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 divide
Ex. 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 genes
Tumor 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 meiosis
Chromosomes 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 mitosis
Tetrads line up at the equator
Anaphase ISame as Anaphase of mitosis
Homologous chromosomes separate and move to the poles
Telophase I
Same as Telophase of mitosis
Instead 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 II
No replication occurs
Mitosis resembles meiosis II more than meiosis I
Sister 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)