Lecture Six… Cell Cycle and Cell Division (a)uowa.edu.iq › filestorage ›...

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Ass. Lec Ghufran S. Salih Cytology/ Second Stage

2018/ 2019 First semester

University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering

Lecture Six… Cell Cycle and Cell Division (a)

1. Cell Cycle or Cell Division

Cell division is a very important process in all living organisms. During

the division of a cell, DNA replication and cell growth also take place.

All these processes, i.e., cell division, DNA replication, and cell growth,

hence, have to take place in a coordinated way to ensure correct division

and formation of progeny cells containing intact genomes. The sequence

of events by which a cell duplicates its genome, synthesizes the other

constituents of the cell and eventually divides into two daughter cells

is termed cell cycle. Although cell growth (in terms of cytoplasmic

increase) is a continuous process, DNA synthesis occurs only during one

specific stage in the cell cycle. The replicated chromosomes (DNA) are

then distributed to daughter nuclei by a complex series of events during

cell division. These events are themselves under genetic control.

Cell Lifespan

Cell pass through life at four stages

- Born

- Differentiate

- Function

- Die or Divide

Lifespan Processes

- Birth…… Mitosis (except germ cells - Meiosis)

- Growth ......Expression of genes and proteins required to grow the

cell, its organelles and cytoskeleton

- Function……Expression of tissue specific genes and proteins

- Division……DNA during cycle, whole cell in Mitosis

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- Death…….Apoptosis (programmed cell death) Necrosis (un

programmed cell death)

Cell Lifespan Examples

Intestinal epithelial cells 3–5 days

Red blood cell 120 days

Brain neuron, heart 50 - 100 years

Neutrophil in circulation 6-7 hours; in tissue 4 days

Chromosomes

A. Composition

1. Chromatin

a. DNA and protein

b. Present in non-dividing cells

c. Chromosomes condense during cell division

2. Nucleosomes

a. Part of the chromatin

b. Packaged when different types of histones interact

- Histone (H1) associates with the linker histone, Creates 30 nm diameter

fibers, Fibers form loops held together by scaffolding proteins, These

interact to form the condensed chromosome seen in metaphase

3. DNA

a. Organized into informational units called genes, Chromosomes contain

hundreds to thousands of genes , Humans have less than 30,000 genes

that code for proteins.

b. DNA in chromosomes is packaged in a highly organized way, which

associated with positively charged histone proteins and form beadlike

nucleosomes. Each nucleosome is composed of 146 base pairs wrapped

around 8 histones.

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B. Chromosomes of different species differ in number and

information content

1. Humans and several other species of organisms have 46 chromosomes

2. The number of chromosomes does not provide information about the

organism

3. chromosomes carry the genetic information of a cell to the next

generation, and to offspring

Figure 6.1: Structure of chromosome and their compositions

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2. Stages of the eukaryotic cell cycle

• The cell cycle describes the status of cells in relationship to growth and division:

1. when cells reach a certain size, growth either stops or the cell must divide

2. most, but not all, eukaryotic cells are capable of dividing

3. cell division is generally a highly regulated process

4. the generation time for eukaryotic cells varies widely, but is usually 8-20 hours

• Cell cycle includes all events from a cell’s formation until it

divides into two major periods (figure 6.2):

a. Interphase

b. Cell division (mitosis: nuclear division and cytokinesis:

cytoplasmic division).

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Figure 6.1: Stages of cell cycle

A. interphase

• Metabolic or growth phase

• From cell formation until cell division

• 90% of the cell cycle

• DNA replication

• Preparation for division

• It is divided into subphases:

1. G0: cells that do not divide become arrested in this phase, G0 can last

minutes to years.

2. S: synthetic phase; the DNA is completely replicated (genetic

information duplicated).

3. G1: growth phase with little cell division related activities

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4. G2: brief period of growth where enzymes and other proteins necessary

for division are synthesized

B. cell division has two main parts – mitosis and cytokinesis

1. mitosis is the process that distributes a complete copy of the duplicated

genetic information to each daughter cell.

2. cytokinesis is the process of dividing the cytoplasm into two separate

cells. some cells can have mitosis without cytokinesis (most common in

fungi and slime molds)

Mitotic Cell Division

A. Characteristics

1. Daughter cells (2) are identical to mother cell

2. No gain or loss of genetic material

3. Series of continuous events

4. Lasts about two hours

B. Phases of mitosis

1. Cellular characteristics at end of G2

a. Nucleus with nuclear membrane

b. Nucleoli present

c. Two centrosomes present

i. Closely affiliated with nucleus

d. Pair of centrioles present in each centrosome

e. Aster is around each centriole pair

i. Array of microtubules

f. Genetic material is still in chromatin form

2. Characteristics of Prophase

a. Nucleus

- Nucleoli disappear

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- Chromatin condenses to form chromosomes

- Each chromosome (duplicated during S phase) forms a pair of sister

chromatids

- Sister chromatids are joined at a centromere by protein tethers

- Centromeres contain a kinetochore where microtubules will bind

- Each sister chromatid has its own kinetochore

b. Cytoplasm

- A system of microtubules, called the mitotic spindle, organizes between

the two poles (opposite ends) of the cell, each pole has a microtubule

organizing center (MTOC), in animals and some other eukaryotes,

centrioles are found in the MTOC.

- Centrosomes migrate toward the poles along the nuclear surface.

- Spindles lengthen.

3. Characteristics of Prometaphase

a. Nuclear membrane disappears and spindles interact with chromosomes

b. Spindles extend from each pole toward the cell’s equator

c. Non-kinetochore microtubules radiate from the centrosomes toward the

metaphase plate, Microtubules overlap with those of the opposite pole

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Figure 6.2: Mitosis cycle (Interphase, Prophase, Prometaphase).

4. Characteristics of Metaphase

- Chromosomes line up along the mid-plane of the cell (the metaphase

plate), chromosomes are most condensed, most visible, and most

distinguishable during metaphase

- The mitotic spindle, now complete, has two types of microtubules

• kinetochore microtubules extend from a pole to a kinetochore

• polar microtubules extend from a pole to the midplane area, often

overlapping with polar microtubules from the other pole

- The mitosis checkpoint appears to be here; progress past metaphase is

typically prevented until the kinetochores are all attached to microtubules.

Figure 6.3.

5. Characteristics of Anaphase

- Centromeres of each chromosome move apart, Sister chromatids split

into separate chromosomes

- Characteristic “V” shape, Kinetochore fibers move ahead of the

remaining chromosome

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- Kinetochore microtubules shorten at the kinetochore end, Chromosomes

move toward poles

- Cell elongates, Poles move farther apart, Non-kinetochore microtubules

push poles apart

- End of Anaphase, two poles have identical chromosomes. figure 6.3.

Figure 6.3: Mitosis cycle (metaphase, Anaphase, Telophase).

6. Telophase the processes of prophase are reversed

- The mitotic spindle is disintegrated

- The chromosomes decondense

- Nuclear membranes reform around the genetic material to form two

nuclei, each with an identical copy of the genetic information

- Nucleoli reappear, and interphase cellular functions resume. Figure 6.3.

B. Cytokinesis

Cell division ends as the cytoplasm divides into two by the process

of cytokinesis (figure 6.4).

https://www.ncbi.nlm.nih.gov/books/n/mboc4/A4754/def-item/A5053/


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Cytokinesis in eucaryotic cells is mediated by a contractile ring, which

is composed of actin and myosin filaments and a variety of other

proteins. Actin contracts until the cells pinch apart.

Cytokinesis occurs by a special mechanism in higher-plant cells in

which the cytoplasm is partitioned by the construction of a new cell

wall, the cell plate, inside the cell. The position of the cell plate

is determined by the position of a preprophase band of microtubules

and actin filaments.

Figure 6.4: Cytokinesis








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Lecture Seven… Cell Cycle and Cell Division (b)

Sexual Life Cycles (Meiosis)

Meiosis is the form of eukaryotic cell division that produces haploid sex

cells or gametes (which contain a single copy of each chromosome) from

diploid cells (which contain two copies of each chromosome). The

process takes the form of one DNA replication followed by two

successive nuclear and cellular divisions (Meiosis I and Meiosis II).

- Meiosis involves two sequential cycles of nuclear and cell division

called meiosis I and meiosis II but only a single cycle of DNA

replication.

- Meiosis I is initiated after the parental chromosomes have replicated to

produce identical sister chromatids at the S phase.

- Meiosis involves pairing of homologous chromosomes and

recombination between them.

- Four haploid cells are formed at the end of meiosis II. Meiotic events

can be grouped under the following phases:

Meiosis I Meiosis II

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I Telophase II

Meiosis I

Meiosis I separates the pairs of homologous chromosomes (Figure 6.5-6).

• Prophase I

Prophase of the first meiotic division is typically longer and more

complex when compared to prophase of mitosis. It has been further

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subdivided into the following five phases based on chromosomal

behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and

Diakinesis.

Leptotene: chromosomes start to condense.

Zygotene: homologous chromosomes become closely associated

(synapsis) to form pairs of chromosomes (bivalents) consisting of four

chromatids (tetrads).

Pachytene: This stage is characterized by the appearance of

recombination nodules (chiasmata), the sites at which crossing over

occurs between non-sister chromatids of the homologous

chromosomes. Crossing over is the exchange of genetic material

between two homologous chromosomes. Crossing over is also an

enzyme-mediated process and the enzyme involved is called

recombinase. Crossing over leads to recombination of genetic

material on the two chromosomes. Recombination between

homologous chromosomes is completed by the end of pachytene,

leaving the chromosomes linked at the sites of crossing over.

Diplotene: homologous chromosomes start to separate but remain

attached by chiasmata.

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Diakinesis: homologous chromosomes continue to separate, and

chiasmata move to the ends of the chromosomes.

• Prometaphase I

Spindle apparatus formed, and chromosomes attached to spindle fibres by

kinetochores.

• Metaphase I

The bivalent chromosomes align on the equatorial plate. The

microtubules from the opposite poles of the spindle attach to the pair of

homologous chromosomes

• Anaphase I

The homologous chromosomes separate, while sister chromatids remain

associated at their centromeres.

• Telophase I

The chromosomes become diffuse and the nuclear membrane reforms.

• Cytokinesis

Cytokinesis follows and this is called as diad of cell. Although in many

cases the chromosomes do undergo some dispersion, they do not reach

the extremely extended state of the interphase nucleus. The stage between

the two meiotic divisions is called interkinesis and is generally short

lived. Interkinesis is followed by prophase II, a much simpler prophase

than prophase I.

Meiosis II

Meiosis II separates each chromosome into two chromatids. The events

of Meiosis II are analogous to those of a mitotic division, although the

number of chromosomes involved has been halved (figure 6.5-6).

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Figure 6.5: Meiosis Cycle in males

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Figure 6.6: Meiosis cycle in female

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Meiosis generates genetic diversity through:

The exchange of genetic material between homologous

chromosomes during Meiosis I.

The random alignment of maternal and paternal chromosomes in

Meiosis I.

The random alignment of the sister chromatids at Meiosis II.

Regulation of the Cell Cycle

The current model of cell cycle regulation involves a highly conserved,

genetically-controlled program that can be influenced by external signals.

Figure 6.7.

1. There are three major checkpoints, found in G1, G2 and mitosis

2. Key regulatory components for checkpoints are cyclins and cyclin-

dependent protein kinases

3. Hormones such as cytokinins in plants and various protein growth

factors in animals can stimulate progression through checkpoints in the

right cells under the right conditions

4. Other factors can serve as suppressors of cell division

5. Cancer cells generally grow without needing stimulation by external

growth factors and fail to respond to normal suppressors of cell division.

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Figure 6.7: Cell Cycle check points

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