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Home , Metaphase

How Cells Divide Chapter 10

Bacterial Division

Bacteria divide by binary fission. -the single, circular bacterial is replicated -replication begins at the origin of replication and proceeds bidirectionally -new are partitioned to opposite ends of the cell -a septum forms to divide the cell into 2 cells

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1 Eukaryotic Chromosomes

Eukaryotic chromosomes – -linear chromsomes -every species has a different number of chromosomes -composed of – a complex of DNA and proteins

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Eukaryotic Chromosomes

Chromosomes are very long and must be condensed to fit within the nucleus. -nucleosome – DNA wrapped around a core of 8 histone proteins -further coiling creates the 30-nm fiber or solenoid

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Eukaryotic Chromosomes

The solenoid is further compacted: -radial loops are held in place by scaffold proteins -scaffold of proteins is aided by a complex of proteins called : the particular array of chromosomes of an organism

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Karyotype

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Eukaryotic Chromosomes

Chromosomes must be replicated before . -Replicated chromsomes are connected to each other at their - – complex of proteins holding replicated chromosomes together -sister chromatids: 2 copies of the chromosome within the replicated chromosome

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Eukaryotic

The eukaryotic cell cycle has 5 main phases:

1. G1 (gap phase 1) 2. S (synthesis)

3. G2 (gap phase 2) 4. M () 5. C () The length of a complete cell cycle varies greatly among cell types. 11

Interphase

Interphase is composed of:

G1 (gap phase 1) – time of

S phase – synthesis of DNA (DNA replication) - 2 sister chromatids are produced

G2 (gap phase 2) – chromosomes condense 12

4 Interphase

Following , the sister chromatids appear to share a . In fact, the centromere has been replicated but the 2 are held together by cohesin proteins. Proteins of the are attached to the centromere. attach to the kinetochore.

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Cohesin Chromatid proteins Centromere region of chromosome

Kinetochore Kinetochore microtubules Metaphase chromosome 14

Interphase

During G2 the chromosomes undergo condensation, becoming tightly coiled.

Centrioles (-organizing centers) replicate and one centriole moves to each pole.

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5 Mitosis

Mitosis is divided into 5 phases: 1. 2. 3. metaphase 4. 5.

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Mitosis

Prophase: -chromosomes continue to condense -centrioles move to each pole of the cell - is assembled -nuclear envelope dissolves

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6 Mitosis

Prometaphase: -chromosomes become attached to the spindle apparatus by their kinetochores -a second set of microtubules is formed from the poles to each kinetochore -microtubules begin to pull each chromosome toward the center of the cell

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Mitosis

Metaphase: -microtubules pull the chromosomes to align them at the center of the cell -metaphase plate: imaginary plane through the center of the cell where the chromosomes align

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Mitosis

Anaphase: -removal of cohesin proteins causes the centromeres to separate -microtubules pull sister chromatids toward the poles

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Mitosis

Telophase: -spindle apparatus disassembles -nuclear envelope forms around each set of sister chromatids -chromosomes begin to uncoil -nucleolus reappears in each new nucleus

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9 Cytokinesis

Cytokinesis – cleavage of the cell into equal halves -in animal cells – constriction of actin filaments produces a cleavage furrow -in plant cells – plasma membrane forms a cell plate between the nuclei -in fungi and some protists – mitosis occurs within the nucleus; division of the nucleus occurs with cytokinesis

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10 Control of the Cell Cycle

The cell cycle is controlled at three checkpoints:

1. G1/S checkpoint -the cell “decides” to divide

2. G2/M checkpoint -the cell makes a commitment to mitosis 3. late metaphase (spindle) checkpoint -the cell ensures that all chromosomes are attached to the spindle 31

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Control of the Cell Cycle – proteins produced in synchrony with the cell cycle -regulate passage of the cell through cell cycle checkpoints -dependent kinases (Cdks) – enzymes that drive the cell cycle -activated only when bound by a cyclin

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Control of the Cell Cycle

Growth factors: -can influence the cell cycle -trigger intracellular signaling systems -can override cellular controls that otherwise inhibit cell division platelet-derived growth factor (PDGF) triggers cells to divide during wound healing

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Control of the Cell Cycle

Cancer is a failure of cell cycle control.

Two kinds of genes can disturb the cell cycle when they are mutated: 1. tumor-suppressor genes 2. proto-

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12 Control of the Cell Cycle

Tumor-suppressor genes: -prevent the development of many cells containing mutations -for example, halts cell division if damaged DNA is detected -p53 is absent or damaged in many cancerous cells

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Control of the Cell Cycle

Proto-oncogenes: -some encode receptors for growth factors -some encode signal transduction proteins -become oncogenes when mutated -oncogenes can cause when they are introduced into a cell

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