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Cell DivisionMs. Rosendo

10-B

Biology

January 10th, 2018Objetive: Describe the steps in the

cell cycle.

Review Concepts

■ What are the known types of cells?

■ How is a multicellular organism formed?

■ What are the differences between the cell of a fly and the cellof a human?

Cells

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Unicellular and Multicellular OrganismsBiological OrganizationLevels

Rudolf Virchow

■ German scientist

■ Part of the Cell Theory Postulates group

■ Established that all cells are derived from pre-existing cells.

– Cellular reproduction = essential for the continuity of life.

Cell cycle

■ The cell cycle is defined as a succession of growth and cell division events.

■ The lapse to complete a cycle includes cell division (mitosis and cytokinesis) and the interface.

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Cell Cycle Cell Cycle Cell CyclePhases

G1

Growth

Increase in size and number of organelles. Checkpoint number 1.

Checkpoint # 1: Identify errors in cell growth.

(Apoptosis = cell death)

S

Synthesis

Create a copy of the nuclear DNA. In this

stage the nucleus contains two copies of

the DNA.

G2

Growth

Continues its normal work. There is a greater

increase in size and includes another

checkpoint.

Checkpoint # 2: Identify DNA errors(eg mutations) before

entering mitosis.

M

Mitosis

It includes mitosis and cytokinesis. The result

is two genetically identical cells.

Assignment # 2: Cell cycle

■ Analyze and answer:

– What would happen if in phase G2 the verification point stopped working?

■ You must write a short paragraph about your analysis.

■ Minimum 7 sentences.

January 11, 2018Objective: Identify the phases of

cell division.

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Cell Division in Prokaryotes

Prokaryotic Cell Cycle

■ Asexual reproduction

■ A bacterial cell is divided by binary fission.

■ Under favorable conditions the cycle of manyprokaryotes occurs rapidly.

– Ex. E. coli - 30min

Prokaryotic Cell Cycle - Steps

1. Before replication, a point of the DNA attaches to themembrane.

2. The chromosome replicates, each copy sticks to themembrane at nearby points, but different

3. The cell lengthens, a new membrane is added betweenthe two junctions and pushes them.

4. Each chromosome moves in the opposite direction andthe membrane grows inward.

5. The cells separate.

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Homework

■ Analyze and answer:

– How is asexual reproduction an advantage in some conditions?

Cell Division in Eukaryotes

Mitosis

■ All cells are formed by the division of pre-existing cells.

■ When a cell divides, the information contained in theDNA must first be duplicated precisely, and the copiesare then transmitted to each daughter cell through a series

of complex processes.

■ This process is called mitosis. This process ensures thateach daughter cell receives a copy of each chromosome.

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Importance of Mitosis

1. Development and growth of multicellular organisms.

2. Biological reproductive mechanism for unicellularorganisms.

3. Tissue repair in multicellular organisms.

4. Preservation of hereditary material

Phases of Mitosis

■ It has 4 phases

– Prophase

– Metaphase

– Anaphase

– Telophase

■ The complete cell cycle includes: mitosis, cytokinesis and

interphase.

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Interphase

■ During the interphase process the cell grows and copies(duplicates) the chromosomes in preparation for theprocess of cell division.

Interphase – G1

■ Interval during which DNA synthesis does not occur.During this period cell growth occurs.

■ This phase is usually of variable and long duration.

Interphase – S

■ During this phase DNA duplication occurs.

■ Other chromosomal components such as proteins are alsosynthesized in this time.

Interphase – G2

■ In this happens an increase in the synthesis of proteins, asthe final steps of the preparation of the cell for divisionare made.

■ The end of the G2 phase is marked by the beginning ofmitosis.

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Prophase

■ Chromatin condenses and becomes a chromosome.

■ The process consists of a coiling of the chromosomes to ashorter and wider shape, which is easier to observe in themicroscope.

■ Each chromosome has been duplicated and consists oftwo sister units called chromatids.

Prophase

■ The chromatids are united in a place called centromere.

■ In animal cells, structures called centrioles stand out,which play an important role in the formation of cellularstructures such as cilia and flagella. The centrioles are

tied by structures called kinetochores.

■ The nuclear envelope begins to disappear. The centriolesmigrate to the poles of the cell.

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Metaphase

■ The chromosomes are aligned in the equatorial plane ofthe cell.

– The chromatids acquire their greatest development.

– It is the ideal phase to observe and photograph thechromosomes.

■ Mooring structures are developed between the poles ofthe cell (centrioles) and the kinetochores of thechromosomes.

Anaphase

■ The forces that hold the sister chromatids together in thecentromeres are released.

– The chromatids become an independent chromosome.

■ The already separated chromosomes begin to moveslowly towards opposite poles by action of thekinetochore microtubules.

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Telophase

■ Final stage of mitosis.

■ Chromosomes are decondensed by unwinding.

■ A new nuclear envelope is formed around each set ofchromosomes.

■ The microtubules of the spindle disappear and thenucleoli rearrange.

Cytokinesis

■ The cytokinesis or division of the cytoplasm to generatethe two daughter cells, often overlaps with mitosis, andusually begins during telophase.

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Differences between the animal and plant mitosis process

■ In plant cells there are no centrioles and the division ofthe cytoplasm occurs by the formation of a new cell walland not by the folding of the membrane inward as occursin cytokinesis of the animal cell.

Homework

■ Analyze and answer:

– Briefly explain why the daughter cells resulting from mitosis are genetically identical to each other and to the original cell.

Cell Cycle Regulation

Loss of control in the cell cycle and development of cancer

■ It has been found that cancer cells do not obey the normalcontrol systems for the cell cycle.

■ Its division process occurs in an uncontrolled way,

invading other tissues (metastasis) causing problems inthe normal functioning of these and eventually killing theorganism.

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Important Factors

■ External factors – include physical and chemical signals.

– Eg. Cell-to-cell contact

– Growth factors are also released signaling that thecell needs to grow.

■ Proteins

■ Internal factors – kinases and cyclins

■ Apoptosis – programmed cell death, triggering self-destructive enzymes.

Cell Division and Cancer

■ Cancer – uncontrolled cell division. They form what isknown as a tumor (cluster of cells).

– Benign tumor – cancer cells typically remain together.Meaning that the tumor is relatively harmless .

– Malignant tumor – some cancer cells can break away

or metastasize from the tumor.

– Cancer cells do not perform the function that theyneed too.

■ These cells are mutated cells.

Carcinogens

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Homework: Cell Regulation

■ Analyze and answer:

– Suppose a child was born with growth hormone receptors that did not work properly. How does this affect the child's development?

January 12, 2018. Objective: Describe the structure

and function of a chromosome.

Cell Division

■ Method of reproduction of cells.

■ It produces two identical cells, which have the same geneticmaterial.

■ Each reproduction cycle is called the cell cycle.

■ In some organisms such as bacteria and Amoebas, the cellcycle is synonymous with the Life Cycle (series of eventsbetween one generation and another).

Cell Division

■ Hereditary information is known as DNA(deoxyribonucleic acid).

– Made of subunits called nucleotides.

– Many DNA segments, which contain thousands ofnucleotides, are known as genes.

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Genes

■ They contain the particular information for thedifferent characteristics of the species.

■ The nucleotide sequence has the information tosynthesize the RNA and protein molecules needed to

produce a cell and perform its metabolic activities.

■ They provide the genetic instructions for the cell tosurvive.

Genes

■ DNA is organized and packaged in structures calledchromosomes.

■ A single circular chromosome of more or less 2mm incircumference, contains all the DNA of a prokaryotic cell.

Eukaryotic Genome

■ The eukaryotic nucleus containsmultiple DNA molecules.

■ Each one is packed with proteinsand organized into structures

called chromosomes.

■ The chromosomes of the variousspecies differ in number andcontent of information.

Genome

■ Before a cell divides, it must duplicate its chromosomes.

■ The two copies are held together by the centromeres.

■ As long as they stay together these copies are calledchromatids.

■ Each chromatid is a DNA band.

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Eukaryotic chromosome

■ Chromosomes consist of two bandsthat come out of a center calledcentromere ("central body").

■ Two sister chromatids, joined bythe centromeres is considered achromosome.

■ Chromosomes are made up of acomplex material called chromatin,which consists of fibers thatcontain proteins anddeoxyribonucleic acid (DNA).

Eukaryotic chromosome

■ Histones - small group of structural proteins with many aminoacids.

■ When the DNA is entangled in groups of histones, shortening it,

then it is called a nucleosome.

Eukaryotic chromosome – Chromatin

■ Heterochromatin - condensed chromatin, unable to transcribe.

■ Eucromatin - loose chromatin, capable of transcription.

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Number of Chromosomes

■ Every individual of a given species has a characteristic numberof chromosomes in the nucleus.

■ There may be different species that have the same number of

chromosomes.

■ It is not the number of chromosomes that makes each speciesunique, but the information specified by the genes of thosechromosomes.

Assignment # 3: Chromosomes

■ Analyze and answer:

– What is the relationship between a DNA molecule and a chromosome?

Meiosis

Species reproduction

■ In eukaryotes we can summarize the reproductivemechanisms to two basic forms: sexual and asexual.

■ Sexual - involves the union of two specialized cells,called gametes, to form a cell called a zygote. The species

would duplicate its chromosomes every time sexualreproduction occurs (polyploids).

■ Asexual - A stem cell divides to produce two or moredaughter cells. In most cases this occurs due to mitosis.The organisms produced in this way are consideredbiological clones.

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Species reproduction

■ The cells called gametes are the ovule and the sperm.

■ They use meiosis.

■ There is a decrease in the number of chromosomes ineach of them.

Meiosis

■ It consists of two successive mitotic phases.

– 4 genetically distinct cells are produced.

– Meiosis I

– Meiosis II

■ If a cell contains 2 of each type of chromosome, it is saidto be diploid.

■ If it has a single set it is haploid.

– In humans diploid is 46 and haploid is 23chromosomes.

Meiosis I – Prophase I

■ Homologous chromosomes synapse to form tetrads; crossingover occurs. The junction points between which DNAexchange occurs between chromosomes are called chiasmas.

■ In the process of cross-linking, the homologous chromatidsexchange genetic material, resulting in an increase in thegenetic variation of the progeny of organisms that reproducesexually.

■ The nuclear envelope is degraded

Meiosis I – Metaphase I

■ The tetrads are aligned in the equatorial plane of the cell,and are held together in the chiasmas (sites of previouscross-linking).

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Meiosis I –Anaphase I

■ The homologous chromosomes separate and move toopposite poles.

■ The sister chromatids remain united in their centromeres.

Meiosis I – Telophase I

■ A homologous chromosome of each pair is in each pole.

■ Cytokinesis occurs.

Interkinesis

■ The DNA does not duplicate.

■ The chromatids are still attached.

■ The chromosomes have not spread completely.

Meiosis II – Prophase II

■ The chromosomes condense again.

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Meiosis II – Metaphase II

■ The chromosomes are aligned along the equatorial planeof the cell.

Meiosis II –Anaphase II

■ The sister chromatids separate, and the chromosomesmove to opposite poles.

Meiosis II – Telophase II

■ Nuclei are formed at opposite poles of each cell andcytokinesis occurs.

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Importance of Meiosis

■ Genetic variation among the progeny of individuals whoreproduce sexually is promoted.

■ The number of chromosomes of the species remains

constant.

Comparison between Mitosis and Meiosis

■ The events of meiosis are similar to those of mitosis, withfour important differences:

– Meiosis involves two successive nuclear andcytoplasmic divisions, which produce up to four cells.

– Although there are two successive nuclear divisions,

DNA and other chromosomal components areduplicated only once, during the interface thatprecedes the first meiotic division.

– Each of the four cells produced in meiosis contains thehaploid number of chromosomes; that is, only one setwith the representative of each peer pair.

– During meiosis, the genetic information of both

parents is mixed, so that each resulting haploid cellhas a virtually unique combination of genes.

Comparison between Mitosis and Meiosis

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Spermatogenesis and Oogenesis

Gamete Formation

SpermatogenesisMales produce sperm

throughout life, after the

onset of puberty, about

1,500 sperm per second.

Oogenesis

Image: Oogenesis, Wiki

Oogenesis in females is

probably complete either

before or shortly after birth.

During oogenesis, three

polar bodies develop as the

mature ovum is generated.

Polar bodies contain little

cytoplasm and eventually

degenerate.