Mitosis and Meiosis Notes

Unit 1.6: Genetic Information Copying and Daughter Cells

Mitosis

• Mitosis and Meiosis are essential processes for cell division.

• Syllabus Objectives:

  • Interphase and the main stages of mitosis.

  • Significance of mitosis: identical gene copies in daughter cells and cytokinesis.

  • Mitosis in damage, disease (renewal, repair, cancer).

  • Main stages of meiosis (excluding prophase 1 subdivisions) and cytokinesis.

  • Mitosis vs. meiosis: genetically identical vs. non-identical daughter cells.

Chromosomes

• Chromosome Structure

  • DNA is in the nucleus of eukaryotic cells.

  • DNA double helix winds around histone proteins.

  • In non-dividing cells, DNA is loosely coiled as chromatin (not visible under a light microscope).

  • In dividing cells, DNA is tightly coiled and dense, forming visible chromosomes.

  • Before DNA replication, a chromosome exists as a single thread called a chromatid.

  • After DNA replication, a chromosome has two chromatids (sister chromatids) with genetically identical alleles/genes.

  • Sister chromatids are joined by a centromere.

  • Centromeres are the attachment site for spindle microtubules during cell division.

  • Centromeric DNA contains short, repeated base sequences for structure, not genetic information.

• Alleles

  • A DNA molecule has many genes along its length. Each gene occupies a specific location (locus) on a specific chromosome.

  • Genes are DNA sections with coded information (base sequences).

  • Genes can have multiple forms called alleles.

  • Each person inherits one allele from their mother (maternal) and one from their father (paternal); alleles can be the same or different.

  • Each allele codes for a different polypeptide and protein.

  • Mutations are changes in an organism’s DNA base sequence and can produce new alleles.

Ploidy Level and Chromosome Number

• Ploidy Level

  • Ploidy is the number of homologous chromosome sets in a cell or organism's genome.

  • Ploidy varies by species.

  • More than two sets of chromosomes are polyploidy.

  • Examples of ploidy levels include haploid (n), diploid (2n), triploid (3n), tetraploid (4n), hexaploid (6n), and octoploid (8n).

  • Humans: Haploid (n = 23) in gametes, Diploid (2n = 46) in body cells.

• Chromosome Number

  • Different species have different numbers of chromosomes in each set.

  • Haploid number in humans is 23.

  • Diploid number in humans is 46 because chromosomes exist in homologous pairs.

  • Homologous pairs:

  • Are identical in size and shape.

  • Carry genes for same characteristics at same loci.

  • Are not genetically identical.

  • One chromosome from each parent.

  • Sex chromosomes (e.g., X and Y in mammals) can differ in size and are not homologous pairs.

  • Meiosis halves the chromosome number, so each daughter cell gets one chromosome from each homologous pair via gametes.

  • The fusion of haploid cells (egg and sperm) restores the diploid state, with paired homologous chromosomes.

The Human Karyotype

• The karyotype is the number and visual appearance of chromosomes in a cell or species.

• There are two classes of chromosomes:

  • Autosomes: not involved in determining sex.

  • Heterosomes: X and Y chromosomes involved in determining sex.

The Cell Cycle Introduction:

• The cell cycle is the series of events that takes place in a cell leading to its division and replication of its DNA to produce daughter cells.

• Divided into three main stages:

  • Interphase

  • Mitosis

  • Cytokinesis

• The Length of cell cycle varies in different tissues:

  • Root tip cell of onions divide every 20 hours

  • Epithelial cell in human gut divides every 10 hours.

• Average mammalian cell takes 24 hours to complete the cell cycle. 90% of this time is spent in Interphase.

Interphase

• The longest stage in the cell cycle.

• Cells are metabolically active, synthesizing new molecules and organelles, and carrying out DNA replication.

• Has three phases:

  • G1 Phase (Growth Phase):

  • The first growth phase.

  • Proteins are produced which will be used to synthesise more of the cells organelles, nucleotides, and histone proteins.

  • The cell doubles in size.

  • S Phase (Synthesis Phase):

  • DNA replication takes place.

  • DNA is unraveled and replicated to double its genetic content. It combines with newly produced histones to double the amount of chromatin in the cell.

  • As chromatin is loosely coiled, it is less visible than chromosomes.

  • Centrioles replicate.

  • G2 Phase (Growth Phase):

  • The second growth phase.

  • Organelles grow and divide and so are replicated.

  • Proteins are synthesised, such as histones and enzymes.

  • Energy stores are also increased, in the form of ATP.

  • Specialist proteins called tubulins are synthesised. These are used to make the spindle fibres, which will separate the chromosomes.

Mitosis

• Following interphase, the nucleus of the cell needs to divide (nuclear division).

• There are two forms of nuclear division:

  • Mitosis – occurs in somatic cells.

  • Meiosis – leads to the formation of gametes.

Prophase

• This is the longest phase of mitosis.

• Therefore, when you observe dividing cells under a microscope most of the cells will be in prophase).

• Early Prophase:

  • DNA coils and condenses to form chromosomes (shorter than chromatin).

  • Each chromosome exists as two genetically identical sister chromatids, joined by a centromere.

  • The nuclear envelope and the nucleolus are present in the early stages of prophase.

• Late Prophase:

  • The nuclear envelope and nucleolus disintegrate.

  • Each pair of centrioles moves to opposite poles of the cell.

  • A network of microtubules form around the centrioles.

  • Spindle fibres extend between the microtubules at each pole and attach to the centromere of each chromosome.

Metaphase

• In Animal cells:

  • Chromosomes align at the equator of the cell at the centre of the spindle fibre.

  • Sister chromatids are still attached by a centromere.

• In Plant cells:

  • Plant cells do not have centrioles, therefore spindle fibres form without focusing on centrioles.

  • The spindle fibres will be straight and parallel to each other.

Anaphase

• This is a rapid stage.

  • The spindle fibres, (microtubules) shorten.

  • The centromeres divide and separate the two sister chromatids of a chromosome.

  • The spindle fibres pull the chromatids to opposite poles of the cell (centromere first).

Telophase

• This is the final stage of mitosis

  • Chromatids have reached the poles and become distinct.

  • They can now be regarded as chromosomes.

  • The chromosomes unwind into longer and thinner chromatin.

  • The nuclear envelope and nucleolus forms around each new nucleus (two).

  • Spindle fibres break down and disintegrate.

Cytokinesis

• The division of the nucleus by mitosis is followed by cytokinesis.

• In animal cells

  • Microfilaments (proteins) attach to the inside of the cell membrane at the centre of the cell.

  • These microfilaments contract and pull the membrane inwards creating a cleavage furrow.

  • The parental cell divides into two genetically identical daughter cells.

• In plant cells

  • Segments of cell wall align at the equator of the cell to form a cell end plate.

  • The segments of cell wall fuse together to form a new cell wall that separates the cell.

  • This results in two genetically identical daughter cells forming.

Mitosis and Cytokinesis in Plant and Animal Cells

The Biological Significance of Mitosis

Genetic stability

• Mitosis produces two nuclei which have the same number of chromosomes as the parent cell.

• Since these were derived from parental chromosomes by the exact replication of their DNA, they will carry the same hereditary information in their genes.

• Daughter cells are genetically identical to the parent cell and no variation in genetic variation can be introduced during mitosis.

• This results in genetic stability within populations of cells derived from the same parental cell.
  Growth

• Multicellular organisms grow by producing new extra cells by mitosis.

• Each new cell is genetically identical to the parent cell so can perform the same functions.

• E.g.

  • Mitosis occurs in the apex of the roots and shoots. These areas are known as the apical meristems and increase in cell numbers here results in growth of the roots or shoots.
    Cell Replacement/Repair of tissues

• The replacement of cells and the repair of tissues involves mitosis.

• E.g.

  • Red and white blood cells are constantly being replaced and are produced in the bone marrow by mitosis.

  • Epithelial cells lining the gut are damaged as food goes down, therefore need constant replacing.

  • Skin cells are damaged and die quickly therefore need to be replaced.
    Regeneration

• Some organisms are able to regenerate whole body parts.

• E.g.

  • Legs in crustaceans

  • Arms in starfish

• The production of these new cells for these body parts occurs by mitosis.
  Asexual reproduction

• Mitosis is basis of asexual reproduction, the production of new individuals of a species by one parent organism.

• The offspring are genetically identical to the parent.

• E.g.

  • Unicellular organism such as yeast and amoeba.

  • Some insects such as greenfly.

  • Some flowering plants with organs such as bulbs, tubers and runners. A bud on the stem grows into a new plant. This then becomes detached from the parent and becomes a new independent plant. This produces large numbers of identical offspring in a short period of time.

• There is no genetic variation between individuals from asexual reproduction because they are genetically identical.

• The ability to generate whole organisms from single cells or small groups of cells is important in biotechnology and genetic engineering.

Cancer

• Cancers are the result of uncontrolled cell division. The type of nuclear division involved is mitosis.

Mutations

• A mutation is a change in the amount, arrangement or structure in the DNA of an organism.

• Mutation rates are generally low.

• They are often spontaneous random events.

• Some mutations are neutral in their effects.

• Some mutations are advantageous in their effects e.g. providing a source of material (new alleles) for natural selection and evolution.

• Some mutations are harmful in their effects e.g. cause cancer.

• Although mutations are often spontaneous and random their frequency can be increased by mutagens.

Mutagens and Carcinogens

• Any agent that causes a mutation is called a mutagen.

• Any mutagen that causes cancer is known as a carcinogen.

• It is thought that the development of a malignant cancer cell involves more than one factor operating over a number of years.

Carcinogens

• lonising radiation

  • Includes X-rays, gamma rays, particles from decay of radioactive elements.

  • They cause formation of damaging ions inside cells which break DNA strands.

• Ultraviolet light does not form damaging ions but can damage genes.

• Chemicals

  • Many chemicals are carcinogenic.

  • They cause damage to DNA.

  • E.g. Soot, tar, coal, asbestos, arsenic compounds, tobacco smoke and food additives.

• Virus infection

  • Some viruses can cause cancer.

  • Usually these viruses carry proto-oncogenes or oncogenes.

  • E.g.:

  • Burkitt's lymphoma - most common cancer in children in parts of Africa caused by virus.

  • Papilloma viruses responsible for some cancers. E.g. two types are linked to cervical cancer, a disease that can be transmitted sexually.

• Hereditary predisposition

  • Some cancers run in families.

  • This may be due to the inheritance of oncogenes or due to mutated tumour suppressor genes.

  • E.g.:

  • The BRCA1 and BRCA2 genes are tumour suppressor genes that contain mutations. This means that they become switched off and so cannot prevent the formation of tumours.

  • Mutations in the BRCA1 and BRCA2 genes increase the likelihood of certain cancers such as breast, ovarian and prostate.

The Genetics of Cancer

• The cell cycle must be carefully regulated by genes which ensures that mitosis happens where and when it is needed.

• If this gene is damaged, cells may fail to divide or may divided too frequently or at the wrong time.
  Tumour-suppressor genes

• There are genes that regulate the cell cycle by producing specific proteins. These genes are called tumour-suppressor genes.

• The proteins they produce detect when cells are dividing uncontrollably and initiate apoptosis (programmed cell death).

• Tumour-suppressor genes must be switched on to prevent the formation of tumours.

• Mutations in tumour-suppressor genes switch them off.

• This prevents the regulation of uncontrolled cell division, therefore leading to cancer.

• E.g. The tumour-suppressor gene TP53 produces a protein called p53.

  • p53 causes apoptosis in abnormal cells.
    Oncogenes

• Proto-oncogenes are genes that normally help cells grow.

• When a proto-oncogene mutates (changes) or there are too many copies of it, it can become permanently switched on when it is not supposed to be.

• A mutated proto-oncogene that has the potential to cause cancer is called an oncogene.

• When this happens, the cell grows out of control.

• Uncontrolled cell division can lead to the formation of a tumour.

• Some tumours are detected and destroyed by the immune system. Those not detected develop into cancers.

Benign and Malignant Tumours

• There are two different classes of tumours:

  • Benign tumours

  • Benign tumours do not spread from their site of origin.

  • They can have no effect or can compress and displace surrounding tissue

  • E.g. warts, ovarian cysts and some brain tumors.

  • Malignant tumours

  • Malignant tumours are cancerous.

  • They affect the normal functioning of the area where they are growing. E.g. block intestines, lungs or blood vessels.

  • Cells can break off and spread throughout the body via the lymphatic or circulatory system, forming secondary growths.

  • The spread of cancers in this way is known as metastasis.

  • The secondary growths are often hard to find and remove.

• Both types of cancers involve a huge drain on the body due to high demand for nutrients because of rapid and continual cell division.

Cancer and its treatment

• The treatment often aims to block the cell cycle of the cancerous cells.

• Chemotherapy uses drugs to treat cancer.

• Some chemicals inhibit the cell cycle and have been developed for use against cancer. They are highly toxic and risk damaging normal cells, as well as killing cancer cells.

• However, these drugs (chemicals) are more effective against rapidly dividing cells (cancerous) than normal cells.

• Therefore, the drugs damage cancer cells more than normal cells.

• There are also other cells that divide rapidly, like hair producing cells. This is why people going through chemotherapy lose their hair.

• Radiotherapy uses radiation to kill cancerous cells in a localised area.

Meiosis

• Takes place in the reproductive organs of plants, animals and some protoctista, prior to sexual reproduction.

• It results in the formation of four haploid daughter gametes.

• It leads to increased genetic variation.

• Haploid gametes are essential so that following fertilisation the diploid number of chromosomes is restored.

• In humans the male gamete – spermatozoa (n), female gamete – ova (n)

• In flowering plants, the male gamete – sperm cell (found in the pollen grain) (n), female gamete – egg cell (n).

Before Meiosis

• Prior to meiosis a diploid (2n) somatic cell will undergo interphase.

The Stages of Meiosis

• Meiosis has two nuclear divisions. These are known as:

  • Meiosis I

  • Meiosis II

Meiosis I

• Meiosis I is sometimes called a reduction division because it halves the chromosome number in each cell.

Prophase I

• The chromatin condenses and becomes supercoiled to form chromosomes (now visible under a light microscope).

• The paternal and maternal chromosomes come together as a pair of homologous chromosomes.

• The pairing of chromosomes is known as synapsis.

• Each homologous pair of chromosomes is called a bivalent. The bivalent has four chromatids (two from each chromosome).

• The nuclear membrane breaks down and the nucleolus disappears.

• In animals and lower plants, where centrioles are present, the centrioles separate and move to the poles of the cell.

• The centrioles organise the polymerisation of microtubules, which radiate out from them and the spindle forms.

• Crossing-over of non-sister chromatids may occur:

  • The non-sister chromatids wrap around each other and then partially repel each other but remain joined at points called chiasma (plural: chiasmata).

  • This is a source of genetic variation as it mixes alleles from the two parents in one chromosome.

  • This produces new combinations of alleles on chromosomes.

  • These new versions of the chromosomes (produced by recombining chromosomes) are known as recombinants.

  • Crossing over can happen at several places along the chromatid. Therefore, large numbers of different genetic combinations can be made.

Metaphase I

• Bivalents line up at the equator of the spindle.

• The maternal and paternal chromosomes of the bivalents will arrange themselves randomly.

• This is known as independent assortment of chromosomes, which gives rise to variation.

Anaphase I

• The homologous chromosomes in each bivalent separate and as the spindle fibres shorten; one chromosome of each pair is pulled to one pole.

• Each pole receives only one of each of the homologous chromosomes.

• Due to their random arrangement at Metaphase I, there is a random mixture of maternal and paternal chromosomes.

Telophase I

• The homologous pairs separate with one chromosome from each going into separate nuclei.

• In some species the chromosomes decondense and are no longer visible.

• The nuclear membrane and nucleoli reform.

• The spindle fibres disintegrate.

• However, in many species the chromosomes stay in their condensed form.

Cytokinesis

• The division of the cytoplasm occurs forming two haploid (n) cells.

• The two cells are genetically different to each other.

Meiosis II

• Meiosis II can be regarded as being the same as Mitosis.

Prophase II

• If a nuclear envelope re-formed it is broken down again.

• The nucleolus disintegrates.

• If the chromosomes decondensed previously, they now become condensed and supercoiled.

• The centrioles separate and organise new spindle fibres at right angles to the old spindles.

• Each cell contains one chromosome from each homologous pair.

Metaphase II

• Chromosomes arrange themselves on the equator of the spindle fibres.

• Each chromosome is attached to the spindle fibre by its centromere.

• Independent assortment occurs because the chromatids of each chromosome can face either pole.

Anaphase II

• The centromere divides, and the spindle fibres shorten, causing the chromosomes to separate.

• The chromatids are pulled to opposite poles.

Telophase II

• At the poles, the chromatids lengthen and uncoil, and can no longer be distinguished under the microscope.

• The spindle fibres disintegrate.

• The nuclear envelope and nucleoli re-form.

• Each cell produces two genetically different haploid nuclei (n).

Cytokinesis

• Following Telophase II cytokinesis takes place.

• This results in four genetically different haploid (n) daughter cells.

How Meiosis and Fertilisation leads to Genetic Variation

Genetic variation is achieved by:

• Crossing over of sections of non-sister chromatids during Prophase I (to produce new recombinants).

• Independent assortment of maternal and paternal chromosomes during Metaphase I.

• Independent assortment of sister chromatids during Metaphase II.

• Random mutations.

• Random fertilisation:

  • Two random gametes fertilise each other to create a unique genotype.

Glossary

• Allele: Gene variant; different forms of a gene.

• Anaphase: Chromosomes move to poles; stage of cell division.

• Autosome: Non-sex chromosome; chromosome not involved in sex determination.

• Bivalent: Paired homologous chromosomes; forms during meiosis I.

• Carcinogen: Cancer-causing agent; substance that promotes cancer development.

• Cell Cycle: Cell division events; sequence of growth and division.

• Centromere: Spindle fiber attachment; region where sister chromatids are joined.

• Chemotherapy: Chemical disease treatment; uses drugs to kill cancer cells.

• Chiasma: Contact point of chromosomes; site of crossing over.

• Chromatid: Chromosome strand after replication; one half of a replicated chromosome.

• Chromatin: DNA & protein in chromosomes; complex of DNA and proteins.

• Chromosome: DNA carrier; structure containing genetic information.

• Crossing Over: Gene exchange; exchange of genetic material during meiosis.

• Cytokinesis: Cytoplasmic division; division of the cell after nuclear division.

• Diploid: Two chromosome sets (2n); cell containing two sets of chromosomes.

• DNA: Genetic material; deoxyribonucleic acid.

• Gamete: Mature sex cell; haploid reproductive cell.

• Gene: Heredity unit; segment of DNA that codes for a trait.

• Haploid: Single chromosome set (n); cell containing one set of chromosomes.

• Heterosome: Sex chromosome; chromosome involved in sex determination.

• Homologous Chromosomes: Similar chromosome pairs; chromosomes with the same genes.

• Interphase: Non-division phase; the phase of the cell cycle when the cell is not dividing.

• Karyotype: Chromosome appearance; the number and appearance of chromosomes in a cell.

• Locus: Gene location; the specific location of a gene on a chromosome.

• Meiosis: Cell division; a type of cell division that produces gametes.

• Metaphase: Chromosome alignment; stage of cell division when chromosomes align at the metaphase plate.

• Mitosis: Cell division; a type of cell division that produces two identical daughter cells.

• Mutagen: Mutation-causing agent; substance that causes mutations