Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
3 Cells: The Living Units: Part D Cell Cycle • Defines changes from formation of the cell until it reproduces • Includes: • Interphase • Cell division (mitotic phase) Interphase • Period from cell formation to cell division • Nuclear material called chromatin • Four subphases: • • • • G1 (gap 1)—vigorous growth and metabolism G0—gap phase in cells that permanently cease dividing S (synthetic)—DNA replication G2 (gap 2)—preparation for division DNA Replication • DNA helices begin unwinding from the nucleosomes • Helicase untwists the double helix and exposes complementary chains • The Y-shaped site of replication is the replication fork • Each nucleotide strand serves as a template for building a new complementary strand DNA Replication • DNA polymerase only works in one direction • Continuous leading strand is synthesized • Discontinuous lagging strand is synthesized in segments • DNA ligase splices together short segments of discontinuous strand DNA Replication • End result: two DNA molecules formed from the original • This process is called semiconservative replication DNA Replication Cell Division • Mitotic (M) phase of the cell cycle • Essential for body growth and tissue repair • Does not occur in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle Cell Division • Includes two distinct events: 1. Mitosis—four stages of nuclear division: • Prophase • Metaphase • Anaphase • Telophase 2. Cytokinesis—division of cytoplasm by cleavage furrow Cell Division Prophase • Chromosomes become visible, each with two chromatids joined at a centromere • Centrosomes separate and migrate toward opposite poles • Mitotic spindles and asters form Prophase • Nuclear envelope fragments • Kinetochore microtubules attach to kinetochore of centromeres and draw them toward the equator of the cell • Polar microtubules assist in forcing the poles apart Metaphase • Centromeres of chromosomes are aligned at the equator • This plane midway between the poles is called the metaphase plate Anaphase • Shortest phase • Centromeres of chromosomes split simultaneously—each chromatid now becomes a chromosome • Chromosomes (V shaped) are pulled toward poles by motor proteins of kinetochores • Polar microtubules continue forcing the poles apart Telophase • Begins when chromosome movement stops • The two sets of chromosomes uncoil to form chromatin • New nuclear membrane forms around each chromatin mass • Nucleoli reappear • Spindle disappears Cytokinesis • Begins during late anaphase • Ring of actin microfilaments contracts to form a cleavage furrow • Two daughter cells are pinched apart, each containing a nucleus identical to the original Control of Cell Division • ―Go‖ signals: • Critical volume of cell when area of membrane is inadequate for exchange • Chemicals (e.g., growth factors, hormones, cyclins, and cyclindependent kinases (Cdks)) Control of Cell Division • ―Stop‖ signals: • Contact inhibition • Growth-inhibiting factors produced by repressor genes Protein Synthesis • DNA is the master blueprint for protein synthesis • Gene: Segment of DNA with blueprint for one polypeptide • Triplets of nucleotide bases form genetic library • Each triplet specifies coding for an amino acid Roles of the Three Main Types of RNA • Messenger RNA (mRNA) • Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm Roles of the Three Main Types of RNA • Ribosomal RNA (rRNA) • A structural component of ribosomes that, along with tRNA, helps translate message from mRNA Roles of the Three Main Types of RNA • Transfer RNAs (tRNAs) • Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis Transcription • Transfers DNA gene base sequence to a complementary base sequence of an mRNA • Transcription factor • Loosens histones from DNA in area to be transcribed • Binds to promoter, a DNA sequence specifying start site of gene to be transcribed • Mediates the binding of RNA polymerase to promoter Transcription • RNA polymerase • Enzyme that oversees synthesis of mRNA • Unwinds DNA template • Adds complementary RNA nucleotides on DNA template and joins them together • Stops when it reaches termination signal • mRNA pulls off the DNA template, is further processed by enzymes, and enters cytosol Translation • Converts base sequence of nucleic acids into the amino acid sequence of proteins • Involves mRNAs, tRNAs, and rRNAs Genetic Code • Each three-base sequence on DNA is represented by a codon • Codon—complementary three-base sequence on mRNA Translation • mRNA attaches to a small ribosomal subunit that moves along the mRNA to the start codon • Large ribosomal unit attaches, forming a functional ribosome • Anticodon of a tRNA binds to its complementary codon and adds its amino acid to the forming protein chain • New amino acids are added by other tRNAs as ribosome moves along rRNA, until stop codon is reached Role of Rough ER in Protein Synthesis • mRNA–ribosome complex is directed to rough ER by a signalrecognition particle (SRP) • Forming protein enters the ER • Sugar groups may be added to the protein, and its shape may be altered • Protein is enclosed in a vesicle for transport to Golgi apparatus Other Roles of DNA • Intron (―junk‖) regions of DNA code for other types of RNA: • Antisense RNA • Prevents protein-coding RNA from being translated • MicroRNA • Small RNAs that interfere with mRNAs made by certain exons • Riboswitches • Folded RNAs that act as switches regulating protein synthesis in response to environmental conditions Cytosolic Protein Degradation • Nonfunctional organelle proteins are degraded by lysosomes • Ubiquitin tags damaged or unneeded soluble proteins in cytosol; they are digested by enzymes of proteasomes Extracellular Materials • Body fluids (interstitial fluid, blood plasma, and cerebrospinal fluid) • Cellular secretions (intestinal and gastric fluids, saliva, mucus, and serous fluids) • Extracellular matrix (abundant jellylike mesh containing proteins and polysaccharides in contact with cells) Developmental Aspects of Cells • All cells of the body contain the same DNA but are not identical • Chemical signals in the embryo channel cells into specific developmental pathways by turning some genes off • Development of specific and distinctive features in cells is called cell differentiation • Elimination of excess, injured, or aged cells occurs through programmed rapid cell death (apoptosis) followed by phagocytosis Theories of Cell Aging • Wear and tear theory: Little chemical insults and free radicals have cumulative effects • Immune system disorders: Autoimmune responses and progressive weakening of the immune response • Genetic theory: Cessation of mitosis and cell aging are programmed into genes. Telomeres (strings of nucleotides on the ends of chromosomes) may determine the number of times a cell can divide.