CELLS: THE LIVING UNITS CHAPTER # 3(d) CELLS: THE LIVING UNITS

Defines changes from formation of the cell until it reproduces Cell Cycle Defines changes from formation of the cell until it reproduces Includes: Interphase Cell division (mitotic phase)

Period from cell formation to cell division 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

S Growth and DNA synthesis G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

Centrosomes (each has 2 centrioles) Interphase Plasma membrane Nucleolus Chromatin Nuclear envelope Interphase Figure 3.33

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 polymerase only works in one direction 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

template for synthesis of new strand Free nucleotides DNA polymerase Old strand acts as a template for synthesis of new strand Free nucleotides DNA polymerase Chromosome Leading strand Two new strands (leading and lagging) synthesized in opposite directions Lagging strand Old DNA Helicase unwinds the double helix and exposes the bases Replication fork Adenine Thymine Cytosine DNA polymerase Old (template) strand Guanine Figure 3.32

DNA Replication PLAY Animation: DNA Replication

Mitotic (M) phase of the cell cycle 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

Includes two distinct events: Cell Division Includes two distinct events: Mitosis—four stages of nuclear division: Prophase Metaphase Anaphase Telophase Cytokinesis—division of cytoplasm by cleavage furrow

S Growth and DNA synthesis G1 checkpoint (restriction point) S Growth and DNA synthesis G2 Growth and final preparations for division G1 Growth M G2 checkpoint Figure 3.31

Cell Division PLAY Animation: Mitosis

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

Early mitotic Early Prophase spindle Aster Chromosome Centromere consisting of two sister chromatids Centromere Early Prophase Figure 3.33

Polar microtubule Spindle pole Late Prophase Fragments of nuclear envelope Kinetochore Kinetochore microtubule Late Prophase Figure 3.33

Metaphase Centromeres of chromosomes are aligned at the equator This plane midway between the poles is called the metaphase plate

Metaphase Spindle Metaphase plate Metaphase Figure 3.33

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

Anaphase Daughter chromosomes Anaphase Figure 3.33

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

Telophase and Cytokinesis Nuclear envelope forming Nucleolus forming Contractile ring at cleavage furrow Telophase and Cytokinesis Telophase Figure 3.33

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

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 PLAY Animation: DNA and RNA

Nuclear envelope DNA Transcription RNA Processing Pre-mRNA mRNA pores Ribosome Translation Polypeptide Figure 3.34

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