The Cell Cycle

Chapter 12

When do cells divide? Reproduction Replacement of damaged cells Growth of new cells In replacement and growth cell divisions, how

should daughter cells compare to parent cell? The daughter cells should be identical copies

of the parent cell

Functions of Cell Division

How can identical daughter cells form? The genome must be copied and then

divided such that each daughter cell gets one of the copies.

Genome = all the genes in an organism

Bacterial Reproduction

How do bacterial

cells reproduce?

Important terms in eukaryotic cell division Chromosome - threadlike structures that are

composed of DNA and protein Replication - process whereby DNA is

identically copied (before cell division) Mitosis - division of the nucleus Cytokinesis - division of the cytoplasm Chromatin - DNA and protein complex that is

thin and fibrous; it will condense into distinct chromosomes during cell division

Important terms in eukaryotic cell division Chromatid - after replication the chromosome

consists of 2 sister chromatids joined at the centromere

Centromere - specialized region of the chromosome, where chromatids are joined. Each chromosome has one centromere

The Cell Cycle

Interphase 90% of cell cycle is spent in this phase G1 = first growth phase S = synthesis phase, DNA synthesis

(replication) occurs here G2 = second growth phase

G2 phase of Interphase in animal cells Nuclear envelope is visible One or more nucleoli are present Centrioles are replicated and the 2 pairs are

near nucleus Aster forms around each pair of centrioles Chromosomes are loosely packed into

chromatin fiber, not distinguishable

General Overview of Mitosis

Prophase: In the Nucleus: Nucleoli disappear Chromosome fibers condense into discrete

chromosomes Each chromosome consists of 2 sister

chromatids joined at the centromere

In the Cytoplasm: Mitotic spindle begins to form Spindle consists of microtubules arranged

between the centrosomes Centrosomes move apart due to lengthening

of microtubules

Prometaphase: Nuclear envelope breaks apart Each chromatid has specialized structure

called kinetochore located at the centromere region

Kinetochore microtubules interact with chromosomes at the kinetochore region

The kinetochore microtubles cause the chromosomes to move

Nonkinetochore microtubules radiate from each pole

Metaphase Chromosomes move to the metaphase plate

and line up there The centromeres of the chromosomes are all

aligned on the metaphase plate Each sister chromatid of one chromosome,

has a kinetochore microtubule attached to it from opposite poles

Kinetochore microtubules + nonkinetochore microtubules = spindle fiber

Anaphase Kinetochore microtubules shorten and non-

kinetochore microtubules lengthen Centromeres divide and each chromosome

has no sister chromatid component The shape of the cell elongates into an ellipse Chromosomes are pulled to the opposite poles

Telophase Nonkinetochore microtubules continue to

elongate the cell New daughter nuclei form at the two poles New nuclear envelopes are formed around the

chromosomes Nucleoli reappear Chromosomes uncoil into chromatin fiber Last phase of mitosis

Cytokinesis Begins before telophase has completed Evidenced by cleavage furrow in animal cells

and cell plate in plant cells

Evolution of mitosis:

Bacterial cells

Dinoflagelates; chromosomes attach to nuclear envelope.

Diatoms; nuclear envelope stays, microtubules inside nucleus

Most other eukaryotes; spindle forms outside of nucleus, and nuclear envelope breaks apart

Cytoplasmic Cell Signals in Regulation of Cell Cycle

Checkpoints in the cell cycle: If it passes the G1 checkpoint cell divides -if not, enters G0 phase and does not divide

Cyclin protein levels fluctuate according to cell cycle stage. When cyclin is high the Cdk attaches and phosphorylation leads to breakdown of nuclear envelope. Later MPF initiates cyclin breakdown

Cancer cells How does abnormal cell division of cancer

cells differ from normal cell division? Cancer cells are not under density dependent

inhibition Continue to grow until all nutrients are used

up Cancer cells are immortal, do not shorten

telomeres