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Cell Division and Genetics – Mechanisms for a Knit of Identity and Thread of Distinction. DNA and Its Faithful Replication – The Knit of Identity.
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Cell Division and Genetics – Mechanisms for a Knit of Identity and Thread of Distinction
DNA and Its Faithful Replication – The Knit of Identity Because DNA stores genetic information and is faithfully replicated, information is passed largely unaltered from cell-to-cell, generation-to- generation.
Cell Division Demands Coordination of DNA Replication, Mitosis and Cytokinesis What’s so important about cell division (3 reasons)?
Cell division requires coordinated division of chromosomes (mitosis) ….. …… and division of the cytoplasm (cytokinesis).
DNA Replication – Simple in Principle, Complicated in Practice
duplicated chromosome chromatin DNA is Packaged into Chromosomes DNA in the cell is virtually always associated with proteins. The packaging is impressive – 2 meters of human DNA fit into a sphere about 0.000005 meters in diameter.
DNA is Condensed into Visible Chromosomes Only For Brief Periods in the Life of a Cell 95% of the time, chromosomes are like this. Easily visible chromosomes are apparent perhaps 5% of the time in an actively growing cell and less in a non-growing cell.
A Karyotype is an Arranged Picture of Chromosomes At Their Most Condensed State Note that almost all chromosomes come in homologous pairs. Boy or girl? A normal human karyotype
From Birth to Rebirth, a Cell Progresses Through Characteristic Stages That Constitute the Cell Cycle In multicellular organisms like us, progress through the cell cycle is carefully regulated.
The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: • Interphase • Mitosis • Cytokinesis
1. Interphase (Cell is not dividing) • G1 Phase – carries out basic functions & performs specialized activities. • duration is extremely variable • contains restriction checkpoint ~ cell “decides” to: • divide • enter a quiescent phase (G0) • die
1. Interphase (Cell is not dividing) • G0 Phase – cell maintains specialized characteristics, but does not divide Ex. neurons & muscle cells
1. Interphase (Cell is not dividing) • S Phase – cell replicates chromosomes & synthesizes proteins • animal cells replicate centrioles as well
1. Interphase (Cell is not dividing) • G2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material
Mitosis (M phase) – Equal distribution of replicated genetic material. • Four steps: • Prophase • Metaphase • Anaphase • Telophase
Mitosis – Prophase • replicated chromosomes condense • centrosomes separate & migrate toward opposite sides of cell • mitotic spindle forms (microtubules grow out from centrosomes) • nucleolus and envelope disappear
Mitosis – Late Prophase (prometaphase) • nuclear membrane breaks down • spindle fibers attach to centromeres of chromosomes
Mitosis – Metaphase • chromosomes are lined up single-file along equator of mitotic spindle
Mitosis – Anaphase • Centromeres part, sister chromatids (now called chromosomes) separate • chromosomes move toward opposite poles • ?
Mitosis – Telophase • mitotic spindle breaks down • chromosomes decondense • nuclear membranes reform around two nuclei • nucleoli reappear
Cytokinesis • Distribution of cytoplasm to daughter cells • begins during anaphase or telophase • differs in animal & plant cells
Cytokinesis in animal cells • Cleavagefurrow (slight indentation) forms around equator of cell • Actin & myosin microfilaments act like a drawstring to pinch the cell in two • Usually an equal division
In Animal Cells, a Cleavage Furrow Forms and Separates Daughter Cells Cleave furrow in a dividing frog cell.
