- Slides: 55
Cell Growth & Division Biology Pearland ISD
Cell Division n There are 2 main reasons cell divides: The cell has more trouble trying to move nutrients and wastes across the cell membrane. 2. The larger a cell becomes, the greater the demand it puts on its DNA. 1. n Bottom line – cells need to be replaced
Cell Cycle n n n The sequence of growth and division of a cell An average cycle may be 22 hours Three general periods: 1. 2. 3. resting phase growth phase division phase
G 0 (Resting phase) Non-dividing, differentiated state. n Most human cells are in G 0 phase. n Liver cells: § Generally in G 0, but can be “called back” to cell cycle by external cues. n Nerve & muscle cells: § Highly specialized. § Arrested in G 0 & can never divide. n
Interphase (Growth phase) n n Most of the cell’s life is spent in interphase Longest phase – (90% of cell’s growth) Centrioles – help to organize cell division Chromatin – DNA bound protein within the nucleus
Interphase (Growth phase) New DNA is formed during 3 phases: G 1 – 1 st period of growth 1. Increase in size. 2. Makes new proteins and organelles. S – DNA is synthesized or replicated 1. Chromosomes are replicated. 2. New DNA molecules are made. G 2 – final cell growth 1. Shortest phase 2. Prepares cell for mitosis
S-Phase of the Cell Cycle SYNTHESIS
DNA § § § Genetic information = genome. Packaged into chromosomes. During the S phase, the chromosomes enter a relaxed state that allows the enzyme DNA polymerase to access the DNA double helix inside each chromosome. 50 µm
Copying & Packaging DNA n When cell is ready to divide… ¨ Copy DNA first, then… ¨ Coil up doubled chromosomes spool… n like thread on a Now move DNA around cell without having it tangle & break. Copying DNA Coil DNA into compact chromosomes
DNA & Chromosomes • • The DNA in a eukaryotic cell is organized into several linear chromosomes, whose organization is much more complex than the single, circular DNA molecule in a prokaryotic cell. All eukaryotic cells store genetic information in chromosomes. Most eukaryotes have between 10 and 50 chromosomes in their body cells. • Human cells have 46 chromosomes. • 23 nearly-identical pairs •
Chromosomes A structure inside the cell’s nucleus containing long, tightly-coiled strands of DNA wrapped around proteins.
Structure of Chromosomes § § § Chromosomes are composed of a complex of DNA and protein called chromatin that condenses during cell division. DNA exists as a single, long, double-stranded fiber extending chromosome’s entire length. Each unduplicated chromosome contains one DNA molecule, which may be several inches long.
Chromosomes Two sister chromatids of one replicated chromosome Maternal set of chromosomes (n = 3) 2 n = 6 Paternal set of chromosomes (n = 3) Centromere Two non-sister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)
Chromosomes n The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores. Pair of sister chromatids Centromere (DNA that is hidden beneath the kinetochore proteins) One chromatid (dark blue) Kinetochore proteins One chromatid (light blue)
Chromosomes • In a diploid cell, the chromosomes occur in pairs. • The 2 members of each pair are called homologous chromosomes or homologues. • In a cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids.
Chromosomes • Non-homologous chromosomes: • • • Look different. Control different traits. Sex chromosomes: Are distinct from each other in their characteristics. • Are represented as X and Y. • Determine the sex of the individual, XX being female, XY being male. •
Chromosome Duplication § § In preparation for cell division, DNA is replicated and the chromosomes condense. Each duplicated chromosome has two sister chromatids, which separate during cell division.
0. 5 µm A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Chromosome duplication (including DNA synthesis) Centromere Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Separation of sister chromatids Sister chromatids Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Centrometers Sister chromatids
Chromosome Duplication § § § Because of duplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere. Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid. Homologous chromosomes are made up of sister chromatids joined at the centromere. Non-sister chromatids Centromere Two Unduplicated Chromosomes Duplication Sister chromatids Two Duplicated Chromosomes
DNA Replication § § § Replication begins when helicase enzymes unzip various locations along the chromosome, separating the two complementary strands of DNA polymerase molecules read each original, or template, strand synthesize new DNA strands containing complementary genetic information. Each replicated double helix consists of a template strand a new strand in a process called semiconservative replication.
Copying DNA n A dividing cell duplicates its DNA. ¨ Creates 2 copies of all DNA. ¨ Separates the 2 copies to opposite ends of the cell. ¨ Splits into 2 daughter cells. ¨ But the DNA stays loosely wound in the nucleus. ¨ If you tried to divide it like that, it could tangle & break. DNA Cell Nucleus
Organizing & Packaging DNA Cell Nucleus s e m so m is ro n h a c g 4 or s i in th DNA has been “wound up” DNA in chromosomes in everyday “working” cell. Cell Nucleus DNA in chromosomes in cell getting ready to divide
Organizing & Packaging Chromosome Nucleosome DNA Double Helix Coils Supercoils Histones
DNA Synthesis n Matching base pairs allows DNA to be easily copied.
Base Pairs § Base-pairing rules § A binds with T § § (A U if DNA-RNA hybrid) G binds with C
DNA Synthesis: Replication DNA starts as a double-stranded molecule with matching bases (A: T, C: G) § Then, it unwinds and unzips… §
DNA Replication n Strands “unzip” at the weak hydrogen bonds between bases. Replication Fork
Replication DNA bases in nucleus Enzymes DNA polymerase • Adds new bases •
DNA Replication § Build daughter DNA strand. § § § Use original parent strand as “template”. Add new matching bases. Synthesis enzyme = DNA polymerase. DNA Polymerase
New Copies of DNA n Get 2 exact copies of DNA to split between new cells.
Double-stranded Human Chromosomes Ready for Mitosis
DNA Must be Duplicated… DNA in Chromosomes Cell Nucleus Duplicated Chromosomes 4 double-stranded chromosomes Cell
Terms to know n n Chromosome – contains genetic information (DNA) passed from one generation to the next Spindle – microtubule that helps separate chromosomes A – centromere: center of chromosome B – chromatids: two identical “sister” parts of the chromosome
Following Interphase. . MITOSIS
Mitosis Takes place within the nucleus of the cell.
Mitosis (Division phase) 4 Phases: (PMAT) 1) 2) 3) 4) Prophase Metaphase Anaphase Telophase
4 phases that blend from one to another
Prophase n n n 1 st and longest phase of mitosis Chromatin become chromosomes Chromatids are joined by centromere Nucleus disappears Centrioles migrate to poles Spindles are formed
Metaphase n n 2 nd phase of mitosis Chromosomes meet in the middle of cell Pulled by spindles Each chromosome is attached to top of spindle
Anaphase n n n 3 rd phase of mitosis Centromeres are split apart Chromatids are pulled apart and begin to drift to opposite poles
Telophase n n n Final phase of mitosis Begins when chromatids reach poles New nucleus starts to form Chromosomes start to unwind Spindles disappear Cytoplasm begins to divide
Cytokinesis n Cytoplasm pinches in half n Each daughter cell has an identical set of chromosomes
Cell Cycle G 0 phase resting
When Cells Don’t Stop … CANCER
Cancer is a disorder where the cell has uncontrolled growth. n Cancer cells do NOT respond to regulators. n This causes the cells to form masses called tumors, which can damage surrounding tissues. n
Cancer Development Cancer develops only after a cell experiences ~6 key mutations (“hits”). n Unlimited growth ¨ n Ignore checkpoints ¨ n Turn ON chromosome maintenance genes. Promotes blood vessel growth ¨ n Turn OFF suicide genes. Immortality = unlimited divisions ¨ n Turn OFF tumor suppressor genes. Escape apoptosis ¨ n Turn ON growth promoter genes. Turn ON blood vessel growth genes. Overcome anchor & density dependence ¨ Turn OFF touch-sensor gene.
Cancer Triggers n Mutations in cells can be triggered by: • • UV radiation Chemical exposure Radiation exposure Heat • • Cigarette smoke Pollution Age Genetics
Growing out of control, cancer cells produce malignant tumors n Cancer cells do not respond normally to the cell cycle control system n ¨ Divide excessively. ¨ Can invade other tissues. ¨ May kill the organism.
n If an abnormal cell avoids destruction by the immune system, it may form a tumor ¨ Benign: abnormal cells remain at original site ¨ Malignant: abnormal cells can spread to other tissues and parts of the body ¨ Metastasis: spread of cancer cells through the circulatory system
n Cancers are named according to location of origin: ¨ Carcinoma: external or internal body coverings ¨ Sarcoma: tissues that support the body ¨ Leukemia and lymphoma: blood-forming tissues n Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division