Cell Division to DNA Replication PDF Lecture Notes

LECTURE NOTES (2ND QUARTER) BY SAFHE MITOSIS ➔ Centromere: region where the sister chromatids are attached, between arms of CELL...

LECTURE NOTES (2ND QUARTER) BY SAFHE MITOSIS ➔ Centromere: region where the sister chromatids are attached, between arms of CELL DIVISION chromatid ➔ Two chromatid separate and move into new ➔ Reproduction of cells nuclei, one at each end forming individual ➔ Asexual Reproduction: Prokaryotic - chromosomes reproducing by giving rise to a new organism ➔ Mitosis: division of genetic material ➔ Sexual Reproduction: Eukaryotic - allows ➔ Cytokinesis: division of the cytoplasm organisms to develop from a single-cell ➔ Function in renewal and repair, replacing cells that die from accidents or normal tear CELL CYCLE ➔ A critical function is the replication of identical genetic material (DNA) ➔ Walther Flemming: behavior of ➔ Growth & development, and tissue renewal chromosomes, mitosis and cytokinesis ➔ Sequence of events in the life of af a cell CELLULAR ORGANIZATION from parent cell to its division into two ➔ Genome: genetic information (Prokaryote: ➔ Interphase and Mitotic phase single DNA molecule, Eukaryote: multiple DNA molecules) INTERPHASE ➔ Chromosomes: structures where DNA ➔ Cell is not dividing, 90% of the cycle molecules are packed ➔ Cellular metabolic activity is high, ➔ Chromatin: complex of DNA and proteins chromosomes and organelles are duplicated, (maintain structure of chromosomes and cell size is increased controls activity of genes) ➔ G1 - first gap, cell grows, metabolic activity, ➔ Somatic Cells: non-reproductive cells, each unduplicated chromosomes contain 46 chromosomes, 2 sets of 23 ➔ S - synthesis, duplication of chromosomes chromosomes (one set from each parent) ➔ G2 - second gap, cell grows more, metabolic ➔ Gametes: reproductive cells, half as many activity, and completes preparation for cell chromosomes as somatic cells (sperm and division, centrosomes (2 centrioles) egg cells) duplicate DISTRIBUTION OF CHROMOSOMES MITOTIC PHASE ➔ Each chromosome is in the form of a ➔ Mitosis: distribution of chromosomes into chromatin fiber two daughter nuclei ➔ Sister Chromatids: two copies of duplicated ➔ Cytokinesis: division of cytoplasm, producing chromosome attached by proteins called two cells cohesins 1  ➔ Once bacteria reaches twice it size, proteins MITOSIS causes its plasma membrane to pinch inward dividing parent cell ➔ Bacteria don’t have visible mitotic spindle or ➔ Prophase: nuceloli disappears, mitotic microtubules spindle forms, chromatin condenses centrosomes move away from each other, EVOLUTION OF MITOSIS asters form (short microtubules and ➔ Mitosis evolved from simpler prokaryotic centrosomes) mechanisms of cell reproduction ➔ Prometaphase: microtubules extends, ➔ Intermediate stages: dinoflagellates, kinetochore forms at the centromere, diatoms and yeasts kinetochore microtubulues (microtubules attached to kinetochore), non-kinetochore microtubules lengthen the cell CELL CYCLE CONTROL SYSTEM ➔ Metaphase: centrosomes at opposite poles of the cell, chromosomes align at metaphase ➔ Set of molecules in the cell that both plate triggers and coordinates key events in the ➔ Anaphase: shortest stage, chromatids part cell cycle from each other ➔ Checkpoint: control point where stop and go ➔ Telophase: nuclear envelope arises, nucleoli signals can regulate the cycle reappears, mitosis is complete CELL CYCLE CLOCK CYTOKINESIS ➔ Regulatory proteins: protein kinases ➔ Division of cytoplasm (enzymes that activate/inactivate other ➔ Formation of cleavage furrow, contracting proteins by phosphorylating) and cyclins ring due to interaction of actin (cyclically fluctuating concentration) microfilaments and myosin (cleavage in ➔ Cyclin-dependent kinases (Cdks): kinase that animals) is active when attached to a cyclin ➔ Vesicles from golgi apparatus move to ➔ Maturation-promoting factor (MPF): middle of cell forming a cell plate (cell wall required for a cell to progress from in plants) interphase to mitosis MITOTIC SPINDLE CHECKPOINTS ➔ Begins to form in cytoplasm during prophase ➔ G1 checkpoint: most important, determines ➔ Elongate by incorporating subunits of whether it will pass other checkpoints protein (polymerization) ➔ G2 checkpoint: prevents cells with damaged ➔ Shorten by losing subunits of protein DNA (depolymerize) ➔ M checkpoint: chromosomes are attached to ➔ Start at the centrosome where centrioles spindle fibers (not essential in cell division) are located ➔ G0 phase: nondividing state ➔ Growth factor: protein that must be present BINARY FISSION in the extracellular environment, stimulates ➔ Asexual reproduction of single-celled other cells to divide eukaryotes ➔ Density Dependent Inhibition: phenomenon ➔ Involves mitosis in eukaryotes but not where crowded cells stop dividing prokaryotes ➔ Anchoarage Dependence: requirement of a ➔ DNA is replicated at the origin of replication, cell to be attached to something in order to producing two origins initiate cell division 2  ➔ Beingin Tumor: remain at the site of tumor’s LOSS OF CELL CYCLE origin ➔ cancer cells do not need growth factors in ➔ Malignant Tumor: cancerous tumor, capable their culture medium to grow and divide of invading and surviving in new sites ➔ Transformation: cell acquires ability to divide ➔ Metastasis: spread of cancer cells to indefinitely like division of cancer cells locations distant from their original site SEXUAL LIFE CYCLES AND MEIOSIS ➔ Heredity: transmission of traits from one ➔ X and Y chromosomes are called se generation to the next chromosomes while the rest are autosomes ➔ Variation: differences between same species ➔ 46 chromosomes in our somatic cells are ➔ Genetics: study of both heredity and actually two sets of 23 chromosomes—a inherited variation maternal set (from our mother) and a ➔ Genes: discrete unit of hereditary paternal set (from our father) information ➔ Diploid cell: 2n in somatic cells (46 ➔ Humans have 46 chromosomes in their chromosomes) somatic cells ➔ Haploid cell: n in gametes (22 autosomes ➔ Locus: gene’s specific location on a and 1 sex chromosomes) chromosome ➔ Human life cycle begins when a haploid sperm fuses with a haploid egg ASEXUAL VS. SEXUAL REPRODUCTION ➔ Fertilization: union of gametes resulting in a ➔ Asexual: produce offsprings that are exact zygote genetic copies of parent, may occur through ➔ Gametes develop from germ cells present in mitosis, gives rise to a clone gonads (ovaries and testes) ➔ Sexual: two parents give rise to offspring that have unique combinations of genes ALTERNATION OF GENERATIONS inherited ➔ Plants and algae ➔ Sporophyte undergoes meiosis forming LIFE CYCLE spores, spores undergo mitosis forming gametophyte, gametophytes undergo mitosis again forming gametes, two gametes ➔ generation-to-generation sequence of stages fuse to form a diploid zygote in the reproductive history of an organism ➔ either haploid or diploid cells can divide by ➔ In humans, each somatic cell has 46 mitosis, depending on the type of life cycle chromosomes ➔ Only diploid cels can undergo meiosis ➔ Karyotype: ordered display of two chromosomes of each of 23 types ➔ Homologous chromosomes: two chromosomes of a pair have the same MEIOSIS length, centromere position, and staining pattern ➔ Preceded by interphase ➔ human females have a homologous pair of X ➔ Two consecutive cell divisions chromosomes (XX) ➔ Produces 4 daughter cells, each with half as ➔ males have one X and one Y chromosome many chromosomes as parent cell (one set) 3 STAGES OF MEIOSIS MITOSIS VS. MEIOSIS ➔ Meiosis I ➔ Meiosis produces 4 cells reducing the ◆ Prophase I: duplicated homologous number of chromosomes from diploid to chromosomes pair up and exchange haploid (cells differ from parent cell and segments, crossing over occurs at each other) chiasmata, microtubules attach at ➔ mitosis produces two cells and conserves kinetochores the number of chromosome sets (cells are ◆ Metaphase I: chromosomes line up genetically identical to parent) by homologous pairs at the ➔ Synapsis and crossing over occurs only in metaphase plate, independent meiosis assortment occur ➔ Alignment of homologous chromosomes at ◆ Anaphase I: two homologous metaphase plate in meiosis 1 while chromosomes separate individual chromosomes in mitosis ◆ Telophase I: two haploid cells form ➔ Separation of homologs occurs at anaphase each with two sister chromatids 1 while sister chromatids separate at mitosis ◆ Cytokinesis: division of cytoplasm ➔ Meiosis II GENETIC VARIATION ◆ Prophase II: spindle apparatus forms, ◆ Metaphase II: sister chromatids position at metaphase plate (not ➔ behavior of chromosomes during meiosis genetically identical due to crossing and fertilization is responsible for most of over) the variation that arises in each generation ◆ Anaphase II: sister chromatids ➔ Independent Assortment separate ◆ random orientation of pairs of ◆ Telophase II: four haploid cells form homologous chromosomes at and are genetically distinct from metaphase of meiosis I each other and the parent cell ◆ Each pair may orient with either its ◆ Cytokinesis: division of cytoplasm maternal or paternal homolog closer to a given pole CROSSING OVER AND SYNAPSIS DURING ➔ Crossing Over PROPHASE I ◆ each chromosome in a gamete is ➔ DNA of nonsister chromatids are broken exclusively maternal or paternal in down origin ➔ Zipper like protein complex, synaptonemal ◆ crossing over produces recombinant complex, attaches one homolog to the other chromosomes ➔ Synapsis: DNA breaks are closed, each ◆ one to three crossover events occurs broken end is joined to the corresponding per chromosome pair segment of the non-sister chromatid, ◆ combining DNA inherited from two producing crossovers parents into a single chromosome ➔ homologs remain attached because sister ➔ Random Fertilization chromatids are still held together by sister ◆ random nature of fertilization adds chromatid cohesion, even though some of to the genetic variation arising from the DNA may no longer be attached to its meiosis original chromosomal DNA 4  NUCLEIC ACIDS AND INHERITANCE ◆ base can be adenine (A), thymine (T), DNA AS GENETIC MATERIAL guanine (G), or cytosine © ◆ Showed evidence of molecular diversity among species (A base was EVIDENCE 32.8% in sea urchins but 24.7% in E. ➔ T.H. Morgan’s group showed that genes exist coli) as part of chromosomes (DNA and protein) ◆ number of adenines approximately ➔ Frederick Griffith (1928) was developing a equaled the number of thymines vaccine against pneumonia ◆ number of guanines approximately ◆ Streptococcus pneumoniae, a equaled the number of cytosines bacterium that causes pneumonia in ➔ Chargaff’s Rules mammals ◆ DNA base composition varies ◆ Mixed the cells remaining from the between species pathogenic bacteria ◆ or each species, the percentages of A (disease-causing) to the and T bases are roughly equal, as are non-pathogenic (harmless) strain those of G and C bases ◆ Some of the living non-pathogenic cells became pathogenic and this STRUCTURAL MODEL OF DNA was inherited by all the descendants ➔ Linus Pauling (California Institute of of the transformed bacteria Technology) ◆ Transformation: change in genotype ➔ Maurice Wilkins and Rosalind Franklin and phenotype due to the (King’s College in London) assimilation of external DNA by a cell ➔ James Watson and Francis Crick ➔ Bacteriophages: viruses that infect bacteria ◆ Watson saw an X-ray diffraction ➔ Virus: little more than DNA (or sometimes image of DNA produced by Wilkins’s RNA) enclosed by a protective coat, which is accomplished colleague Rosalind often simply protein (capsid), must infect a ◆ spots in the image were produced by cell and take over the cell’s metabolic X-rays that were diffracted as they machinery to produce passed through aligned fibers of ➔ Alfred Hershey and Martha Chase purified DNA ◆ DNA is the genetic material of a ◆ the photo that Wilkins showed him phage known as T2 (phages that confirmed that DNA was helical in infect Escherichia coli) shape ◆ used a radioactive isotope of sulfur ◆ pattern in this photo implied that the to tag protein in one batch of T2 helix was made up of two strands, ◆ Used a radioactive isotope of contrary to a three-stranded model phosphorus to tag DNA in a second proposed by Linus Pauling batch ◆ Franklin’s X-ray data indicated that the helix makes one full turn every ADDITIONAL EVIDENCE 3.4 nm along its length ➔ Erwin Chargaff ➔ Purines: Adenine/Gunanine, nitrogenous ◆ DNA is a polymer of nucleotides bases with two organic rings (nitrogenous (nitrogen-containing) ➔ Pyrimidines: Cytosine/Thymine, single ring base, a pentose sugar called deoxyribose, and a phosphate group) 5  ➔ Pairing a purine with a pyrimidine is the only ➔ Replication of DNA then proceeds in both combination that results in a uniform directions until the entire molecule is copied diameter for the double helix ➔ Multiple replication bubbles form and ➔ Adenine can form two hydrogen bonds with eventually fuse thymine ➔ Guanine forms three hydrogen bonds with PARTS cytosine ➔ Replication fork at the end of each replication bubble; y-shaped region where PROTEIN IN DNA REPLICATION AND REPAIR parental strands are unwound ➔ Helicases: enzymes that untwist the double helix at replication forks ➔ nucleic acids are unique in their ability to ➔ Single-strand binding proteins: bind to the dictate their own replication from unpaired DNA strands, keeping them from monomers re-pairing ➔ specific complementary pairing of ➔ Topoisomerase: helps relieve this strain by nitrogenous bases in DNA has a functional breaking, swiveling, and rejoining DNA significance strands ➔ Primer: RNA chain synthesized by primase BASE PAIRING TO A TEMPLATE STRAND ➔ DNA polymerase: catalyzes the addition of ➔ prior to duplication the hydrogen bonds are each monomer to the growing end of a DNA broken, and the two chains unwind and strand by a condensation reaction in which separate two phosphate groups are lost as a molecule ➔ each chain then acts as a template for the of pyrophosphate formation on to itself of a new companion ➔ dATP: adenine nucleotide used to make DNA chain, so that eventually we shall have two pairs of chains ANTIPARALLEL ELONGATION ➔ two strands are complementary; each stores ➔ the two strands of DNA in a double helix are the information necessary to reconstruct the antiparallel other ➔ the two new strands formed during DNA ➔ Semiconservative model: each of the two replication must also be antiparallel to their daughter molecules will have one old strand, template strands from the parental molecule, and one newly ➔ Along one template strand, DNA polymerase made strand III can synthesize a complementary strand ➔ Matthew Meselson and Franklin Stahl: continuously by elongating the new DNA in experiment that distinguished difference the mandatory 5' → 3' direction between the conservative, ➔ Leading Strand: new complementary strand semiconservative, and dispersed model synthesized along the template strand ➔ DNA replication is semiconservative towards the replication fork in mandatory 5’ to 3’ direction DNA REPLICATION ➔ Lagging Strand: elongates by means of ➔ most of the process is fundamentally similar Okazaki fragments in 5’ to 3’ direction away for prokaryotes and eukaryotes from replication fork ➔ Begins at sites called origins of replication ➔ Okazaki fragments: after Reiji Okazaki, ➔ Proteins that initiate DNA replication segment of DNA synthesized away from the recognize this sequence and attach to the replication fork DNA, separating the two strands and ➔ RNA primer is made, DNA pol III creates the opening up a replication “bubble” leading strand that elongates along the 5’ to 6  3’ direction with the helicase opening the ◆ prevent the staggered ends of the fork daughter molecule from activating the cell’s systems for monitoring DNA DNA REPLICATION COMPLEX damage ➔ Trombone Model: two DNA polymerase ◆ acts as a kind of buffer zone that molecules, one on each template strand, provides some protection against the “reel in” the parental DNA and extrude organism’s genes shortening newly made daughter DNA molecules ➔ Whether the complex moves along the DNA CHROMOSOME IS MADE OF DNA MOLECULES or whether the DNA moves through the AND PROTEINS complex, either anchored or not, are still open ➔ main component of the genome in most PROOFREADING AND REPAIRING DNA bacteria is a double-stranded, circular DNA ➔ many DNA polymerases proofread each molecule that is associated with specific nucleotide against its template as soon as it proteins is covalently bonded to the growing strand ➔ a eukaryotic chromosome consists of a ➔ Mismatched nucleotides sometimes evade single linear DNA molecule associated with a proofreading by a DNA polymerase large number of proteins ➔ Mismatch repair: uses specific enzymes to ➔ certain proteins cause the chromosome to replace incorrectly paired nucleotides coil and “supercoil,” densely packing it so ➔ these changes in DNA are usually corrected that it fills only part of the cell before they become permanent ➔ Euchromatin: less compacted, more changes—mutations dispersed interphase chromatin ➔ Nuclease: DNA-cutting enzyme; segment of ➔ Heterochromatin: more compacted, the strand containing the damage is cut out denser-appearing ➔ Nucleotide Excision Repair: removes and ➔ The histones bind to each other and to the correctly replaces a damaged segment of DNA to form nucleosomes, the most basic DNA using undamaged strand as a guide units of DNA packing REPLICATING ENDS OF DNA MOLECULES ➔ Telomeres: repetitive DNA at the end of eukaryotic chromosomes EXPRESSION OF GENES ➔ Proteins are the link between genotype and GENES SPECIFY PROTEINS BY TRANSCRIPTION phenotype AND TRANSLATION ➔ Gene expression: process by which DNA directs the synthesis of proteins; includes 2 stages (transcription and translation) ➔ The DNA inherited by an organism leads to specific traits by dictating the synthesis of EVIDENCE FROM METABOLIC DEFECTS proteins and of RNA molecules involved in ➔ Archibald Garrod protein synthesis ◆ suggest that genes dictate phenotypes through enzymes 7  ◆ symptoms of an inherited disease ◆ Messenger RNA: resulting RNA reflect an inability to make a molecule is a faithful transcript of particular enzyme (inborn errors of the gene’s protein-building metabolism) instructions ➔ one gene–one enzyme hypothesis ➔ Translation ◆ cells synthesize and degrade most ◆ synthesis of a polypeptide using the organic molecules via metabolic information in the mRNA pathways ◆ cell must translate the nucleotide ➔ George Beadle and Boris Ephrussi sequence of an mRNA molecule into ◆ each mutation affecting eye color the amino acid sequence of a blocks pigment synthesis at a specific polypeptide step by preventing production of the ◆ Ribosomes: sites of translation enzyme that catalyzes that step ◆ nuclear membranes do not separate (Drosophila) bacterial DNA and mRNA from ➔ George Beadle and Edward Tatum ribosomes and the other ◆ Neurospora crassa: a haploid specie protein-synthesizing equipment; no ◆ They caused mutations in genes by nuclei (bacteria) bombarding Neurospora with X-rays ◆ The presence of a nuclear envelope ➔ Adrian Srb and Norman Horowitz separates transcription from ◆ used a collection of translation in space and time; arginine-requiring mutants to Transcription occurs in the nucleus, investigate the biochemical pathway but the mRNA must be transported for arginine synthesis in Neurospora to the cytoplasm for translation ➔ suggests that each class was blocked at a (eukaryotic) different step in this pathway because ◆ Primary Transcript: initial RNA mutants in that class lacked the enzyme that transcript from gene, pre-mRNA catalyzes the blocked step ➔ Central Dogma: flow of information (DNA to RNA to Protein) by Francis Crick PRODUCTS OF GENE EXPRESSION ➔ proteins that are not enzymes are GENETIC CODE nevertheless gene products ➔ Codons: triplets of nucleotides ➔ Ex: Hemoglobin contains two kinds of ➔ Triplet code: flow of information from gene polypeptide, thus two genes code for this to protein protein ➔ genetic instructions for a polypeptide chain are written in the DNA as a series of BASIC PRINCIPLES OF TRANSCRIPTION AND nonoverlapping, three-nucleotide words TRANSLATION ➔ Template strand: only one of the two DNA ➔ gene does not build a protein directly strands is transcribed ➔ bridge between DNA and protein synthesis is ➔ RNA molecule is synthesized in an the nucleic acid RNA antiparallel direction to the template strand ➔ RNA: contains ribose instead of deoxyribose, of DNA amd follow the base-pairing rule (U uracil rather than thymine, single strand will partner with A instead of T) ➔ Transcription ➔ Coding strand: nontemplate DNA strand ◆ synthesis (production) of RNA using ➔ the number of nucleotides making up a information in the DNA genetic message must be three times the ◆ information is simply transcribed, or number of amino acids in the protein “rewritten,” from DNA to RNA product 8  ➔ AUG: start signal or initiation codon ◆ Eukaryotes: RNA polymerase II ➔ there is redundancy in the genetic code, but transcribes a sequence on the DNA no ambiguity called the polyadenylation signal ➔ Reading frame: triplet grouping during sequence polypeptide synthesis AAUAAA ➔ 3 of the 64 codons function as stop signals pre-mRNA then undergoes (UAA, UAG, UGA) processing TRANSCRIPTION EUKARYOTIC CELLS MODIFY RNA MOLECULAR COMPONENTS ➔ RNA processing: Enzymes in the eukaryotic ➔ RNA polymerase: pries the two strands of nucleus modify pre-mRNA DNA apart and joins together RNA ➔ both ends of the primary transcript are nucleotides complementary to the DNA altered template strand ➔ Initiation: RNA polymerase binds to DNA ALTERATION OF MRNA ENDS ➔ Elongation: RNA elongates from 5’ to 3’ ➔ 3’ end receives a 5/ cap ➔ Termination: RNA transcript is released, ➔ An enzymes adds more adenine forming polymerase detaches poly-A tail ➔ Promoter: DNA sequence where RNA ➔ 5ʹ cap and poly-A tail share several polymerase attaches/start point important functions ➔ Terminator: signals end of transcription ◆ export of the mature mRNA from the (bacteria) nucleus ➔ Transcription unit: stretch of DNA ◆ help protect the mRNA from downstream from the promoter that is degradation transcribed into an RNA molecule ◆ help ribosomes attach to the 5ʹ end ➔ Bacteria: single type of RNA (messenger and of the mRNA ribosomal) ➔ UTRs: untranslated regions for ribosome binding SYNTHESIS OF RNA TRANSCRIPT ➔ Initiation RNA SPLICING ◆ Eukaryotic promoter (TATA box) ➔ portions of the RNA primary transcript ◆ Transcription factors (collection of molecules are removed proteins) bind to TATA box ➔ Introns: noncoding, intervening sequence ◆ More transcription factors and RNA that is removed polymerase II (transcription initiation ➔ Exons: remains in RNA that will be expressed complex) bind to DNA ➔ Spiceosome: complex that accomplishes ➔ Elongation removal of introns ◆ As transcription proceeds forward, ➔ Ribozymes: RNA molecules that function as the newly synthesized RNA molecule enzymes behind the RNA polymerase peels ➔ Alternative RNA splicing: different mRNA away from its DNA template molecules are produced from the same ◆ DNA double helix re-forms primary transcript (depends on which is ➔ Termination treated as exons and introns) ◆ Bacteria: transcribed terminator ➔ Domain: functional region of protein functions as the termination signal 9  ◆ Peptide bond formation TRANSLATION ◆ Translocation ➔ Termination ◆ Elongation continues until a stop ➔ Transfer RNA (tRNA): transfer an amino acid codon in the mRNA reaches the A from the cytoplasmic pool of amino acids to site a growing polypeptide in a ribosome ➔ Anticodon: nucleotide triplet that base-pairs PROTEIN FOLDING AND POST-TRANSLATIONAL with a codon on mRNA (3’ to 5’) MODIFICATIONS ➔ aminoacyl-tRNA synthetase: carries correct ➔ a polypeptide chain begins to coil and fold matching up of tRNA and amino acid spontaneously ➔ Amino acid and correct tRNA enter active ➔ two or more polypeptides that are site of the specific synthase synthesized separately may come together ➔ ATP will catalyze covalent bonding of amino ➔ Certain amino acids may be chemically acid to its tRNA modified by the attachment of sugars, lipids, ➔ Synthase releases tRNA with its amino acid phosphate groups, or other additions ➔ Small subunits and large subunits make up a ➔ Signal peptide: marks polypeptides of functional ribosome when attached to proteins destined for the endomembrane mRNA system or for secretion ➔ Each ribosome has 3 tRNA binding sites: ➔ Polyribosomes: group of ribosomes ◆ Aminoacyl-tRNA Binding Site (A Site): attaching to mRNA to make many copies of a holds the tRNA carrying the next polypeptide simultaneously amino acid to be added ◆ Peptidyl-tRNA Binding Site (P-Site): holds the tRNA carrying the growing MUTATIONS polypeptide chain ◆ Exit Site (E-Site): discharged tRNAs ➔ Mutations: changes to the genetic leave the ribosome from this site. information of a cell ◆ ultimate source of new genes BUILDING A POLYPEPTIDE ◆ responsible for the huge diversity ➔ require protein “factors” that aid in the ➔ Point mutations: changes in a single translation process nucleotide pair of a gene ➔ Initiation ◆ brings together an mRNA, a tRNA TYPES OF SMALL-SCALE MUTATIONS bearing the first amino acid of the ➔ Single nucleotide-pair substitution polypeptide, and the two subunits of ◆ replacement of one nucleotide and a ribosome its partner with another pair of ◆ Small ribosomal subunit binds to nucleotides mRNA, base-pairs with AUG ◆ Silent Mutation: a change in a ◆ Translation Initiation Complex: arrival nucleotide pair may transform one of large ribosomal subunit, hydrolysis codon into another that is translated of GTP into the same amino acid ➔ Elongation ◆ Missense Mutation: change one ◆ amino acids are added one by one to amino acid to another one the previous amino acid ◆ Nonsense Mutation: point mutation ◆ Codon recognition can also change a codon for an 10  amino acid into a stop codon; ead to ➔ Mutagens: physical and chemical agents, nonfunctional proteins interact with DNA in ways that cause ➔ Nucleotide-pair insertions or deletions mutations ◆ additions or losses of nucleotide pairs in a gene CRISPR ◆ have a disastrous effect on the ➔ Gene editing: altering genes in a specific, resulting protein predictable way ◆ Frameshift Mutations: may alter the ➔ CRISPR-Cas9 system reading frame of the genetic ◆ technique for gene editing message, the triplet grouping of ◆ Cas9 is a bacterial protein that helps nucleotides on the mRNA that is read defend bacteria against the viruses during translation that infect them NEW MUTATIONS AND MUTAGENS ➔ Mutations can occur by: ◆ Errors during DNA replication ◆ recombination can lead to nucleotide-pair substitutions, insertions, or deletions 11