Cell Growth and Division:
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Transcript of Cell Growth and Division:
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Cell Growth and Division:
Mitosis vs. Meiosis and What’s going on the rest of the time
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Why does a cell need to divide?• A little geometry: As an object grows, the volume
increases at a faster rate than the surface area
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The same happens with a cell• What is the surface area?
• What is the volume?
• The more cytoplasm there is the more materials are needed.
• How do the materials enter?
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Oh no… diffusion is back!• By limiting the ratio of membrane to cytoplasm you limit the “doorways” into the cell.
• Cell can’t get enough materials to support its large size
• Cell dies, unless it divides in half!
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What do our cells need to do before they can divide?
• Get bigger• Make another copy of DNA• Make more organelles
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Cell Cycle
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Cell CycleG1 – growth and protein synthesis
S – DNA replication (copying the DNA)
G2 – Make organelles
M – Mitosis (Nuclear division) and
Cytokinesis – division of cytoplasm and membrane
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What is DNA again?
• A Chain of nucleotides
• Twisted into a double helix (spiral)
• VERY LONG
• Contains ALL the recipes for every protein our body needs
• Recipes are called genes
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Understanding DNA structure
• Most of the time out DNA is in the form of chromatin: strings of DNA wrapped around proteins called histones
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Understanding DNA structure
• DNA is in chromatin form through G1.
• In S phase, each strand of chromatin is duplicated and the duplicated copies remain attached together at the centromere.
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Understanding DNA structure
• During M phase the chromatin is folded into chromosomes
• DNA remains in chromosome form until cell division is over
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Understanding DNA structure
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Remember DNA contains the information needed to build an
organism
• Each chromosome contains some of the information.
• Each organism has a specific number of chromosomes.
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Humans have 23 types of chromosomes and 2 of each type = total of 46 chromosomes
• One of each type came from your mom the other from your dad.
• Every cell in your body has all 46 chromosomes with the exception of egg/sperm cells
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• Cells that contain two of each chromosome are called Diploid cells
• Cells that contain one of each chromosome are called Haploid cells
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Chromosome Analogy
• Think of the Information in a cell as an Encyclopedia - Each chromosome is one book– Haploid = one set of info / one encyclopedia
A BCD E
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Chromosome Analogy
Diploid = two different sets of info/ two different encyclopedias!
A B CD E A B CD E
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Chromosome Analogy• Genome = total information in cell
– If a cell is haploid, the genome consists of all the information in one encyclopedia
–If Diploid, all the info in both encyclopedias
A B C D E ABCDE
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Chromosome Analogy• Chromosome = One volume
– Genes: segments of DNA; each contains a specific message
–Genes are like Articles in encyclopedia
AF jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
F jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
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Chromosome Analogy• Genes can have different
variations. The variations are called Alleles.
• Think of the alleles as two different articles on the aardvark!
AF jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
F jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
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Chromosome Analogy• Homologous Chromosomes :
–Contain same genes, but may contain different alleles
–Example: both might contain Hair color gene but one might have brown hair info the other blonde
–One from Mom, one from Dad
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Chromosome Analogy• Homologous Chromosomes =
A AHomologousChromosomes
Volume “A” from each encycl.
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Chromosome Analogy
When DNA duplicates itself before cell division it makes two identical copies of each of chromosome
Identical Copies are Sister Chromatids
A AA A
SisterChromatids
SisterChromatids
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Single, unduplicated Chromosome
1 duplicated chromosome, 2 sister chromatids
2 separated chromatids become individual chromosomes
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Telomeres
Centromere
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Mitosis
• One Fluid Event; no stopping and starting.
• BUT: for ease of study, we break it into 4 stages
• REMEMBER: all phases are continuous and may, in part, overlap
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1. Prophase• Longest phase of Mitosis
• Chromatin folds up into Chromosomes which can now be seen
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1. Prophase• Centrosomes separate and move
toward opposite poles
• These are parts of the cell that make microtubules
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1. Prophase• Centrosomes start to form the
Mitotic spindle
–Made of microtubules
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1. Prophase• Nuclear membrane breaks down
• Nucleolus disappears
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2. Metaphase• Chromosomes line up along
equator• Spindle fibers attach to the
centromeres of chromosomes
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3. Anaphase• Centromeres connecting sister
chromatids separate and one chromatid of each chromosome moves toward each poles
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4. Telophase• Chromosomes begin to
unwind back into chromatin
• Nuclear Envelope reforms around chromosomes
• Spindle breaks down
• Nucleolus reappears
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Cell Division is not yet Done!!
• What have we made?
– One Cell with Two complete Nuclei
What is left to do?
- Cytokinesis: Divide the cytoplasm and separate the cells
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Cytokinesis
• Animal cells: Membrane pinches inward forming a Cleavage Furrow until it divides the cytoplasm into two equal parts
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Cytokinesis• Plant cell: Cell
plate forms in the middle of the cytoplasm and extends toward the edges. Cell wall forms from this cell plate
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Cell division is done!
• Now you have Two Identical daughter cells
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Controlling Cell Division
• Cells know when they need to divide
–When?
•During growth
•Repair – injury
•Replacement (cells are not immortal!)
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Controlling Cell Division• Cells know when they don’t need
to divide– Cells stop when they reach other cells
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How do cells “know”?• Cells communicate by releasing
molecules
• To control cell division cells release proteins called “cyclins”
• Some cyclins are “Go” signals
–Trigger the cell division process
• Some cyclins are “Stop” signals
– shut down the cell division process
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Loss of control
• What happens if the control signals don’t work?!?
– cells divide uncontrollably
–Pile up on top of each other
–Form big balls of cells called?????
»TUMORS!!!
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Tumors
• Tumor cells do not respond to (or do not have) the body’s control signals
–missing a “stop” signal so cell division doesn’t stop
–Hyperactive “go” signal so cell is constantly dividing
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Tumors Vs. Cancer
• Tumor = uncontrolled but isolated growth of cells
• Tumor cells become cancer when they start to invade healthy tissue–What if 1 cancer cell breaks off and enters the blood stream?
–Where ever it “lands” = new tumor = metastasis
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That’s how Somatic (or body) cells divide!
• What would happen if we made Egg and Sperm cells this way?
–Way too much DNA
• What do we have to do when forming these cells?
–Reduce the amount of DNA
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Meiosis
• Happens ONLY in sex cells
• Reduces information by ½
• Requires two different divisions
–How many cells at the end??
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Meiosis begins the same as Mitosis
• Cell in G1 enters S phase.
• ALL DNA is copied
• Chromatin folds up to form 46 duplicated chromosomes
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Meiosis IProphase I
- homologous chromosomes pair up forming tetrad; 4 chromatids together;
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Meiosis IProphase I
- Centrosomes separate to poles
- Nuclear envelope breaks down
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Meiosis IProphase I
- Crossing occurs between homologous chromosomes
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Meiosis IMetaphase I
- Spindle fibers attach to chromosomes at metaphase plate
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Meiosis IAnaphase I
- Fibers pull apart homologous chromosomes toward opposite poles
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Meiosis IAnaphase I
- Fibers pull apart homologous chromosomes toward opposite poles
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Meiosis ITelophase I
- Two cells are formed
- only half of the info of the Original cell
- 2N (diploid at start) N (haploid at end of meiosis I)
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Meiosis IIProphase II
- Spindle reforms in each haploid daughter cell
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Meiosis IIMetaphase II
- Chromosomes line up at the equator
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Meiosis IIAnaphase II
- Sister chromatids are pulled to opposite poles
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Meiosis IITelophase II
- Four haploid (N) cells created
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