Cell Growth & Division

Cell Unit III: Cell Division, Cell Cycle, Transcription and TranslationChapters 12, 13, 16, 17Limits to Cell GrowthThe larger a cell becomes, the more demands a cell places on its DNAIf extra copies of DNA are not made, an information crisis would occurThe cell also has more trouble moving nutrients and wastes across the cell membraneFood, oxygen, water, and wastes move through the cell membraneThe rate at which the exchange takes place depends on the surface area of the cellThe rate at which food and oxygen are used up and wastes produced depends on the cells volume Ratio of Surface Area to VolumeVolume increases much more rapidly than surface area causing the ratio of surface area to volume to decreaseThis decrease creates serious problems for the cell such as:Inability to remove wastes from the cellLack of sufficient oxygen and food entering through the cell membraneDivision of the CellThe process by which a cell divides into two new daughter cells is called cell divisionBefore cell division occurs, the cell replicates, or copies, all of its DNAEach daughter cell gets one complete set of genetic informationEach daughter cell also has an increased ratio of surface area to volume

Cell DivisionEach cell has only one set of genetic informationmust be copied before cell division beginsThe first stage, division of the cell nucleus, is called mitosisThe second stage, division of the cytoplasm, is called cytokinesisReproduction by mitosis is classified as asexualMitosis is the source of new cells when a multicellular organism grows and develops

ChromosomesChromosomes are made of DNA (genetic information) and proteins (histones)The cells of every organism have a specific number of chromosomesFruit flies = 8, human = 46, carrots = 18Chromosomes are not visible in most cells except during cell divisionEach chromosome consists of two identical sister chromatids which separate during cell divisionEach pair of chromatids is attached in an area called the centromere

The Cell CycleInterphase is the period in between periods of cell divisionThe cell cycle is the series of events that cells go through as they grow and divideDuring the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle againThe cell cycle consists of four phasesM, S, G1, and G2The Cell Cycle

The Cell Cycle

Events of the Cell CycleDuring the normal cell cycle, interphase can be quite long, whereas the process of cell division takes place quicklyThe G1 phase is a period in which cells do most of their growingIn the S phase, chromosomes are replicated and the synthesis of DNA molecules takes placeDuring the G2 phase, many of the organelles and molecules required for cell division are produced

MitosisProphase: Chromosomes become visible, centrioles begin to organize the spindle and move to opposite ends of the cell, fibers attach to centromeres, nucleolus and nuclear envelope disappearMetaphase:Chromosomes line up across the center of the cellAnaphase:Centromeres split and individual chromatids are separated into two groups near the polesTelophase:Chromosomes disperse, nuclear envelope and nucleolus re-form, spindle breaks apart

MitosisCytokinesisCytokinesis is the division of the cytoplasm itself and usually occurs at the same time as telophaseIn most animal cells, the cytoplasm is drawn inward until the cytoplasm is pinched into two nearly equal parts. This is called a cleavage furrow.In plants, a structure known as the cell plate forms midway between the divided nuclei

Cytokinesis in Animal Cells

Controls on Cell DivisionWhen placed on a petri dish with a thin layer of nutrient solution, cells will grow until they form a thin layer on the bottom of the dishWhen cells come into contact with other cells, they respond by not growingIf cells are removed from the center of the dish, the cells bordering the open space will divide until they have filled the spaceControls on cell growth and division can be turned off and onCell Cycle RegulatorsSeveral scientists discovered that cells in mitosis contained a protein that when injected into a nondividing cell, would cause a mitotic spindle to formThey called this protein cyclin because it seemed to regulate the cell cycleCyclins regulate the timing of the cell cycle in eukaryotic cellsProteins that respond to events inside the cell are called internal regulatorsExternal regulators respond to events outside of the cellCell Cycle Regulators

Uncontrolled Cell GrowthCancer is a disorder in which some of the bodys own cells lose the ability to control growthCancer cells do not respond to the signals that regulate the growth of most cellsThey divide uncontrollable and form masses of cells called tumors that can damage the surrounding tissuesCauses include smoking, radiation, and viral infectionsDamaged or defective p53 genes cause the cells to lose the information needed to respond to signals that would normally control their growthp53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe, this protein can cause apoptosis (cell death). p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle. A p53 mutation is the most frequent mutation leading to cancer. p27 is a protein that binds to cyclin and cdk blocking entry into S phase. Recent research (Nature Medicine 3, 152 (1997)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.

Uncontrolled Cell Growth

CHAPTER 13: MEIOSIS AND SEXUAL CYCLESMeiosis - cell division that reduces the diploid # to the haploid # in the formation of sex cells (gametes).Example (Humans) - 46 chromosomes is reduced to 23.MOST IMPORTANT - the cells produced at the end of meiosis contain one chromosome of each homologous (matching) pair.GENE - HEREDITARY INFORMATION, IN A SECTION OF A DNA MOLECULE ON A CHROMOSOME.LOCUS (LOCI) - A GENES SPECIFIC LOCATION ON A CHROMOSOME.TERMS:CLONE - A GROUP OF GENETICALLY IDENTICAL INDIVIDUALS ( WHAT MITOSIS PRODUCES)ASEXUAL REPRODUCTION - REPRODUCTION W/O SEX (NO MALE/FEMALE; 1 PARENT; OFFSPRING IS A CLONE OF PARENT. HOMOLOGOUS CHROMOSOMES - A MATCHING PAIR ALWAYS ONE FROM EACH PARENT.(one paternal/ one maternal.)AUTOSOMES - CHROMOSOMES NOT DIRECTLY INVOLVED IN DETERMINING SEX. (IN HUMANS: 22 HOMOLOGOUS PAIR).SEX CHROMOSOMES - THE CHROMOSOMES DIRECTLY INVOLVED IN DETERMINING SEX (IN HUMANS THE LAST HOMOLOGOUS PAIR).(a) CALLED (X) & (Y) CHROMOSOMES.(b) XX = FEMALE & XY = MALE.FERTILIZATION (or SYNGAMY) - UNION OF GAMETES.KARYOTYPE: DISPLAY OF AN INDIVIDUALS CHROMOSOMES. CHROMOSOMES ARE COLLECTED DURING METAPHASE. THIS IS DONE BY NUMBER, SIZE & TYPE CHROMOSOME.(c) In other organisms: (1) Insects (Grasshoppers, Roaches): X-O sex chromosomes. O represents no sex chromosome = Male(2) Birds, Butterflies and some fish: Z-W sex chromosomes. Female gamete determines sex. Males are ZZ, Females are ZW(3) Parthenogenesis wasps, bees and ants. If the egg is fertilized it becomes a female and is diploid. If the egg is unfertilized it is male and haploid.

THE HUMAN LIFE CYCLE:(characteristic of most animals)Gametes are the only haploid cells.The diploid zygote divides by mitosis producing a diploid organism.MEIOSIS STEPS:( FIG. 13.5.)

(a) Each chromosome replicates. (This shows 1 homologous pair). Remember - sister chromatids & centromere.(b) Meiosis I segregates the homologous pair into 2 different cells (each new daughter cell is in HAPLOID).(c ) Meiosis II separates sister chromatids into chromosomes. No chromosome duplication) MEIOSIS TERMS:Synapsis - ( in prophase I ) - the duplicated chromosomes pair with their Homologues). This is a PROCESS. Homologous chromosomes made of two sister chromatids come together as pairs.Homologue - one of a homologous pair.Tetrad - the four closely associated chromatids of a homologous pair together. This happens during synapsis.Crossing over - (a process) reciprocal exchange of genetic material between nonsister chromatids.

COMPARING MITOSIS & MEIOSIS.MEIOSIS - Prophase I with -(a) Tetrad & synapsis making a synaptonemal complex (b) Crossing over with the chiasma. MITOSIS- No tetrads, synapsis, or crossing over.DAUGHTER CELL DIFFERENCE - Mitosis has produced 2 identical cells. Meiosis produced daughter cells with one of each homologous pair.

SUMMARY: differences between Mitosis & Meiosis.

FIG. 13.8 - This shows the most important concept of meiosis (how it produces genetic variation in organisms).INDEPENDENT ASSORTMENT: At the end of meiosis chromosome pairs distribute themselves independently of one another. This causes 4 different combinations of chromosomes with 2 homologous pair.1st MEIOTIC DIVISION RESULTS IN INDEPENDENT ASSORTMENT OF MATERNAL & PATERNAL CHROMOSOMES IN DAUGHTER CELLS. FORMULA: The number of combinations possible when chromosomes assort independently into gametes during meiosis is 2n, where (n) is the haploid # in the organism.EXAMPLE - Human haploid (n) is 23. 223 is over 8 million. A male can produce 8 million genetically different combinations of sperm & a female 8 million combinations of eggs.RANDOM FERTILIZATION then would produce 8 million x 8 million(over 64 Trillion) possibly different genetic combinations in the offspring.

Crossing Over - produces individual chromosomes that combine genes inherited from our two parents.Independent Assortment, Random Fertilization, & Crossing Over - result ways that genetic variation can be produced.SUMMARY:Prophase I & Anaphase I produce the most variation in the 4 new daughter cells.If clones were genetically different, this would be due to mutation (change in the code of DNA).

Remember these !!!!Which might be a daughter cell of meiosis I ?Which might be a daughter cell of meiosis II?CHAPTER 16 - THE MOLECULAR BASIS OF INHERITANCEDNA - most celebrated molecule of all time. It is made of nucleic acids that have the unique ability to direct their own replication. PROBLEM: Since a chromosome is made of protein & DNA which one is carrying the genetic material?There can be an infinite # of proteins so it would be a prime candidate to carry genetic material.

JAMES WATSON CO-FOUNDER OF THE STRUCTURE OF DNA

Watson & Crick working on the DNA structure model. (April 1953)

Transformation of Bacteria - Frederick Griffin.(1928)The captions under the picture is all that is needed to explain this experiment.TRANSFORMATION - the change in genotype & phenotype due to the assimilation of external DNA by a cell.EVIDENCE THAT VIRAL DNA CAN PROGRAM CELLS (FIG. 16.2)

Virus is made of a protein coat & DNA core. Virus injects DNA into a Bacteriophage. DNA coat has radioactive protein coat (S35) while DNA is radiated with (P32).HERSHEY-CHASE EXPERIMENTThat the bacteria are called T2 Phages.ADDITIONAL EVIDENCE THAT DNA IS THE GENETIC MATERIAL OF CELLSErwin Chargaff - Said that the bases of DNA (A, T, C, G) vary from one species to another. He also found a regular ratio of bases. (A approximately = T; and G approx. = C). This was known as Chargaffs Rules.NOTE: All these discoveries were before Watson & Crick discovered the double helix structure of DNA.

Structure of a DNA strandDNA is composed of nucleotides ( 5 carbon sugar, phosphate & a nitrogenous base (A,T,C,G). Phosphate of one nucleotide is attached to the sugar of the next nucleotide.

Fig. 16.5 (a) The Double Helix Structure of DNA.Adenine (A) is always paired with Thymine (T) & Guanine (G) is always paired with Cytosine (C). The nitrogenous bases are held together with Hydrogen bonds (weak).We even know the distances between steps of the DNA rungs. Whats a nm?

Notice the strands are oriented in opposite directions.This entire structure was worked out by Watson & Crick in 1953 with help from Rosalind Franklins x- ray diffraction photo of DNA

Base Pairing in DNA.A & G are double ring compounds called Purines. T & C are single ring compounds called Pyrimidines.Each rung of DNA is made of a Purine attached to a Pyrimidine. Held together by H bonds.

The SEMICONSERVATIVE MODEL - DNA replication model

Meselson & Stahl tested the three hypothesis's on DNA replication

Beginnings of how DNA Replicates.Elongation of DNA at a replication fork is catalyzed by a enzyme called DNA polymerase.Rate of elongation in humans is approx.50/sec.

Adding a Nucleotide:A similar molecule to ATP (NTP) is used to link the new nucleotide to the proper position.The enzyme that catalyzes the reaction is DNA POLYMERASE.

THE TWO STRAND OF DNA ARE ANTIPARALLELKnow: Where the 5 & 3 end are.PROBLEM: DNA polymerase can ONLY add nucleotides to the free 3 end of a growing DNA strand.So..A new DNA strand can only elongate in the 5 to 3 direction.

SYNTHESIS OF LEADING & LAGGING STRANDS DURING DNA REPLICATION.DNA polymerase is adding new DNA fragments in a 5 to 3 direction continuously along a replication fork, adding to the 3 end. Lagging strand is synthesized in segments called Okazaki fragments. DNA ligase joins the fragments into a single DNA strand.Okazaki fragments are about 100 -200 nucleotides long in eukaryotes.

PRIMING DNA SYNTHESISDNA polymerase cannot initiate a polynucleotide strand; it can only add to the 3 end of an already-started strand. The primer is a short segment of RNA synthesized by the enzyme primase. Each primer is eventually replaced by DNA.

FIG. 16.15 - THE MAIN PROTEINS OF REPLICATION & THEIR FUNCTIONS.DNA must also be able to form complementary base pairs with both DNA & RNA nucleotides. The sequence of nucleotides will be decoded into a sequence to make amino acids into proteins.Replication -> Transcription -> TranslationEnzymes must proofread DNA during its Replication and repair damage in existing DNA.Mismatch Repair fixes mistakes made in DNA. DNA polymerase itself carries out the mismatch repair.Telomeres - special sequences of DNA nucleotides found at the end of the DNA molecule. They do not contain genes. They protect the organisms genes from being eroded through successive rounds of DNA replication.Secret to aging? http://www.youtube.com/watch?v=J9QApCHsrJk&feature=related

Image of Telomere squeneces (yellow) on chromosomes

Chapter 17 From Gene to ProteinTranscription - the synthesis of mRNA (messenger RNA) under the direction of DNA. This is a code to make a polypeptide (protein). This is also the synthesis of any RNA from DNA.Translation - the actual synthesis of a polypeptide (which occurs at the ribosomes.) The difference in Eukaryotic & Prokaryotic cells.Gene to RNA to Protein.Basics of the Genetic Code:1. There is a total of 20 amino acids possible in any protein.2. 3 Nucleotides on mRNA code for an amino acid. This is called the triplet code.3. Only one strand of DNA is transcribed into mRNA. This strand is called the TEMPLATE strand. The other strand is called the complementary strand.4. All Translation & Transcription occur in a 3 to 5 direction.5. The mRNA is in triplet bases called CODONS.

mRNA is only a single helix & that Uracil (U) is a substitute for Thymine (T).The number of nucleotides making up a genetic message must be 3 times the number of amino acids making up the protein. EXAMPLE - 4 amino acids = 12 nucleotides.Amino Acids are connected by polypeptide bonds.

Learn to read this!!!AUG codon is a start codon & the amino acid Methionine (Met). Start Codon begins the sentence & UAA,UAG & UGA = no amino acid but stops the amino acid chain (read in a 5 to 3 direction)= STOP CODON, like the period at the end of a sentence

Fig. 17.6 The Stages of Transcription1. RNA binds to the promoter region of DNA (several dozen nucleotides upstream from the transcription startpoint).2. RNA moves downstream from promoter, unwinding DNA & elongating RNA at the 3 end (5 to 3 direction).

3. RNA polymerase transcribes a terminator (this sequence of nucleotides along DNA signals the end of transcription unit.)4. Eventually RNA is released & the polymerase moves from DNA.5. Prokaryotes - RNA transcript immediately used to make protein.6. Eukaryotes - mRNA will undergo additional processing.Progresses at about 60 nucleotides/sec in Eukaryotes.

RNA Processing 1st step:Enzymes modify 2 ends of a eukaryote pre-mRNA molecule.Cap made of modified guanosine triphosphate added to the 5 end of RNA.A Poly(A) tail consiting of 200 adenine nucleotides attached to 3 end. ( may helps export mRNA from the nucleus.)***Role of Cap and Tail - protect RNA from degradation**** The leader, trailer & termination signal. Leader & trailer are not translated.

RNA processing (splicing).Pre-mRNA - Exons (Expressed sequence) are keep & the Introns (Intervening sequence) are removed (both by enzymes). Exons are then spliced together. We now have the processed RNA ready to leave the nucleus & go to the ribosome for translation.

Translation - Basic Concept:1) tRNA picks up amino acids & transport them to the ribosome 2) Each tRNA has an anticodon (3 letters) that pick up one of the twenty amino acids.3) When the tRNAs deliver their amino acid, they add them to a growing polypeptide chain. tRNAs are now available to pick up another amino acid to repeat the process.4) New Polypeptide chain added in the 5 to 3 direction.

The Anatomy of a Ribosome:Ribosomes are made of 2 subunits each made of many molecules or rRNA (ribosomal RNA) and proteins.The sites on the ribosome: (1) P site - holds the tRNA attached to the growing polypeptide (2) A site - holds the tRNA carrying the amino acid to be added to the polypeptide chain(3) Discharged tRNA leaves via the E site.Peptide bonding between amino acids maintains the shape of tRNA.

Fig. 17.15 Initiation of Translation1. Small ribosomal subunit binds to molecule of mRNA.2. Initiator tRNA with anticodon UAC base-pairs with the start codon, AUG carrying the amino acid Met.3. A large ribosomal unit arrives & completes the initiation complex.4. Initiator tRNA is in the P site. A site is available to tRNA carrying the next amino acid.5. Proteins called initiation factors bring translation components together. GTP provides the energy for all this.

GTP & proteins called elongation factors needed to drive this process.

Termination of Translation1. When ribosome reaches a termination codon on mRNA, the (A) site of ribosome accepts a protein called a release factor instead of tRNA.2. Release factor hydrolyzes the bond between tRNA in the P site & the last amino acid of the chain. This frees the polypeptide from the ribosome.3. The 2 ribosomal subunits dissociate

Fig. 17.18 PolyribosomesA. An mRNA molecule is generally translated together with several ribosomes in clusters called polyribosomes.B. This enables a single mRNA to make many copies of a polypeptide simultaneously.Proteins can be chemically modified by attachment of sugars, lipids, phosphate groups etc.Example: Enzymes may remove leading amino acids from a chain. Sometimes several proteins will join together to allow them to function or one protein may split into several proteins.Proteins formed here are only the primary structure & must develop a secondary, tertiary, or even Quaternary structure.

Transcription & Translation in Bacteria:Bacteria (Prokaroytes) have no nucleus, so mRNA does not need to move through the membrane to the ribosome. Streamlined operations here - Transcription & Translation can be occurring at the same time.3. There is not RNA processing in bacteria. (All exons).MUTATIONS: Changes in the genetic code of DNA.Point Mutations: Chemical changes in just one or a few base pairs in a single gene. If a point mutation occurs in a gamete or cells giving rise to them, it could be transmitted to offspring & future generations.TYPES OF MUTATIONS:1. Base-pair substitution - replacement of one nucleotide & its partner in the complementary DNA strand with another pair of nucleotides.Some substitutions are silent mutations since genetic code is redundant, there may be no change in the amino acid coded for.EXAMPLE: CCG mutated to CCA would make mRNA GGC become GGU which is still glycine.2. Missense Mutation - altered codon still codes for an amino acid & makes sense although not necessarily the RIGHT sense. (Make a protein, just not the correct one)3. Nonsense mutation - Alterations that change an amino acid code to a stop codon. Almost always leads to a nonfunctional protein.4. Insertions & deletions are additions or losses of one or more nucleotide pairs in a gene.a. Note this can cause missense or nonsense. Where the amino acid is incorrect in a chain can be important or not.Frameshift mutation - alters reading frame of message (# of nucleotides inserted or deleted is not a multiple of 3. a. ( the big cat) remove the h = teb igc at_.) This will make all amino acids downstream from this incorrect.What can cause Mutations?:Mutagens - Physical & chemical agents that cause mutations or increase the mutation rate.Examples - X-rays, Radiation, UV light, chemicals (pesticides, radon), Viruses & Bacteria