Stages of Meiosis We will look at the details later The key is that the four daughter cells produced are NOT identical

Asexual & Sexual Reproduction Asexual reproduction is any reproduction that does NOT involve meiosis. Sexual reproduction is any reproduction that does involve meiosis.

Asexual Reproduction 1. A single parent gives rise to offspring that are genetically identical to the original parent (clones) 2. Often produces many offspring rapidly 3. There are no specialized structures required by the parent.

For example: Binary fission of Amoeba; Budding of yeast;

fragmentation of sea stars; spore formation by ferns vegetative propagation by strawberries;

Sexual Reproduction Genetic information from parent cells are combined to produce a new organism (offspring are genetically different from parent) Requires more energy and time than asexual reproduction

Sexually reproducing organisms are better able to adapt to changing environments because of differences between individuals Individuals that are better adapted will survive and perpetuate the species

Parthenogenesis Sexual reproduction involving only one individual Egg is made by meiosis and then it duplicates its chromosomes without being fertilized common in lizards & salamanders in deserts