1.6. Topic 1: Cell Biology (Teacher) Essential Idea: Cell division is essential but must be controlled. 1.6 Cell Division

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1 Tpic 1: Cell Bilgy (Teacher) 1.6 Essential Idea: Cell divisin is essential but must be cntrlled. 1.6 Cell Divisin Why d cells divide: - Sa:Vl Rati - Allws fr grwth f the rganism - Allws fr cell differentiatin - Replace dead, damaged, r infected cells - Embrynic develpment Cell Cycle: series f events thrugh which cells pass t divide and create tw identical daughter cells. Chrmsmes: - Eukarytes have genes n chrmsmes - Each chrmsme has a partner (ne frm each parent) that cntains same genes Different species have different chrmsme numbers Humans have 23 pairs (n=23) r 46 ttal Diplid cells have bth cpies (2n), example smatic cells r bdy cells Haplid cells have just ne (n), example gametes r sex cells

2 vi. Interphase is a very active phase f the cell cycle with many prcesses ccurring in the nucleus and cytplasm. Interphase - is the lngest part f the cell cycle which cnsists f 3 stages G1, S, G2 Interphase is an active perid in the life f the cell in which many metablic reactins ccur, including prtein synthesis, DNA replicatin and prductin r mitchndria and/r chlrplasts. Prtein synthesis - synthesis f prteins and enzymes (Gap 1), many f which are required fr the synthesis phase during DNA replicatin and the prductin f micrtubules and prteins in Gap 2; needed fr mitsis. DNA replicatin - Fundamental prcess in which the cell replicates its DNA befre it divides (prtein synthesis and transcriptin is lw in the S phase. Mitchndria and/r chlrplasts number increases during interphase in preparatin fr divisin. Cellular respiratin als takes place interphase and mitsis G1 This stage is called Gap 1 in which the cell grws. After a checkup by the cell, if they are nt ready t divide they g int G0 i. Mitsis is divisin f the nucleus int tw genetically identical daughter nuclei. During cell divisin (mitsis and cytkinesis) the cell divides int tw genetically identical daughter cells. During S phase in the cell cycle, the cell will replicate its chrmsmes t create t identical sets f chrmsmes (nw called chrmatids) attached in the middle by a centrmere. Replicatin is semi-cnservative, meaning that each strand f the riginal duble-stranded DNA mlecule serves as template fr the prductin f the new cmplementary strand. Thereby insuring tw identical cpies f the DNA are created. Prfreading and errr-checking mechanisms during replicatin ensure near perfect cpies f DNA. Each chrmsme nw cntains duble the genetic material as it enters mitsis. During mitsis, the pairs f sister chrmatids line up alng the metaphase plate, where each chrmatid is attached t a spindle fiber cnnected t ppsite ples in the cell. During anaphase each identical chrmatid is pulled twards ppsite ples resulting in tw genetically identical nuclei at ppsite ples in the cell. ii. Chrmsmes cndense by superciling during mitsis. During mitsis chrmsmes cndense int visible structures due t a prcess called superciling Since a nucleus is generally less than 5 µm in diameter and sme f the DNA mlecules are ver 50,000 µm in length. They have t cndense and cil arund histne prteins making the chrmsme much shrter and fatter. The nuclesmes (made f histnes) will interact further with each ther causing the chrmsmes t supercil. This superciling helps regulate transcriptin because nly certain areas f the DNA are accessible fr the prductin f mrna by transcriptin. This regulates the prductin f a plypeptide. iii. Cytkinesis ccurs after mitsis and is different in plant and animal cells. Cytkinesis is the prcess in which the cytplasm f a single eukarytic cell is divided t frm tw daughter cells after mitsis is cmplete. In plant cells tubular structures are frmed by vesicles alng the equatr f the cell

3 This cntinues until tw layers f membrane exist acrss the equatr, which develp int the plasma membrane f the tw new cells Vesicles bring pectin and ther substances and depsit these between the tw membranes thrugh excytsis frming the middle lamella Cellulse is then brught and depsited by excytsis between the membranes as well, frming the new cell walls In animal cells a cleavage furrw frms when the plasma membrane is pulled inwards arund the equatr by the cntractile prteins actin and mysin Once the invaginatin reaches the center the membrane pinches ff and t new cells are frmed Skill: Identificatin f phases f mitsis in cells viewed with a micrscpe r in a micrgraph. Mitsis Descriptin Diagrams Prphase Chrmsmes becme shrter and mre cndensed in the prcess called superciling. The nuclear envelpe begins t break dwn and disintegrate. Micrtubules that frm the mittic spindle begin t develp frm the centrsmes in the cell. Centrsmes mve twards the ples as the spindle grws and lengthens. Metaphase The spindle fibers are attached t the centrmeres f the chrmsmes. Chrmsmes mve twards the equatr f the cell and line up alng the metaphase plate. The ther ends f the micrtubules f the spindle are attached t ples f the cell. Anaphase The pairs f sister chrmatids are pulled apart by the spindle fibers twards the ples. The chrmatids are nw cnsidered chrmsmes. The chrmsmes mve t the ples as a result f the shrtening f the micrtubule. After anaphase the cell nw has tw genetically identical nuclei at each end f the cell. Telphase Nuclear membranes nw begin t frm arund each set f chrmsmes. Chrmsmes begin t uncil t frm chrmatin again. The spindle fibers break dwn and nucleli refrm in each nucleus. The cell elngates and gets ready fr cytkineis.

4 interphase prphase metaphase anaphase telphase Skill: Determinatin f a mittic index frm a micrgraph. The Mittic index = number f cells cntaining visible chrmsmes (in mitsis) divided by the ttal number f cells in field f view. What is the Mittic index fr this picture? Label the parts f the cycle. A: B: C: D: Fr part D label the steps f mitsis and cytkinesis n the picture t the right.

5 iv. Cyclins are invlved in the cntrl f the cell cycle. Cyclins are a family f prteins that help regulate the cell cycle. These prteins ensure that task are perfrmed at the crrect time and the cells mve t the next stage at the apprpriate time. They bind t enzymes called cyclin-dependent kinases, activating these cdk enzymes causing them t attach phsphates t ther prtein in the cell, which triggers ther prteins t becme active and carry ut specific tasks. 4 main types f cyclin in human cells. Cyclin D causes G0 t mve t G1 and G1 t mve t S phase Cyclin E causes the cell t prepare fr replicatin in S phase Cyclin A activates DNA replicatin in S phase Cyclin B causes the mittic spindle t begin t frm and ther tasks needed in the preparatin f mitsis v. Mutagens, ncgenes and metastasis are invlved in the develpment f primary and secndary tumurs. Tumrs are the result f uncntrlled cell divisin, which can ccur in any rgan r tissue. These abnrmal grwths can either be lcalized (primary tumurs), meaning they d nt mve t ther part f yur bdy. These tumurs are benign. If the cancer cells detach and mve elsewhere int the bdy (secndary tumurs), they are called malignant and are mre lifethreatening Diseases due t malignant tumurs are knwn as cancer Metastasis is the mvement frm a primary tumur t set up secndary tumurs in ther parts f the bdy Cancer is usually caused by genetic abnrmalities due t a variety f different surces called carcingens r due t inheritance r errrs in DNA replicatin. Carcingens are agents that can cause cancer, such as viruses, X-Rays, UV Radiatin and many chemical agents Mutagens are agents that can cause mutatins in ne s DNA which can lead t cancer

6 Mutagens and carcingens are strngly crrelated and many mutagens can be carcingens In cancer tw types f genes are usually affected, ncgenes and tumr suppressr genes. Oncgenes are mutated frms f prt-ncgenes (which typically cntrl synthesis f prteins invlved in cell signaling r cell divisin). These cells with activated ncgenes cause uncntrlled grwth and cell divisin, prevent the cancer cell frm dying and allw them t invade ther tissues. Tumr suppressr genes usually cntrl replicatin and the cell cycle. In cancer cells these genes are generally inactivated causing a lss f nrmal functin. Applicatin: The crrelatin between smking and incidence f cancers. A crrelatin is a relatinship between tw variable factrs There is a strng psitive crrelatin between smking and cancer Surveys have shwn that the mre cigarettes that ne smkes per day, the higher the death rate due t cancer. The main cancers invlved are cancer f the muth, pharynx, larynx, esphagus and lungs