Genetics and Cellular Function Genes and nucleic acids

Genetics and Cellular Function • • Genes and nucleic acids Protein synthesis and secretion DNA replication and the cell cycle Chromosomes and heredity 1

Organization of the Chromatin • Threadlike chromatin = chromosomes = 46 DNA molecules and associated proteins • Nondividing state = DNA molecules compacted – coiled around core particle (histone protein) – zig-zagged, looped and coiled onto itself • Preparing to divide – DNA copies itself to form 2 parallel sister chromatids 2

Chromatin Structure 3

DNA Structure: Twisted Ladder DNA molecule described as double helix. 4

Nucleotide Structure • DNA = polymer of nucleotides • Each nucleotide consist of – phosphate group – sugar • ribose (RNA) • deoxyribose (DNA) – nitrogenous base • in this picture = adenine 5

Nitrogenous Bases • Purines - double ring – guanine – adenine • Pyrimidines - single ring – uracil - RNA only – thymine - DNA only – cytosine – both • DNA bases =CTAG • RNA bases = CUAG 6

Complementary Base Pairing • Nitrogenous bases united by hydrogen bonds • DNA base pairings – A-T and C-G • Law of complementary base pairing – one strand determines base sequence of other Segment of DNA 7

DNA Function • Code for protein synthesis • Gene - sequence of DNA nucleotides that codes for one protein • Genome - all the genes of one person – humans have estimated 30 -35, 000 genes – other 98% of DNA noncoding – “junk” or regulatory 8

Discovery of the Double Helix • By 1900: components of DNA were known – sugar, phosphate and bases • By 1953: x ray 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 getting the x ray data that provided the answers. 9

RNA: Structure and Function • RNA smaller than DNA (fewer bases) – transfer RNA (t. RNA) 70 - 90 bases – messenger RNA (m. RNA) 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 10

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 11

Preview of Protein Synthesis • Transcription – messenger RNA (m. RNA) is formed next to an activated gene – m. RNA migrates to cytoplasm • Translation – m. RNA code is “read” by ribosomal RNA as amino acids are assembled into a protein molecule – transfer RNA delivers the amino acids to the ribosome 12

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) – nucleotides that stand for 1 amino acid • Codon: – “mirror-image” sequence of nucleotides found in m. RNA (ex AUG) – 64 possible codons (43) • often 2 -3 codons represent the same amino acid • start codon = AUG 13 • 3 stop codons = UAG, UGA, UAA

Transcription • Copying 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-m. RNA • if C on DNA, G is added to m. RNA • if A on DNA, U is added to m. RNA, etc. – rewinds DNA helix • Pre-m. RNA is unfinished – “nonsense” (introns) removed by enzymes – “sense” (exons) reconnected and exit nucleus 14

Alternative Splicing of m. RNA • One gene can code for more than one protein • Exons can be spliced together into a variety of different m. RNAs. 15

Translation of m. RNA • m. RNA begins with leader sequence – binding site for ribosome • Start codon AUG 16

Steps in Translation of m. RNA • Converts alphabet of nucleotides into a sequence of amino acids to create a specific protein • Ribosome in cytosol or on rough ER – small subunit attaches to m. RNA leader sequence – large subunit joins and pulls m. RNA along as it “reads” it • start codon (AUG) where protein synthesis begins – small subunit binds activated t. RNA with corresponding anticodon – large subunit enzyme forms peptide bond 17

Steps in Translation of m. RNA • Growth of polypeptide chain – next codon read, next t. RNA attached, amino acids joined, first t. RNA released, process repeats and repeats • Stop codon reached and process halted – polypeptide released and ribosome dissociates into 2 subunits 18

Transfer RNA (t. RNA) • 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 19 m. RNA

Polyribosomes 20

Polyribosomes and Signal Peptides • Polyribosome – cluster of 10 -20 ribosomes reading m. RNA at one time – horizontal filament - m. RNA – large granules - ribosomes – beadlike chains projecting out - newly formed proteins • takes 20 seconds to assemble protein of 400 amino acids • cell may produce > 150, 000 proteins/second • Signal peptide = beginning of chain of amino acids – determines protein’s destination within cell 21

DNA and Peptide Formation 22

Protein Packaging and Secretion 23

Posttranslational Modification in Rough ER • Proteins destined for secretion or packaging are assembled on rough ER and sent to Golgi complex • Signal peptide – drags new protein from ribosome through pore into cisterna of ER • Posttranslational modification of protein in ER – remove some amino acids, fold the protein adding disulfide bridges or adding carbohydrates • Rough ER pinches off transport vesicles – fuse with and empty into nearest Golgi complex 24

Posttranslational Modification in Golgi Complex • Protein modified in cisterna, passed to next cisterna • Last golgi cisterna releases finished product as membrane bound vesicles – secretory vesicles • migrate to plasma membrane and release product by exocytosis – lysosomes • vesicles that remain in cell 25

DNA Replication 1 26

DNA Replication 2 • Law of complimentary base pairing allows building of one DNA strand based on the bases in 2 nd strand • Steps of replication process – DNA helicase opens short segment of helix • replication fork is point of separation of 2 strands – DNA polymerase assembles new strand of DNA next to one of the old strands • 2 DNA polymerase enzymes at work simultaneously 27

DNA Replication 3 • Semiconservative replication – each new DNA molecule contains one new helix and one conserved from parent DNA • Additional histones made in cytoplasm • Each DNA helix winds around histones to form nucleosomes • 46 chromosomes replicated in 6 -8 hours by 1000’s of polymerase molecules 28

Errors and Mutations • Error rates of DNA polymerase – in bacteria, 3 errors per 100, 000 bases copied • Proofreading and error correction – a small polymerase proofreads each new DNA strand makes corrections – results in only 1 error per 1, 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 29

Cell Cycle • G 1 phase, the first gap phase – accumulates materials needed to replicate DNA • S phase, synthesis phase – DNA replication • G 2 phase, second gap phase – replicates centrioles – synthesizes enzymes for division • M phase, mitotic phase – nuclear and cytoplasmic division • G 0 phase, cells that have left the cycle • Cell cycle duration varies between cell types 30

Mitosis • one cell divides into 2 daughter cells with identical copies of DNA • Functions of mitosis – embryonic development – tissue growth – replacement of dead cells – repair of injured tissues • Phases of mitosis (nuclear division) – prophase, metaphase, anaphase, telophase 31

Mitosis 32

Mitosis: Prophase 1 • Chromatin coils into genetically identical, paired, sister chromatids – each chromatid contains a DNA molecule – remember: genetic material (DNA) was doubled during S phase of interphase • Thus, there are 46 chromosomes with 2 chromatids/chromosome and 1 molecule DNA per chromatid. 33

Mitosis: Prophase 2 • Nuclear envelope disintegrates • Centrioles sprout microtubules that push them apart and towards each pole of the cell – spindle fibers grow towards chromosomes • attach to kinetochore on side of centromere – spindle fibers pull chromosomes towards cell equator 34

Mitosis: Metaphase • Chromosomes line up on one equator • Mitosis spindles finished – spindle fibers (microtubules) attach centrioles to long centromere – shorter microtubules anchor centrioles to plasma membrane (aster) 35

Mitosis: Anaphase • Enzyme splits 2 chromatids apart at centromere • Daughter chromosomes move towards opposite poles of cells with centromere leading the way – motor proteins in kinetochore move centromeres along spindle fibers as fibers are disassembled 36

Mitosis: Telophase • New nuclear envelopes formed by rough ER • Chromatids uncoil into chromatin • Mitotic spindle breaks down • Nucleus forms nucleoli 37

Cytokinesis • Division of cytoplasm into 2 cells – overlaps telophase • Myosin pulls on microfilaments of actin in the membrane skeleton – creates crease around cell equator called cleavage furrow • Cell pinches in two – interphase has begun 38

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 due to neighboring cell death Cells stop dividing when: • Loss of growth factors or nutrients • Contact inhibition 39

Chromosomes and Heredity • Heredity = transmission of genetic characteristics from parent to offspring – karyotype = chart of chromosomes at metaphase • 23 pairs homologous chromosomes in somatic cells (diploid number of chromosomes) – 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 contain only 23 chromosomes – fertilized egg has diploid number of chromosomes 40

Karyotype of Normal Male 41

The Genome • Human Genome project (1990 -2003) – mapped entire base sequence (A, T, C, G) of 99% of our DNA • Genomics – study of how your DNA affects structure and function – Homo sapiens have 35, 000 genes – these genes generate millions of different proteins with alternative splicing • All humans 99. 99% genetically identical • Genomic medicine – Prediction, diagnosis and treatment of disease using knowledge of genome • Gene-substitution therapy 42

Genes and Alleles • Locus = location of particular gene • Alleles – different forms of gene at same locus on 2 homologous chromosomes • Dominant allele (D) – produces protein responsible for visible trait • Recessive allele (d) – expressed only when both alleles are recessive 43

Genetics of Earlobes 44

Genetics of Earlobes • Genotype – alleles for a particular trait (DD) • Phenotype – trait that results (appearance) • Homozygous – 2 identical alleles at a particular gene • Heterozygous – different alleles for a particular gene • Carriers of hereditary disease (cystic fibrosis) – heterozygous individual Punnett square 45

Multiple Alleles and Dominance • Gene pool – collective genetic makeup of 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 46

Polygenic Inheritance • 2 or more loci contribute to a single phenotypic trait (skin and eye color, alcoholism and heart disease) 47

Pleiotropy • One gene produces multiple phenotypic effects – Alkaptonuria = mutation that blocks the breakdown of tyrosine 48

Sex-Linked Inheritance • Recessive hemophilic allele on X, no gene locus for trait on Y, so hemophilia more common in 49 men (mother is carrier)

Penetrance and Environmental Effects • Penetrance – % of population expressing predicted phenotype • Role of environment – brown eye color requires phenylalanine from diet to produce melanin pigment 50

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 51

Cancer • Tumors (neoplasms) – abnormal growth, cells multiply faster than they die – oncology = study of tumors • Benign – connective tissue capsule, slow growth, stays local – potentially lethal by compression of vital tissues • Malignant tumor = cancer – unencapsulated, fast growing, metastatic (spreading), stimulate angiogenesis 52

Causes of Cancer • Carcinogens - estimates of 60 - 70% of cancers from environmental agents – chemical = cigarette tar, food preservatives, industrial chemicals – radiation – Viruses = type 2 herpes simplex uterus, hepatitis C - liver 53

Carcinogens (Mutagens) • Trigger gene mutations – cell may die, be destroyed by immune system or produce a tumor • Defenses against mutagens and tumors – – scavenger cells - remove mutagens peroxisomes - neutralize mutagens nuclear enzymes - repair damaged DNA macrophages and monocytes secrete tumor necrosis factor (TNF) - destroys tumors – natural killer cells destroy malignant cells during immune surveillance 54

Malignant Tumor Genes • Oncogenes – mutated form of normal growth factor genes called proto-oncogenes – sis oncogene causes excessive production of growth factors – ras oncogene codes for abnormal growth factor receptors • Tumor suppressor genes – inhibit development of cancer – damage to one or both removes control of cell division 55

Effects of Malignancies • Displaces normal tissue and organ function deteriorates – cell growth of immature nonfunctional cells • Block vital passageways – block air flow 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 56 and adipose tissue