Chapter 4 Genetics and Cellular Function Nucleus and nucleic acids Protein synthesis and secretion...
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Transcript of Chapter 4 Genetics and Cellular Function Nucleus and nucleic acids Protein synthesis and secretion...
![Page 1: Chapter 4 Genetics and Cellular Function Nucleus and nucleic acids Protein synthesis and secretion DNA replication and the cell cycle Chromosomes and heredity.](https://reader036.fdocuments.in/reader036/viewer/2022062518/56649e7d5503460f94b7f899/html5/thumbnails/1.jpg)
Chapter 4Genetics and Cellular Function
• Nucleus and nucleic acids
• Protein synthesis and secretion
• DNA replication and the cell cycle
• Chromosomes and heredity
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The Nucleic Acids (medical history)• Discovery of DNA
– named by biochemist Johann Friedrich Miescher (1844-1895) and his student
– isolated an acidic substance rich in phosphorus from salmon sperm
– believed it was heriditary matter of cell, but no real evidence
• Discovery of the double helix– by 1900:components of DNA were known
– by 1953: xray diffraction determined geometry of DNA molecule
– Nobel Prize awarded in 1962 to 3 men: Watson, Crick and Wilkins but not to Rosalind Franklin who died of cancer at 37 from the xray data that provided the answers.
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Organization of the Chromatin
• 46 Molecules of DNA and their associated proteins form chromatin– looks like granular thread
• DNA molecules compacted– coiled around
nucleosomes (histone clusters) like a spool
– twisted into a coil that supercoils itself in preparation for cell division
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Nucleotide Structure
• Nucleic acids like DNA are polymers of nucleotides
• Nucleotides consist of
– sugar• RNA - ribose
• DNA - deoxyribose
– phosphate group– nitrogenous base
• next slide
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DNA Structure: Twisted Ladder
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Nitrogenous Bases
• Purines - double carbon-nitrogen ring– guanine– adenine
• Pyrimidines - single carbon-nitrogen ring– uracil - RNA only– thymine - DNA only– cytosine - both
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Complementary Base Pairing
• Nitrogenous bases form hydrogen bonds
• Base pairs– A-T and C-G
• Law of complementary base pairing– one strand determines
base sequence of other
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Segment of DNA
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DNA Function
• Serves as code for protein (polypeptide) synthesis
• Gene - sequence of DNA nucleotides that codes for one polypeptide
• Genome - all the genes of one person– humans have estimated 35,000 genes– other 97% of DNA is noncoding – either “junk” or
organizational– human genome project completed in 2000
• mapped base sequence of all human genes
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RNA: Structure and Function
• RNA much smaller than DNA (fewer bases)– transfer RNA (tRNA) has 70 - 90 bases– messenger RNA (mRNA) has over 10,000 bases– DNA has over a billion base pairs
• Only one nucleotide chain (not a helix)– ribose replaces deoxyribose as the sugar– uracil replaces thymine as a nitrogenous base
• Essential function– interpret DNA code– direct protein synthesis in the cytoplasm
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Genetic Control of Cell Action through Protein Synthesis
• DNA directs the synthesis of all cell proteins– including enzymes
that direct the synthesis of nonproteins
• Different cells synthesize different proteins– dependent upon
differing gene activation
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Preview of Protein Synthesis
• Transcription– messenger RNA (mRNA) is formed next to an
activated gene– mRNA migrates to cytoplasm
• Translation– mRNA code is “read” by ribosomal RNA as
amino acids are assembled into a protein molecule– transfer RNA delivers the amino acids to the
ribosome
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Genetic Code
• System that enables the 4 nucleotides (A,T,G,C) to code for the 20 amino acids
• Base triplet: – found on DNA molecule (ex. TAC)
– sequence of 3 nucleotides that codes for 1 amino acid
• Codon:– “mirror-image” sequence of nucleotides in mRNA (ex AUG)
– 64 possible codons (43)
• often 2-3 codons represent the same amino acid
• start codon = AUG
• 3 stop codons = UAG, UGA, UAA
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Transcription• Copying genetic instructions from DNA to RNA
– RNA polymerase binds to DNA • at site selected by chemical messengers from cytoplasm
– opens DNA helix and transcribes bases from 1 strand of DNA into pre-mRNA
• if C on DNA, G is added to mRNA• if A on DNA, U is added to mRNA, etc.
– rewinds DNA helix
• Pre-mRNA is unfinished– “nonsense” portions (introns) removed by enzymes– “sense” portions (exons) reconnected and exit nucleus
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Steps in Translation of mRNA
• Converts language of nucleotides into sequence of amino acids in a protein
• Ribosome in cytosol or on rough ER– small subunit attaches to mRNA leader sequence– large subunit joins and pulls mRNA along as it “reads” it
• start codon (AUG) begins protein synthesis– small subunit binds activated tRNA with corresponding
anticodon– large subunit enzyme forms peptid bond
• Growth of polypeptide chain– next codon read, next tRNA attached, amino acids joined, first
tRNA released, process repeats and repeats
• Stop codon reached and process halted– polypeptide released and ribosome dissociates into 2 subunits
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Transfer RNA (tRNA)
• Activation by ATP binds specific amino acid and provides necessary energy to join amino acid to growing protein molecule
• Anticodon binds to complementary codon of mRNA
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Translation of mRNA
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Polyribosomes and Signal Peptides
• Polyribosome– cluster of 10-20 ribosomes reading mRNA at one time– horizontal filament - mRNA– large granules - ribosomes– beadlike chains projecting out - newly formed proteins
• takes 20 seconds to assemble protein with 400 amino acids• average gene 1200 nucleotides long (3 for each amino acid)
• Signal peptide = beginning of chain of amino acids– determines protein’s destination within cell
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Review: DNA & Peptide Formation
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Chaperones and Protein Structure
• Newly forming protein molecules must coil, fold or join with another protein or nonprotein moiety
• Chaperone proteins– prevent premature folding of molecule– assists in proper folding of new protein– may escort protein to destination in cell
• Stress or heat-shock proteins– chaperones produced in response to heat or stress – help protein fold back into correct functional shapes
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Posttranslational Modification
• Proteins destined for secretion or packaging are modified by rough ER or golgi
• Signal peptide – causes polyribosome to migrate to rough ER and drags protein
through pore into cisterna
• Possible modifications in RER cisterna– remove signal peptide, remove some amino acids, fold the
protein adding disulfide bridges or add CHO moieties
• Transport vesicles– leave rough ER and fuse to golgi complex
• Golgi complex– further modifies protein in cisterna, forms tranport vesicles to
pass to next cisterna
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Golgi Packaging and Secretion
• Last golgi cisterna releases finished product as membrane bound golgi vesicles
• Secretory vesicles– golgi vesicles that migrate to plasma membrane and
release product by exocytosis
• Lysosomes– golgi vesicles that remain in cell
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Protein Packaging & Secretion
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DNA Replication
• Law of complimentary base pairing allows building of one DNA strand based on the bases in 2nd strand
• Steps of replication process– DNA helicase opens short segment of helix
• point of separation called replication fork
– DNA polymerase • strands replicated in opposite directions
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DNA Replication• Semiconservative replication
– each new DNA molecule has one new helix with the other helix conserved from parent DNA
• Each new DNA helix winds around new histones formed in the cytoplasm to form nucleosomes
• 46 chromosomes replicated in 6-8 hours by 1000’s of polymerase molecules
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DNA Replication: Errors and Mutations
• Error rates of DNA polymerase– in bacteria, 3 errors per 100,000 bases copied
– every generation of cells would have 1,000 faulty proteins
• Proofreading and error correction– a small polymerase proofreads each new DNA strand and
makes corrections
– results in only 1 error per 1,000,000,000 bases copied
• Mutations - changes in DNA structure due to replication errors or environmental factors– some cause no effect, some kill cell, turn it cancerous or cause
genetic defects in future generations
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Cell Cycle• G1 phase, the first gap phase
– normal cellular functions
– begins to replicate centrioles
• S phase, synthesis phase– DNA replication
• G2 phase, second gap phase – preparation for mitosis
• replicates centrioles, synthesizes enzymes for cell division
• M phase, mitotic phase– nuclear and cytoplasmic division
• G0 phase, cells that have left the cycle
• Cell cycle duration varies between cell types
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Mitosis
• Process by which one cell divides into 2 daughter cells with identical copies of DNA
• Functions of mitosis– embryonic development – tissue growth– replacement of old and dead cells– repair of injured tissues
• Phases of mitosis (nuclear division)– prophase, metaphase, anaphase, telophase
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Mitosis: Prophase
• Chromatin supercoils into chromosomes– each chromosome = 2 genetically identical sister
chromatids joined at the centromere– each chromosomes contains a DNA molecule
• Nuclear envelope disintegrates
• Centrioles sprout microtubules pushing them apart towards each pole of the cell
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Prophase Chromosome
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Mitosis: Metaphase
• Chromosomes line up on equator• Spindle fibers (microtubules) from centrioles attach to centromere• Asters (microtubules) anchor centrioles to plasma membrane
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Mitosis: Anaphase
• Centromeres split in 2 and chromatids separate • Daughter chromosomes move towards opposite poles of
cells• Centromeres move down spindle fibers by kinetochore
protein (dynein)
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Mitosis: Telophase
• Chromosomes uncoil forming chromatin
• Nuclear envelopes form
• Mitotic spindle breaks down
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Cytokinesis
• Division of cytoplasm / overlaps telophase
• Myosin pulls on microfilaments of actin in the membrane skeleton
• Causes crease around cell equator called cleavage furrow
• Cell pinches in two
• Interphase has begun
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Timing of Cell Division
Cells divide when:• Have enough cytoplasm for 2 daughter cells• DNA replicated• Adequate supply of nutrients• Growth factor stimulation• Open space in tissue due to neighboring cell
deathCells stop dividing when:• Loss of growth factors or nutrients• Contact inhibition
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Chromosomes and Heredity
• Heredity = transmission of genetic characteristics from parent to offspring
• Karyotype = chart of chromosomes at metaphase • Humans have 23 pairs homologous chromosomes in
somatic cells (diploid number)– 1 chromosome inherited from each parent– 22 pairs called autosomes– one pair of sex chromosomes (X and Y)
• normal female has 2 X chromosomes• normal male has one X and one Y chromosome
• Sperm and egg cells contain 23 haploid chromosomes– paternal chromosomes combine with maternal chromosomes
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Karyotype of Normal Human Male
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Genes and Alleles
• Gene loci– location of gene on chromosome
• Alleles– different forms of gene at same locus on 2
homologous chromosomes
• Dominant allele– produces protein responsible for visible trait
• Recessive allele– expressed only when both alleles are recessive– ususually produces abnormal protein variant
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Genetics of Earlobes
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Genetics of Earlobes• Genotype
– alleles for a particular trait (DD)
• Phenotype– trait that results (appearance)
• Dominant allele (D)– expressed with DD or Dd
– Dd parent ‘carrier’ of recessive gene
• Recessive allele (d)– expressed with dd only
• Heterozygous carriers of hereditary disease – cystic fibrosis Punnett square
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Multiple Alleles, Codominance, Incomplete Dominance
• Gene pool– collective genetic makeup of whole population
• Multiple alleles– more than 2 alleles for a trait– such as IA, IB, i alleles for blood type
• Codominant– both alleles expressed, IAIB = type AB blood
• Incomplete dominance– phenotype intermediate between traits for each allele
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Polygenic Inheritance
• 2 or more genes combine their effects to produce single phenotypic trait, such as skin and eye color, alcoholism and heart disease
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Pleiotropy
• Single gene causes multiple phenotypic traits (ex. sickle-cell disease)– sticky, fragile, abnormal shaped red blood cells at
low oxygen levels cause anemia and enlarged spleen
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Sex-Linked Inheritance
• Recessive allele on X, no gene locus for trait on Y, so hemophilia more common in men (mother must be carrier)
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Penetrance and Environmental Effects
• Penetrance– % of population to
express predicted phenotype given their genotypes
• Role of environment– brown eye color
requires phenylalanine from diet to produce melanin, the eye pigment
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Alleles at the Population Level
• Dominance and recessiveness of allele do not determine frequency in a population
• Some recessive alleles, blood type O, are the most common
• Some dominant alleles, polydactyly and blood type AB, are rare
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Cancer
• Tumors (neoplasms)– abnormal growth, when cells multiply faster than
they die– oncology is the study of tumors
• Benign– connective tissue capsule, grow slowly, stays local – potentially lethal by compression of vital tissues
• Malignant– unencapsulated, fast growing, metastatic (causes 90%
of cancer deaths)
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Causes of Cancer
• Carcinogens - estimates of 60 - 70% of cancers from environmental agents
– chemical • cigarette tar, food preservatives
– radiation• UV radiation, particles, rays, particles
– viruses• type 2 herpes simplex - uterus, hepatitis C - liver
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Mutagens• Trigger gene mutations
– cell may die, be destroyed by immune system or produce a tumor
Defenses against mutagens• Scavenger cells
– remove them before they cause genetic damage
• Peroxisomes – neutralize nitrites, free radicals and oxidizing agents
• Nuclear enzymes– repair DNA
• Tumor necrosis factor (TNF) from macrophages and certain WBCs destroys tumors
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Malignant Tumor (Cancer) Genes
• Oncogenes– mutated form of normal growth factor genes called
proto-oncogenes– sis oncogene causes excessive production of growth
factors• stimulate neovascularization of tumor
– ras oncogene codes for abnormal growth factor receptors
• sends constant divide signal to cell
• Tumor suppressor genes– inhibit development of cancer– damage to one or both removes control of cell division
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Effects of Malignancies• Displaces normal tissue, organ function
deteriorates– rapid cell growth of immature nonfunctional cells– metastatic cells have different tissue origin
• Block vital passageways– block air flow and compress or rupture blood vessels
• Diverts nutrients from healthy tissues– tumors have high metabolic rates– causes weakness, fatigue, emaciation, susceptibility to
infection– cachexia is extreme wasting away of muscle and
adipose tissue