Cellnuclear division

Introduction• Living cells grow in size and volume and then undergo division

• Cell division includes both nuclear division and cytokinesis (division ofthe cytoplasm)

• Two types of nuclear division• Mitosis: occurs in both somatic and reproductive cells, results in formation of

2 diploid daughter cells both of which are genetically identical

• Meiosis: occurs only in reproductive cells, results in the formation of 4haploid daughter cells, all of which are not genetically identical

The cell cycle • The sequence of events that a cell

undergoes from one cell division to thenext cell division is called the cell cycle:Mitosis and Cytokinesis leads toInterphase, and then back to Mitosisand Cytokinesis.

• Interphase is marked by a period ofrapid growth, duplication ofchromosomes(DNA replication), andanother period of rapid growth

• The cycle: Cytokinesis results in theformation of a new cell and it isfollowed by G1 phase, then S phase,the G2 phase and then Mitosis

Growth

DNA Replication

Growth 2Mitosis

Cytokinesis

Outline the cell cycle, including interphase (growth and DNA replication), mitosis and cytokinesis

Structure of a chromosome

• A chromosome is a structure made up of two chromatids joined together at the centromere.

• The two chromatids are genetically identical, containing the same number of genes, in the same sequence, at the same loci.

• Each chromatid is composed of DNA tightlywound around histone proteins to form acompactly packed structure that can readily bemoved within the cell during cell division.

Describe the structure of a chromosome, limited to DNA, histone proteins, chromatids, centromere and telomeres

Telomeres• Each chromatid possess telomeres on both ends.

• Telomeres are short sequences of DNA that are repeated multipletimes. It functions to ensure continual cell division and to prevent theloss of genes during DNA replication.

• Everytime the cell divides, the telomere gets shortened a little bit andonce it is completely removed from the ends of the chromatid, thecell stops dividing.

• Thus, the older we are, the shorter our telomere and the fewer timesour cells have left to divide before they die.

Outline the significance of telomeres in permitting continued replication and preventing the loss of genes

Chromosomes

• Each species has a characteristic number of chromosomes in each cell

• Humans: 46, fruitflies: 8, cats: 38, dogs: 78.

• Genes are arranged along each chromosomes

• Chromosomes exist in cells in pairs. These pairs are known as homologues, they are similar in structure. One set comes from the male parent, the other from the female parent

• The sperm and egg contain one set of chromosomes, (haploid) and when fertilisation occurs, the fertilised cell contains two sets of chromosomes (diploid)

Homologous Chromosomes, Genes, and Alleles

•Chromosome pair:• “homologous

chromosomes”

•pair has genes at the same loci• “alleles”• may be the same or

different

Chromosomes and Genes• one long strand of DNA

• associated proteins• “chromatin”

• thousands of genes

• gene• discrete unit of heredity

• locus• physical location of a gene

Importance of mitosis • Production of genetically identical cells,

• Growth: it is responsible for the growth and development of multicellular organisms from a single-celled zygote. It results in an increase in size and growth of an organism

• Repair: it restores the wear and tear in body tissues, replaces damaged or lost parts, healing of wounds, and regeneration of detached parts

• Asexual reproduction: it is the method by which unicellular organisms reproduce themselves and as vegetative reproduction in plants

Explain the importance of mitosis in the production of genetically identical cells, growth, cell replacement, repair of tissues and asexual reproduction

Significance of Mitosis

• Mitosis allows the formation of genetically identical cells from pre-existing cells.

• This allows for: • Growth in multicellular organisms

• Production of rapid and amplified immune response

• Asexual reproduction; e.g. in unicellular organisms

• Repair and replacement of worn out tissues; e.g. skin and lining of the gut

Outline the significance of mitosis in cell replacement and tissue repair by stem cells and state that uncontrolled cell division can result in the formation of a tumour

Stem cells • These are cells capable of dividing to form different types of cells

• There are three types: • Totipotent stem cells: these are cells of the zygote from the one-cell stage up

to the 16-cell stage • Pluripotent stem cells: these are the cells of the embryo; capable of dividing

to form any type of body tissue or organ • Multipotent stem cells: this are stem cells that divide to form specific types of

cells. E.g. stem cells of the bone marrow divide to form red blood cells, lymphocytes, monocytes and neutrophils. These multipotent stem cells are also found in the skin, gut, brain, and heart.

• Stem cells rapidly divide by mitosis to replace damaged cells in different parts of the body and repair damaged tissues.

Tumours• A tumour is a mass of undifferentiated cells that have been formed as a result

of uncontrolled cell division. • The division of living cells is regulated and controlled to ensure that errors are

minimized and cells are dividing at the right time. • Uncontrolled cell division results in the formation of a tumour• It occurs as a result of a damage to one or more of the genes responsible for

regulating the process of cell division leading to the formation of oncogenes. • Some of the mutated cells may die or they may be destroyed by the immune

system. However, others can continue to grow uncontrollably until they become a tumour.

• Tumours that do not spread from their site of origin are known as benign, those that spread are called malignant. Cells can break off from malignant tumours and be transported by the blood or lymph to other parts of the boy they they can continue to grow (a process called metastasis).

Outline the significance of mitosis in cell replacement and tissue repair by stem cells and state that uncontrolled cell division can result in the formation of a tumour

Uncontrolled cell division • Mutation or abnormal activation of genes which control cell division

lead uncontrolled cell division

• This can lead to the formation of irregular mass of relatively undifferentiated cells called tumours

• Tumour cells can break away and spread to other parts of the body and cause secondary tumours or metastases.

• Tumours that spread are called malignant

• Tumours that don’t‘ spread are called benign

Factors that increase the chances of cancerous growth Anything that causes change in the structure of a gene is called a mutagen.

An agent which causes cancer is called a carcinogen

Factors that can cause cancer include: • Retroviruses

• DNA viruses

• Heriditary predisposition

• Ionising radiation

• Ultraviolet light

• Radon gas

• Chemical mutagens

Interphase• No nuclear or cell division

• DNA replication, chromosome duplication, and cell growth

• Genetic material appears as chromatin material

• but can see nucleolus inside nucleus

• As nuclear division starts, the chromatin material thickens and coils up to form a chromosome.

• Each chromosome appears as a double structure containing two chromatids that are held together at a point called the centromere. The two chromatids contain two identical DNA molecules

• Centrioles also replicate during interphase

Describe with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle

Mitosis • Mitosis is the process by which a cell nucleus

divides to produce two daughter nuclei containingidentical sets of chromosomes to the parent cell

• Four phases of mitosis:

• prophase• metaphase• anaphase• telophase

• End result of mitosis: 2 daughter cells

Describe with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle

Prophase • Longest phase

• Duplicated chromosomes condense and become visible as sister chromatids joined at the centromere

• nuclear membrane disintegrates; and nucleoli disperses

• Centrioles move to opposite poles of the cell and short microtubules radiate from the centrioles

• Spindle fibres are formed

Describe with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle

Early prophase

Prophase

Prophase

• Chromatin condenses into discrete chromosomes• two identical “sister

chromatids”• joined at the centromere

• Spindle fibres produced by the centrioles (or MTOC) attach to the chromosomes at their centromere

Metaphase

Chromosomes convene on the metaphase plate, an imaginary “equator” located between the two poles of the spindle. Centromeres are aligned, and sister chromatids of each chromosome straddle the metaphase plate.

Metaphase plate

Spindle fibers

Chromosomes are aligned along the cell’s equator, and this alignment is facilitated by the spindle fibres which are attached

to the chromosomes at their centromeres

AnaphaseThe sister chromatids separate from each other and move by the spindle fibers towards opposite poles.

Each chromatid is now considered a chromosome. The spindle fibres contract and pull on the sister chromatids, moving them towards opposite poles

Early anaphase

Anaphase

Spindle poles

•Migration of the chromosomes: •centromeres split •move by the spindle fibers towards opposite poles •pulling the sister chromatid

•Each chromatid -- a chromosome

Anaphase

Late anaphase

By the end of anaphase, each pole has an equivalent and complete set of chromosomes.

Telophase• Chromatid are aggregated at the poles and begin to uncoil and extend in

length to form chromatin material

• new nuclear membrane forms; nucleolus and nucleus begin to reappear

• spindle fibres disintegrates

• After this phase, cytoplasm divides (cytokinesis occurs); daughter cells begin

to form

Describe with the aid of photomicrographs and diagrams, the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, cell surface membrane and the spindle

Telophase

Late telophase

Cytokinesis begins, separating the cytoplasm into 2 “daughter cells”.

Telophase

• The “daughter” nuclei begin to form at the two poles

• Nuclear membranes begin to form

• chromosomes lose their distinct appearance.

Daughter Cells

Telophase

• formation of 2

daughter cells

• Each daughter cell

has same genetic

material as parent

Checkpoints during the mitotic cell cycle…

Practicals

• Behaviour of chromosomes during the mitotic cell cycle

• Behaviour of • Nuclear envelope

• Cell membrane

• Centrioles

• Spindle

during the mitotic cell cycle