Chapter 5: Genetics and Cellular Function

• Nucleus and nucleic acids• Protein synthesis and secretion• DNA replication and the cell

cycle• Chromosomes and heredity

Nuclear Structure

• Most cells have single nucleus• Enucleate - no nucleus (mature RBC’s) • Multinucleate - 2 - 50 nuclei (skeletal muscle)• Nuclear envelope - surrounds nucleus, has two

unit membranes • Nuclear pores - perforate nuclear envelope• Nucleoplasm - material within nucleus• Chromatin - DNA and associated proteins• Nucleoli-dark staining, produce ribosome

subunits

Nucleus TEM

Nuclear Envelope, SEM

• Nuclear pores– formed by a ring

shaped complex of proteins

– regulates traffic through envelope

– holds nuclear envelope together

Chromatin

• DNA and associated proteins, looks like granular thread

• Nucleosomes - cluster of eight proteins, histones, serve as spools to protect and organize DNA

• Supercoils - preparation for cell division

Chromatin Structure

Nucleotide Structure

• Nucleic acids are polymers of nucleotides

• Nucleotides consist of– sugar

• RNA - ribose• DNA - deoxyribose

– phosphate group– nitrogenous base

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Nitrogenous Bases

• Purines - double carbon-nitrogen ring

• Pyrimidines - single carbon-nitrogen ring– uracil - RNA only– thymine - DNA only

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

Sugar-phosphate backbone

Sugar-phosphate backbone

Segment of DNA

DNA Structure: Twisted Ladder

DNA Function

• Serves as code for protein synthesis, cell replication and reproduction

• Gene - sequence of DNA nucleotides that codes for one polypeptide

• Genome - all the genes of one person

RNA Structure and Function

• Only one nucleotide chain• Much smaller

– transfer RNA (tRNA) has 70 - 90 bases– messenger RNA (mRNA) has over 10,000 bases– DNA has over a billion base pairs

• Ribose replaces deoxyribose as the sugar• Uracil replaces thymine as a nitrogenous base• Essential function

– interpret DNA code– direct protein synthesis

Control of Protein Synthesis

• DNA directs the synthesis of enzymes and other proteins– other substances synthesized depend on enzymes – thus indirectly under genetic control

Indirect Control of Nonprotein Synthesis by DNA

Genetic Code• System that enables the 4 nucleotides (A,T,G,C)

to code for the 20 amino acids• Base triplet: TAC

– sequence of 3 DNA nucleotides that codes for 1 amino acid

• Codon: AUG – “mirror-image” sequence in mRNA – there are 64 codons (43)

• AUG codes for methionine, also the start codon• there are 3 stop codons • often 2-3 codons represent the same amino acid

Transcription

• Copy genetic instructions from DNA to RNA• RNA polymerase

– binds to DNA • at site selected by chemical messengers from cytoplasm

– opens DNA helix– transcribes bases from 1 strand of DNA into pre-

mRNA– rewinds DNA helix

Posttranscriptional Modification

• Pre-mRNA contains – exons -“sense” portions– introns -“nonsense portions”

must be removed• Modification

– enzymes remove introns and splice exons together

• Functional mRNA leaves nucleus to be translated

Transfer RNA (tRNA)

• Activation by ATP binds specific amino acid• Anticodon binds to complementary codon of mRNA

Translation of mRNA• Ribosome

– attaches to mRNA– reads mRNA

• start codon (AUG) begins protein synthesis– binds activated tRNA

• Growth of polypeptide chain– reads next codon– binds next tRNA– links amino acids on tRNA’s– releases first tRNA– repeats until stop codon reached

Translation of mRNA Flow Chart

Polyribosome Formation

• Polyribosome– cluster of 10-20 ribosomes reading mRNA at once

• Horizontal filament - mRNA• Large granules - ribosomes• Beadlike chains projecting out - newly formed

proteins

DNA & Peptide Formation

Chaperones and Protein Structure

• As proteins assembled may associate with other proteins - chaperones

• Assists in proper folding of new protein• May escort protein to destination in cell• Stress proteins or heat-shock proteins

– chaperones produced in response to heat or stress – help protein fold back into correct functional shapes

Posttranslational Modification• Signal peptide

– amino acid sequence that causes polyribosome to migrate to rough ER and enters into cisterna

• Modifications in RER cisterna– signal peptide removed, may remove amino acids, fold

protein, form disulfide bridges or add CHO moieties • Transport vesicles

– when modifications are finished, RER pinches off clathrin-coated transport vesicles

• Golgi complex– removes clathrin, further modifies protein in cisterna,

forms tranport vesicles to pass to next cisterna

Packaging and Secretion

• Golgi vesicles– last golgi cisterna releases finished product in

membrane bound golgi vesicles– secretory vesicles

• some golgi vesicles become secretory vesicles, migrate to plasma membrane and release product by exocytosis

– lysosomes• some remain in cell and become lysosomes

Protein Packaging & Secretion

DNA Replication

• DNA unwinds from histones• DNA helicase opens short segment

– point of separation called replication fork• Replication by DNA polymerase

– one strand replicated from replication fork– other strand replicated from opposite direction

DNA Replication

• Semiconservative replication– each new DNA has one new helix and the other helix

conserved from parent DNA

• Each new DNA helix winds around new histones to form nucleosomes

DNA Replication Errors and Mutations

• Error rates– in bacteria 3 errors per 100,000 bases copied– every generation of cells would have 1,000 faulty

proteins• Proofreading and error correction

– after DNA polymerase replicates strand, a smaller polymerase proofreads it 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

Cell Cycle

• G1 phase, the first gap phase– normal cellular functions

• 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

Functions of Mitosis

• Embryonic development • Tissue growth• Replacement of old and dead cells• Repair of injured tissues

Mitosis: Prophase

• Chromatin supercoils into chromosomes• Nuclear envelope disintegrates• Centrioles sprout microtubules, mitotic spindle• Centrioles move to poles

Metaphase Chromosome

Mitosis: Metaphase

• Chromosomes line up on equator• Spindle fibers attach to centromere• Asters anchor centrioles to plasma membrane

Mitosis: Anaphase

• Centromeres divide• Spindle fibers pull sister chromatids to opposite

poles of cell

Mitosis: Telophase

• Chromatin uncoils• Nuclear envelopes form• Mitotic spindle breaks down

Cytokinesis

• Division of cytoplasm• Myosin pulls on actin in the membrane skeleton• Causes crease around cell equator called

cleavage furrow• Cell pinches in two

Timing of Cell Division

Cells divide when:• Cells large enough• DNA replicated• Adequate supply of nutrients• Growth factor stimulation• Open space in tissueCells stop dividing when:• Loss of growth factors or nutrients• Contact inhibition

Cancer

• Tumors– 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)

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 B - liver

Mutagens

• Trigger gene mutations– cell may die, be destroyed by immune system or

produce a tumorDefenses against mutagens• Scavenger cells

– remove them• Peroxisomes

– neutralize nitrites, free radicals and oxidizing agents• Nuclear enzymes

– repair DNA• Tumor necrosis factor (TNF) destroys tumors

Cancer Genes

• Oncogenes– mutated form of 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

Effects of Malignancies

• Displaces normal tissue, function deteriorates– rapid cell growth of immature nonfunctional cells– metastatic cells different tissue origin

• Block vital passageways– respiratory or vascular

• Diverts nutrients from other tissues– tumors have high metabolic rates– causes weakness, fatigue, emaciation, infection

Chromosomes

• Karyotype– chart of chromosomes at metaphase by size, structure

• Homologous chromosomes– 2 chromosomes in each pair, 1 from each parent– autosomes (22 pairs)– sex chromosomes (X and Y)

• Germ cells - sperm and egg cells, haploid• Somatic cells - all other cells, diploid

Genes and Alleles

• Gene loci– location of gene on chromosome

• Alleles– two homologous chromosomes have same gene at

same locus, may be different forms of gene• Dominant allele

– produces normal, functional protein• Recessive allele

– when both alleles are recessive produces abnormal protein or no protein

Genetics of Earlobes

• Genotype– alleles for a trait (DD)

• Phenotype– trait that results

• Dominant allele (D)– expressed with DD or Dd– Dd parent ‘carrier’ of

recessive gene• Recessive allele (d)

– expressed with dd onlyPunnett square

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

Polygenic Inheritance

• 2 or more genes combine their effects to produce single phenotypic trait, such as skin color

Pleiotropy

• Single gene causes multiple phenotypic traits, as in sickle-cell disease

Sex-Linked Inheritance

• Recessive allele on X, no gene locus for trait on Y, so hemophilia more common in men

Penetrance and Environmental Effects

• Penetrance– % of population to

express predicted phenotype

• Role of environment– brown eye color

requires phenylalanine from diet to produce melanin, the eye pigment

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, are rare

Recombinant DNA Technique