The Cell CycleChapter 12

Why are cells small?• Ascellsgetbiggertheydon’tworkaswell

• WHY?

• DifficultiesLargerCellsHave:• MoredemandsonitsDNA

• Lessefficientinmovingnutrients/wasteacrossitscellmembraneà Diffusion

Function of Cell Division• Unicellular organisms → reproduction of organism,

increase in population

• Multicellular organisms → development, growth, repair

• Involves distribution of identical genetic material to two daughter cells

The Cell Cycle

• Interphase – cell grows by producing proteins & organelles, replicates DNA, prepares for cell division. Accounts for 90% of cell cycle

• G1 phase – growth, cell performs it’s normal function

• This phase varies most in length among different types of cells

• S phase – DNA synthesis – chromosomes are duplicated

• G2 phase – growth and preparation for cell division

• Mitotic phase: includes mitosis & cytokinesis• Mitosis = the formation of two daughter nuclei

• Cytokinesis = division of the cytoplasm

Replication of DNAduring the S phase

• DNA molecules are packaged into chromosomes

• Prokaryotes – genome is a single circular chromosome

• Eukaryotes – genome consists of several chromosomes

• Every species has a characteristic number of chromosomes in the nucleus of each cell.

• Human somatic (body) cells have 46 chromosomes or two sets of 23.

• Human gametes (sperm or egg) have 23 chromosomes

Learning Check

• A single gamete can grow into a zygote through the process of mitosis:

• True

• False

• Eggs and sperm are what type of cells?

• Somatic Cells

• Gametes

• Germ Cells

Learning Check

• A zygote grows into an organism through what process? • Mitosis• Meiosis

• Gametes are produced from what type of cells?• Somatic Cells• Gametes• Germ Cells

Learning Check• A skin cell is an example of:

• Gamete• Germ Cell• Zygote• Somatic Cell

• A skin cell would divide to make more skin cells using the process of: • Mitosis• Meiosis• Fertilization

Learning Check• Why must somatic cells be produced

through mitosis? • Because mitosis produces four genetically

distinct daughter cells.• Because somatic cells are used for sexual

reproduction• To ensure that old, damaged, or growing cells

are replaced with cells that are different from the parent cell

• To ensure that old, damaged, or growing cells are replaced with the cells that are the same type and can perform the same function

Replication of DNAduring the S phase

• Eukaryotic chromosomes are made of chromatin – a complex of DNA & protein.

• Proteins maintain the structure and help control gene activity

• In a non-dividing cell, each chromosome is in the form of a long thin chromatin fiber.

DNA replication (continued)

•Before cell division, chromatin condenses•Each duplicated chromosome consists of two sister chromatids = identical copies of DNA• Centromere = region where

chromatids connect• During mitosis, the sister

chromatids are pulled apart.

Chromosomes

Subphases of Mitosis

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

Learning Check

• During the S phase of the cell cycle, DNA is replicated. What form would you expect the DNA to be in?• Tightly wound around histone proteins as

chromosomes• Separate nucleotides just floating around the nucleus• In a loose pile

• During the M phase of the cell cycle, what form would you expect the DNA to be in?• Tightly wound around histone proteins as

chromosomes• Separate nucleotides just floating around the nucleus• In a loose pile

Learning Check• In which phase of the cell cycle does DNA replication

take place?• G1• S• G2• M

• Interphase is the part of the cell cycle when the cell: _____________________ • Grows and duplicates its DNA• Undergoes mitosis• Ceases to function• Divides its cytoplasm

Learning Check• In which phase of the cell cycle are new

daughter cells formed?• G1 S G2 M• Cytokinesis

• In which stage of the cell cycle would you expect the growing of the cell to occur?• G1 S G2 M• Cytokinesis

• In which stage of the cell cycle would checking for DNA integrity occur?• G1 S G2 M• Cytokinesis

Learning Check

• In the above diagram, what do the yellow circles represent?• Chromosomes

• Homology

• Chromatids

• centromeres

Learning Check

• What do these two chromosomes together represent?• Homologous Chromosomes

• Histones

• Chromatids

• centromeres

Learning Check

• How many chromatids are shown in the diagram above?

• Centromeres connect how many chromatids?

Role of mitotic spindle

• Mitotic spindle - originates from centrosomes, composed of microtubules and associated proteins (tubulin)

• In animal cells each centrosome has a pair of centrioles at the center.

• As mitosis starts, the two centrosomes are located near the nucleus

• As the spindle fibers grow from them the centrioles are pushed apart towards opposite ends of the cell

Role of spindle fibers (cont.)

• Each sister chromatid has a kinetochore – a region of proteins and DNA at the centromere.

• During prometaphase, some spindle microtubules attach to the kinetochores.• the chromosome will move

toward the pole from which the microtubules come.

• Eventually the chromosomes settle along the metaphase plate.

• Nonkinetochore microtubules from opposite poles overlap and interact with each other

Role of spindle fibers (cont.)

• During anaphase the proteins that have held the sister chromatids together are inactivated and each chromatid becomes a full fledged chromosome

• Chromosomes move toward opposite poles of the cell

• Hypothesis: motor proteins on the kinetochore “walk’the attached chromosome along the microtubule. Excess microtubule sections depolymerize at the end of the kinetochore

Cytokinesis

• Division of the cytoplasm; typically follows mitosis

• Animal cells the process involves cleavage

• First sign of cleavage = cleavage furrow

• Cleavage furrow is formed by a ring of contractile actin microfilaments + molecules of the motor protein myosin

• Contraction of the ring pinches the cell in two.

Cytokinesis in Plant Cells

• Plant cells – cell wall necessitates a different mechanism →cellplate• Duringtelophase,vesicles

fromtheGolgiapparatusorganizeatthemetaphaseplateformingthecellplate

• Theplateenlargesuntilitsmembranesfusewiththeplasmamembrane

• Thecontentsofthevesiclesformnewcellwallmaterialbetweenthedaughtercells

Cell Division in Prokaryotes

• Mitosis in eukaryotes may have evolved from binary fission in prokaryotes

• Bacterial chromosome is a circular DNA molecule with associated proteins.

• In binary fission, chromosome replication begins at one point (the origin of replication) and proceeds in both directions around the circle

The Evolution of Mitosis• Since prokaryotes preceded eukaryotes by

billions of years

• It is likely that mitosis evolved from bacterial cell division

• Certain protists

• Exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells

• A hypothetical sequence for the evolution of mitosis

Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms.

Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division.Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei.

Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane.

Bacterialchromosome

Microtubules

Intact nuclear envelope

Chromosomes

Kinetochore microtubules

Intact nuclearenvelope

Kinetochore microtubules

Fragments ofnuclear envelope

Centrosome