Cell Structure and Genetic Control Chapter 03 Cell Structure and Genetic Control Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I. Plasma Membrane and Associated Structures

Overview Cells are the basic functional units of the body. They come in a variety of shapes and sizes. This diversity reflects their diverse functions. Principal parts of cells Plasma membrane – selectively permeable, gives form, and separates from the external environment Cytoplasm and organelles – fluid part of cell and little organs that do the functions Nucleus – contains DNA and directs cell activities

Cell Golgi complex Secretory vesicle Nuclear envelope Centriole Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Golgi complex Secretory vesicle Nuclear envelope Centriole Mitochondrion Nucleolus Lysosome Chromatin Plasma membrane Nucleus Microtubule Granular endoplasmic reticulum Cytoplasm (cytosol) Agranular endoplasmic reticulum Ribosome

Summary of Cell Components

Structure of the Plasma Membrane Phospholipid barrier separates the intracellular and extracellular environments Hydrophobic center of the double membrane restricts the movement of water, water-soluble molecules, and ions. Many substances are selectively allowed to pass through protein channels. Proteins and phospholipids are not trapped in the membrane but constantly move laterally; known as the fluid mosaic model.

Plasma Membrane Structure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Extracellular side Carbohydrate Glycoprotein Glycolipid Nonpolar end Polar end Phospholipids Proteins Cholesterol Intracellular side

Membrane Proteins Integral proteins are embedded into the membrane. Peripheral proteins are attached to just one side of the membrane. Functions: Structural support Transport (channels) Enzymatic control of cell processes Receptors for hormones and other molecules “Self” markers for the immune system

Other components of the membrane Carbohydrates – attached to lipids (glycolipids) and to proteins (glycoproteins); serve as antigens and interactions with regulatory molecules Cholesterol – gives flexibility to the membrane

Phagocytosis (cell eating) Bulk transport of large extracellular substances into the cell Some cells, like neutrophils and macrophages, can perform amoeboid movement by extending pseudopods to pull the cell forward. Relies on the bonding of proteins called integrins with extracellular proteins Pseudopods engulf bacteria, dead cells, or other organic materials and then fuse together to form a food vacuole. The food vacuole fuses with a lysosome, and the material is digested. Very important for body defense, inflammation, and apoptosis

(both): © Kwang Jeon/Visuals Unlimited, Inc. Phagocytosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pseudopod Pseudopods forming food vacuole (a) (b) (both): © Kwang Jeon/Visuals Unlimited, Inc.

Endocytosis Another process for bringing large materials into the cell The plasma membrane furrows inward rather than extending outward. A small part of the membrane surrounding the substance pinches off and is brought in as a vesicle. Pinocytosis (nonspecific) Receptor-mediated endocytosis (specific) Has receptor proteins in the membrane that will bind to the substance to be brought in Ex – cholesterol, AIDS virus, hepatitis virus

Endocytosis Extracellular Plasma membrane (pit forming) Membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Extracellular Plasma membrane (pit forming) Membrane pouching inward Cytoplasm (1) (2) Extracellular Cytoplasm Vesicle within cell Vesicle (3) (4) (all): From M.M. Perry and A.B. Gilbert. Journal of Cell Science 39: 257–272, 1979. Reprinted by permission Company of Biologists, Ltd.

Exocytosis Process of moving large cellular products (proteins) out of the cell. The Golgi apparatus packages proteins into vesicles that fuse to the plasma membrane, and the contents spill out of the cell.

Cilia & Flagella Cilia - tiny, hairlike structures composed of microtubules that project from the plasma membrane Primary cilium – most cells have this nonmotile cilium with “9+0” structure; may have a sensory function in some cells Motile cilia beat in unison to move substances through hollow organs. Found in respiratory tract and uterine tubes Have a “9+2” arrangement

Cilia Base of each cilium has a pair of centrioles that are perpendicular to each other and are parts of a structure called the centrosome; One centriole is parallel to the cilium and called the basal body

Cilia Cilia (a) 10 µm 0.15 µm (b) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cilia (a) 10 µm 0.15 µm (b) a: © Science Photo Library RF/Getty Images; b: © Biophoto Associates/Photo Researchers, Inc.

Flagellum A single whip-like structure that can propel a cell forward Composed of microtubules with a “9+2” arrangement The sperm is the only cell in the human body with a flagellum.

Microvilli Folds in the plasma membrane that increase the surface area for chemical reactions and rapid diffusion Examples: Digestive tracts and kidney tubules Microvilli Lumen Junctional complexes

II. Cytoplasm and Organelles

Cytoplasm & Cytoskeleton Structures within a cell Includes organelles, a fluid called cytosol, the cytoskeleton, and inclusions. Inclusions – stored chemical aggregates such as glycogen, melanin, and triglycerides

Cytoskeleton Organized system of microtubules and microfilaments throughout the cytoplasm Proteins of the cytoskeleton are mobile. They organize the intracellular environment and allow movement of muscle cells and phagocytic cells. They form the spindle apparatus that pulls chromosomes apart in mitosis. They also serve as a “railway” system for vesicles and organelles to move along using molecular motors of myosin, kinesins, and dyneins

Cytoskeleton Microtubules Microfilaments Plasma membrane Mitochondrion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plasma membrane Mitochondrion Polysome Microtubules Endoplasmic reticulum Microtubule Ribosome Microfilaments Nuclear envelope

Lysosomes Organelles filled with digestive enzymes Fuse with vacuoles that contain engulfed extracellular particles (proteins, lipids, polysaccharides, viral proteins, bacterium, etc) Primary lysosome: only contains digestive enzymes Secondary lysosome: contains the partially digested contents of the food vacuole or worn-out organelles Residual body: a lysosome filled with waste, which can be expelled through exocytosis

Lysosomes Besides digesting bacteria, lysosomes are also responsible for: Autophagy – process of digesting worn-out or damaged organelles and proteins in the cell Apoptosis – programmed cell death. The lysosome spills its contents, killing the cell.

Peroxisomes Contain enzymes specific to certain oxidative reactions Found in most cells but most numerous in the liver; often oxidize toxic molecules (such as alcohol) Enzymes used to remove hydrogen from a molecule and transfer it to O2, forming hydrogen peroxide Also contain the enzyme catalase, which converts hydrogen peroxide into water and O2

Mitochondria Sites of energy production through aerobic cell respiration Structure Have an inner membrane and an outer membrane separated by an intermembranous space Inner membrane is folded into cristae to increase surface area for reactions Central area is fluid and called the matrix

a: Courtesy Keith R. Porter Endowment Mitochondria Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inner mitochondrial membrane Outer mitochondrial membrane Matrix Cristae (a) (b) a: Courtesy Keith R. Porter Endowment

Mitochondria Most cells have mitochondria, and there can be thousands of mitochondria in a single cell. Mitochondria can migrate around the cell and can make copies of themselves. Have their own DNA, all derived from mother Mutations of mitochondrial DNA may contribute to aging and disease

Ribosomes Protein factories of the cell Messenger RNA takes genetic information to the ribosome so a protein can be assembled. Very small; made of 2 subunits of ribosomal RNA and protein Found free in the cytoplasm or associated with the endoplasmic reticulum Serves as enzymes called ribozymes that are needed for protein synthesis

Ribosome

Endoplasmic Reticulum (ER) System of membranous passageways whose membrane are continuous with the nuclear membrane Rough ER (Granular ER) Has ribosomes embedded on the outer surface Functions in protein modification Smooth ER (Agranular ER) Has many functions, depending on the cell Steroid hormone metabolism, detoxification, Ca2+ storage

a: Courtesy Keith R. Porter Endowment Endoplasmic Reticulum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Nucleus Tubule Membrane Ribosome (b) (c) a: Courtesy Keith R. Porter Endowment

Golgi Complex (Apparatus) Consists of stacks of hollow, flattened sacs; cavities are called cisternae One side receives proteins from the ER These are packaged in vesicles called endosomes, that bud off to (1) fuse with the plasma membrane for exocytosis, (2) fuse and become integral proteins, (3) fuse with lysosomes inside the cell Proteins are modified within the cisternae based on the type of protein Retrograde transport – extracellular proteins brought in by endocytosis and then through the Golgi complex to the ER

Golgi Complex (Apparatus) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rough endoplasmic reticulum Protein Plasma membrane Secretion Cisternae Nucleus Secretory storage Ribosomes Cytoplasm Golgi complex (a) Lysosome (b) a: Courtesy of Mark S. Ladinsky and Kathryn D. Howell, University of Colorado

III. Cell Nucleus and Gene Expression

Cell Nucleus The nucleus is enclosed by a double membrane nuclear envelope: Outer membrane continuous with ER Inner membrane often fused to outer by nuclear pore complexes, which allow movement of small molecules and RNA into/out of the nucleus Contains genetic materials stored in form of chromatin, which are highly coiled DNA

Cell Nucleus Inner and outer nuclear membranes Nucleus Nucleus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inner and outer nuclear membranes Nucleus Nucleus Chromatin Nucleolus Pore Outer membrane Ribosome Inner membrane Pore complex

DNA and Genes The nucleus contains DNA. A gene is a length of DNA that codes for a specific protein. Transcription – the gene on the DNA is copied as messenger RNA (mRNA), which then leaves the nucleus to go to the rough ER Translation – the mRNA is processed at the ribosomes to assemble the proper amino acid sequence to form a protein These two steps can be called Genetic expression.

Nucleolus The nucleus also has one or more darker regions not surrounded by a membrane; these are called nucleolus. The nucleolus contain DNA that codes for the production of ribosomal RNA (rRNA).

Genome and Proteome The genome is all the genes in a particular individual or all the genes of a particular species. Researchers believe humans have ~25,000 different genes. The proteome is all the proteins that are produced from the genome. More than 100,000 proteins are produced in the human body.

Genome and Proteome How can a gene code for more than one protein? mRNA is altered after transcription by cutting and splicing different ways Proteins are made of many polypeptide chains that can associate in different combinations Post-translational modification by: Glycosylation Methylation Phosphorylation Cleavage of larger into smaller units

Chromatin DNA in the nucleus is packaged with proteins called histones to form chromatin. Histones are positively charged and will interact with negatively charged DNA to cause spooling Two turns of DNA around 8 histone proteins create structure called nucleosomes Euchromatin: active in transcription, looser; chemical changes in histones (such as acetylation) allow molecules access to the DNA during gene expression. Heterochromatin: inactive regions, highly condensed; much of the DNA is inactive.

Chromatin Chromosome Region of euchromatin with activated genes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromosome Region of euchromatin with activated genes O O O O Nucleosome DNA O O O O

Chromatin and Gene Expression Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Condensed chromatin, where nucleosomes are compacted Acetylation Acetylation of chromatin produces a more open structure Transcription factor Transcription factors attach to chromatin, activate genes (producing RNA) DNA region to be transcribed Deacetylation Deacetylation causes compaction of chromation, silencing genetic transcription

RNA Synthesis Transcription (DNA-directed RNA synthesis) Occurs inside the nucleus Involves: Start and stop regions at the beginning and end of the gene Promoters, areas of DNA that are not part of the gene but tell enzymes involved where to begin Transcription factors that bind to the promoter to begin transcription

RNA Synthesis C- G- A- U- C- G-C-T-A-G-T-C-G-G RNA polymerase, breaks the hydrogen bonds between the base pairs of DNA and assembles the appropriate RNA nucleotide RNA nucleotides pair up to the DNA template Assembly is complementary. RNA has uracil instead of thymine. Forms precursor messenger RNA that detaches from the DNA template Only 1 freed DNA strand is transcribed C- G- A- U- C- mRNA- G-C-T-A-G-T-C-G-G DNA-

Types of RNA Precursor messenger RNA (pre-mRNA) – made directly by transcription Messenger RNA (mRNA) – modified pre-mRNA; contains the code to make a specific protein Transfer RNA (tRNA) – carries amino acids to mRNA for translation Ribosomal RNA (rRNA) – along with protein, forms ribosomes; site of translation; acts as an enzyme

Pre-mRNA modification Precursor messenger RNA is altered in the nucleus before it leaves as messenger RNA (mRNA). Introns – portions of the Pre-mRNA that do not actually code for proteins. These must be spliced out. Introns may regulate the expression of the area that do code for protein Exons – portions of Pre-mRNA that contain codes for proteins.

Pre-mRNA modification Alternative splicing allows one Pre-mRNA to be used to make multiple proteins. Exons are joined together by spliceosomes and snRNPs (snurps) which are small nuclear ribonucleoproteins, to form mRNA

Messenger RNA Synthesis and Splicing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A T C G Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T A DNA DNA (gene) G C C A T RNA C G U Introns A A C C G G C G A T U Transcription A U Pre-mRNA T C G G C G C A U A A Exon Intron Exon Intron Exon G G C A U C C G U T C G A T G C G Exons spliced together mRNA C G C A G A C T

RNA Interference RNA molecules that don’t code for proteins may prevent some mRNA molecules from being translated. Two types: siRNA (short interfering) Formed from double-stranded RNA and cut by Dicer enzyme into short segments miRNA (micro interfering) Formed from introns of Pre-mRNA and cut by Dicer enzyme into short segments

RNA interference One of the strands of siRNA and miRNA enter a protein particles called RNA-induced silencing complex (RISC) Can bind to mRNA where it is complementary Prevents tRNA from bringing amino acids to mRNA, so translation is prevented – the gene is silenced RNA interference may help in cancer treatments as well as other genetic conditions

IV. Protein Synthesis and Secretion

Protein Synthesis Translation mRNA attaches to a string of ribosomes to form a polyribosome. Codon - a group of three bases on mRNA that provide a code for an amino acid The order of the codons gives the order of amino acids in a polypeptide

Newly synthesized protein Polyribosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Ribosomes Newly synthesized protein mRNA © E. Kiselva-D. Fawcett/Visuals Unlimited, Inc.

Triplets,Codons, Amino Acids

Amino Acids

Protein Synthesis T G C G T A C G A C C G T A T G G C C C G DNA double Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T G C G T A C G A C C G T A T G G C C C G DNA double helix C A G C G G G C DNA coding strand T A C C C G A G G T A G C C G C G T C G T Transcription A U G G G C U C C A U C G G C G C A G C A Messenger RNA Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6 Codon 7 Translation Methionine Glycine Serine Isoleucine Glycine Alanine Alanine Protein

Transfer RNA (tRNA) A single strand of RNA bent into a cloverleaf shape One end has the anticodon, which is three nucleotides that will be complementary to the proper codon. The other end has the appropriate amino acid bonded by aminoacyl-tRNA synthetase enzyme A Amino acid- accepting end C C Loop 3 Loop 1 Loop 2 U U A Anticodon (a) C C A Loop 3 Amino acid- accepting end Loop 1 Loop 2 A U U Anticodon

Formation of a Polypeptide The mRNA moves through the ribosome, with the proper tRNA attaching at each codon. Amino acids attached to the tRNAs form peptide bonds to each other and disassociate from the tRNA. tRNAs disassociate from the mRNA as they lose their amino acids. This continues until a stop codon is encountered and the whole complex disassociates. Interactions between amino acids of the growing polypeptide chain cause bends and folds into secondary and tertiary structures Chaperone proteins aid the formation of correct interactions and folds

Translation Codons Anticodons 3 mRNA C Next amino acid 6 U A I E C G Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Codons Anticodons 3 mRNA C Next amino acid 6 U A I E C G tRNA Codons 1 C G H I tRNA A U G D U A tRNA H C G F G G E F tRNA 5 Next amino acid D 2 E tRNA tRNA C 4 D 5 B C 3 Growing polypeptide chain 4 A B A 2 3 tRNA 1 2 1 Ribosome

Functions of the ER and Golgi Complex Newly formed proteins destined to leave the cell are made on the rough (granular) ER. The first ~30 amino acids (leader sequence) are hydrophobic and are attracted to the lipid portion of the membrane of the granular ER. The growing polypeptide chain enters the cisternae of the ER. The leader leader sequence is removed and other portions may be removed or added.

endoplasmic reticulum Granular ER Role in Protein Synthesis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cytoplasm Ribosome Free ribosome Granular endoplasmic reticulum mRNA Leader sequence Leader sequence removed Protein Cisterna of endoplasmic reticulum Carbohydrate

Functions of the ER and Golgi Complex Secretory proteins are next sent to the Golgi complex. Proteins may be further modified, including the production of glycoproteins. Proteins are separated according to destination. Proteins are packaged and shipped in vesicles to their destinations.

Protein Degradation Regulatory proteins are rapidly degraded, making their effects short-lived. This allows for greater control of cell functions. In the lysosome, proteins are digested by proteases. Outside the lysosome, proteins to be destroyed are bonded by a molecule called ubiquitin, which marks them for degradation by a proteasome (large protease complex). Ubiquitin may also tag membrane proteins and organelles for destruction

V. DNA Synthesis and Cell Division

DNA Replication Before cell division, each DNA molecule must replicate itself so that one of each copy can be distributed to the two new cells. Involves two types of enzymes: Helicases break hydrogen bonds between the DNA strands. This creates a fork in the double-stranded molecule where nucleotides can be added to both strands.

DNA Replication Enzymes DNA polymerase attaches complementary nucleotides to the exposed strand. Two new molecules are being made from the original one, each with half old and half new DNA. This is called semiconservative replication.

DNA Replication A T C G Region of parental DNA helix. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A T C G Region of parental DNA helix. (Both backbones are light.) A C G G C A T A T G G C Region of replication. Parental DNA is unzipped and new nucleotides are pairing with those in parental strands. C C G C A G T A T G G C A C T A T C G A T A Region of completed replication. Each double helix is composed of an old parental strand (light purple) and a new daughter strand (blue). The two DNA molecules formed are identical to the original DNA helix and to one another. G C C G T C G T A T C G C G G C A T C G G C

The Cell Cycle Divided into interphase, mitosis, and cytokinesis Interphase is divided into G1, S, and G2 Mitosis is divided into Prophase, Metaphase, Anaphase, and Telophase Cytokinesis overlaps the last parts of mitosis Mitotic Phase Anaphase Metaphase Telophase Prophase Cytokinesis Mitosis G2 G1 Final growth and activity before mitosis Centrioles replicate S DNA replication Interphase

Interphase Lots of RNA synthesis occurring G1 Phase: The cell is performing the functions characteristic of cells in that tissue. Cyclin D moves the cell quickly through G1. Overactivity of the gene for cyclin D has been implicated in some cancers. p53 is a transcription factor that can stall a gene at the G1/S checkpoint, repair DNA damage or promote apoptosis If a cell does not divide, it remains in a modified G1 phase its whole life.

One (duplicated) chromosome Interphase S phase: If a cell is going to divide, it performs DNA replication at this time G2 Phase: Chromosomes start to condense; consist of two strands called sister chromatids joined by a centromere. One (duplicated) chromosome Centromere Chromatid DNA Histone

Interphase Interphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chromatin Nucleolus Centrosomes Interphase • The chromosomes are in an extended form and seen as chromatin in the electron microscope. • The nucleus is visible. (all): © Ed Reschke

Mitosis Prophase: Chromosomes become visible, nuclear membrane and nucleolus disappear, centrioles move apart, spindle fibers form Chromatid pairs Spindle fibers Prophase • The chromosomes are seen to consist of two chromatids joined by a centromere. • The centrioles move apart toward opposite poles of the cell. • Spindle fibers are produced and extend from each centrosome. • The nuclear membrane starts to disappear. • The nucleolus is no longer visible.

Mitosis Metaphase: Chromosomes line up in the center of the cell and have attached to spindle fibers Spindle fibers Metaphase • The chromosomes are lined up at the equator of the cell. • The spindle fibers from each centriole are attached to the centromeres of the chromosomes. • The nuclear membrane has disappeared.

Mitosis Anaphase: Centromeres split as the spindle fibers shorten and pull chromatids to opposite sides. Anaphase • The centromeres split, and the sister chromatids separate as each is pulled to an opposite pole.

Mitosis Telophase: Cytoplasm is divided (cytokinesis) and cells separate, new nuclear membrane and nucleolus appears, chromosomes lengthen Furrowing Nucleolus Telophase • The chromosomes become longer, thinner, and less distinct. • New nuclear membranes form. • The nucleolus reappears. • Cell division is nearly complete.

Role of the Centrosome Located near the nucleus of a nondividing cell. Centrosome contains: Two centrioles at the center, which replicate in interphase and move away from each other in prophase. Pericentriolar materials form around the centrioles. Spindle fibers form from this and attach to the centromere of the replicated sister chromatids Other tubules cause the formation of the cleavage furrow for cytokinesis

Role of the Centrosome In nondividing cells, the centrosome migrates to the plasma membrane and forms the nonmotile primary cilium. In ciliated cells, hundreds of centrosomes form and become the basal bodies of the cilia (a) (b)

Telomeres and Cell Division Telomeres – protective cap on the ends of DNA that prevents DNA degradation. Each time DNA is replicated, a little more telomere is lost, which will cause the cell to eventually lose its ability to divide. Damaged telomeres activate p53 which induces cell cycle arrest, senescence, and apoptosis Cells that can divide indefinitely, such as those in bone marrow, have an enzyme called telomerase that replicates the telomere.

Hypertrophy and Hyperplasia Hyperplasia: growth due to an increase in the number of cells; responsible for the growth of most body regions Hypertrophy: growth due to an increase in cell size; responsible for increase in skeletal muscle size

Cell Death Necrosis: Cell dies pathologically due to deprivation of blood supply. Apoptosis: Programmed cell death is performed by enzymes called caspases. Extrinsic: “Death ligands” attach to the cell and mark it for destruction. Intrinsic: Intercellular signals trigger death due to DNA damage, cancer, infection, or oxidative stress. “Knocked-out” mice with their p53 gene removed are used to study cancer and apoptosis

Meiosis Homologous Chromosomes Humans have 23 pairs of chromosomes, one set from each parent. 22 pairs are autosomes and 1 pair are sex chromosomes Each pair is called homologous chromosomes, and they have the same genes on them (but not identical DNA). Meiosis – process by which two cell division steps produce gametes (ova and sperm); only occurs in the gonads (ovaries and testes)

Homologous Chromosomes

Meiosis I – reduction division Prophase I: Homologous chromosomes pair up; parts are often swapped in a process called crossing-over. Metaphase I: Homologous chromosomes randomly line up in the center of the cell; maternal and paternal chromosomes are shuffled.

Crossing-over (a) First meiotic prophase Chromosomes pairing Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) First meiotic prophase Chromosomes pairing Chromosomes crossing-over (b) Crossing-over

Meiosis I Anaphase I: Homologous chromosomes are pulled apart. Telophase I: Homologous chromosomes are separated. This results in two daughter cells with 23 chromosomes each (haploid). This is reduction division since each cell now has half as many chromosomes. Necessary for sexual reproduction Crossing-over and shuffling of chromosomes in metaphase 1 result in genetic recombination and genetic diversity

Meiosis II Proceeds like mitosis with phases prophase II through telophase II. Sister chromatids line up in the center of the cell. Centromeres are broken and pulled to opposite poles. Results in 4 cells with 23 chromosomes each.

Meiosis Prophase I Tetrad Metaphase I Anaphase I Telophase I Daughter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prophase I Tetrad Metaphase I Anaphase I Telophase I Daughter cell Daughter cell Prophase II Metaphase II Anaphase II Telophase II Daughter cells Daughter cells

Meiosis

Epigenetic Inheritance Silenced genes are passed to daughter cells during mitosis and meiosis without a change in DNA base sequence Mechanisms of epigenetic inheritance Posttranslational modifications of histone proteins Methylation of cytosine bases in DNA that precede guanine Problems with epigenetic inheritance contribute to cancer, fragile X syndrome, and systemic lupus erythematosus