Cell Division

Multicellular life starts as a single cell Growth, development and reproduction require cells to divide and replicate themselves

Types of Cell Division Binary Fission –Replication and division in Prokaryotes Mitosis –Replication of a cell to produce identical daughter cells Meiosis –The creation of gametes for sexual reproduction

Quick Refresher – Binary Fission The prokaryote replicates its single chromosome The cell continues to grow and eventually splits into two cells

Cell Cycle The process of cells growing, replicating and dividing Can take minutes, hours, or up to a year to cycle through (most about 1 day) Some cells cease to grow and divide and are no longer part of the cycle –Ex. Heart, eye, nervous system

Cell Cycle Consists of two main stages –Interphase –Mitotic Phase

Cell Cycle The interphase is divided into 3 parts –Gap 1 (G1) –Synthesis (S) –Gap 2 (G2)

Cell Cycle - Interphase G1 –The cell grows in size and produces RNA and synthesizes proteins S –The cell continues to grow, and replicates (synthesizes) it’s DNA G2 –The cell continues to grow and prepare for mitosis

Cool Fact It takes approximately six hours to replicate the entire human genome That is 6,000,000,000 (six billion) nucleotides that must be kept exactly the same as the parent cell Mistakes in this replication lead to mutations

Mitotic Phase Divided into two main parts –Mitosis Division of the nuclear material –Cytokinesis Division of the cytoplasm

Cell Cycle Control Errors in cells growth can lead to uncontrolled division and development –This is how tumors can develop Proteins in the cytoplasm of a cell regulate the cell cycle When working properly, this prohibits the cell from replicating if conditions are not ideal, or there are errors in DNA

Cell Cycle Control There are three main checkpoints in the cell cycle –G1 Checkpoint –G2 Checkpoint –M Checkpoint There are also control mechanisms in the S phase

Cell Cycle Control Main proteins involved –Cyclins –CDKs –MPF (maturation promoting factor) –p53 –p27

Cyclins Levels within the cell rise and fall through the cycle G1 (cyclin D) S (cyclin E and cyclin A) M (cyclin B and cyclin A)

CDKs Levels remain relatively constant in the cell Bind with cyclins to be activated G1 to S CDK4 S CDK2 G2 to M CDK1

Other Control Proteins MPF – promotes movement from G2 to M p53 – can stop cell cycle if damage is present for either repair or apoptosis (cell death) p27 – a protein that can bind to cyclin and CDK stopping entry to S phase

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Cell Cycle Homework Draw the cell cycle and label each stage and checkpoint Describe in your own words what is happening at each stage, and each checkpoint

The Chromosome Chromosomes are the condensed form of the nuclear Chromatin Eukaryotes generally have two copies of each chromosome In humans that is 46 total chromosomes We call a cell with two full sets of chromosomes 2N or Diploid

The Chromosome During cell division chromosomes replicate When they are replicated each chromosome is called a chromatid The region of DNA that joins them is called the centromere Covering the centromere is the kinetochore

Label the Following Diagram Kinetochore Centromere Chromatid

Important Structures Spindle – a collection of microtubules that connect the centrosome to the kinetochore Aster – a collection of microtubules that connect the centrosome to the cell membrane

Important Structures Centriole – a cylindrical rod of microtubules found only in animal cells Centrosome – a condensed portion of cytoplasm that organizes microtubules. In animal cells it contains the centrioles.

Mitosis The separation of the nuclear material in a cell Divided into 5 phases –Prophase –Prometaphase –Metaphase –Anaphase –Telophase

Prophase Chromatin condenses to chromosomes Nuclear envelope dissolves Centrioles divide and migrate to opposite ends of the cell Microtubules begin to form from the centrosome

Prometaphase Centrioles finish their migration Proteins attach to the centromere to create the kinetochore Microtubules emerge from the kinetochore and connect to the centrosome Microtubules connect each centrosome creating the spindle Microtubules connect the centrosome to the cell membrane creating the aster

Metaphase Spindle fibers line up sister chromatids along the center of the cell called the Metaphase Plate The M Checkpoint happens at this time, ensuring the chromosomes are properly lined up so one chromosome enters each daughter cell

Anaphase Spindle fibers pull apart the sister chromatids Chromatids migrate to opposite sides of the cell

Telophase Chromatids arrive at opposite ends of the cell Chromatids disperse back to chromatin Spindle disperse Cleavage furrow (animal cells) begins to form Cytokinesis begins

In animals, a ring of microfilaments contracts until it finally pinched the cell in half. In plants, portions of cell wall contained in membrane vescicles (circles) condense to form a plate (or wall).

Stages of M phase

Early Prophase

Prometaphase

Metaphase

Early Anaphase

Late Anaphase

telophase

Daughter cells

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