Cell Division

Chapter 11AP

Division in Prokaryotes

• Binary Fission– Lack a nucleus– Circular DNA attached to

plasma membrane– At replication site 22 proteins

begin replication– When complete, daughter

DNAs attached to PM next to each other

– Plasma membrane grows between DNA until divided in two

Eukaryotic Chromosomes

• Discovered during mitosis• Varied number between organisms– Primitive plant (fern) has 500 pairs, but advanced

flowering plant has 1 pair• Humans have 23 almost identical pairs– Loss of one chromosome = monosomy (usually

death)– Gain of one chromosome = trisomy (sometimes

death or developmental problems)

Structure of Chromosomes

• Chromatin– Complex of DNA and protein

• Chromosome– Composed of chromatin– Long unbroken strands of DNA– Can contain 140 million nucleotides– Super-super-super-coiled

Supercoiling

• “String-of-beads”– Every 200 nucleotides is wrapped around 8 histone proteins =

nucleosome• DNA attracted to histones by opposite charge (“+” histones to “-”

phosphates)

– Heterochromatin• Highly condensed chromatin that

does not uncoil, thus is never expressed

– Euchromatin• Condensed only during cell

division, but is uncoiled when not dividing so genes can be expressed

Karyotypes

• Particular array of chromosomes for an individual• Chromosomes differ from each other within the same

cell– Size– Staining properties– Location of centromere– Length of arms on either side of

centromere

• To view karyotype– Induce cell division, stop cell

division, lyse cells, stain chromosomes, take picture, cut out, then order largest to smallest

Phases of Cell Cycle

• Phases– Interphase

• G1 phase: primary growth– G0 phase: resting phase

• S phase: synthesis of entire genome• G2 phase: prep for cell division

– (M) Mitosis• Prophase• Metaphase• Anaphase• Telophase

– (C) Cytokinesis—division of cytoplasm

Duration of Cell Cycle

• Cell cycle lengths– Most embryos = 20 minutes– Fruit fly embryo = 8 minutes– Dividing mammalian cell = 24 hours– Human liver = 1 year

– M phase of cell cycle only about an hour for regular cells

– Some cells enter G0 phase for days to years before entering cellular division (some stay there indefinitely)

Interphase

• G1

– Major cell growth– Occurs directly after cell division, so cell must get bigger and

mature before dividing again– Protein synthesis

• S phase– Each chromosome is replicated producing two sister chromatids

attached by a centromere

• G2

– Second growth– Mitochondria and other organelles are replicated– Chromosomes are condensed (supercoiled)– Centrioles replicate (animal cells)

Mitosis

• Prophase– Formation of mitotic apparatus

• Begins as chromosomes become visible w/ light microscope• Condensation continues

– Assembling spindle apparatus• Begins at end of G2 phase and continues into prophase• Centrioles begin to move apart forming an axis of microtubules

called spindle fibers– Bridge of spindle fibers called spindle apparatus– Also in plant cells, but no centrioles

• Nuclear envelope disappears

– Microtubules from opposite poles begin to connect to kinetochores of sister chromatids

Mitosis (cont.)

• Metaphase– Chromosomes line up in center of cell at

metaphase plate– Positioned by microtubules attached at

kinetochores• Anaphase– All centromeres divide at same time– Separates sister chromatids– Pulled apart to different poles– Poles themselves move apart– Centromeres move toward the poles

Mitosis (cont.)

• Telophase– Spindle apparatus disassembles– Nuclear envelope reassembles around each set of

sister chromatids (chromosomes, now)– Chromosomes begin to uncoil– rRNA genes expressed and nucleolus will soon be

reformed

Cytokinesis

• Mitosis over• Nuclei at opposite ends of cell• Actual cell division not over until two new

daughter cells separate = cytokinesis• Involves cleavage of cell into two equal halves• In animal cells– Belt of actin filaments constricts and pinches creating

a cleavage furrow. Constriction continues until cells separate

Cytokinesis

• In plant cells– Cell wall too rigid to be pinched– Assemble membrane components

from within– Called cell plate– Cellulose then placed between

two new membranes making cell wall

• In fungi and protists– When mitosis is complete, nucleus

divides into daughter nuclei

Cell Cycle Control

• Need sufficient time for events to occur– Internal clock– End of last phase starts next phase– “go/no-go” switches

• Control system– Growth is assessed at G1 checkpoint• Key decision whether cell should divide or not– If favorable, cell goes into S phase– If not, cell will continue to grow

Cell Cycle Control (cont.)

– DNA replication success assessed at G2 checkpoint• Problems with DNA synthesis are fixed• If passes inspection cell will enter M phase

– Mitosis assessed at M checkpoint• At metaphase checkpoint initiates exit from mitosis and

cytokinesis and beginning of G1

Mechanisms of Cell Control

• Cyclin control system– Cyclin-dependent protein kinases (Cdks)

• Phosphorylate particular amino acids in proteins important to cellular division

• This initiates the cell cycle past the checkpoint

– G2 checkpoint is better understood• G2 cyclin gradually increases• Cyclin binds to Cdk forming MPF (mitosis promoting

factor)• As more MPF accumulates there is a positive feedback

that phosphorylates more MPF• The MPF threshold is reached and triggers mitosis and

the end of G2

Mechanisms of Cell Control

– MPF also activates proteins that destroy the very cyclin that started the whole process

– As cyclin becomes less available to make MPF it initiates the end of mitosis

• G1 checkpoint– Less understood than G2– Thought to be regulated like G2– Cell size triggers DNA replication– Determinant for S phase is ratio of amount of cytoplasm

to genome size– This triggers production of more cyclins, then triggering S

and G2 phases

Cell Cycle in Eukaryotes

• To maintain organization, only certain cells can divide at certain times

• Cells use regulatory signals called growth factors• Growth factors fit cell surface receptor– Some very specific, others not so specific– Many cells need many different types of growth

factors in order to overcome the controls that inhibit cell division

• If cells do not get growth factors, they stop after G1 and go into G0 phase (non-growing stage)

Cancer

• Unrestrained, uncontrolled cell growth• Read about the p53 gene • Proto-oncogenes (must be turned on to cause cancer)

– Genes that stimulate cell division– If damaged they lead to oncogenes (cancer)– myc, fos, jun

• myc expression prevents cell division even in presence of growth factors

• Tumor-suppressor genes (must be turned off or mutated to cause cancer—recessive: both copies must be bad)– Suppress cell division by preventing cyclins from binding to Cdk– If mutated, can lead to uncontrolled cell division, but is recessive– Read about retinoblastoma gene