Midterm Final Review Part II. Somatic CellsGametes Body cells Diploid (2n): 2 of each type of...
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Transcript of Midterm Final Review Part II. Somatic CellsGametes Body cells Diploid (2n): 2 of each type of...
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Midterm Final Review
Part II
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Somatic Cells Gametes• Body cells• Diploid (2n): 2 of each
type of chromosome• Divide by mitosis
• Humans: 2n = 46
• Sex cells (sperm/egg)• Haploid (n): 1 of each
type of chromosome• Divide by meiosis
• Humans: n = 23
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Phases of the Cell Cycle
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Phases of the Cell Cycle The mitotic phase alternates with interphase:
G1 S G2 mitosis cytokinesis Interphase (90% of cell cycle)G1 Phase: cell grows and carries out normal functionsS Phase: duplicates chromosomesG2 Phase: prepares for cell division M Phase (mitotic)Mitosis: nucleus dividesCytokinesis: cytoplasm divides
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Mitosis: Prophase Metaphase Anaphase Telophase
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Cell Cycle Control System• Checkpoint = control point where stop/go signals
regulate the cell cycle
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Major Checkpoints1.1. GG11 checkpoint checkpoint (Most important!)
– Controlled by cell size, growth factors, environment– “Go” completes whole cell cycle– “Stop” cell enters nondividing state (G0 Phase)
• Nerve, muscle cells stay at G0; liver cells called back from G0
2.2. GG22 checkpoint checkpoint• Controlled by DNA replication completion, DNA mutations, Controlled by DNA replication completion, DNA mutations,
cell sizecell size
3.3. M-spindle (Metaphase) checkpointM-spindle (Metaphase) checkpoint– Check spindle fiber (microtubule) attachment to chromosomes
at kinetochores (anchor sites)
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Internal Regulatory Molecules
• Kinases (cyclin-dependent kinase, Cdk): protein enzyme controls cell cycle; active when connected to cyclin
• Cyclins: proteins which attach to kinases to activate them; levels fluctuate in the cell cycle
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Internal Regulatory Molecules
MPF = maturation-promoting factor• specific cyclin-Cdk complex which allows cells
to pass G2 and go to M phase
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• Growth Factor: proteins released by other cells to stimulate cell division
• Density-Dependent Inhibition: crowded cells normally stop dividing; cell-surface protein binds to adjoining cell to inhibit growth
• Anchorage Dependence: cells must be attached to another cell or ECM (extracellular matrix) to divide
External Regulatory Factors
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Cancer Cells
Cancer: disorder in which cells lose the ability to control growth by not responding to regulation.
• multistep process of about 5-7 genetic changes (for a human) for a cell to transform
• loses anchorage dependency and density-dependency regulation
Normal Cells Cancer Cells
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Cancer cells• Some have abnormal #’s of chromosomes
Karyotype of Metastatic Melanoma
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Types of Reproduction
ASEXUAL• Produces clones
(genetically identical)• Single parent• Little variation in
population - only through mutations
• Fast and energy efficient• Eg. budding, binary
fission
SEXUAL• Meiosis produces
gametes (sex cells)• 2 parents: male/female• Lots of
variation/diversity• Slower and energy
consumptive• Eg. humans, trees
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Homologous Chromosomes in a Somatic Cell
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Meiosis = reduction divisionMeiosis = reduction division
• Cells divide twicetwice• Result: 4 daughter
cells, each with half as many chromosomes as parent cell
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Events Unique to Meiosis I (not in mitosis)
1. Prophase I: Synapsis and crossing over
2. Metaphase I: pairs of homologous chromosomes line up on metaphase plate
3. Anaphase I: homologous pairs separate sister chromatids still attached at centromere
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Sources of Genetic Variation:1. Crossing Over
– Exchange genetic material
– Recombinant chromosomes
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Sources of Genetic Variation:2.Independent Assortment of Chromosomes
– Random orientation of homologous pairs in Metaphase I
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Sources of Genetic Variation:3. Random Fertilization
– Any sperm + Any egg– 8 million X 8 million = 64 trillion combinations!
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Mitosis Meiosis Both are divisions of cell nucleus
• Somatic cells• 1 division• 2 diploid daughter cells• Clones• From zygote to death• Purpose: growth and repair• No synapsis, crossing over
• Gametes• 2 divisions• 4 haploid daughter cells• Genetically different-less than
1 in 8 million alike• Females before birth follicles
are formed. Mature ova released beginning puberty
• Purpose: Reproduction
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MENDELMENDEL’’S PRINCIPLESS PRINCIPLES
1. Alternate version of genes (allelesalleles) cause variations in inherited characteristics among offspring.
2. For each character, every organism inherits one allele from each parent.
3. If 2 alleles are different, the dominantdominant allele will be fully expressed; the recessiverecessive allele will have no noticeable effect on offspring’s appearance.
4.4. Law of SegregationLaw of Segregation: the 2 alleles for each character separate during gamete formation.
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– dominant (P), recessive (p)• homozygous = 2 same alleles (PP or pp)• heterozygous = 2 different alleles (Pp)– Phenotype: expressed physical traits– Genotype: genetic make-up
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TestcrossTestcross: : determine if dominant trait is homozygous or heterozygous by crossing with recessive (pp)
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Law of Independent Assortment:Law of Independent Assortment:• Each pair of alleles segregates (separates) independently
during gamete formation• Eg. color is separate from shape
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Extending Mendelian GeneticsThe relationship between genotype and phenotype is rarely simple
Complete Dominance: heterozygote and homozygote for dominant allele are indistinguishable•Eg. YY or Yy = yellow seed
Incomplete Dominance: F1 hybrids have appearance that is between that of 2 parents•Eg. red x white = pink flowers
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Blood Typing
Phenotype(Blood Group) Genotype(s)
Type A IAIA or IAi
Type B IBIB or IBi
Type AB IAIB
Type O ii
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Mendelian Inheritance in Humans
Pedigree: diagram that shows the relationship between parents/offspring across 2+ generations
Woman = Man = Trait expressed:
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Sex-linked genes
• Sex-linked gene on X or Y• Females (XX), male (XY)
– Eggs = X, sperm = X or Y• Fathers pass X-linked genes to daughters, but not
sons• Males express recessive trait on the single X
(hemizygous)• Females can be affected or carrier
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X-InactivationX-InactivationBarr body = inactive X chromosome; regulate gene dosage in females during embryonic development
• Cats: allele for fur color is on X
• Only female cats can be tortoiseshell or calico.
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Genetic Recombination: production of offspring with new combo of genes from parents
• If offspring look like parents parental types• If different from parents recombinants
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Linked genes: located on same chromosome and tend to be inherited together during cell division
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Crossing over: explains why some linked genes get separated during meiosis
• the furtherfurther apart 2 genes on same chromosome, the higher higher the probability of crossing over and the higherhigher the recombination frequency
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Calculating recombination frequency
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Linkage Map: genetic map that is based on % of cross-over events
• 1 map unit = 1% recombination frequency• Express relative distances along chromosome• 50% recombination = far apart on same chromosome
or on 2 different chromosomes
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NondisjunctionNondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II
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NondisjunctionNondisjunction
• Aneuploidy: incorrect # chromosomes– Monosomy (1 copy) or Trisomy (3 copies)
• Polyploidy: 2+ complete sets of chromosomes; 3n or 4n– Rare in animals, frequent in plants
A tetraploid mammal. Scientists think this species may have arisen when an ancestor doubled its chromosome # by errors in mitosis or meiosis.
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Chromosomal Mutations
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Chromosomal Mutations
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Exceptions to Mendelian inheritance• Genomic imprinting: phenotypic effect of gene
depends on whether from M or F parent• Silence genes by adding methyl groups to DNA
(methylation)
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Exceptions to Mendelian inheritance
• Some genes located in organelles– Mitochondria, chloroplasts,
plastids– Contain small circular DNA
• Mitochondria = maternal inheritance (eggs)
Variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from
the maternal parent.
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Frederick Griffith (1928)
Conclusion: living R bacteria transformed into deadly S bacteria by unknown, heritable substance
Avery, McCarty, McLeod (1944)– Tested DNA, RNA, & proteins in heat-killed
pathogenic bacteria– Discovered that the transforming agent was
DNA
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Hershey and Chase (1952)
• Bacteriophages: virus that infects bacteria; composed of DNA and protein
Protein = radiolabel SProtein = radiolabel SDNA = radiolabel PDNA = radiolabel P
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Edwin Chargaff (1947)
Chargaff’s Rules:• DNA composition varies
between species• Ratios:
– %A = %T and %G = %C
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Rosalind Franklin (1950’s)
• Worked with Maurice Wilkins• X-ray crystallography = images of DNA• Provided measurements on chemistry of DNA
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James Watson & Francis Crick (1953)
• Discovered the double helix by building models to conform to Franklin’s X-ray data and Chargaff’s Rules.
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Structure of DNA
DNA = double helix– “Backbone” = sugar +
phosphate– “Rungs” = nitrogenous
bases
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Structure of DNA
Nitrogenous Bases– Adenine (A)– Guanine (G)– Thymine (T)– Cytosine (C)
• Pairing:– purine + pyrimidine– A = T– G Ξ C
purine
pyrimidine
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Structure of DNA
Hydrogen bonds between base pairs of the two strands hold the molecule together like a zipper.
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Structure of DNA
Antiparallel: one strand (5’ 3’), other strand runs in opposite, upside-down direction (3’ 5’)
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DNA Comparison
• Double-stranded• Circular• One chromosome• In cytoplasm• No histones• Supercoiled DNA
• Double-stranded• Linear• Usually 1+ chromosomes• In nucleus• DNA wrapped around histones
(proteins)• Forms chromatin
Prokaryotic DNA Eukaryotic DNA
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Replication is semiconservative
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DNA Replication
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Leading strand vs. Lagging strand
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Proofreading and Repair
• Mismatch repair: special enzymes fix incorrect pairings
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Nucleotide Excision Repair
• DNA polymerases proofread as bases added
• Nucleotide excision repair:– Nucleases cut damaged
DNA– DNA poly and ligase fill in
gaps
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Telomeres: repeated units of short nucleotide sequences (TTAGGG) at ends of DNA
• Telomeres “cap” ends of DNA to postpone erosion of genes at ends (TTAGGG)
• Telomerase: enzyme that adds to telomeres– Eukaryotic germ cells, cancer cells
Telomeres stained orange at the ends of mouse chromosomes
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Telomeres & Telomerase
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1 Gene = 1 polypeptide or RNA molecule
A Summary of Protein Synthesis
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1 gene = 1 polypeptide or 1 RNA molecule1 gene = 1 polypeptide or 1 RNA molecule
DNA RNA• Nucleic acid composed of
nucleotides• Double-stranded• Deoxyribose=sugar• Thymine• Template for individual
• Nucleic acid composed of nucleotides
• Single-stranded• Ribose=sugar• Uracil• Helper in steps from DNA to
protein• Types: mRNA, pre-mRNA,
tRNA, rRNA, snRNA, srpRNA, siRNA
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The Genetic Code
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The Genetic Code
64 different codon combinations
This code is universal: all life forms use the same code.
Reading frame: groups of 3 must be read in correct groupings
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1. Initiation
Transcription factors must recognize TATA box before RNA polymerase can bind to DNA promoter
Eukaryotes:TATA box = DNA sequence (TATAAAA) upstream from promoter
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2. Elongation
• RNA polymerase adds RNA nucleotides to the 3’ end of the growing chain (A-U, G-C)
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3. Termination
RNA polymerase transcribes a terminator sequence in DNA, then mRNA and polymerase detach.
It is now called pre-mRNA for eukaryotes.
Prokaryotes = mRNA ready for use
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Additions to pre-mRNA:• 5’ cap (modified guanine) and 3’ poly-A tail (50-520
A’s) are added
• Help export from nucleus, protect from enzyme degradation, attach to ribosomes
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RNA Splicing
• Pre-mRNA has introns (noncoding sequences) and exons (codes for amino acids)
• Splicing = introns cut out, exons joined together
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RNA Splicing
• SpliceosomeSpliceosome = snRNP + Proteins• Remove intronsintrons and join exonsexons• RibozymeRibozyme = RNA acts as enzyme
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Why have introns?• Some regulate gene activity
• Alternative RNA SplicingAlternative RNA Splicing: produce different combinations of exons– One gene can make more
than one polypeptide!– 20,000 genes 100,000
polypeptides
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Translation:1. Initiation
• Small subunit binds to start codon (AUG) on mRNA• tRNA carrying Met attaches to P site• Large subunit attaches
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2. Elongation
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3.Termination• Stop codon reached and translation stops• Release factor binds to stop codon;
polypeptide is released• Ribosomal subunits dissociate
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Protein Folding• During synthesis, polypeptide chain coils and
folds spontaneously• Chaperonin: protein that helps polypeptide
fold correctly
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Types of Ribosomes• Free ribosomes: synthesize proteins that stay
in cytosol and function there• Bound ribosomes (to ER): make proteins of
endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vacuoles, plasma membrane) & proteins for secretion– Uses signal peptide to target location
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Cellular “Zip Codes”• Signal peptide: 20 AA at leading end of
polypeptide determines destination• Signal-recognition particle (SRP): brings
ribosome to ER
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The Central Dogma
Mutations happen here
Effects play out here
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Mutations = changes in the genetic material of a cell
• Large scale mutations: chromosomal; always cause disorders or death– nondisjunction, translocation, inversions,
duplications, large deletions• Point mutations: alter 1 base pair of a gene
1. Base-pair substitutions – replace 1 with another• Missense: different amino acid• Nonsense: stop codon, not amino acid
2. Frameshift – mRNA read incorrectly; nonfunctional proteins
• Caused by insertions or deletions
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Sickle-Cell Disease = Point Mutation
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Prokaryotes vs. Eukaryotes
Prokaryotes Eukaryotes• Transcription and
translation both in cytoplasm
• DNA/RNA in cytoplasm• RNA poly binds directly to
promoter• Transcription makes mRNA
(not processed)• No introns
• Transcription in nucleus; translation in cytoplasm
• DNA in nucleus, RNA travels in/out nucleus
• RNA poly binds to TATA box & transcription factors
• Transcription makes pre-mRNA RNA processing final mRNA
• Exons, introns (cut out)