Presentation is loading. Please wait.

Presentation is loading. Please wait.

THE CELL CYCLE Karyokinesis & cytokinesis can overlap in time, depending upon the species.

Similar presentations

Presentation on theme: "THE CELL CYCLE Karyokinesis & cytokinesis can overlap in time, depending upon the species."— Presentation transcript:

1 THE CELL CYCLE Karyokinesis & cytokinesis can overlap in time, depending upon the species.

2 Events that occur in the life of a cell. Includes 3 major stages:
The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: Interphase Mitosis Cytokinesis Karyokinesis & cytokinesis can overlap in time, depending upon the species.


4 1. Interphase (Cell is not dividing)
G1 Phase – carries out basic functions & performs specialized activities. duration is extremely variable contains restriction checkpoint ~ cell “decides” to: divide enter a quiescent phase (G0) die “G” stands for “gap” or “growth”


6 1. Interphase (Cell is not dividing)
G0 Phase – cell maintains specialized characteristics, but does not divide Ex. neurons & muscle cells “G” stands for “gap” or “growth”


8 1. Interphase (Cell is not dividing)
S Phase – cell replicates chromosomes & synthesizes proteins animal cells replicate centrioles as well “G” stands for “gap” or “growth”


10 1. Interphase (Cell is not dividing)
G2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material “G” stands for “gap” or “growth”


12 Mitosis (M phase) – Equal distribution of replicated genetic material.
Five steps: Prophase Prometaphase Metaphase Anaphase Telophase “G” stands for “gap” or “growth”

13 replicated chromosomes condense
Mitosis – Prophase replicated chromosomes condense centrosomes separate & migrate toward opposite sides of cell mitotic spindle forms (microtubules grow out from centrosomes) nucleolus disappears “G” stands for “gap” or “growth”


15 Mitosis – Prometaphase nuclear membrane breaks down
spindle fibers attach to centromeres of chromosomes “G” stands for “gap” or “growth”


17 chromosomes are lined up single-file along equator of mitotic spindle
Mitosis – Metaphase chromosomes are lined up single-file along equator of mitotic spindle “G” stands for “gap” or “growth”


19 Centromeres part, sister chromatids (now called chromosomes) separate
Mitosis – Anaphase Centromeres part, sister chromatids (now called chromosomes) separate chromosomes move toward opposite poles “G” stands for “gap” or “growth”


21 mitotic spindle breaks down chromosomes decondense
Mitosis – Telophase mitotic spindle breaks down chromosomes decondense nuclear membranes reform around two nuclei nucleoli reappear “G” stands for “gap” or “growth”


23 Distribution of cytoplasm to daughter cells
Cytokinesis Distribution of cytoplasm to daughter cells begins during anaphase or telophase differs in animal & plant cells “G” stands for “gap” or “growth”


25 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 “G” stands for “gap” or “growth”

26 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 “G” stands for “gap” or “growth”


28 Review of the M-phase

29 Review of the M-phase

30 Review of the M-phase

31 Review of the M-phase

32 Review of the M-phase

33 Review of the M-phase

34 Review of the M-phase

35 Review of the M-phase

36 Review of the M-phase

37 Review of the M-phase


39 Does cytokinesis always accompany karyokinesis?
Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells). Usually, but not always. Examples: human skeletal muscle tissue (pictured) endosperm tissue in some plants (nourishes the developing embryo in a seed).

40 Control of the Cell Cycle
Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence. Various mechanisms interact to regulate the cell cycle. Are numerous checkpoints - 4 are described in figure. Restriction checkpoint - cell “decides” whether to divide, enter G0 phase, or die. Most important checkpoint. DNA damage checkpoint - turns on genes that manufacture proteins that repair damaged DNA. Apoptosis checkpoint - necessary to override a signal for the cell to die. Spindle assembly checkpoint - necessary for construction of spindle & binding of chromosomes to it.

41 Shortening of telomeres - loss of telomere DNA signals cell to stop dividing.
Some cells produce telomerase (enzyme that continually adds telomere DNA). Telomeres are repeating nucleotide sequences found at the tips of chromosomes. Every time a cell divides, some of its telomere DNA is lost. After about 50 divisions (Hayflick limit), a key amount of lost telomere DNA signals cell division to cease. NOTE: Cells that produce telomerase are able to divide beyond the 50-division limit. They include: plant cells cells lining small intestine bone marrow cells certain immune system cells sperm-forming cells cancer cells

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

43 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.

44 B. Apoptosis Programmed cell death; part of normal development.
Example of apoptosis: Feet of embryonic chickens & ducks have webbing between the toes. Webbing vanishes as the chicken’s foot develops (due to apoptosis). Webbing is retained as duck’s foot develops. Apoptosis fine-tunes the human immune system - in the fetus, it destroys T cells that do not recognize self. If they were not destroyed, these T cell would begin attacking the body (autoimmune disease).

45 Steps of Apoptosis: Death receptors activate enzymes called capases.
Capases destroy proteins & various cell components & ready the cell for phagocyte destruction. From outside, cell rounds up, forms bulges called blebs and fragments (fragments are surrounded by cell membrane, so they don’t initiate an inflammatory response). Cell death in response to an injury is called necrosis. The cell swells & bursts, initiating an inflammatory response.

46 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 Cancer cells can divide eternally because they produce telomerase. Cervical cancer cells (HeLa) of Henrietta Lacks who died in 1951 are used extensively in cancer research all over the world today. Heritability - when a cancer cell becomes cancerous, it passes its loss ov cell cycle control to its descendants. Transplatability - cancer cells can be injected into a healthy animal; animal will develop cancer because those cancer cells divide to form more cancerous cells. Cancer cells lack contact inhibition - they tend to form masses called tumors. Metastasize = ability to spread Angiogenesis = ability to induce local blood vessel formation. Cancer cells mutate - a treatment that shrank the original tumor may have no effect on its subsequent growth.

47 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. Oncogenes are normally switched off in most cells. They are activated only under certain circumstances (ex. in cells at wound site). Tumor suppressor genes are normally switched on in most cells. If they are inactivated or removed cells will divide continually or apoptosis does not occur (ex. a childhood kidney cancer is caused by absence of a tumor suppressor gene that normally halts mitosis in kidney tubule cells in the fetus) Factors that turn on oncogenes or turn off tumor suppressor genes at inappropriate times would lead to development of tumors.

48 Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors:
carcinogens radiation viruses diet exercise habits Carcinogens = chemicals that cause cancer (tobacco, asbestos, insecticides, saccharine) Radiation - UV & x-ray photons have enough energy to cause DNA damage.

49 Meiosis - formation of gametes
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

50 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

51 Meiosis - formation of gametes
A cell that contains both sets of chromosomes (1 from each parent ) is said to be diploid (2n) Cells containing 1 set of chromosomes are said to be haploid (n)

52 Meiosis 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

53 Prophase I Everything 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

54 Metaphase I Same as Metaphase of mitosis
Tetrads line up at the equator

55 Anaphase I Same as Anaphase of mitosis
Homologous chromosomes separate and move to the poles

56 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)

57 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

58 Prophase II Same as prophase of mitosis

59 Metaphase II and Anaphase II
Double stranded (not homologous) chromosomes align along the equator in Metaphase II

60 Telophase II and cytokinesis
At the end of meiosis, there are four haploid daughter cells

61 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.

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


64 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

65 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

66 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)

Download ppt "THE CELL CYCLE Karyokinesis & cytokinesis can overlap in time, depending upon the species."

Similar presentations

Ads by Google