THE CELL CYCLE

The Cell CycleEvents that occur in the life of a cell.Includes 3 major stages:1. Interphase 2. Mitosis3. Cytokinesis

1. Interphase (Cell is not dividing) G1 Phase – carries out basic

functions & performs specialized activities.

duration is extremely variablecontains restriction checkpoint ~ cell “decides” to:

divideenter a quiescent phase (G0)die

1. Interphase (Cell is not dividing) G0 Phase – cell maintains specialized

characteristics, but does not divideEx. neurons & muscle cells

1. Interphase (Cell is not dividing) S Phase – cell replicates

chromosomes & synthesizes proteins

animal cells replicate centrioles as well

1. Interphase (Cell is not dividing) G2 Phase - cell synthesizes

additional proteins (ex. tubulin) & assembles/stores

membrane material

2. Mitosis (M phase) – Equal distribution of replicated genetic material.

Five steps: Prophase Prometaphase Metaphase Anaphase Telophase

2. Mitosis – Prophase replicated chromosomes condense centrosomes separate & migrate

toward opposite sides of cell

mitotic spindle forms (microtubules grow out from centrosomes)

nucleolus disappears

2. Mitosis – Prometaphase nuclear membrane breaks down spindle fibers

attach to centromeres of chromosomes

2. Mitosis – Metaphase chromosomes

are lined up single-file along equator of mitotic spindle

2. Mitosis – Anaphase Centromeres part,

sister chromatids (now called chromosomes)

separate chromosomes

move toward opposite poles

2. Mitosis – Telophase mitotic spindle

breaks down chromosomes

decondense nuclear

membranes reform around two nuclei

nucleoli reappear

3. Cytokinesis Distribution of cytoplasm to

daughter cells begins during anaphase or

telophase differs in animal & plant cells

3. Cytokinesis in animal cells Cleavage furrow (slight indentation)

forms around equator of cell Actin & myosin

microfilaments act like a drawstring to pinch the cell in two

Usually an equal division

3. Cytokinesis in plant cells phragmoplast (microtubule

structure) forms in cytoplasm & traps vesicles containing cell wall material

vesicles fuse, forming a cell plate across midline of cell

cell plate gives rise to two primary cell walls

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Review of the M-phase

Does cytokinesis always accompany karyokinesis?

Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells).

Control of the Cell CycleCheckpoints - groups of interacting

proteins that ensure cell cycle events occur in the correct sequence.

Shortening of telomeres - loss of telomere DNA signals cell to stop dividing.

Some cells produce telomerase (enzyme that continually adds telomere DNA).

Contact Inhibition - healthy cells stop dividing when they come in contact with other cells.

Hormones - stimulate cell division.Ex. Estrogen stimulates uterine cell division

Growth factors - proteins that stimulate local cell division.Ex. Epidermal growth factor (EGF) stimulates

epithelial cell division filling in new skin underneath a scab

Interaction of kinases & cyclins - activate genes that stimulate cell division.

B. ApoptosisProgrammed cell

death; part of normal development.

Steps of Apoptosis:

C. Cancer (loss of cell cycle control) Condition resulting from excess cell division

or deficient apoptosis.Characteristics of Cancer Cells: can divide uncontrollably & eternally are heritable & transplantable lack contact inhibition readily metastasize exhibit angiogenesis exhibit genetic mutability

Causes of Cancer: Over-expression of oncogenes

Oncogenes are genes that trigger limited cell division.

Inactivation of tumor suppressor genesTumor suppressor genes prevent a cell from

dividing or promote apoptosis.

Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors: carcinogens radiation viruses diet exercise habits

• Somatic cells – body cells• In contrast to mitosis (occurs in somatic cells),

gametes (eggs or sperm) are produced only in gonads (ovaries or testes).

• In the gonads, cells undergo a variation of cell division (meiosis) which yields four daughter cells, each with half the chromosomes of the parent.• In humans, meiosis reduces the number of

chromosomes from 46 to 23• Chromosomes #1 through 22 – autosomal• Chromosome #23 – sex

Meiosis - formation of gametes

Meiosis - formation of gametes • Fertilization fuses two gametes together and

doubles the number of chromosomes to 46 again.

• Organisms inherit single copy of each gene from each parent

• These copies are segregated from each other during formation of the gametes

• Homologous – corresponding male and female chromosomes

Meiosis - formation of gametes

• A cell that contains both sets of chromosomes (1 from each parent ) is said to be diploiddiploid (2n)

• Cells containing 1 set of chromosomes are said to be haploidhaploid (n)

• It produces 4 haploid cells that are genetically different from each other and from the diploid parent

• 2 parts:• Meiosis I – separation of homologues• Meiosis II – separation of

sister chromatids

Meiosis

Prophase IEverything that happens in Prophase of mitosis also happens in Prophase I of meiosisChromosomes find their pairs to form a tetrad (process called synapsis)They can exchange genetic info (crossing over)Site of crossing over is the chiasmata

Metaphase ISame as Metaphase of mitosisTetrads line up at the equator

Anaphase ISame as Anaphase of mitosisHomologous chromosomes separate and move to the poles

Telophase ISame as Telophase of mitosisInstead of having two genetically identical cells, the chromosomal number has been halved (2n to n)Chromosomes are still double stranded (sister chromatids still attached)

Meiosis IINo replication occursMitosis resembles meiosis II more than

meiosis ISister chromatids are separated to make

daughter cells that have a single set (n) of single stranded chromosomes

Prophase IISame as prophase of mitosis

Metaphase II and Anaphase IIDouble stranded (not homologous)

chromosomes align along the equator in Metaphase II

Telophase II and cytokinesisAt the end of meiosis, there are four haploid

daughter cells

• Mitosis and meiosis have several key differences.• The chromosome number is reduced

by half in meiosis, but not in mitosis.• Mitosis produces daughter cells that

are genetically identical to the parent and to each other.

• Meiosis produces cells that differ from the parent and each other.

Mitosis produces two identical daughter cells, but meiosis produces 4 genetically different cells.

Sexual vs. Asexual Reproduction• In asexual reproduction, a single individual

passes along copies of all its genes to its offspring• Single-celled eukaryotes reproduce asexually

by mitotic cell division to produce two identical daughter cells

• Even some multicellular eukaryotes, like hydra, can reproduce by budding cells produced by mitosis

Sexual vs. Asexual Reproduction• Sexual reproduction results in greater variation

among offspring than does asexual reproduction

• Offspring of sexual reproduction vary genetically from their siblings and from both parents

Sexual vs. Asexual Reproduction• Three mechanisms contribute to genetic

variation:• independent assortment

• crossing over (Prophase I)• random fertilization – each zygote is the result

of 1 of 70 trillion possible chromosomal combos (223 x 223)