A.Introduction 1.Cells are the basic functional units of the body. 2.They come in a variety of shapes and sizes. This diversity reflects their diverse functions. 3.Principal parts of cells a.Plasma membrane – selectively permeable, gives form, and separates from the external environment b.Cytoplasm and organelles – fluid part of cell and little organs that do the functions c.Nucleus – contains DNA and directs cell activities
B.Structure of the Plasma Membrane 1.Phospholipid barrier between the intracellular and extracellular environments a.Hydrophobic center of the double membrane restricts the movement of water, water-soluble molecules, and ions. b.Many substances are selectively allowed to pass through protein channels. c.Proteins and phospholipids are not trapped in the membrane but constantly move laterally; known as the fluid mosaic model.
2.Membrane Proteins a.Integral proteins span the membrane. b.Peripheral proteins are embedded on just one side of the membrane. c.Functions: 1)Structural support 2)Transport 3)Enzymatic control of cell processes 4)Receptors for hormones and other molecules 5)“Self” markers for the immune system
3.Other components of the membrane a.Carbohydrates – attached to lipids (glycolipids) and to proteins (glycoproteins); serve as antigens and interactions with regulatory molecules b.Cholesterol – gives flexibility to the membrane
C.Phagocytosis (cell eating) 1.Bulk transport or large extracellular substances into the cell 2.Some cells, like neutrophils and macrophages, can perform amoeboid movement by extending pseudopods to pull the cell forward. a.Relies on the bonding of proteins called integrins with extracellular proteins 3.Pseudopods engulf bacteria, dead cells, or other organic materials and then fuse together to form a food vacuole. 4.The food vacuole fuses with a lysosome, and the material is digested. 5.Very important for body defense, inflammation, and apoptosis
D.Endocytosis 1.Another process for bringing large materials into the cell 2.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. a.Pinocytosis: nonspecific b.Receptor-mediated endocytosis: specific 1)Has receptor proteins in the membrane that will bind to the substance to be brought in 2)Ex – cholesterol, AIDS virus, hepatitis virus
E.Exocytosis 1.Large cellular products (proteins) are moved out of the cell. 2.The Golgi apparatus packages proteins into vesicles that fuse to the plasma membrane, and the contents spill out of the cell.
F.Cilia & Flagella 1.Cilia - tiny, hairlike structures composed of microtubules that project from the plasma membrane a.Primary cilium – most cells have this nonmotile cilium with “9+0” structure; may have a sensory function in some cells b.Motile cilia beat in unison to move substances through hollow organs. 1)Found in respiratory tract and uterine tubes 2)Have a “9+2” arrangement
Cilia, cont c.Base of each cilium has a pair of centrioles that are perpendicular to each other and called the centrosome; one centriole is parallel to the cilium and called the basal body
A.Cytoplasm & Cytoskeleton 1.Material within a cell a.Includes organelles, a fluid called cytosol, the cytoskeleton, and inclusions. b.Inclusions – stored chemical aggregates such as glycogen, melanin, and triglycerides
2.Cytoskeleton a.Organized system of microtubules and microfilaments throughout the cytoplasm b.Proteins of the cytoskeleton are mobile. c.They organize the intracellular environment and allow movement of muscle cells and phagocytic cells. d.They form the spindle apparatus that pulls chromosomes apart in mitosis. e.They also serve as a “railway” system for vesicles and organelles to move along using molecular motors of myosin, kinesins, and dyneins
B.Lysosomes 1.Organelles filled with digestive enzymes a.Fuse with food vacuoles after an immune cell engulfs a bacterium or dead cell b.Primary lysosome: only contains digestive enzymes c.Secondary lysosome: contains the partially digested contents of the food vacuole or worn- out organelles d.Residual body: a lysosome filled with waste, which can be expelled through exocytosis
Lysosomes, cont 2.Besides digesting bacteria, lysosomes are responsible for: a.Autophagy: process of digesting worn-out or damaged organelles and proteins in the cell b.Apoptosis: programmed cell death. The lysosome spills its contents, killing the cell.
C.Peroxisomes 1.Contain enzymes specific to certain oxidative reactions 2.Found in most cells but most numerous in the liver; often oxidize toxic molecules (such as alcohol) 3.Enzymes used to remove hydrogen from a molecule and transfer it to O 2, forming hydrogen peroxide 4.Also contain the enzyme catalase, which converts hydrogen peroxide into water and O 2
D.Mitochondria 1.Sites of energy production through aerobic cell respiration 2.Structure a.Have an inner membrane and an outer membrane separated by an intermembranous space b.Inner membrane is folded into cristae to increase surface area for reactions c.Central area is fluid and called the matrix
Mitochondria, cont 3.Most cells have mitochondria, and there can be thousands of mitochondria in a single cell. 4.Mitochondria can migrate around the cell and can make copies of themselves. a.Have their own DNA, all derived from mom b.Mutations of mitochondrial DNA may contribute to aging and disease
E.Ribosomes 1.Protein factories of the cell 2.Messenger RNA takes genetic information to the ribosome so a protein can be assembled. 3.Very small; made of 2 subunits of ribosomal RNA and protein 4.Found free in the cytoplasm or associated with the granular endoplasmic reticulum 5.Serves as enzymes called ribozymes that are needed for protein synthesis
F.Endoplasmic Reticulum (ER) 1.System of membranous passageways from the nuclear membrane to the the plasma membrane 2.Granular ER is also called rough ER. a.Has ribosomes embedded on the outer surface b.Functions in protein modification 3.Agranular ER is also called smooth ER. a.Has many functions, depending on the cell
G.Golgi Complex (Apparatus) 1.Consists of stacks of hollow, flattened sacs; cavities are called cisternae a.One side receives proteins from the ER b.These are packaged in vesicles called endosomes, that bud off to fuse with the plasma membrane for exocytosis. 2.Proteins are modified within the cisternae based on the type of protein 3.Retrograde transport – extracellular proteins brought in by endocytosis and then through the Golgi complex to the ER
A.Cell Nucleus 1.Most cells have one nucleus. a.Muscle cells have hundreds; mature RBCs have none. 2.The nucleus is enclosed by the nuclear envelope made of two membranes: a.Outer membrane continuous with rough ER b.Inner membrane often fused to outer by nuclear pore complexes, which allow small molecules and RNA to move into/out of the nucleus through pores
3.DNA and Genes a.The nucleus contains DNA. A gene is a length of DNA that codes for a specific protein. b.The gene on the DNA is transcribed as messenger RNA, which can leave the cell – genetic transcription c.The messenger RNA is then translated at the ribosome to assemble the proper amino acid sequence – genetic translation d.These two steps can be called genetic expression.
4.Nucleoli a.The nucleus also has one or more darker regions not surrounded by a membrane; these are called nucleoli. b.The nucleoli contain the DNA that codes for the production of ribosomal RNA.
B.Genome and Proteome 1.The genome is all the genes in a particular individual or all the genes of a particular species. a.Researchers believe humans have ~25,000 different genes. 2.The proteome is all the proteins that are produced from the genome. a.More than 100,000 proteins are produced in the human body.
Genome and Proteome, cont 3.How can a gene code for more than one protein? a.mRNA is altered after transcription by cutting and splicing different ways b.Proteins are made of many polypeptide chains that can associate in different combinations c.Protein modification can occur by adding a lipid or carbohydrate d.Posttranslational modification by 1)Methylation 2)Phosphorylation 3)Cutting into small units
C.Chromatin 1.DNA in the nucleus is packaged with proteins called histones to form chromatin. a.Histones are positively charged and will interact with negatively charged DNA to cause spooling b.Creates particles called nucleosomes 2.Euchromatin: active in transcription, looser; chemical changes in histones (such as acetylation) allow molecules access to the DNA during gene expression. 3.Heterochromatin: inactive regions, highly condensed; much of the DNA is inactive.
D.RNA Synthesis 1.DNA-directed RNA synthesis a.Also called transcription. Involves: 1)Start and stop regions at the beginning and end of the gene 2)Promoters, areas of DNA that are not part of the gene but tell enzymes involved where to begin 3)Transcription factors that bind to the promoter to begin transcription
RNA Synthesis, cont b.RNA polymerase, breaks the hydrogen bonds between the base pairs of DNA and assembles the appropriate RNA nucleotide 1)RNA nucleotides pair up to the DNA template 2)Assembly is complementary. If the DNA is GCTA, the RNA will be CGAU. 3)RNA has uracil instead of thymine. c.Forms precursor messenger RNA that detaches from the DNA template d.Only 1 freed DNA strand is transcribed
2.Types of RNA a.Precursor messenger RNA (pre-mRNA) – made directly by transcription b.Messenger RNA (mRNA) – modified pre-mRNA; contains the code to make a specific protein c.Transfer RNA (tRNA) – carries amino acids to mRNA for translation d.Ribosomal RNA (rRNA) – along with protein, forms ribosomes; site of translation; acts as an enzyme
3.Pre-mRNA modification a.Precursor messenger RNA is altered in the nucleus before it leaves as messenger RNA (mRNA). b.Portions of the gene that do not actually code for proteins are called introns and must be spliced out. Introns may regulate the expression of the area that do code for protein c.The portions that are kept are called exons because they are “expressed”
Pre-mRNA modification, cont d.Alternative splicing allows one precursor mRNA to be used to make multiple proteins. e.Exons are joined together by spliceosomes and snRNPs (snurps) which are small nuclear ribonucleoproteins
E.RNA Interference 1.RNA molecules that don’t code for proteins may prevent some mRNA molecules from being translated. a.Two types: siRNA (short interfering) and miRNA (micro interfering) b.The expression of at least 30% of genes is regulated in this way.
2.siRNA a.Formed from a larger double-stranded RNA molecule that leaves the nucleus b.An enzyme in the cytoplasm called Dicer cuts the strand into short segments 21-25 nucleotides long
3.miRNA a.Formed form a single-stranded RNA that doubles over in hair-pin loops of double- stranded RNA b.Made from introns removed from pre-mRNA c.Processed in the nucleus by enzymes and by Dicer in the cytoplasm into short (22 nucleotides) double-stranded segments
RNA interference, cont 4.One of the strands of siRNA and miRNA enter a protein particles called RNA-induced silencing complex (RISC) a.Can bind to mRNA where it is complementary b.Prevents tRNA from bringing amino acids to mRNA, so translation is prevented – the gene is silenced 5.RNA interference may help in cancer treatments as well as other genetic conditions
A.Protein Synthesis 1.Also called translation 2.mRNA attaches to a string of ribosomes to form a polyribosome. 3.A group of three bases on DNA, called a triplet, gives the complementary three base sequence in mRNA, called a codon 4.The codon codes for an amino acid, so the order of the codons gives the order of amino acids in a polypeptide
B.Transfer RNA (tRNA) 1.A single strand of RNA bent into a cloverleaf shape 2.One end has the anticodon, which is three nucleotides that will be complementary to the proper codon. 3.The other end has the appropriate amino acid bonded by aminoacyl-tRNA synthetase enzyme
C.Formation of a Polypeptide 1.The mRNA moves through the ribosome, with the proper tRNA attaching at each codon. 2.Amino acids attached to the tRNAs form peptide bonds to each other and disassociate from the tRNA. 3.tRNAs disassociate from the mRNA as they lose their amino acids. 4.This continues until a stop codon is encountered and the whole complex disassociates. 5.Interactions between amino acids of the growing polypeptide chain cause bends and folds into secondary and tertiary structures 6.Chaperone proteins aid the formation of correct interactions and folds
D.Functions of the ER and Golgi Complex 1.Newly formed proteins destined to leave the cell are made on the granular ER. a.The first ~30 amino acids are hydrophobic and are attracted to the lipid portion of the membrane of the granular ER. b.The growing polypeptide chain enters the cisternae of the ER. c.The hydrophobic leader sequence is removed and other portions may be removed or added. d.Ex - In the conversion of preproinsulin to insulin, the middle region is removed and the ends bonded together.
Functions of the ER and Golgi Complex, cont 2.Secretory proteins are next sent to the Golgi complex. a.Proteins may be further modified, including the production of glycoproteins. b.Proteins are separated according to destination. c.Proteins are packaged and shipped in vesicles to their destinations.
E.Protein Degradation 1.Regulatory proteins are rapidly degraded, making their effects short-lived. This allows for greater control of cell functions. a.Proteases in the lysosome digest proteins. b.Proteins outside the lysosomes are tagged by a molecule called ubiquitin, which marks them for degradation by a proteasome. c.Ubiquitin may also tag membrane proteins and organelles for destruction
A.DNA Replication 1.Before cell division, each DNA molecule must replicate itself so that one of each copy can be distributed to the two new cells. 2.Involves two types of enzymes: a.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, cont b.DNA polymerase attaches complementary nucleotides to the exposed strand. 1)Two new molecules are being made from the original one, each with half old and half new DNA. 2)This is called semiconservative replication.
B.The Cell Cycle 1.Introduction a.Divided into interphase, mitosis, and cytokinesis b.Interphase is divided into G 1, S, and G 2 c.Mitosis is divided into Prophase, Metaphase, Anaphase, and Telophase d.Cytokinesis overlaps the last parts of mitosis
2.Interphase a.Lots of RNA synthesis occurring b.G 1 phase: The cell is performing the functions characteristic of cells in that tissue. 1)Cyclin D moves the cell through G 1. 2)Overactivity of the gene for cyclin D has been implicated in some cancers. 3)p53 is a transcription factor that can stall a gene at the G 1 /S checkpoint, repair DNA damage or promote apoptosis 4)If a cell does not divide, it remains in a modified G 1 phase its whole life.
Interphase, cont c.S phase: If a cell is going to divide, it performs DNA replication at this time d.G 2 phase: Chromosomes start to condense; c onsist of two strands called sister chromatids joined by a centromere.
3.Cell Death a.Necrosis: Cell dies pathologically due to deprivation of blood supply. b.Apoptosis: Programmed cell death is performed by enzymes called caspases. 1)Extrinsic: “Death ligands” attach to the cell and mark it for destruction. 2)Intrinsic: Intercellular signals trigger death due to DNA damage, cancer, infection, or oxidative stress. c.“Knocked-out” mice with their p53 gene removed are used to study cancer and apoptosis
C.Mitosis 1.Prophase: Chromosomes become visible, nuclear membrane and nucleolus disappear, centrioles move apart, spindle fibers form 2.Metaphase: Chromosomes line up in the center of the cell and have attached to spindle fibers
Mitosis, cont 3.Anaphase: Centromeres split as the spindle fibers shorten and pull chromatids to opposite sides. 4.Telophase: Cytoplasm is divided (cytokinesis) and cells separate, new nuclear membrane and nucleolus appears, chromosomes lengthen
5.Role of the Centrosome a.Structure located near the nucleus of a nondividing cell. b.At the center are two centrioles. They replicate in interphase and move away from each other in prophase. c.Spindle fibers form from the pericentriolar materials around the centrioles and attach to the centromere of the replicated sister chromatids d.Other tubules cause the formation of the cleavage furrow for cytokinesis
Centrosome, cont e.In nondividing cells, the centrosome migrates to the plasma membrane and forms the nonmotile primary cilium. f.In ciliated cells, hundreds of centrosomes form and become the basal bodies of the cilia
6.Telomeres and Cell Division a.Cells can only divide for so long. Loss of the ability to divide may be due to the loss of regions at the end of the DNA called telomeres. b.Each time DNA is replicated, a little more telomere is lost. c.Damaged telomeres activate p53 which induces cell cycle arrest, senescence, and apoptosis d.Cells that can divide indefinitely, such as those in bone marrow, have an enzyme called telomerase that replicates the telomere.
7.Hypertrophy and Hyperplasia a.Hyperplasia: growth due to an increase in the number of cells; responsible for the growth of most body regions b.Hypertrophy: growth due to an increase in cell size; responsible for increase in skeletal muscle size
D.Meiosis 1.Homologous Chromosomes a.Humans have 23 pairs of chromosomes, one set from each parent. 22 pair are autosomes and 1 pair are sex chromosomes b.Each pair is called homologous chromosomes, and they have the same genes on them (but not identical DNA). 2.Meiosis - Process by which two cell division steps produce gametes (ova and sperm); only occurs in the gonads (ovaries and testes)
a.Meiosis I – reduction division 1)Prophase I: Homologous chromosomes pair up; parts are often swapped in a process called crossing-over. 2)Metaphase I: Homologous chromosomes line up in the center of the cell; cells line up at random; maternal and paternal chromosomes are shuffled.
Meiosis I, cont 3)Anaphase I: Homologous chromosomes are pulled apart. 4)Telophase I: Homologous chromosomes are separated. This results in two daughter cells with 23 chromosomes each. a)This is reduction division since each cell now has half as many chromosomes. b)Necessary for sexual reproduction c)Crossing-over and shuffling of chromosomes in metaphase 1 result in genetic recombination and genetic diversity
b.Meiosis II 1)Proceeds like mitosis with phases prophase II through telophase II. 2)Sister chromatids line up in the center of the cell. Centromeres are broken and pulled to opposite poles. 3)Results in 4 cells with 23 chromosomes each.