Cells ch3

defines the changes from the formation of the cell until it (the cell) reproduces

This includes: Interphase and Cell division (mitotic phase)

• -period from cell formation to cell division
• -thought of as "resting" only from dividing
• -nuclear material is 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 helices begin unwinding from the nucleosomes
• helicase untwists the double helix and exposes complementary chains
• the Y-shaped site of replication fork
• each nucleotide strand serves as a template for building a new complementary strand

• 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
• End Result: two DNA molecules formed from the original
• this process is called semiconservative replication

• -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

Includes 2 distinct events: Mitosis and Cytokenesis

Mitosis: four stages of nuclear division

• 1) Prophase =
• -chromosomes become visible, each with 2 chromatids joined at centromere
• -centrosomes separate and migrate toward opposite poles
• -mitotic spindles and asters form
• -nuclear envelope fragments
• -kinetochore microtubules attach to kinetochore of centromeres and draw them toward equator
• -polar microtubules assist in forcing poles apart

• 2) Metaphase=
• -centromeres of chromosomes are aligned at equator
• -this plane midway b/w poles is called the metaphase plate

• 3) Anaphase=
• -shortest phase
• -centromeres of chromosomes split at same time; each chromatid now becomes a chromosome
• -chromosomes (V shaped) are pulled toward poles by motor proteins of kinetochores
• -polar microtubules continue forcing poles apart

• 4) Telophase=
• -begins when chromosome movement stops
• - the 2 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

• "Go" signals:
• -critical volume of cell when area of membrane is inadequate for exchange
• -chemicals (eg growth factors, hormones, cyclins...)

• "Stop" signals:
• -contact inhibition
• -growth-inhibiting factors produced by repressor genes

• 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

• Messenger RNA (mRNA)
• -carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm

• Ribosomal RNA (rRNA)
• -a structural component of ribosomes that, along with tRNA, helps translate message from mRNA

• Transfer RNAs (tRNAs)
• -bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis

transfers DNA gene base sequence to a complementary base sequence of an mRNA

• 3 basic phases:
• initiation, elongation, and termination

1) initiation - once properly positioned, RNA polymerase pulls apart strands of DNA double helix so transcription can begin at the start point in promoter

• 2) elongation - RNA aligns nucleotides w/complementary DNA bases on template strand and links them together.  RNA polymerase elongates the mRNA strand one base at a time, unwinds the DNA helix and rewinds helix behind it. At any given time, 16-18 base pairs of DNA are unwound
• DNA-RNA hybrid is up to 12 pairs long

3) termination - transcription ends and newly formed mRNA separates from DNA template

• 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

• 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

at any given moment, 16-18 base pairs of DNA are unwound and the most recently made RNA is still bound to DNA

• converts base sequence of nuceic acids into the amino acid sequence of proteins
• involves mRNA, tRNAs, and rRNAs
• 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

• mRNA/ribosome complex is directed to rough ER by a singal-recognition 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

• nonfunctional organelle proteins are degraded by lysosomes
• ubiquitin tags damaged or unneeded soluble proteins in cytosol; they are digested by enzymes of proteasomes

• 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

• all cells of the body contain the same DNA but are not identical
• chemical signals in the emryo channel cells into specific development pathways by turning some genes off
• development of specific and distinctive features in cell is called cell differentiation
• elimination of excess, injured, or aged cells occurs through programmed rapid cell death (apoptosis) followed by phagocytosis

• 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 # of times a cell can divide