4.-cell-division.docx

**Cell Division** - **All cells are derived from pre-existing cells** - **New cells are produced for growth and to replace damaged or old cells** - **Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)** **Keeping Cells Identical** **The instructi...

**Cell Division** - **All cells are derived from pre-existing cells** - **New cells are produced for growth and to replace damaged or old cells** - **Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)** **Keeping Cells Identical** **The instructions for making cell parts are encoded in the DNA, so each new cell must get a complete set of the DNA molecules** **DNA Replication** - **DNA must be copied or replicated before cell division** - **Each new cell will then have an identical copy of the DNA** **Prokaryotic Chromosome** - **The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane** **Eukaryotic Chromosomes** - **All eukaryotic cells store genetic information in chromosomes** - **Most eukaryotes have between 10 and 50 chromosomes in their body cells** - **Human body cells have 46 chromosomes or 23 identical pairs** - **Each chromosome is composed of a single, tightly coiled DNA molecule** - **Chromosomes can't be seen when cells aren't dividing and are called chromatin** **Compacting DNA into Chromosomes** - **DNA is tightly coiled around proteins called histones** **Chromosomes in Dividing Cells** - **Duplicated chromosomes are called chromatids & are held together by the centromere** **Karyotype** - **A picture of the chromosomes from a human cell arranged in pairs by size** - **First 22 pairs are called autosomes** - **Last pair are the sex chromosomes** - **XX female or XY male** **Cell Reproduction** **Types of Cell Reproduction** - **Asexual reproduction involves a single cell dividing to make 2 new, identical daughter cells** - **Mitosis & binary fission are examples of asexual reproduction** - **Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells** - **Meiosis is an example** **Cell Division in Prokaryotes** - **Prokaryotes such as bacteria divide into 2 identical cells by the process of binary fission** - **Single chromosome makes a copy of itself** - **Cell wall forms between the chromosomes dividing the cell** **The Cell Cycle** **Five Phases of the Cell Cycle** - **G~1~ - primary growth phase** - **S -- synthesis; DNA replicated** - **G~2~ - secondary growth phase** ***collectively these 3 stages are called interphase*** - **M - mitosis** - **C - cytokinesis** **Interphase - G~1~ Stage** - **1^st^ growth stage after cell division** - **Cells mature by making more cytoplasm & organelles** - **Cell carries on its normal metabolic activities** **Interphase -- S Stage** - **Synthesis stage** - **DNA is copied or replicated** **Interphase -- G~2~ Stage** - **2^nd^ Growth Stage** - **Occurs after DNA has been copied** - **All cell structures needed for division are made (e.g. centrioles)** - **Both organelles & proteins are synthesized** **What's Happening in Interphase?** **Sketch the Cell Cycle** **Mitosis** - **Division of the nucleus** - **Also called karyokinesis** - **Only occurs in eukaryotes** - **Has four stages** - **Doesn't occur in some cells such as brain cells** **Four Mitotic Stages**  **Spindle Fibers** - **The mitotic spindle form from the microtubules in plants and centrioles in animal cells** - **Polar fibers extend from one pole of the cell to the opposite pole** - **Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach** - **Asters are short fibers radiating from centrioles** **Metaphase** - **Chromosomes, attached to the kinetochore fibers, move to the center of the cell** - **Chromosomes are now lined up at the equator** **Anaphase** - **Occurs rapidly** - **Sister chromatids are pulled apart to opposite poles of the cell by kinetochore fibers** **Telophase** - **Sister chromatids at opposite poles** - **Spindle disassembles** - **Nuclear envelope forms around each set of sister chromatids** - **Nucleolus reappears** - **CYTOKINESIS occurs** - **Chromosomes reappear as chromatin**  **Cytokinesis** - **Means division of the cytoplasm** - **Division of cell into two, identical halves called daughter cells** - **In plant cells, cell plate forms at the equator to divide cell** - **In animal cells, cleavage furrow forms to split cell** **Daughter Cells of Mitosis** - **Have the same number of chromosomes as each other and as the parent cell from which they were formed** - **Identical to each other, but smaller than parent cell** - **Must grow in size to become mature cells (G~1~ of Interphase)** **Uncontrolled Mitosis** - **If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors** - **Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell** **Meiosis\ *Formation of Gametes* (Eggs & Sperm)** **Facts About Meiosis** - **Preceded by interphase which includes chromosome replication** - **Two meiotic divisions \-\-- Meiosis I and Meiosis II** - **Called Reduction- division** - **Original cell is diploid (2n)** - **Four daughter cells produced that are monoploid (1n)** - **Daughter cells contain half the number of chromosomes as the original cell** - **Produces gametes (eggs & sperm)** - **Occurs in the testes in males (Spermatogenesis)** - **Occurs in the ovaries in females (Oogenesis)** **More Meiosis Facts** - **Start with 46 double stranded chromosomes (2n)** - **After 1 division - 23 double stranded chromosomes (n)** - **After 2nd division - 23 single stranded chromosomes (n)** - **  Occurs in our germ cells that produce gametes** **Why Do we Need Meiosis?** - **It is the fundamental basis of sexual reproduction** - **Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote** **Fertilization -- "Putting it all together"** **Replication of Chromosomes** - **Replication is the process of duplicating a chromosome** - **Occurs prior to division** - **Replicated copies are called sister chromatids** - **Held together at centromere** **Meiosis Forms Haploid Gametes** - **Meiosis must reduce the chromosome number by half** - **Fertilization then restores the 2n number** **Meiosis: Two Part Cell Division** Prophase I (Gr. pro = before) is divided into:\ **[Leptotene,](http://www.vcbio.science.ru.nl/en/virtuallessons/cellcycle/meiostage/leptotene/) [Zygotene,](http://www.vcbio.science.ru.nl/en/virtuallessons/cellcycle/meiostage/zygotene/) [Pachytene,](http://www.vcbio.science.ru.nl/en/virtuallessons/cellcycle/meiostage/pachytene/) [Diplotene,](http://www.vcbio.science.ru.nl/en/virtuallessons/cellcycle/meiostage/diplotene/) [Diakinesis](http://www.vcbio.science.ru.nl/en/virtuallessons/cellcycle/meiostage/diakinesis/)**\ Meiosis I begins with the condensation of the chromosomes (**B**) in the leptotene during prophase I. During the pachytene (another stage of prophase I) (**C**) the pairs of homolog chromosomes align to form tetrads in a process called synapsis. Corresponding segments of DNA of sister chromatids of two homolog chromosomes twist and cross, forming so-called **chiasmata**. In these regions exchange of DNA between homologe chromosomes (**C**) can occur: **crossing-over**). Crossing-over (or crossover) requires that the chromosomes break and reconnect to the other chromosome. **Crossing-Over** - **Homologous chromosomes in a tetrad cross over each other** - **Pieces of chromosomes or genes are exchanged** - **Produces Genetic recombination in the offspring** **Results of Meiosis** **Gametes (egg & sperm) form** **Four haploid cells with one copy of each chromosome** **One allele of each gene** **Different combinations of alleles for different genes along the chromosome** **Gametogenesis\ Spermatogenesis** - **Occurs in the testes** - **Two divisions produce 4 spermatids** - **Spermatids mature into sperm** - **Men produce about 250,000,000 sperm per day** - **Occurs in the ovaries** - **Two divisions produce 3 polar bodies that die and 1 egg** - **Polar bodies die because of unequal division of cytoplasm** - **Immature egg called oocyte** - **Starting at puberty, one oocyte matures into an ovum (egg) every 28 days**