AP Biology

The Cell CyclePart 2

Important concepts from previous units:1) Centrioles are components of the cytoskeleton that are composed of microtubules. (Pg. 114)2) Microtubules also compose the spindle fibers of mitosis and meiosis. (pg. 113)3) Proteins are the work horses of cells. They help move things within cells.

Centrioles

CELL CYCLE-IPMATC

• The Cell Cycle Phases for Eukaryotic Cells are:– Interphase

• Cells spend 90% of there existence in this phase.• This phase consists of three parts:• G1 (Primary or “first” growth)– This is ordinary, everyday growth, activity, or repair of

the cell.– Organelles begin replicating.– First checkpoint (called “point of no return”) is the

barrier to the rest of the cycle.

• S (synthesis)– The DNA replicates or is synthesized during this phase.– In humans, we go from 46 Chromosomes “2n” to 92

chromosomes “4n”. (make correction!!!!!!)

• G2 (Secondary or “second” growth)– The organelles mainly enlarge or complete replication.– The newly synthesized DNA is checked for errors.– Second checkpoint occurs after this “part”. (Do we have

everything for TWO cells? If yes, then proceed to dividing; if no, then make what is missing.)

• Human cells NEVER normally contain 92 chromosomes. – A diploid human cell prior to replicating its DNA has 23 pairs of

non-replicated chromosomes (46 total). – A diploid human cell after replicating its DNA has 23 pairs of

replicated chromosomes (46 total). – The difference is that each replicated chromosome consists of

two DNA molecules called 'sister chromatids'. – Non-replicated chromosomes consist of only one DNA molecule

• (it is not called a chromatid because chromatids are like twins--if you only have one you don't say there is one twin.

– Thus, you can say that a human cell following DNA replication has 92 chromatids, but never 92 chromosomes.

Before and after the S phase

Interphase cell (Look at the chromatin in the blue nucleus and the

yellow cytoskeleton.)

– Mitosis - means “nucleus division” (First divide the DNA; then secondly the cytoplasm.)– This process has four parts:

• Prophase (“pro” means “first”)– Nuclear envelope is broken down and

rearranged to make the spindle apparatus.– The chromatin condenses to form “X” shaped

chromosomes. (Two chromatids)– Centrioles move toward the poles. (In animal

cells only…plants use the cell wall.)

Cell in Prophase

Start of Mitosis

• Metaphase (“meta” means “middle”)– The replicated chromosomes line up on the

metaphase plate. (Middle of cell.)– The spindle apparatus attaches to the

kinetochore (a part of the centromere) AND centrioles (the anchors).

– Third checkpoint occurs here. (Are all the chromosomes attached and lined up and ready to “divide/separate” or “segregate”?)

Cell in Metaphase

• Anaphase (“ana” means “separate”)– Replicated chromosomes are pulled apart into sister

chromatids and each chromatid moves toward opposite poles (ends) of the cell.

– The spindle apparatus is being broken down as the two sister chromatids are “walked” toward the poles by the motor protein using ATP.

Cell in Anaphase

• Telophase (“telo” means “last”)– The nuclear envelope is rebuilt by using broken down

spindle apparatus pieces.– The Chromatids begin to decondense back to their

chromatin state.– A cleavage furrow begins to form using actin and

myosin microfilaments.

Cell in Telophase and starting Cytokinesis

Mitosis “Division of the nucleus”

– Cytokinesis (Cleavage means “split”) This is the division of the cytoplasm.– The cytoplasm and cell organelles are separated to produce

two daughter cells.

– G0 (Zero growth phase)– The cells are tired and take a brief break and rest or they

stop adult development.

One cell becoming two

• Spindle Apparatus– These structures are formed from the broken down of

the cytoskeleton and nuclear envelope. (recycled)– The construction starts at the centrosome (where the

centrioles are) and works toward the chromosomes.– They attach to the kinetochore on the centromere of

the replicated chromosomes.– Motor Protein “walks” the sister chromatids toward

the opposite poles (ends) using ATP by phosphorylation.

– Non-kinetochore spindles are used to “push” the poles farther apart to help produce the cleavage furrow.

Motor Protein and the Spindle Fibers

Chromosomemovement

Microtubule Motorprotein

Chromosome

Kinetochore

Tubulinsubunits

• Cell Plate – Remember Plant cells DO NOT have centrioles

because the have cell walls to anchor to.– The NEW cell wall “Plate” develops, using small

segments of cellulose, instead of a cleavage furrow.

Animal vs. Plant

• Regulation “control” of the Cell Cycle.• Regulation is crucial for normal growth and development.• Regulation varies for each different type of cell.• The regulation is controlled by protein molecules called

Cyclins. (They control the cell CYCLE.) • Three checkpoints exist: (Checkpoints are “stopping points to

make sure everything is correct before going on to the next phase.)– First – It is at the end of G1. (Called the Restriction point.) “point

of no return”– Second – It is at the End of G2. (Do we have 2 sets of DNA and 2

sets of organelles?)– Third – It is at the End of Metaphase. (Are all the replicated

chromosomes in the middle of the cell and are they ALL attached to the spindle fibers?)

Checkpoints(Is all going according to plan?)

• What is the difference between a theta chromosome and Eukaryotic chromosomes:

– More genes are present on Eukaryotic chromosomes than on theta chromosomes.– This means there exists more genetic variation exists in Eukaryotic

organisms.– There also exist more possible genetic combinations that can be

inherited.

– There exists more genetic stability in eukaryotic chromosomes.– Offspring receive the same number of chromosomes most of the

time.– Important linked genes tend to be inherited together.

– Allows for diploid cells to exist, as a result of sexual reproduction and one half of the DNA to come from each parent.– Also increases variation among a species.

LE 12-10

NucleusCell plateChromosomesNucleolus

Chromatincondensing 10 µm

Prophase. The chromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.

Prometaphase. Wenow see discrete chromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelope will fragment.

Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten.

Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

Microscopic view of Mitosis in Onion root tips.Can you identify the stages?

Onion root tip mitosis