Cell and nuclear division
Duplicated chromosome (known as replicated chromosome)
- An X-shaped structure visible under light microscope -Consists of two genetically identical DNA molecules -Consists of two genetically identical sister chromatids joined at the centromere (constricted region of DNA) -bound to kinetochore(proteins) which allow for attachment of spindle fibres -Centromeres hold 2 genetically identical sister chromatids together & are involved in chromosome movement during nuclear division -Genetically identical sister chromatids are formed due to DNA replication during interphase
Mitosis Anaphase stage
- Centromeres separate and two genetically identical sister chromatids separate, thus becoming two daughter chromosomes -Daughter chromosomes migrate toward opposite poles of the cell, with the centromere leading the way as spindle fibres shorten.
DNA during nuclear division
- Condenses into chromosomes, which may exist as unduplicated chromosomes or duplicated chromosomes -Duplicated : sister chromatids -Unduplicated: chromosome
Significance and functions of meiosis (Genetic Variation )
- Meiosis creates genetic variation, allows for new combinations of alleles -Genetic variation ensures that species constantly change and adapt when environmental conditions alter
Significance and functions of mitosis (4. Asexual Reproduction)
- Mitosis is the basis of asexual reproduction, the production of new individuals of a species by one parent organism -Ensures that offspring are genetically identical to the parent for continued survival of the species and retain advantage of the organism in adapting to its environment.
Characteristics of homologous chromosomes
- Similar in size, shape and length -Same position of centromeres -Genes for the same characters(traits) at corresponding loci(position on chromosome): -Alleles present may not be identical -Alleles are alternative forms of the same gene with a different nucleotide sequence -pair with each other during prophase 1 of meiosis -Order of genes on homologous chromosome are the same
DNA in chromosomes
-Appears as condensed rod-like structures -Exists when cell is undergoing cell division
DNA in chromatin
-Appears as uncondensed threadlike fibres -Exists when cell is not undergoing cell division
Mitosis Prophase stage
-Chromatin condense and shorten to become chromosomes -Centrosome/centrioles organize microtubules into spindle fibres -Centrosomes/centrioles duplicate and migrate to opposite poles of the cell by lengthening of spindle fibres. -Nucleolus disappears -Nuclear envelope disintegrates
How does chromosomes exist in a diploid cell?
-Chromosomes exists as a pair of homologous chromosomes -Each chromosomes known as a homolog -In HUMAN diploid cell, we inherit one chromosome of each pair from each parent -46 chromosomes in our somatic cells are actually two sets of 23 chromosomes: -A maternal set and a paternal set
Unduplicated chromosome
-Consists of a single DNA molecule
Significance and functions of mitosis (3. Repair tissues or organs / Cell Replacement and Regeneration )
-Ensures that damaged cells lost in normal processes of wear and tear are replaced with exact copies of the original cells in order for the tissue to function properly. -When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced.
Where do meiosis occur in human?
-In human, meiosis occurs in germs cells - cells that can produce gametes, which are found in the reproductive organs (ovaries and testes). -Germs cells are diploid, with 2 sets of chromosomes -Germ cells undergo meiosis to form haploid gametes
Significance and functions of meiosis (Sexual reproduction)
-Meiosis produces haploid gametes like sperm for sexual reproduction -Haploid gametes are a result of separation of homologous chromosomes during anaphase I -During Fertilisation, the nuclei of male and female gametes fuse and produce a zygote with a diploid number of chromosomes, restoring the diploid condition -If meiosis did not occur, fusion of male and female gametes would result in a doubling of the number of chromosomes for each successive sexually reproduced generation
Significance and functions of mitosis (1.Maintaining Genetic stability)
-Mitosis produces two daughter nuclei which are genetically identical to the parent -The 2 daughter nuclei have Sam number and type of chromosomes as the parent nucleus due to DNA replication in S phase -This results in genetic stability within populations of cells
Mitosis Telophase stage
-Nuclear envelopes reform to form two nuclei. • Nucleolus reappears in each nucleus. • (Unduplicated) Chromosomes become decondensed to form back chromatin. • Spindle fibres break down (or: depolymerise).
Cytokinesis
-Occurs after mitosis, dividing the cytoplasm between the two daughter cells. - In animal cells, 1. Cell surface membrane invaginates towards the metaphase plate 2.The cleavage furrow deepens until the parent cell is pinched into two, producing two daughter cells -In plant cells, 1. Golgi vesicles that contain cell wall material ( cellulose ) move towards the metaphase plate and fuse together to form a cell plate 2. Cell plate enlarges as more Golgi vesicles fuse with it, until its surrounding membrane fuses with cell surface membrane along the perimeter of the cell 3. A new cell wall is formed between the two daughter cells
M phase
-Represents two types of nuclear division: 1. Mitosis for all somatic cells and, 2. Meiosis in sexual reproductive organs to form gametes/sex cells
Mitosis Metaphase stage
-Spindle fibres from centrosome attach to kinetochore (proteins) at the centromere of each sister chromatid of chromosome. - Centromeres of chromosomes are aligned along the metaphase plate (an imaginary plane equidistant between the two poles of the cell) by spindle fibres.
Meiosis
-The nuclear division where a cell nucleus divides to produce 4 genetically non-identical daughter nuclei, each containing half the number of chromosomes as the parent nucleus. -Also known as a reductive division - vital for sexual reproduction and takes place in the reproductive organs of plants and animals. -Prior to meiosis, interphase compromising of G1, S and G2 takes place
Cell division in sexual reproducing organisms
-Undergo sexual reproduction -Meiosis results in formation of haploid gametes (sperm and egg) - Fertilisation of sperm and egg give rise to diploid zygote - Zygote undergoes multiple rounds of mitosis as it develops -After complete growth of organism, mitosis continues to function in renewal and repair of tissue by replacing cells that die from normal wear and tear or accidents.
Haploid cells
-contains a single set of chromosomes -haploid number of chromosomes(n) -23 different chromosomes have 22 autosomes (non-sex cells) and a single sex chromosome -Human egg contains 22 autosomes and an X chromosome -Human sperm contains 22 autosomes and either an X or Y chromosome
Diploid cells
-contains two sets of chromosomes (2n) where n represents the number if chromosomes in a single set -Number of chromosomes in somatic cells ( ie. non-sex cells) are fixed for a species -Each sexually reproducing species has a characteristic diploid number and haploid number -Human: 46 chromosomes, diploid number is 46(2n) -Can be clearly seen in a karyotype, ordered display of chromosomes
Significance and functions of mitosis (2. Growth )
-the number of cells within an organism increases by mitosis and new cells are identical to existing cells -mitosis in cell division is the basis of growth in multi-cellular organisms
Stages of Meiosis II (Anaphase II)
1. Centromere divides and separates 2. Genetically unidentical non-sister chromatids separate and migrate to opposite poles as spindle fibers shorten, each becoming a full-fledged chromosome - Each daughter chromosome is an unduplicated chromosome( ie, contains a single condensed DNA molecule)
Existence of 2 forms of DNA in the cell cycle
1. Chromatin 2.Chromosomes , depending on the stage of cell cycle
Stages of Meiosis I (Prophase I)
1. Chromatin condense and shorten to become chromosomes 2. Each chromosome is made up of two genetically identical sister chromatids joined at their centromeres 3. Nucleolus disappears 4. Nuclear envelope disintegrates 5. Centrosome/centrioles organism microtubules into spindle fibers 6. Centrosomes/centrioles migrate to opposite poles of the cell by lengthening of spindle fibers 7. Homologous chromosomes pair up to form bivalents 8. Crossing over occurs between non-sister chromatids of homologous chromosomes resulting in chiasma (plural: chiasmata) formation 9. Crossing over refers tot the exchange of genetic material between non-sister chromatids of homologous chromosomes
Different ways of Genetic variation in Meiosis
1. Crossing Over during Prophase I : -crossing over between non-sister chromatids of homologous chromosomes occur during prophase I -leads to new combinations of alleles on the chromosomes of the gametes 2. Independent Assortment of Chromosomes in metaphase I -the arrangement and separation of homologous chromosome is independent of another during metaphase I and anaphase I -Results in the arrangement and separation of sister chromatids is independent of another sister chromatids during metaphase II and anaphase II -Leads to variety of allele combination in the gametes 3. Random fertilization: -No. of possible gamete combinations (2^n) -Due to independent assortment of chromosome at metaphase I -E.G. Humans = 2^23 = 8 million possible gametes -Possible zygotes ( assuming no crossing over) = 2^n X 2^n = 8 mil X 8 mil = 64 trillion -Random fusion results in genetic variation in zygote
What happens after Telophase II of Meiosis II?
1. Cytokinesis occurs, cells divide through cleavage/cell way formation to give a total of four daughter cells per parent cell 2. Each daughter cell has half the number of chromosomes of the original diploid parent cell 3. As each gamete contains half the number of chromosomes as the parent cell, this allows 2 gametes to fuse together to produce a new diploid organism 4. Since each gamete carries different combination of genes, fusion of the 2 gametes will then result in an offspring with new genetic variation
Stages of Meiosis II (Prophase II)
1. DNA coils and condense into chromosomes with two non-sister chromatids joined at their centromeres 2. If crossing over had occurred at prophase I, the two sister chromatids are non-identical 3. Nucleolus disappears 4. Nuclear envelope disintegrates 5. Centrosome/centrioles organism microtubules into simple fibers 6. Centrosomes/ centrioles migrate to opposite poles of the cell by lengthening of spindle fibers
Independent assortment of homologous chromosomes in meiosis I
1. During prophase I: Bivalents are aligned where the maternal homologous and paternal homologous faces each side of the pole 2. At metaphase I: The two pairs of homologous chromosomes randomly orientate themselves along the metaphase plate. Thus, the first arrangement is equally as likely as the second arrangement 3. During anaphase I: Homologous chromosomes separate independently of other bivalents - Only 2 of 4 combinations of daughter cells shown would result from meiosis of a single diploid cell - A single parent cell would have 1 or the other possible chromosomal arrangement at metaphase I, but not both - Population of daughter cells resulting from meiosis of a large number of diploid cells contains all 4 types of gametes in approximately equal numbers - Number of possible chromosomal combinations in a gamete is 2^n, n=haploid number of the organism - In humans, no. of possible chromosomal combination is 2^23, thus 2^23 different combinations of gametes can be produced by an individual
Stages of Meiosis I (Anaphase I)
1. Homologous chromosomes separate 2. Centromeres in chromosome remains together, hence sister chromatids remain joined at their centromeres as duplicated chromosomes 3.Homologous chromosomes migrate toward opposite poles of the cell, with the centromere leading the way as spindle fibers shorten
Stages of the Cell cycle
1. Interphase - cell growth and synthesis of cell components: -G1 and G2 phase involve synthesis of organelles and other cell material -S phase involves DNA replication -M phase: Nuclear division - nucleus divides to form two nuclei: -Nucleus of parent cell divides once to produce two genetically identical daughter nuclei with the same number and types of chromosomes as parent nucleus -Cytokinesis - cytoplasm divides to form two daughter cells
Significance and functions of mitosis
1. Maintaining Genetic stability 2. Growth 3. Repair tissues or organs (Cell Replacement and Regeneration) 4.Asexual reproduction
2 Successive nuclear divisions of Meiosis
1. Meiosis I involving separation of homologous chromosomes 2. Meiosis II involving separation of identical/non-identical sister chromatids
Stages of Meiosis II (Telophase II)
1. Nuclear envelopes reform 2. Nucleolus reappears in each nucleus 3. (Unduplicated) Chromosomes become decondensed to form back chromatin fibers 4. Spindle fibers break down
Stages of Meiosis I (Telophase I)
1. Nuclear envelopes reforms, forming two nuclei 2. Nucleolus reappears in each nucleus 3. (Duplicated) Chromosomes become decondensed to form back DNA 4. Spindle fibers break down/ depolymerise 5. Each nucleus has a haploid set of duplicated chromosomes
What happens in meiosis I?
1. Reduction division occurs as the number of chromosome sets per cell is already halved here 2. Cytokinesis could/might occur after Telophase I, forming two haploid daughter cells 3. No replication of DNA occurs between meiosis I and meiosis II
Significance and functions of meiosis
1. Sexual Reproduction 2.Genetic Variation -Crossing over during Prophase I -Independent Assortment of Chromosomes in metaphase I -Random fertilization
Stages of Meiosis II (Metaphase II)
1. Spindle fibers from centrosome attach to kinetochore (proteins) at the centromeres of each non-sister chromatids of each chromosome 2. Centromeres of chromosomes are aligned along the metaphase plate by spindle fibers
Two features of mitosis that ensure that the chromosome constitution is preserved from one generation of cells to another:
1. The chromosomes of the parental nucleus replicate during S phase before mitosis begins 2. The arrangement of the chromosomes on the spindle during metaphase ensures that hate chromosomes are shared equally between the two daughter nuclei
Stages of Meiosis I (Metaphase I)
1. The pairs of homologous chromosomes align themselves at the metaphase plate, with a chromosome of each pair facing each pole. 2. Independent assortment of homologous chromosomes occurs (ie. The arrangement of chromosomes of each bivalent is independent of the arrangement of the other bivalents) 3. Each one homologous is attached to spindle fibers from one of the poles of the cell; the other homologous are attached to spindle fibers from the opposite pole
Cell division in unicellular organisms
Bacteria and amoeba for example, undergoes aesexual reproduction known as binary fission to reproduce
Diploid and number of chromosomes in a gamete of a Human
Diploid number: 46 Number of chromosomes in a gamete: 23
Diploid and number of chromosomes in a gamete of a Dog
Diploid number: 78 Number of chromosomes in a gamete: 39
Diploid and number of chromosomes in a gamete of a Fruit Fly
Diploid number: 8 Number of chromosomes in a gamete: 4
Fertilisation definition
Fusion of nucleus from sperm and nucleus from egg
Genetic locus
Location of a particular gene on a chromosome - at each genetic locus, an individual has two alleles, one on each homologous chromosome
Difference between Mitosis and Meiosis (Feature: Number of daughter cells)
Mitosis: 1. 2 genetically identical daughter cells 2. Genetically identical to parent 3. Same number of chromosomes as parent cell Meiosis: 1. 4 genetically non-identical daughter cells 2. Genetically different from parent cell 3. Half as many chromosomes as parent cell
Difference between Mitosis and Meiosis (Feature: Significance)
Mitosis: 1. Basis of asexual reproduction 2. Maintain genetic stability 3. Growth of multicellular organism 4. Repair Meiosis: 1. Creates genetic variation via; crossing over during prophase I, Independent assortment of chromosome in metaphase I 2. To produce gametes
Difference between Mitosis and Meiosis (Feature: Anaphase)
Mitosis: 1. Centromeres split and sister chromatids move to opposite Meiosis: Anaphase I; A. Centromeres remain joined B. Homologous chromosomes separate Anaphase II; A. Centromeres split and non-sister chromatids move to opposite poles
Difference between Mitosis and Meiosis (Feature: Metaphase)
Mitosis: 1. Chromosome forms a single row at metaphase plate/equator 2. Centromere of each chromosome is attached to spindle fibers extending from both poles of the cell Meiosis: Metaphase I; A. Homologous chromosome forms two rows at metaphase plate/equator B. Centromere of each chromosome is attached to spindle fibers extending from one poles of the cell Metaphase II; A. Chromosome forms a single row at metaphase plate/equator
Difference between Mitosis and Meiosis (Feature: Prophase )
Mitosis: 1. No synapsids - homologous chromosomes do not pair up 2. No crossing over/ no formation of chiasma Meiosis: Prophase I; A. Synapsis occurs during prophase I - homologous chromosomes pair up to form bivalent B. Crossing over/formation of chiasma during prophase I Prophase II; A. No synapsis - homologous chromosomes do not pair up B. No crossing over/ no formation of chiasma
Difference between Mitosis and Meiosis (Feature: Location)
Mitosis: In somatic cells Meiosis: In germ cells (precursor of gametes) found in reproductive organs
Difference between Mitosis and Meiosis (Feature: Number of nuclear divisions)
Mitosis: Once Meiosis: Two successive nuclear division (meiosis I and meiosis II)
Difference between Mitosis and Meiosis (Feature: Dividing cell)
Mitosis: can be diploid or haploid Meiosis: diploid
Where do Mitosis occur?
Occurs in all somatic cells in humans and plants
4 phases Mitosis can be divided into:
Prophase, Metaphase, Anaphase, and Telophase
Definition of cell cycle
Refers to the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells.
Definition of cell division
Separation of a cell into daughter cells
What is responsible for carrying out cell and nuclear division?
The nucleus
Definition of Mitosis
The process by which a cell nucleus divides to produce 2 daughter nuclei each containing identical sets of chromosomes to the parent nucleus
