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:
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•
•
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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.