Unit 2: Molecules and Cells Cell Growth and Division

Cell Growth Cells cannot continue to grow indefinitely because: The genetic information in a single cell cannot serve the needs of a giant cell due to its complexity and many needs. A giant cell has too great a volume compared to its surface area for material exchange.

Cell Growth In order for a cell to continue living, it must form 2 “daughter cells” from itself in the process of Cell Division. During Cell Division, the cell replicates its entire DNA sequence so that each daughter cell will get a complete set of genetic information.

Cell Division in Prokaryotes Most Prokaryotic Cells divide through binary fission, during which 2 identical cells are created from 1 cell.

Cell Division in Eukaryotes Eukaryotes divide by a process called MITOSIS. A eukaryotic cell lives through a Cell Cycle with different phases: Interphase = the time between cell divisions G1 Phase – cell growth S Phase – DNA is copied G2 Phase – growth and preparation for cell division M Phase = Mitosis – division of nucleus and contents Cytokinesis – division of the cytoplasm and cell

The Cell Cycle A Pie Chart of the Cell Cycle with times for each phase in a typical animal cell: Complete Cell Cycle = about 24 hours (in typical animal cells) 12 hr 0.5 hr M Phase Mitosis 6 hr 6 hr

Cell Cycle Control The Cell Cycle is regulated by certain genes coding for proteins called cyclins, which monitor “checkpoints” during the cycle. There are 3 checkpoints: the G1, G2 and Mitosis checkpoints If conditions are not favorable for the cell to undergo the following phase, cell division will stop. Cancer is the result of uncontrolled cell growth/division from speeding up the cycle or inactivating the proteins that slow or stop the cycle.

Chromosomes Genetic Information in the form of DNA is held in Chromatin inside the nucleus. When the DNA needs to replicate and divide during mitosis, this chromatin condenses to form Chromosomes. Chromosomes are composed of 2 sister chromatids (contains DNA) and a centromere, which holds the 2 chromatids together

Mitosis After Interphase (cell growth, DNA replication), a eukaryotic cell undergoes Mitosis. There are 4 Steps of Mitosis – Prophase Metaphase Anaphase Telophase

Prophase Chromatin condenses into chromosomes. Nuclear Envelope breaks down. The Centrioles (microtubule “organizing centers” in animal cells) move to opposite “poles” of the cell and begin sprouting spindle fibers (microtubules) that will later help separate chromosomes.

Metaphase Spindle Fibers from centrioles either form a cage around the chromosomes or attach directly to the centromeres. Microtubules pull chromosomes to line up along cell’s “equator,” known as the Metaphase Plate.

Anaphase Sister chromatids are separated into individual chromosomes and transported by spindle fibers to opposite poles.

Telophase Chromosomes at each end begin to unwind and lose distinct shape. Spindle Fibers dissolve. 2 new nuclear membranes are formed.

Cytokinesis After Mitosis, the cell must complete the division with the dividing of cytoplasm during Cytokinesis. Cell membrane creates a cleavage furrow. Cell membrane pinches together completely and fuses One cell becomes two, and each new cell begins the cycle over starting with G1.

New Topic: Meiosis All animal somatic (body) cells divide through Mitosis. However, reproductive cells, called gametes, are created through Meiosis by specialized cells.

Some Prerequisites for Meiosis Normal body cells contain 2 of each type of chromosome; a pair of these similar chromosomes are called homologous chromosomes. A cell having 2 copies of each chromosome contains a Diploid (2n) Number of chromosomes (in humans, this number is 46). Gametes contain only 1 type of each chromosome; this is considered a Haploid (n) Number of chromosomes (in humans, this number is 23).

Meiosis Meiosis consists of 2 separate cell divisions, Meiosis I and Meiosis II. Meiosis I Prophase I Metaphase I Anaphase I Telophase I/Cytokinesis Meiosis II Prophase II Metaphase II Anaphase II Telophase II/Cytokinesis

Meiosis I Prophase I – homologous chromosomes form a tetrad where they can exchange genetic information in a process known as “crossing over.” Metaphase I – Homologous chromosomes line up at metaphase plate Anaphase I – Homologous chromosomes separate, leaving neither daughter cell with a complete set of genetic information Telophase I/Cytokinesis – 2 daughter cells are formed, each with a Haploid (n) Number of chromosomes.

Meiosis I

Meiosis II Prophase II – The haploid daughter cells do not replicate their DNA Metaphase II – The chromosomes line up at the metaphase plate. Anaphase II – The paired chromatids separate and move to opposite poles Telophase II/Cytokinesis – a total of 4 new cells are formed each containing a Haploid (n) Number of chromosomes.

Meiosis II (Haploid cell) (Haploid cell)

Formation of Gametes The production of Sperm, called Spermatogenesis, produces 4 sperm cells. The production of Eggs, called Oogenesis, produces 1 Egg cell and 3 polar bodies (which degenerate) due to unequal cytoplasm division during meiosis.

Mitosis versus Meiosis Begins with 1 DIPLOID cell Produces 2 genetically identical DIPLOID cells Occurs in Somatic (body) cells MEIOSIS Begins with 1 DIPLOID cell Produces 4 genetically different HAPLOID cells Occurs in Reproductive “precursor” cells