Cells: the Living Units: Part D

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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 cyclin-dependent 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 signal-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

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.