1.6. cell division

1.6. Cell division

Miltiadis-Spyridon Kitsos Platon IB Diploma

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The official IB Diploma Biology guideEssential idea: Cell division is essential but must be controlled.

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MitosisMitosis is division of the nucleus into two genetically identical daughter nuclei

Mitosis is a type of cell division happening in eukaryotic cells, which creates two genetically identical daughter cells.

when does mitosis takes place?TISSUE GROWTHIn multicellular organisms mitosis increases the number of cells, and hence, the size of the organisms.

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ASEXUAL REPRODUCTION OF UNICELLULAR EUKARYOTESYeast cells, reproduce asexually, via budding. The formation of the bud is a result of mitosis and cytokinesis.

MitosisMitosis is division of the nucleus into two genetically identical daughter nuclei.

TISSUE REPAIRDamaged tissue repair happens through mitosis.

EMBRYONIC DEVELOPMENTDuring sexual reproduction a zygote is formed. The zygote undergoes mitosis to produce the first embryonic cells and consequently, the whole embryo.

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when does mitosis takes place?

MitosisMitosis is division of the nucleus into two genetically identical daughter nuclei.

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The cell cycle Mitosis is division of the nucleus into two genetically identical daughter nuclei.

The cell cycle is a series of events between one cell division and the next. The result of thecell cycle is the formation of two genetically identical cells.

InterphaseInterphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.

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Supercoiling of chromosomesChromosomes condense by supercoiling during mitosis.

Chromosomes (Greek: chroma (colour) + soma (body) are characteristic structures which become visible during the different stages of mitosis. They are made of proteins and DNA and they may gradually supercoil or uncoil.

http://www.gistsupport.org/media/cells%20%20DNA/DNA-Marina-3.jpg

https://youtu.be/OjPcT1uUZiE

Getting things straightChromosomes condense by supercoiling during mitosis.

This is a chromatin fiber. It is made of DNA coiled around clusters of proteins called histones.

This is how DNA looks at the beginning of interphase. The genome is the form of multiple chromatin fibers forming a network.

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During the S phase of interphase DNA undergoes replication. At the end of replication each chromatin fiber has replicated. The two identical molecules are now called sister chromatids and they are attached to the centromere.

Getting things straightChromosomes condense by supercoiling during mitosis.

At the start of mitosis, sister chromatids start to supercoil with the aid of the histones and other enzymes. Supercoiling completes during metaphase when the chromosomes assume their maximum degree of coiling.

http://www.wiley.com/college/trefil/0470118547/animations/ch23/dna_coiling/09_02.html

ProphaseIdentification of phases of mitosis in cells viewed with a microscope.

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Nuclear membrane starts tobreak down

Chromosomes consist of the two sister chromatids attached to the centromere Shorter and fatter.

Centrosomes (and not centromeres) start moving towards the opposite poles of the cell forming the Microtubule organizing centers (MTOC’s) . Microtubules start growing from the MTOC’s

Late ProphaseIdentification of phases of mitosis in cells viewed with a microscope.

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Nuclear membrane has disintegrated

Spindle microtubules

MTOC

Mitotic spindle

MetaphaseIdentification of phases of mitosis in cells viewed with a microscope.

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The centromeres of the chromosomes attach to microtubules from the two opposing MTOC’s

Contraction of the microtubules tests if the attachment is correct. Contraction of the microtubules is happening through shortening of the microtubules at the centromere. The chromosomes align to the equator of the cell.

MetaphaseIdentification of phases of mitosis in cells viewed with a microscope.

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The centromeres of the chromosomes attach to microtubules from the two opposing MTOC’s

Contraction of the microtubules tests if the attachment is correct. Contraction of the microtubules is happening through shortening of the microtubules at the centromere. The chromosomes align to the equator of the cell.

AnaphaseIdentification of phases of mitosis in cells viewed with a microscope.

Contraction of the microtubules leads to the breaking of centromeres.

Each of the two sister chromatids is pulled to opposite poles.

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TelophaseIdentification of phases of mitosis in cells viewed with a microscope.

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• Chromosomes uncoil and they are no longer distinguishable under the microscope.

• Nuclear envelopes reform around the tightly packed chromatids.

• Mitotic spindle disintegrates.

• Cytokinesis is about to start.

The mitotic indexDetermination of a mitotic index from a micrograph.

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Mitotic index =Number of cells in mitosis

Total number of cells observed

In the image below you need to (a) count the number of cells in

different stages of mitosis(b) and then, calculate the mitotic

index.

The mitotic indexDetermination of a mitotic index from a micrograph.

CytokinesisCytokinesis occurs after mitosis and is different in plant and animal cells.

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Cytokinesis is the process of cell division that follows mitosis.

Animal cellsPlasma membrane is pulled inwards with the aid of a ring of contractile proteins (actin and myosin).When cleavage furrow reaches the centre, the two cells pinch apart.

https://youtu.be/H2HmRnYLDhA

CytokinesisCytokinesis occurs after mitosis and is different in plant and animal cells.

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Cytokinesis is the process of cell division that follows mitosis.

Plant cells• Vesicles are transferred to the area of the

equator and two layers of membrane are developed.

• Vesicles are transferred in the area between the two membranes and the middle lamella is developed.

• The two daughter cells deposit cellulose on both sides of lamella adjacent to the cell membrane and the two new cell walls are formed.

CytokinesisCytokinesis occurs after mitosis and is different in plant and animal cells.

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Plant cells

Cell plate

Middle lamella

CytokinesisCytokinesis occurs after mitosis and is different in plant and animal cells. Media

https://highered.mheducation.com/sites/9834092339/student_view0/chapter10/animation_-_cytokinesis.html

http://www.sumanasinc.com/webcontent/animations/content/mitosis.htmlhttps://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html

http://www.johnkyrk.com/mitosis.html

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CyclinsCyclins are involved in the control of the cell cycle.

Cyclins are a group of proteins that ensures:• tasks in mitosis will happen on the correct time and• cell advances to the next stage when appropriate

Their action is concentration-dependent. In high concentrations, they bind to specific enzymes called cyclin dependent kinases. These enzyme will in turn add phosphate groups to other proteins, activating them. These activated proteins are responsibly for many tasks during mitosis

P

CDK

Mitosis promoting factor

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Triggers cells to move from G0 to G1 and from G1 into S phase.

prepares the cell for DNA replication in S phase.

activates DNA replication inside the nucleus in S phase.

promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.

CyclinsCyclins are involved in the control of the cell cycle.

There are four different kinds of cyclins.

CyclinsSerendipity and scientific discoveries: the discovery of cyclins was accidental.

Tim Hunt discovered cyclins during a series of experiments on the synthesis of proteins in sea urchin fertilized eggs. He traced a protein that would be synthesized for 30 minutes and then being broken down.

Tim Hunt’s experiments initiated a series of innovative breakthroughs that would explain the fundamental mechanism determining cell division. For this discovery, Tim Hunt—together with Lee Hartwell and Paul Nurse—would receive the 2001 Nobel Prize in Physiology or Medicine

Tim Hunt has always emphasized on the effect of serendipity on his scientific discoveries. For example, in his Nobel prize lecture he noted that he was lucky enough to select 10-minute intervals in his experiments. This selection revealed the true dynamics in the protein synthesis

http://www.cell.com/cell/pdf/S0092-8674(08)00888-X.pdfRead more https://blogs.biomedcentral.com/on-biology/wp-content/uploads/sites/5/2015/08/Untitled-design-620x200.jpg

Tumour formation and cancerMutagens, oncogenes and metastasis are involved in the development of primary and secondary tumours

Tumours results from the uncontrolled division of a group of cells. Tumour cells may stay adhered to the initial tissue and may not invade neighbouring tissues. These tumours are called benign and most likely will not cause serious health problems.

However, in many cases, cells from the tumours travel through the blood and lymph circulation and invade other organs or tissues creating secondary tumours . This tumours are called malignant and may cause severe health problems

http://ib.bioninja.com.au/_Media/metastasis_med.jpeghttps://youtu.be/ggwTjOsN0_w

Causes of tumour formationMutagens, oncogenes and metastasis are involved in the development of primary and secondary tumours

These are different factors associated with tumour formation and cancer (disease related to malignant tumours). These factos are known as carcinogens.

• Air pollution • Smoking (chemical produced from the combustion of

tobacco)• Specific strains of viruses (e.g., strains of HPV)• Different forms of radiation (X-rays, short-wave UV)

Read more here

http://www.cancer.org/cancer/cancercauses/othercarcinogens/generalinformationaboutcarcinogens/known-and-probable-human-carcinogens

In most cases carcinogens cause mutations (this is why they are also called mutagens). Mutations are changes in the sequence of genes related to the regulation of cell division (proto-oncogenes, promoting cell division) and (Tumour suppressor genes, repressing cell division)

Smoking and cancer The correlation between smoking and incidence of cancers.

A study was carried out into the number of cigarettes smoked by men per year and the number of deaths from lung cancer. The graph below shows the results of this study.

EDEXCEL Biology AS Paper 1 January 2009 http://qualifications.pearson.com/en/support/support-topics/exams/past-papers.html

(a) Describe the changes in the number of deaths from lung cancer between 1920 and 1975.(b) Explain the meaning of the term correlation.(c) Describe the evidence shown in this graph that suggests there is a correlation between cigarette smoking and the number of deaths from lung cancer.(d) Give two additional pieces of information that would increase the validity of any conclusions made from this study.

Smoking and cancer The correlation between smoking and incidence of cancers.

A study was carried out into the number of cigarettes smoked by men per year and the number of deaths from lung cancer. The graph below shows the results of this study.

EDEXCEL Biology AS Paper 1 January 2009 http://qualifications.pearson.com/en/support/support-topics/exams/past-papers.html

Remember that: (a) Correlation is a relation

between two variables (b) May be positive or

negative(c) There is a positive

correlation between cigarette consumption and lung cancer

(d) However, correlation does not always imply causality.

(e) However, in the case of smoking 20 different chemicals have been identified to be causing tumours in lab animals, while another 40 have been identified as carcinogenic.

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