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Cytogenetics Lesson 8: Mitosis and Cytokinesis - Meiosis Events in the M Phase, Study notes of Cell Biology

Silliman University (SU)Cell Biology

The process of mitosis and cytokinesis in the M phase of the cell cycle. It describes the stages of mitosis and the role of cytoskeletal mediators in animal cells. It also briefly covers meiosis and the types of reproduction. definitions, diagrams, and explanations of each stage of mitosis and cytokinesis.

Typology: Study notes

2022/2023

Available from 01/23/2024

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CYTOGENETICS
Lesson
8
[TRANS] LESSON 08: MITOSIS & CYTOKINESIS - MEIOSIS
EVENTS IN THE M PHASE
MITOSIS
● Accurate segregation of chromosomes
●“In mitosis, only the nucleus in the cell is
dividing. The actual cell division is actually in the
cytokinesis, whereas mitosis is just the
segregation of chromosomes.”
CYTOKINESIS
●Division of the cell into two
●“During cytokinesis of an animal cell, the
cytoplasm is divided in two by a contractile ring
of actin and myosin filaments, which pinches the
cell into two daughters, each with one nucleus.”
STAGES OF THE M PHASE
- The division of a cell into two daughters occurs in the M
phase of the cell cycle. M phase consists of nuclear
division, or mitosis, and cytoplasmic division, or
cytokinesis. In this figure, the M phase has been greatly
expanded for clarity. Mitosis is itself divided into five
stages, and these, together with cytokinesis, are
described
CYTOSKELETAL MEDIATORS IN THE M PHASE
●Two transient cytoskeletal structures
mediate M phase in animal cells. The mitotic
spindle assembles first to separate the
duplicated chromosomes. Then, the contractile
ring assembles to divide the cell in two.
Whereas the mitotic spindle is based on
microtubules, the contractile ring is based on
actin and myosin.
(1) PROPHASE
●Mitotic spindle begins to form
○ Centrosomes become the 2 spindle
poles
● Chromosomes start condensing into chromatids
○ 2 copies are present
○ Held together by cohesins
● “At prophase, the duplicated chromosomes,
each consisting of two closely associated sister
chromatids, condense. Outside the nucleus, the
mitotic spindle assembles between the two
centrosomes, which have begun to move apart.
For simplicity, only three chromosomes are
drawn.”
(2) PROMETAPHASE
●Breakdown of the nuclear envelope
●Chromosomes attach to opposite poles of the
spindles
●“Prometaphase starts abruptly with the
breakdown of the nuclear envelope.
Chromosomes can now attach to spindle
microtubules via their kinetochores and undergo
active movement.”
(3) METAPHASE
●Chromosomes are aligned
● “At metaphase, the chromosomes are aligned at
the equator of the spindle, midway between the
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CYTOGENETICS

Lesson

[TRANS] LESSON 08: MITOSIS & CYTOKINESIS - MEIOSIS

EVENTS IN THE M PHASE

MITOSIS

● Accurate segregation of chromosomes ● “In mitosis, only the nucleus in the cell is dividing. The actual cell division is actually in the cytokinesis, whereas mitosis is just the segregation of chromosomes.” CYTOKINESIS ● Division of the cell into two ● “During cytokinesis of an animal cell, the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell into two daughters, each with one nucleus.” STAGES OF THE M PHASE

  • The division of a cell into two daughters occurs in the M phase of the cell cycle. M phase consists of nuclear division, or mitosis, and cytoplasmic division, or cytokinesis. In this figure, the M phase has been greatly expanded for clarity. Mitosis is itself divided into five stages, and these, together with cytokinesis, are described CYTOSKELETAL MEDIATORS IN THE M PHASE ● Two transient cytoskeletal structures mediate M phase in animal cells. The mitotic spindle assembles first to separate the duplicated chromosomes. Then, the contractile ring assembles to divide the cell in two. Whereas the mitotic spindle is based on microtubules, the contractile ring is based on actin and myosin. (1) PROPHASE ● Mitotic spindle begins to form ○ Centrosomes become the 2 spindle poles ● Chromosomes start condensing into chromatids ○ 2 copies are present ○ Held together by cohesins ● “At prophase, the duplicated chromosomes, each consisting of two closely associated sister chromatids, condense. Outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to move apart. For simplicity, only three chromosomes are drawn.” (2) PROMETAPHASE

● Breakdown of the nuclear envelope

● Chromosomes attach to opposite poles of the

spindles

● “Prometaphase starts abruptly with the

breakdown of the nuclear envelope. Chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement.” (3) METAPHASE ● Chromosomes are aligned ● “At metaphase, the chromosomes are aligned at the equator of the spindle, midway between the

spindle poles. The kinetochore microtubules on each sister chromatid attach to opposite poles of the spindle.” (4) ANAPHASE

● Sister chromatids continuously separate

○ Cohesin destruction via separase ○ Kinetochore microtubules get shorter ● “At anaphase, the sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached. The kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosome segregation.” (5) TELOPHASE ● Two sets of chromosomes arrive at the poles of the spindle ● A new nuclear envelope reassembles ○ Two nuclei are formed ● End of mitosis ● Contractile ring starts to assemble ● “During telophase, the two sets of chromosomes arrive at the poles of the spindle. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with the assembly of the contractile ring.” CYTOKINESIS CLEAVAGE FURROW ● Formation begins in the anaphase ● Occurs along the long axis of the mitotic spindle ● Ensures that the cell membrane and chromosomes are equally divided

CONTRACTILE RING

● An overlapping array of actin and myosin filaments ● Formation begins after the 2 sister chromatids separate ● Force is generated by the sliding motion of actin and myosin MEIOSIS TYPES OF REPRODUCTION ASEXUAL REPRODUCTION ● Genetically identical offspring ● Used by most bacteria and single celled organisms SEXUAL REPRODUCTION ● Genetically distinct offspring ● Used by most plants and animal cells TYPES OF CELLS IN SEXUAL REPRODUCTION DIPLOID CELLS ● Has 2 sets of similar chromosomes (homologs), except for the sex chromosomes ● Known as somatic cells

can also delay the start of the M phase to ensure that DNA replication is complete. ● Centrosomes duplicate during S phase and separate during G2. Some of the microtubules that grow out of the duplicated centrosomes interact to form the mitotic spindle. ● When the nuclear envelope breaks down, the spindle microtubules capture the duplicated chromosomes and pull them in opposite directions, positioning the chromosomes at the equator of the metaphase spindle. ● The sudden separation of sister chromatids at anaphase allows the chromosomes to be pulled to opposite poles; this movement is driven by the depolymerization of spindle microtubules and by microtubule- associated motor proteins. ● A nuclear envelope re-forms around the two sets of segregated chro- mosomes to form two new nuclei, thereby completing mitosis. ● In animal cells, cytokinesis is mediated by a contractile ring of actin filaments and myosin filaments, which assembles midway between the spindle poles; in plant cells, by contrast, a new cell wall forms inside the parent cell to divide the cytoplasm in two. ● In animals, extracellular signals regulate cell numbers by controlling cell survival, cell growth, and cell proliferation. ● Most animal cells require survival signals from other cells to avoid apoptosis—a form of cell suicide mediated by a proteolytic caspase cascade; this strategy helps ensure that cells survive only when and where they are needed. ● Animal cells proliferate only if stimulated by extracellular mitogens produced by other cells; mitogens release the normal intracellular brakes that block progression from G1 or G0 into S phase. ● For an organism or an organ to grow, cells must grow as well as divide; animal cell growth depends on extracellular growth factors that stimulate protein synthesis and inhibit protein degradation. ● Some extracellular signal molecules inhibit rather than promote cell survival, cell growth, or cell division. ● Cancer cells fail to obey these normal “social” controls on cell behavior and therefore outgrow, out-divide, and out-survive their normal neighbors.